School Science Lessons
2024-05-25
Magnetism
(UNPh29)
Table of contents
29.1.0 Electromagnetic field, EM Field
29.2.0 Force on current-carrying wire
29.3.0 Magnetic fields and forces
29.4.0 Magnetization
29.5.0 Solenoids
29.1.0 Electromagnetic field, EM Field
A "field" is a property or a condition of space.
The field concept was introduced into physics in 1845 by Michael Faraday as an explanation for electric and magnetic forces.
His experiment with iron filings that align themselves in the region around a magnet forming "lines of force" about a magnet led Faraday to believe that there must be a field present in the space around the magnet.
However, the idea that fields can exist by themselves as properties of space was not accepted by physicists, who invented an invisible substance called "ether" to carry the EM oscillations.
Although belief in the ether prevailed for decades, but when no evidence for its existence could be found, despite many attempts, the ether was finally abandoned and physicists accepted that the EM field has an existence in itself.
29.2.0 Force on current-carrying wire
29.2.2.01 Magnetic force on current-carrying wire
29.2.7.10 Force between current-carrying wires
Experiments
29.2.6.13 Ampère's motor, Ampère's frame
29.2.6.9 Barlow's wheel
29.2.6.7 Current balance
29.2.6.2 Interacting coils
29.2.6.11 Magnetic grapevine
29.2.6.8 Maxwell's rule
29.2.6.1 Parallel conductors
Experiments
29.2.4.6 Ampère's ants
29.2.2.10 Demountable Helmholtz coils
29.2.7.6 Force on rectangular current loop
29.2.7.1 Galvanometer coil and magnet
29.2.7.3 Interacting coils
29.2.7.4 Interacting solenoids
29.2.4.1 Interaction of magnet and magnetizing coil
29.2.2.7 Iron filings and a solenoid
29.2.2.8 Length of a solenoid
29.2.4.3 Jumping magnet
29.4.69 Magnetizing coil
29.2.2.9 Small coils in a solenoid
29.4.0 Magnetic fields and forces
29.2.7.8 Ampère's law, permeability of free space, µ0 = 4π × 10-7henry/metre
29.2.2.6 Biot-Savart law, Ampère's law, Ampère-Laplace law
Ferrites
29.2.5.02 Lorentz force law
29.1.1.7 Magnetic force and magnetic field lines (lines of force)
29.2.7.11 Magnetic field of current loop, B = Nµ0I/2r
29.2.7.12 Magnetic field of solenoid, B = mu 0(N/L)I
29.2.2.11 Magnetic field of a toroid, B = mu 0NI/2πr
29.5.1.02 Magnetic field of current loop, Earth's magnetic field
29.2.5.01 Magnetic field, tesla
29.2.7.9 Magnetic field of current-carrying long straight wire, B = mu 0I/2πrT
Experiments
29.2.5.3 Bending of an electron beam
29.2.5.1 Cathode ray tube, CRT
29.2.5.4 Crookes tube
29.2.5.5 CRT and earth's magnetic field
29.2.5.2 e / m for electrons, measurement of e / m
29.2.7.7 Direction of a magnetic field caused by a current-carrying wire
29.5.4 Direction of magnetic field using iron filings
29.5.3 Direction of magnetic field using plotting compass
29.5.5 Forces between conductors carrying a current in same direction
29.5.6 Forces between conductors carrying a current in opposite directions
29.5.7 Forces between permanent magnet and conductor carrying a current
29.2.1.4 Hysteresis
29.1.2.2 Induced magnetic poles, plotting compass
29.2.3.4 Inverse square law, inverse fourth power, inverse seventh power
29.2.2.12 Iron filings on overhead projector show magnetic field
29.4.68 Magnetic dip, north magnetic pole, south magnetic pole
29.4.81 Magnetic field of electric current in a wire
29.4.82 Magnetic field of open coil, open solenoid
29.2.2.13 Magnetic fields around bar magnet, magnetic field flies (lines of force)
29.4.76 Magnetic fields in two dimensions
29.4.77 Magnetic fields in three dimensions
29.2.5.0 Magnetic force on a charged particle moving in a magnetic field
29.2.5.6 Magnetic forces on an electron beam, magnetic deflection of cathode rays
29.2.3.2 Magnetic levitation, refrigerator door magnet, Halbach array, Maglev train
29.2.5.8 Magnetic pump, ion motor force on conducting field
29.2.2.5 Open right-hand rule, force on charges moving through magnetic field
29.4.74 Magnetic poles and pin chains
29.1.1.6 Magnetic poles, magnetize wire, freely suspended magnets
29.1.2.11 Magnetism along a bar magnet, pole to pole
29.2.1.2 Oersted effect, magnetic field around a current-carrying wire
29.4.2.4 Substances magnetic fields can pass through
29.4.0 Magnetization
See: Magnets (Commercial).
29.1.2.1 Magnetization, magnetic moment
29.1.2.9 Barkhausen effect
29.1.2.8 Permalloy bar
Experiments
29.1.2.4 Demagnetize a specimen
29.2.4.5 Float magnetized needles
29.1.2.3 Hammer iron bar, magnetization in the Earth's field
29.1.2.2 Induced magnetic poles, magnetic induction
29.2.4.1 Interaction of magnet and magnetizing coil
29.1.1.6 Magnetic poles, magnetize wire, freely suspended magnets
29.1.2.11 Magnetism along a bar magnet, pole to pole
29.1.2.10 Magnetizing by touch
29.1.1.4 Magnetized test
4.69 Magnetizing coil
29.2.1.5 Magnetometer, measure magnetization, strength of magnetic field
29.5.0 Solenoids
29.2.7.4 Interacting solenoids
29.2.2.7 Iron filings and a solenoid
29.2.2.8 Length of a solenoid
29.2.2.9 Small coils in a solenoid
29.2.3.0 Magnetic
See: Magnets (Commercial).
Magnetic Accelerator, Gaussian rail gun, conservation of energy and momentum, (toy product)
Experiment
29.4.68 Magnetic dip, north magnetic pole, south magnetic pole
29.4.73 Magnetic substances
29.4.2.4 Substances magnetic fields can pass through
See diagram 4.2.4: Substances magnetic fields can pass through.
Experiment
Collect some thin and small things in different materials such as pieces of wood, pieces of metal, slice of plastic, paper, glass, iron sheet, piece of cloth and sponge.
Can they stop the magnetic field or can the magnetic field go through them?
Hypothesis: They all allow the magnetic field go through.
Design a experiment to verify the hypothesis.
Set up the magnet and paper clip tied to the thread and stone in a proper place on the table.
The clip attracted by the magnet maintains a distance from the magnet due to tying to the thread so there is a magnetic field between the magnet and clip.
Insert the materials you have prepared between the magnet and clip in turn.
If the clip falls, the material there stops the magnetic field.
29.1.1.4 Magnetized test
See diagram 29.1.1.4: Like and unlike poles of a bar magnet.
Experiment
1. Two bars look alike, but one is a magnet and the other is not a magnet.
With two similar bars of iron one magnetized, use the end of one to lift the middle of the other.
2. Many iron and steel objects are magnetized without you knowing it.
Detect this magnetism with a compass.
If a rod is magnetized, it must, like the compass needle, have a north pole and a south pole.
The rule of magnets is that two unlike poles attract and two like poles repel.
So one pole of the needle will be attracted to the end of the rod and the other repelled.
If the rod is not magnetized, both poles of the needle are weakly attracted to the end.
Collect objects made of paper, wax, brass, zinc, iron, steel, glass, cork, rubber, aluminium, copper, gold, silver, wood, tin.
Test each object with a magnet to see which are attracted to the magnet and which are not.
Bring a soft iron wire and hard steel or piano wire near a compass needle to see if it is affected by a magnetic field.
3. Bring the north poles of 2 bar magnets near each other.
Feel the force of repulsion between them.
Reverse one magnet and feel the force of attraction.
Reverse the other magnet and feel the force of repulsion when the 2 north poles come together.
29.1.1.6 Magnetic poles, magnetize wire, freely suspended magnets
Magnets always have two magnetic poles.
If a bar magnet is broken in two each half has two poles.
Magnetism is the strongest at the poles of a magnet.
Use a bar magnet, a horseshoe-shaped magnet, a magnetized needle and other magnetized objects.
Immerse them in fine iron filings then take them out.
Note that most filings are at the poles.
Scatter iron filings or iron powder over every part of a magnet and note that some filings slip off the magnet and some filings are attracted to its poles.
Experiment
1. Magnetize a piece of iron wire or a needle by rubbing with a bar magnet.
Find its poles with iron filings.
Cut into two the magnetized iron wire or needle with pliers then test it with filings again.
Each piece still has two poles.
Cut each piece into two parts again then test them with filings.
Each small piece has two poles.
No matter how short the remaining wire is, it has two poles.
Outside magnetic effects can cause the domains in a ferromagnetic material to act strongly in one direction to magnetize the substance temporarily in soft iron and permanently in hard steel.
Soft iron is used in electromagnets so the magnetism can be "turned on" and "turned off".
Permanent magnetism is necessary for compasses and permanent magnets, but heating or hammering can destroy it.
2. Use a 6 cm length of steel wire or piano wire.
Draw one end of a steel magnet along it once only and in one direction from end to end.
Lay the wire on a piece of paper then test for magnetism by sprinkling iron filings over it.
The iron filings are not attracted equally along its whole length.
They call the areas of strongest attraction the "magnetic poles".
3. To isolate a magnetic pole pass a long magnetized knitting needle through a cork and float it on water.
4. Make a freely suspended magnet.
Use loops of cotton to suspend two magnets freely.
Bring each pole of the two magnets close to, but not touching, each other.
Show that like poles repel and unlike poles attract.
29.1.1.7 Magnetic force and magnetic field lines (lines of force)
See diagram 29.1.1.7: Magnetic field lines (lines of force).
Experiment
Place a glass plate over a bar magnet and sprinkle iron filings over the glass.
Tap the glass plate a few times to overcome friction and the iron filings arrange themselves in a pattern of arcs
from one pole of the magnet to the other.
This pattern is a map of the unseen magnetic force that permeates space.
Magnetic fields show the effect a magnet would have in that region.
Magnetic fields are usually represented by drawing magnetic field lines in the same way that electric field may be shown.
The closer the magnetic field lines the stronger the magnetic field.
Magnetic field lines point away from the north poles towards the south poles in closed loops that never cross over.
29.2.4.5 Float magnetized needles
See: Magnetic fields, magnetic "levitation stands", (Commercial).
See diagram 29.1.1.8: Float magnetized needles.
Experiment
1. Stroke pins with the north pole of a bar magnet.
Very carefully lower the pins into water so that they float.
Note how they line up end to end.
Move the pins to make circles, north to south poles.
2. Stroke three pins many times with the north pole end of a magnet in the same direction so that their points attract each other.
Put each pin in a little paper boat made of greaseproof paper.
Put the boats in a dish of water.
The boats will line up end to end in a north south direction.
3. Rub eight needles on one pole of a magnet to magnetize them and make the sharp end of them being the same pole.
Push each needle through a cork leaving only one cm length in the cork.
Float the magnetized needles on the surface of water in a plastic bowl.
Put one pole of a strong magnet above the floating magnet needles and the floating needles will change their positions to form a certain picture.
Increase or decrease the numbers of the magnet needles, change the poles of the magnet needles, change the distance from the pole to magnet needles and observe if the shape of the picture changes.
29.1.2.1 Magnetization, magnetic moment
Magnetic moment is the measure of magnetic strength.
It is the torque on a magnet placed at right angles to a magnetic field.
The magnetic moment is a vector.
As the magnetic moments of molecular current inside matter are in the same direction, the matter shows a character that can attract the things made by iron,
cobalt, nickel, and metallic oxides called magnetism.
29.1.2.2 Induced magnetic poles, magnetic induction
See diagram 29.1.2.2: Induced magnetism.
Experiment
1. A chain of nails is supported by a magnet each becoming a magnet by induction.
A soft iron bar held collinear with a permanent magnet will become magnetized by induction.
Use a compass needle to show the far pole of the bar is the same as the near pole of the magnet.
2. Induced magnetism with iron bar on block of wood
Put an iron bar on a block of wood.
Hold an iron nail near one of its ends vertically.
The nail can drop down when you release it showing that the iron bar has not magnetized.
Hold a strong magnet near the other end of the iron bar.
The nail does not drop when you release it, showing that the magnetic induction from the magnet has magnetized the iron bar.
Remove the magnet and check if the iron bar is still magnetized by dropping the nail again.
3. Put a compass on the table.
Hold an iron nail 15 cm in length with its sharp end near the north pole of the compass.
Bring the north pole of a bar magnet near the other end of the iron nail, but do not let them touch each other.
Observe if the north pole of the compass moves.
Remove the magnet after the compass points to the direction of north again.
Bring the south pole of the magnet near the other end of the nail.
Observe how the compass moves.
29.1.2.3 Hammer iron bar, magnetization in the earth's field
See diagram 32.163.2: Plotting compass in a matchbox.
Experiment
1. Hammer the end of a soft iron bar in the earth's magnetic field.
Pound a soft iron bar held in the earth's field.
A permalloy bar does not need to be pounded.
Hammer a soft iron bar held parallel to the field of the earth.
A bar of permalloy is magnetized by simply holding it in the earth's field.
2. Temporary magnetism lasts only if the external source of magnetism lasts.
However, even permanent magnetism can be lost by hammering or heating.
3. Hold a soft iron bar pointing to the north and sloping downwards with the lower end against a thick piece of plastic.
Hammer it down into the plastic.
Lay the iron bar on the plastic, put a piece of paper over it and sprinkle iron filings on the paper.
The iron filings move into a pattern showing that the iron bar has become slightly magnetic.
4. Hold a plotting compass near the iron bar and notice any movement of the compass needle.
When you hammer the iron, some of its particles line up with the earth's
magnetic field lines so that they point to the north.
29.1.2.4 Demagnetise a specimen
Experiment
1. Place an iron core in a solenoid.
Magnetize with direct current and demagnetize by reducing alternating current to zero.
2. Magnetize iron by contact and demagnetization.
Stroke a nail on a permanent magnet and it will pick up iron filings.
Magnetize an iron bar in a solenoid then pound it to demagnetize.
Stroke a steel needle with a permanent magnet to magnetize it and pass it through an AC solenoid to demagnetize it.
3. Demagnetize a steel knitting needle, using a solenoid carrying an alternating current or by heating the specimen to dull red heat along its whole length and plunging it into cold water.
Fix the specimen under a brass drawing pin stuck into the bench.
Stroke the specimen ten times in the same direction with one pole of a permanent bar magnet with marked poles.
Note the pole used and mark the end of the specimen where the pole first meets the specimen.
Tests for magnetism in the specimen using iron filings.
Find the polarity of the specimen with a plotting compass.
29.1.2.8 Permalloy bar
Permalloy is an alloy with magnetic high permeability (easily forms internal magnetic fields), low hysteresis loss (not easily becomes demagnetized), near zero magnetostriction (not much change in shape during magnetization).
It contains 78.5% Ni and 21.5% Fe + possibly other elements, e.g. Cu, Cr, Co, which affect the resistivity.
Permalloys are used in magnetic shields and computer memory chips.
Iron filings stick to a permalloy bar held parallel to the earth's magnetic field, but fall off when it is held perpendicular.
A small strip of iron sticks to a permalloy rod when it is held in the direction of the Earth's field.
29.1.2.9 Barkhausen effect
The Barkhausen effect is observed when the steady increase in a magnetizing flux produces jumps in the magnetization of ferromagnetic materials.
Magnetic domains in the core of a small coil can be heard flipping as a magnet is moved by using and an audio amplifier.
29.1.2.10 Magnetizing by touch
See diagram: 29.4.1: Magnetizing by touch.
Experiment
1. Fix the specimen under a brass drawing pin stuck into the bench.
Stroke the specimen by using opposite poles of two permanent bar magnets with marked poles.
Stroke the specimen ten times.
Note the poles used and mark the end where the pole first meets the specimen.
Test for magnetism in the specimen using iron filings.
Find the polarity of the specimen with a plotting compass.
2. Adjust the sliding contact of the rheostat to half the resistance.
Put the specimen in the solenoid.
Close the switch.
Note the direction of flow of the current, from positive through the circuit to negative.
Note the direction of winding of the solenoid.
Open the key.
Remove the specimen from in the solenoid.
Mark it to show its position in the solenoid.
Test for magnetism in the specimen using iron filings.
Find the polarity of the specimen with a plotting compass.
29.1.2.11 Magnetism along a bar magnet, pole to pole
See diagram 29.1.2.11: Variation of magnetism along a bar magnet.
Experiment
Place a bar magnet on a piece of squared paper.
Tie a soft iron nail to the hook of a spring balance.
Let the bar magnet attract the nail then try to pull the nail off the magnet.
Record the needed pulling force.
Start the experiment from one pole of the bar magnet and test every 2.5 cm.
Show the readings of the spring balance on a graph.
Let the distance at the first end be zero, graph the distance on the horizontal axis, the needed pulling force on the vertical axis.
Draw a graph to show the distribution of the magnetism in a bar magnet.
Magnetism is strongest at the magnetic poles.
29.2.1.3 Current through an electrolyte
Experiment
Use a compass needle to detect the magnetic field from 2 amps flowing in an electrolyte.
Detect a magnetic field produced current in copper electrolyte and a gas discharge tube with a large compass needle.
29.2.1.4 Hysteresis
See diagram 29.2.1.4: Hysteresis.
Hysteresis is the lag of an effect behind the cause of the effect.
A hysteresis loop is a closed figure obtained by plotting magnetic flux density, B, against magnetizing field, H, when H increases and decreases.
The area of the loop measures the energy lost during magnetization.
Ferromagnets can retain a memory of an applied field once it is removed.
Hysteresis loops for laminated steel and ferrite cores as saturation is reached can be displayed on an oscilloscope.
The hysteresis loop for the iron core of a transformer is shown on an oscilloscope.
Parallel iron bars suspended in a coil show hysteresis when slowly magnetized.
Water can be boiled by magnetic hysteresis waste heat.
Hysteresis depends on grain size, domain state, stresses, and temperature.
Hysteresis parameters are useful for magnetic grain sizing of samples.
29.2.1.5 Magnetometer, measure magnetization, strength of magnetic field
See diagram 29.1.3: Sensitive magnetometer.
Experiment
Use a large test-tube and a rubber stopper.
Push a copper wire through the centre of the stopper.
Use a small magnetic needle that can fit into the large test-tube and spin freely in the test-tube.
Punch a hole exactly at the centre of the magnetic needle, originally the point of support, with a high speed drill.
The size of the hole is to just allow the copper wire to go through it.
After pushing the copper wire through the hole, weld a copper tail surface in a triangular shape on one end of the copper wire.
The welding point should be at centre of one side of the triangular and the copper wire is vertical to this side.
Drop a soldering tin on copper wire, between the tail surface and magnetic needle and two to three cm from the upper of the tail surface to support the magnetic needle.
Then hang the copper wire, in the case of ensuring the magnetic needle being in a horizontal state paste the place that the copper wire going through until it is dry.
Now use a small slice of mirror, tape the copper wire on it's back along it's long axis.
The position of the mirror is two to three cm above the magnetic needle.
The action of the mirror is to reflect the beams of light.
Pour about three cm depth of oil into the test tube as a damping.
Put the copper wire that hang mirror, magnetic needle and tail surface into the test-tube.
Adjust the length of the copper wire from centre of the stopper until the tail surface goes just into the oil after cover the stopper on the test-tube.
Adjust the depth of the tail surface in oil, that is adjusting the sensitivity of the apparatus.
Fix and seal the copper wire by dropping wax into the hole on top of the stopper.
The whole apparatus can be fixed on a wooden bottom.
Make an incident light by a torch to the mirror, then hold a magnet near the magnetic needle from the test-tube.
Even a small turning around of magnetic needle can make the reflected light ray from the mirror to have a larger moving from the original position.
29.2.1.9 Magnetic field and area of contact
If one end of a magnet 1 cm in diameter is reduced to 0.5 cm in diameter, the small end lifts a much larger piece of iron than the large end.
An electromagnet supports less weight when the face of the ring is against the pole than when the curved edge is against the pole.
A soft iron truncated cone will support less weight when the large end is in contact with the face of an electromagnet.
29.2.1.10 Magnetic field strength and gap
Experiment
Vary the gap of a magnet and measure the field with a gauss meter (Name in USA term for instrument that measures magnetic flux density.)
29.2.1.11 Magnetic field strength shunt
Experiment
Pick up a steel ball with a bar magnet then slide a soft iron bar along the magnet towards the ball until it drops off.
29.2.1.12 Magnetic field shielding, magnetic screening
Slide sheets of copper aluminium and iron between an electromagnet and an acrylic sheet separating nails from the magnet.
Experiment
Displace a hanging soft iron bar by attraction to a magnet then interpose a sheet of iron.
Use a test magnet to show the shielding properties of a soft iron tube with various magnetic field generators.
Hold a magnet above a nail attached to the table by a string then interpose a sheet of iron.
Two horizontal sheets of glass separated by and air space intervene between an electromagnet and collection of nails being held up.
Insert a sheet of iron into the space and the nails drop.
29.2.1.13 Magnetic moments of two bar magnets using a deflection magnetometer (null method)
See diagram 29.4.5: Deflection magnetometer.
Experiment
Find the magnetic lengths of two bar magnets 2L1 and 2L1, using a plotting compass.
Place the magnetometer so that the pointer indicates 0o at one end and the arms lie magnetic east-west.
Place magnet 1 at a distance d, to give a deflection of 30o.
Adjust the position of magnet 2 in the other arm of the magnetometer so that there is no deflection of the pivoted magnet P.
Record d1 and d2.Repeat the experiment twice, with d1 so that magnet 1 alone gives initial deflections of 45o and 60o and record d1 and d2.
When there is no deflection of the pivoted magnet P, the magnetic fields at P from the two magnets, must be equal and opposite.
Taking each pair of readings of d1, and d2, calculate M1 / M2 each of the three cases from the formula above.
Calculate the mean value M1 / M2.
If the magnets are short and powerful so that L is small compared with d, then L2 is negligible compared with d2.
29.2.1.14 Vibrator with a magnet
See diagram: 29.3.7: Needle stands vertically.
Experiment
Place an U-shape magnet with one pole up, the other pole down, at the edge of the table.
Put a needle or a razor blade on the pole that is down.
The needle or razor blade will stand vertically between the two poles.
Beat the needle in the centre of the magnetic field slightly by a pencil at right angles to the magnetic lines of force.
Note how the needle moves.
Move the needle up or down, i.e. change the length of the needle in the magnetic field.
Repeat the experiment, observe the variations of the vibrating frequency of the needle.
29.2.1.15 Substances magnetic lines of force can pass through
Experiment
Put a bar magnet on the table and cover with a piece of paper.
Put different substances, e.g. wood, glass, copper, zinc, cardboard, paper, plastic, iron, aluminium, on the paper over the bar magnet.
Put iron filings on a piece of stiff white paper.
Hold the paper over the substances and tap the paper from the side until some pattern forms.
You can distinguish which substances can allow magnetic lines of force to pass through them by observing the pattern of iron filings on the paper.
A magnetic field acts though all these materials except iron.
29.2.2.0 Magnetic force on moving charge, F = qvB sin θ newton, N
A magnetic field exerts a force on a particle only if the particle has an electrical charge and is moving.
However, the particle's charge and speed are not sufficient when calculating the magnetic force on the particle, because the relative directions of the particle's motion and magnetic field must also be known.
Magnitude of the magnetic force, F = qvB sin θ newton, N.
Assume a magnetic field B--> has direction from left to right in the plane of the screen, and assume a particle charge q moves with velocity V-->.
Let the angle between V--> and B--> = θ (theta).
The force, F--> on this charged particle = qvBsin θ (theta).
The force, F--> is maximum when velocity V--> is at right angles to B--> because θ (theta) = 90o, so sin θ (theta) = 1.
The force, F--> is minimum (0), when velocity V--> is in same direction as B-->, because θ (theta) = 0o, so sin θ (theta) = 0.
The charged particle experiences no force so moves with constant velocity.
The force, F--> is between minimum (0), and maximum when velocity V--> is at angle θ theta with B-->, so sin θ (theta) = between 0 and 1 (0 ≤ θ (theta) ≤ 180o), because F--> q vBsin θ (theta).
29.2.2.01 Magnetic force exerted on a current-carrying wire
The movement of electric charges along a wire causes electric current.
So a current-carrying wire will experience magnetic forces in the same way as the moving charges in the wire.
The magnetic force on a current-carrying wire is at right angles to both the direction of the wire and the direction of the magnetic field.
The direction of the force is given by the open right-hand rule.
From magnetic force on moving charge, previous paragraph, F = q vB sin θ.
In the equation, divide q by t = q/t, and multiply v by t = (vt).
Now F = q/t(vt)B sin θ - the equation has not changed!
But q/t is charge passing in unit time = current, I.
Also (vt) = distance or length of wire travelled by the charge = length L.
So F = ILB sin θ newton, N.
Magnetic force, F = ILB sin θ newton, N, where I = current, L = length of the conductor, e.g. wire, B = magnetic field, θ = angle between the direction of the conductor and the direction of the magnetic field.
Maximum force is when direction of current is at right angles to direction of magnetic field (θ = 90o, sin θ = 1).
So F = ILB.
Minimum force, is when direction of current is in same direction as magnetic field (θ = 0o, sin θ = 0).
So F = 0 (no force).
29.2.7.7 Direction of a magnetic field caused by a current-carrying wire
Experiment
Use the right-hand rule:
Point the thumb along the wire in the direction of the current, I.
The fingers curl around the wire in the direction of the magnetic field.
Plotting compasses placed around a vertical long straight wire carrying a current, point along tangents to a circle with the wire as the centre of the circle.
29.2.2.5 Open right-hand rule, force on charges moving through magnetic field
See diagram 29.2.1.5: Open right-hand rule.
1. A positive charge crossing a magnetic field line experiences a force in the direction found by using the open right-hand rule:
Fingers point in the direction of the magnetic field (north to south).
Thumb points in the direction of movement of the positive charge.
Palm of the hand pushes in direction of force on the positive charge.
2. The size of the force on the positive charge depends on the product of four factors:
* the size of the charge, q,
* the velocity of the charge, v, in m / s,
* the strength of the magnetic field, B,
* the angle between the direction of movement of the positive charge and the direction of the field lines, θ
So F = q v B sin.
3. The right-hand rule shows the direction of a magnetic force by the Lorentz force law.
It shows the force on a moving positive charge and is in opposite direction for a negative charge moving in the same direction.
So the magnetic force is perpendicular to both the magnetic field and the charge velocity.
If the right hand is held open.
The extended thumb points in the direction of the conventional current, I.
The fingers point in the direction of the magnetic field, B, north to south.
The push of the palm indicates the direction of magnetic force, F.
4. Induced emf and the open right-hand rule
A conducting rod, assumed to be like a tube of positive and negative charges, is moving to the right, perpendicular to a magnetic field, B, has an induced emf across its ends.
Using the open right-hand rule:
* The thumb is pointed in the direction of movement of the rod, because positive charges will be moved in that direction, so this is the initial direction of movement of conventional current.
* The fingers point in the direction of the magnetic field B.
* The push of the palm indicates which end of the rod will become positive.
Electrons will move to the other end and make it negative.
The resulting potential difference across the ends is the induced emf in volts.
3.A positive charge moving parallel to field lines experiences no force.
Experiment
Move a compass around a vertical wire carrying a current, then reverse the current.
See diagram 30.1.3.0.1: Force on a current-carrying wire in a magnetic field.
For applications to current-carrying wires, substitute the conventional electric current direction for velocity v in diagram RHR above.
29.2.2.6 Biot-Savart law,
Ampère's law, Ampère-Laplace law
The Biot-Savart law expresses the intensity of magnetic flux density produced at a point at a distance from a current-carrying conductor.
It gives Ampère's law, Ampère-Laplace law, that expresses the force between parallel current-carrying conductors in free space.
29.2.2.7 Iron filings and a solenoid
See diagram 29.2.2.7: Solenoid and iron filings.
Experiment
1. Wind solenoid through a piece of Plexiglas.
Sprinkle iron filings on the Plexiglas.
Use iron filings on an overhead projector to show the field of a solenoid wound through a sheet of Plexiglas.
Hold down the tap switch briefly connecting the bottom while tapping the Plexiglas to encourage alignment.
2. Insert into a solenoid a glass cylinder filled with iron filings in a solution of glycerine and alcohol.
29.2.2.8 Length of a solenoid
Experiment
Construct a large solenoid to make it easy to change the spacing of turns and its length.
Use a magnetometer or coil to show field strength.
29.2.2.9 Small coils in a solenoid
Experiment
Mount an array of small coils inside a large solenoid.
Small springs keep the small coils aligned randomly when no current is applied.
29.2.2.10 Demountable Helmholtz coils
Helmholtz coils are a pair of compact identical coaxial coils separated by a distance equal to their radius.
Use Helmholtz coils to generate a large uniform magnetic field at a midway position.
29.2.2.11 Magnetic field of a toroid
A toroid is a coil in the shape of a ring, in geometry, a torus.
Iron filings show the field of a toroid, which is wound through a sheet of Plexiglas.
29.2.2.12 Iron filings on the overhead projector show magnetic field
Experiment
Sprinkle iron filings on the Plexiglas plate of an overhead projector.
Tap the Plexiglas to encourage alignment.
Use iron filings in a viscous liquid, e.g. castor oil, to show magnetic field configurations.
Sprinkle iron filings on plastic sheets that have a single wire, parallel wires and a solenoid passing through holes.
29.2.2.13 Magnetic fields around bar magnet, magnetic field flies (lines of force)
"Magnets Discovery Set" (toy product)
See diagram 29.2.1.0: Lines of force of bar magnet.
See diagram 29.2.2: Bar magnet on compasses.
See diagram 29.173.3: Magnetic field of a bar magnet.
See diagram 30.3.3: Magnetic field of a solenoid.
A magnetic field, B, exists where a charge experiences a force, because of its motion.
Detect a magnetic field by using a compass needle that aligns itself in the direction of the magnetic field at that place.
A magnetic field refers to where a magnetic force is found, i.e. magnetic flux is present.
The force found in a magnetic field has a direction at any point in the magnetic field found by putting small pieces of iron in the magnetic field.
The direction is called a line of force.
In a strong magnetic field, many lines of force are found in a very small space, the flux density is high.
Magnetic field lines, lines of force, can be drawn to show the direction a compass needle would have at any place in the magnetic field.
Magnetic field lines never cross, because at any point the compass needle can point only in one direction in space.
Assume that the direction of the magnetic field is the direction of a compassneedle, so magnetic field lines leave north poles and enter south poles.
Like magnetic poles repel, i.e. NN or SS.
Unlike poles attract, i.e. NS or SN.
Magnetic field lines, lines of force, can be drawn to show the direction a compass needle would have at any place in the magnetic field.
The pattern of a magnetic field about a bar magnet produced by sprinkling iron filings on a piece of paper over it.
Lines of magnetic force are unbroken, pass through the magnet, never cross and have the same strength.
A strong magnetic field has more lines of force in an area than a weak field.
If you bring the north pole of one magnet close to the south pole of another magnet, the two magnets will attract each other.
If you bring the north pole of one magnet near to the north pole of another magnet, these poles will repel each other.
The lines of force repel each other, and the two magnets push each other away.
If you turn around the magnets so that the two south poles brought together, the poles will repel each other.
Like poles repel and unlike poles attract.
29.2.3.2 Magnetic levitation, refrigerator door magnet, Halbach array, Maglev train
Experiment
1. Suspended needle
See diagram: 29.3.6: Suspended needle.
Tie one end of a light thread through the eye of a needle.
Hold on to the other end of the thread and pull up to lift and suspend the needle.
Fix an U-shaped magnet vertically on the table.
Lower the needle over the north pole of the magnet and pull the "eye" end of the needle over that end of the magnet.
Hold the thread steady and move the magnet horizontally so the needle drags across the pole until the sharp end of the needle separates from the pole.
Move the magnet away then bring the magnet back with the south pole end below the sharp end of the needle.
The magnetized needle floats in the air above the south pole of the magnet.
2. Refrigerator door magnet
See diagram 29.177: Refrigerator door magnet, "fridge magnet".
In a refrigerator door magnet, the magnetic field lines are more intense on one side than the other, because they contain side-by-side, parallel horseshoe magnets.
Refrigerator door magnets are made of soft white plastic.
One side has a message, e.g. Jo's Pizza Parlour Telephone, and only the other side is magnetic and can stick to the refrigerator door.
The magnetic side of the plastic contains magnetic ferrite stripes of opposite polarity that together act like a row of side by side horseshoe magnets.
This arrangement, called a Halbach array (Klaus Halbach, 1924-2000, USA), causes a stronger magnetic field on one side and almost no magnetic field on the other side.
Magnetic flux cancels below the plane and reinforces above the plane.
A magnetization pattern where the two components are π / 2 out of phase causes a one-sided flux.
So the side with the message cannot stick to the refrigerator door.
Press the magnetic sides of two fridge magnets together.
Rub them forwards and backwards using the forefinger and thumb to feel alternate attraction and repulsion.
3. Magnetic levitation, maglev, is where an object is suspended only by magnetic fields.
In a maglev train, linear motors use a force from a moving linear magnetic field that react with a conducting rail.
Electromagnets in the train lift it and act as the rotor of an electric motor.
Eddy currents induced in the rail create an opposing magnetic field.
The two opposing magnetic fields repel each other and force the conductor away from the stator in the direction of the moving magnetic field.
This principle is used in the Shanghai Maglev Train and other magnetic levitation trains.
A Halbach array is used to suspend in the air a speeding Inductrack Maglev train, e.g. in Shanghai.
In a speeding train, the Halbach arrays repels loops of wire forming the track to lift the train.
29.2.3.4 Inverse square law, Inverse fourth power, Inverse seventh power
Experiment
Use a balance to measure the repulsion of two bar magnets.
Make a balance out of a meter stick with a magnet on one end facing the pole of another similar magnet.
Adjust the distance between the magnets and slide the counter balance along the meter stick until equilibrium is reached.
Use a bar magnet brought near a second bar magnet counter weighted and on a knife edge to roughly verify the inverse square law.
Use three simple variations of magnets levitating in a glass tube to show a force varying with the inverse of the distance squared.
Apparatus shows the force between two dipoles varies as the inverse fourth power of the separation.
Apparatus shows the force between a magnet and a piece of soft iron varies with the inverse seventh of the separation.
29.2.4.1 Interaction of magnet and magnetizing coil
See diagram 28.180: Magnet and coil.
Experiment
Make a magnetizing coil by using a glass tube wound with close turns of insulated copper wire to magnetize steel knitting needles.
A solenoid on a pivot and a magnet on a pivot interact.
A bar magnet is mounted in a large flat coil.
The deflection of a compass needle in the centre of a large coil placed in the plane of the magnetic meridian is proportional to the tangent of the current.
29.2.4.2 Solenoid and bar magnet
See diagram 30.3.2: Suspended solenoid.
A suspended solenoid reacts with a bar magnet only when the current is on.
A magnet oscillates in a coil proportional to the square of the current in the coil.
When a solenoid is energized an iron core is violently drawn into the coil.
When a bar magnet is suspended freely it will come to rest along the direction of the earth's magnetic field.
The magnetic field lines that are produced by the solenoid are similar to that of the bar magnet and it behaves like a bar magnet.
29.2.4.3 Jumping magnet
Experiment
Place a bar magnet in a vertical transformer and apply DC with a tap switch.
29.2.4.6 Ampère's ants
See: Magnetic stirrers, (Commercial).
An amusement park display where a magnetic stirrer controlled by a push button is under a dish of iron filings.
29.2.5.0 Magnetic force on a charged particle moving in a magnetic field
A particle with charge q moves with velocity v--> through a magnetic field B--> from left to right across the page.
The angle between v--> and B--> = θ.
Magnetic force, F =q v B sin θ newton, N
So magnetic force is maximum when the direction of v is at right angles to the direction of the magnetic field (θ = 90o and sin θ = 1), and is minimum, zero,
when the direction of v is parallel to the direction of the magnetic field (θ = 0 and sin θ = 0).
29.2.5.01 Magnetic field, tesla
F =q v B sin θ, so B = F / q v sin θ tesla, T (Nicola Tesla, 1865-1943)
1 tesla of magnetic field = 1 T
1 gauss, 1 G = 10-4 T
The magnetic field on the surface of the Earth = about 5.0 X 10-5 T = 0.5 G.
A bar magnet is about 100 G.
29.2.5.02 Lorentz force law
Lorentz force (Hendrik Antoon Lorentz, 1853-1928), is the force on a point charge due to electromagnetic field.
It is the force exerted on a charged particle by a magnetic field through which it is moving.
If a charged particle is moving in the presence of an electric field and a magnetic field, F = (qE) + (qv × B), where F = force, E = electric field, q = charge, v = velocity, B = magnetic field
The first term, qE, is contributed by the electric field.
The second term, qv × B, is the magnetic force and has a direction perpendicular to both the velocity and the magnetic field.
The magnetic force is proportional to q and to the magnitude of the vector cross product v × B.
If the angle between v and B =α, the force = qvB sin α.
For a charged particle moving in a uniform magnetic field,
If angle α = 90 (v is perpendicular to B), the particle will follow a circular trajectory, radius, r = mv / qB.
If angle α < 90, the particle orbit will be a helix with an axis parallel to the field lines.
If angle α = 0, there will be no magnetic force on the particle, so it continue to move undeflected along the field lines.
Charged particle accelerators like cyclotrons make use of the fact that particles move in a circular orbit when v and B are at right angles.
29.2.5.1 Cathode ray tube, CRT
Experiment
Deflect the beam in an open CRT with a magnet.
A magnet or battery connected to the plates is used to deflect the beam of an open CRT.
29.2.5.2 e / m for electrons, measurement of e / m
Experiment
Deflect the beam in an open CRT with a magnet.
Use the earth's field to deflect the beam in an oscilloscope.
Deflect the beam of an oscilloscope with large solenoids.
Deflect the beam of an oscilloscope by current in wires parallel to the axis of the tube.
29.2.5.3 Bending of an electron beam
An electron beam hitting a fluorescent screen in a tube is bent by a magnet.
A thin beam along a fluorescent screen is bent by a magnet or charged rod.
A thin electron beam made visible by a fluorescent screen is bent when a magnet is brought near.
29.2.5.4 Crookes tube
See: Crookes Tubes, (Commercial).
The Crookes tube was an improved gas discharge tube, vacuum tube, that showed a striped positive column, Faraday dark space.
Crookes dark space, negative glow, cathode glow.
Unwanted deflections of the beam in the Crookes tube are caused by induced charge.
29.2.5.5 CRT and earth's magnetic field
A CRT is mounted so it can be oriented in any direction and rotated about its axis.
Experiment
Find the position that results in no deflection from the Earth's field turn 90o.
29.2.5.6 Magnetic forces on an electron beam, Magnetic deflection of cathode rays
A beam of free electrons is bent in a circle by large Helmholtz coils.
A beam from a lime spot cathode in a large bulb is made circular by Helmholtz coils.
29.2.5.8 Magnetic pump, ion motor force on conducting field
* Copper (II) sulfate solution flows in a circle when placed between the poles of a magnet with a current from the centre to edge.
* An ion motor for the overhead projector with cork dust in a copper (II) sulfate solution.
Cork dust floating on a solution of zinc chloride in a circular container rotates when current is passed through the solution in the presence of a magnetic field.
Cork dust shows the motion of copper (II) sulfate an ion motor.
* Salt solution rotates when placed in a circular dish over a magnet with electrodes at the centre and edge.
29.2.6.1 Parallel conductors
(The use of open surface mercury is illegal in some school systems!)
Long vertical parallel wires attract or repel depending on the current direction.
Experiment
Use two heavy vertical wires 1 cm apart and pass 20 amps in the same or opposite direction.
Use rectangular loops of solid wire hanging on pivots from two stands.
Use parallel wires with one being a loop free to turn in a pool of mercury.
Radial wires (like clock hands) spring apart when current is passed through them.
29.2.6.2 Interacting coils
Experiment
Two hanging loops attract or repel depending on current direction.
A narrow loop formed by hanging a flexible wire opens when current is passed.
Two loops in proximity attract or repel depending on current direction.
29.2.6.7 Current balance
See: Current Balance (Modern Teaching Aids).
In a current balance a balancing mass measures the force required to prevent the movement of one current-carrying coil in the magnetic field of a second coil carrying the same current.
Current balance has a rectangular coil on knife edges and stationary windings with parallel conductors.
An open rectangle of aluminium wire is balanced between the poles of a U-magnet until current is passed through the part perpendicular to the field.
Hang a triangular loop of wire from a spring scale in the mouth of an electromagnet and the current in the loop is varied.
29.2.6.8 Maxwell's rule
(The use of open surface mercury is illegal in some school systems!)
Maxwell's rule states each part of an electric circuit the circuit experiences a force causing it to tend to move in such a direction as to enclose the maximum possible magnetic flux.
Show an electric circuit that can change shape to include the maximum possible magnetic flux.
A heavy wire connects two metal boats floating in mercury troughs with electrodes at one end.
29.2.6.9 Barlow's wheel
See: Barlow's Wheel (Commercial).
(The use of open surface mercury is illegal in some school systems!)
A copper disc with current flowing from the centre to a pool of mercury at the edge rotates when placed between the poles of a horseshoe magnet.
A potential is applied from the axle of a wheel to a pool of mercury at the rim while the wheel is between the poles of a magnet.
Current passes from the bearings of a copper wheel mounted vertically to a pool of mercury at the base.
A U-shaped magnet is mounted so the current is perpendicular to the magnetic field.
The copper disc in Barlow's wheel is replaced by a cylindrical Alnico magnet with the field parallel to its axis.
For a variation of Barlow's wheel, an Alnico disc magnetized in the direction of the axis rotates around the axis when a current is made to flow from the axis to the rim.
29.2.6.11 Magnetic grapevine
Experiment
A very flexible wire suspended alongside a vertical bar magnet will wrap itself around the magnet when there is a current in the wire.
29.2.6.13 Ampère's motor, Ampère's frames
A coil on a reversing switch is placed between the poles of strong magnets.
A magnet is brought near and rotates a large current-carrying loop.
A copper rod rolls along two electrified rails over ring magnets sandwiched between steel plates.
A wheel on electrified rails over a large vertical field produced by electromagnets rolls back and forth depending on the current direction.
As the current is reversed in a rod rolling horizontally on a track between the poles of a strong magnet, the direction of motion reverses.
29.2.7.1 Galvanometer coil and magnet
See diagram 9.2.6.5: Magnet, coil and galvanometer
1. Use a large working model of a galvanometer with a large coil and magnet to show the essentials.
Construct a large model d'Arsonval galvanometer from a coil and a large U-shaped magnet.
A galvanometer contains a coil with attached spring and pointer, a magnetic field, an electric field between the poles of a magnet.
When electric current passes through the coil, a torque on it causes the coil to rotate.
The torque opposes the motion so that the angle of rotation of the coil, i.e. the angle travelled by the pointer, is proportional to the current passing through the coil.
2. Wind insulated wire around an iron core.
Connect the coil to a galvanometer.
Move a bar magnet back and forth above the coil and observe movement of the needle of the galvanometer.
Reverse the magnetic poles and repeat the experiment.
Observe the movement of the needle again.
Remove the iron core, repeat the steps above and observe what happens.
29.2.7.3 Interacting coils
A small free turning coil is mounted in a larger coil.
Two horizontal coaxial coils the inner stationary and the outer larger coil suspended freely interact when currents are passed through in like or opposite directions.
A solenoid attached to a battery is mounted in a large open Helmholtz coils assembly.
29.2.7.4 Interacting solenoids
(The use of open surface mercury is illegal in some school systems!)
Two heavy copper horizontal solenoids pivot in mercury cups about a vertical axis.
Suspend a solenoid and show the effects of a bar magnet on it.
A vertical coil energized by a flashlight cell floats in a large pan.
Use a bar magnet to move the coil.
29.2.7.6 Force on rectangular current loop
Rectangular current loop, abcd, has length ab or cd and width bc or da is in a magnetic field B --> parallel to the plane of the current loop.
Torque on a rectangular loop, τ, = IAB sin θ, newton.metres, N.m.
Torque on n turns of a rectangular loop τ, = nIAB sin θ, newton.metres, N.m.
nIA is the magnetic moment of the loop.
29.2.7.8 Ampère's law, permeability of free space
See diagram 29.2.7.8: Ampère's law.
The magnetic field in space around an electric current is proportional to the electric current that serves as its source.
Ampère's law relates the magnetic field along a closed path to the electric current enclosed by the path.
Assume a circular path exists around a long straight wire carrying an electric current.
The magnetic field is always parallel to this closed path and always has magnitude B at all points on the closed path.
In a closed loop path, the sum of the length elements × magnetic field in the direction of the length element = magnetic constant X current enclosed in the loop.
The permeability of free space (magnetic constant, vacuum permeability), measures the resistance when forming a magnetic field in a vacuum.
29.2.7.9 Magnetic field of current-carrying long straight wire, B = muoT / 2 pi r
29.2.7.10 Force between current-carrying wires
A current-carrying wire exerts a force on another current-carrying wire.
If two wires parallel to each other, carrying a current I1 and I2, and distance s apart, are lying on the desk before you, the magnetic field from the first wire circulates vertically around the first wire from above to below it and vertically cuts across the second wire.
So the second wire experiences a magnetic field down across it.
So wires carrying a current is the same direction attract each other and wires carrying a current in opposite directions repel each other.
So we can calculate, in a vacuum, the force per metre (F/L), between two parallel conductors one metre apart (s) carrying a one amp current.
F/L = (muo/ 2 pi)( I1 X I2 / s)
29.2.7.11 Magnetic field of current loop
At the centre of the loop, where N = number of turns of the wire, r = the radius of the current loop, I = current.
The magnetic field is proportional to the current in the loop and inversely proportional to the radius of the loop.
29.2.7.12 Magnetic field of solenoid, B
See diagram 30.3.9: Solenoid behaves like a bar magnet.
A solenoid is a long wire wound into successive helical loops to produce a strong magnetic field inside the solenoid parallel to its axis.
Each loop carries current in the same direction and the wire successive loops are almost parallel so the magnetic force between loops is attractive.
The magnetic field outside the solenoid is weak and may be assumed to be zero.
So the magnetic field lines, lines of force are close together inside the solenoid and far apart outside the solenoid.
29.4.68 Magnetic dip, north magnetic pole, south magnetic pole
See diagram 29.165: Magnetic dip needle.
Experiment
1. Use a large compass needle or dip needle as an indicator of magnetic field.
Construct a magnetoscope by hanging needles from the edge of a small brass disc.
Use a dip needle to show the inclination and local direction of the earth's magnetic field.
Explore the magnetic field around a long wire with a compass needle or dip needle.
2. Push a steel knitting needle through cylindrical cork at right angles to its long axis.
Push a pin into the centre of each end of the cork to act as an axle.
Balance the cork through its axle of pins on knife edges.
Magnetize the steel knitting needle using a magnetizing coil.
Balance the cork again.
The earth's magnetic field pulls one end of the needle downwards.
Fix a spirit level, or a glass tube containing a bubble in water, above the knitting needle.
Use a protractor to measure the angle of dip between the horizontal spirit level and the knitting needle.
At the north magnetic pole or at the south magnetic pole, the needle should point straight down.
At the equator the knitting needle will be about parallel to the spirit level.
29.4.69 Magnetizing coil
See diagram 29.166: Magnetizing coil.
Experiment
Use glass tubing wound with close turns of insulated copper wire to magnetize steel knitting needles.
29.4.73 Magnetic substances
Experiment
Collect objects made of different substances, e.g. paper, wax, brass, zinc, iron, steel, glass, cork, rubber, aluminium, copper, gold, silver, wood, tin.
Test each object with a magnet to see which objects a magnet attracts or does not attract.
Bring a soft iron wire and hard steel or piano wire near a compass needle to see if a magnetic field affects it.
29.4.74 Magnetic poles and pin chains
See diagram 29.2.3.2: Pin chain.
Experiment
1. Pin chain
Pick up a pile of pins with the magnet.
Leave one pin attached to the magnet.
Take off another pin and bring it close to the end of the first pin.
They will stick together by magnetic force.
Connect all the pins to make a magnetic pin chain.
2. Estimate the strength of bar magnets by using a magnetized object to attract pins or paper clips and estimate this object's magnetization effect by the number of attracted pins or paper clips.
3. Use light thread to attach a paper clip to the desk with adhesive tape.
Hold a strong magnet above the paper clip and see it rise.
29.4.76 Magnetic fields in two dimensions
See diagram 29.2.1.0: Lines of force of bar magnet.
In the following experiments, the iron filings tend to line up in "lines of force" ("magnetic field lines", "field lines").
The magnetic field lines are closely spaced where the magnetic field is the strongest.
By convention, magnetic field lines exit from the north pole of a bar magnet and enter through the south pole.
Magnetic field lines never cross.
Experiment
1. Sprinkle iron filings on a glass sheet, or a sheet of Plexiglas, over a magnet
2. Sprinkle iron filings on a magnet between two glass plates.
Repeat the experiment with two bar magnets in different positions.
3. Iron filings on a card
See diagram 29.173.1: Iron filings on a card.
Sprinkle iron filings evenly on a thin card.
Hold the card high over a bar magnet then carefully lower it until it almost touches the magnet.
Tap the card gently with the end of a pencil.
The iron filings move into a pattern showing the magnetic field lines.
Hold a plotting compass above the lines of force and compare their direction with the direction of the compass needle.
4. Put an unmagnetized piece of soft iron near two bar magnets on the desk and observe the magnetic fields formed.
5. Iron filings patterns
See diagram 29.173.2: Iron filings over magnets.
Put two bar magnets, a piece of soft iron, and wood blocks thicker than the magnets and iron on the table and cover with a piece of white cardboard.
Scatter fine iron filings on the cardboard.
Tap the cardboard from the side and note how the fine iron filings settle into a pattern.
6. Magnet circles
Push a wire up through a piece of thin cardboard.
Drop iron filings evenly over the cardboard.
Connect the wire to a dry cell battery.
Tap the cardboard and the iron filings form circles around the wire, because of the magnetic field from the flow of electric current in the wire.
7. Make permanent records of the magnetic field as follows:
* spray over the iron filings with a paint sprayer,
* use photographic paper instead of the thin card in a darkroom, shine a bright light on it and develop the print,
* dip a white sheet of paper in melted wax.
Let it cool then sprinkle iron filings on the solid wax.
Hold the paper over a strong magnet to allow the iron filings to move into lines of force patterns.
Hold a hot iron over the iron filings to let them sink into the wax.
Photocopy iron filings on transparent paper, but do not use a strong magnet near a photocopy machine!
29.4.77 Magnetic fields in three dimensions
See: Magnetic fields, (Commercial).
Experiment
1. Add oil to iron filings in a container.
Shake to see if the filings will go into suspension in the oil.
Use a concentration of oil that allows the iron filings to remain suspended then bring a magnet to the container to develop a pattern of iron filings in three dimensions.
Make a permanent record using water glass or liquid plastic.
2. Pour a spoon of fine iron filings into a bottle then 3 / 4 fill with sticky liquid, e.g. water glass or oil.
Close the bottle then forcibly shake it to make the iron filings suspend evenly in the liquid.
Place a strong magnet near the bottle.
Note the distribution of the iron filings.
You may save the distribution of the filings after it cools and solidifies if the liquid is transparent plastic in liquid state.
3. Use a suspension of carbonyl nickel powder in
silicon oil as an indicator of magnetic field.
4. Make a sandwich of iron filings in glycerine between two glass plates.
Soft iron bars extend the poles of a permanent magnet into a projection cell with iron filings in an equal mixture of glycerine and alcohol.
5. Make a 3-D view of magnetic fields by sprinkling iron filings on a series of stacked glass plates.
29.4.81 Magnetic field of electric current in a wire
Experiment
Pull 25 cm of insulated copper wire through a hole in the centre of a small white piece card.
Connect the ends of the wire to a battery through a car headlight bulb.
Fix the card in a horizontal position.
Fix the wire in a vertical position.
Sprinkle iron filings evenly on the card.
Switch on the current.
Tap the card gently with the end of a pencil.
The iron filings move into a pattern showing the magnetic field.
Switch off the current.
Repeat the experiment using a small plotting compass instead of iron filings.
Compare the directions of the compass needle to the patterns of iron filings on the card.
Repeat the experiment with the direction of current reversed.
29.4.82 Magnetic field inside an open coil, open solenoid
See: Magnetic fields, Solenoid, air cored, (Commercial).
See diagram 29.179: Open solenoid.
Experiment
Wind five evenly spaced turns of bell wire around a wooden cylinder.
Slide the coil off the cylinder.
Fit the cylinder into slots in a piece of cardboard so that the cardboard appears to cut the coil in half lengthways.
Connect the coil to the terminals of a dry cell or lead cell accumulator or low voltage power supply, using a car headlight bulb in series.
Sprinkle iron filings evenly on the card.
Switch on the current.
Tap the card gently with the end of a pencil.
The iron filings move into a pattern showing the magnetic field.
Note the pattern inside the coil and outside the coil.
Switch off the current.
Repeat the experiment using a plotting compass instead of iron filings.
29.2.1.2 Oersted effect, magnetic field around a current-carrying wire
See: Oersted's Apparatus (Commercial).
See diagram 29.2.1.2: Oersted's effect.
See diagram 30.01: Right-hand grip rule.
H. C. Oersted used to demonstrate no connection between electricity and magnetism when he would place a compass needle at right angles to a current-carrying wire to show that there was no effect on each the other.
But a student asked, "What would happen if the compass needle was placed parallel to the current carrying wire?"
In 1820, Hans Christian Oersted, Danish physicist, noticed that the needle of a compass near a wire carrying a current turned so that the needle at right angles to the wire.
Perhaps he had heard that when lightning strikes the masts of tall sailing ships, the ships compass spins wildly.
Realising that moving electricity created a magnetic force, he invented the term "electromagnetism".
Experiment
1. Show Oersted's effect with a compass needle and a long wire carrying a heavy current to explore the magnetic field around a long wire.
A compass deflects above and below a current-carrying wire.
Hold a current-carrying wire over a bar magnet on a pivot and the magnet moves perpendicular to the wire.
Arrange four compass needles around a vertical wire running through Plexiglas.
Pass a current of 50 amps through a heavy vertical wire and investigate the magnetic field with a compass needle.
Pass a heavy current from a storage cell through a long wire and use a compass needle to investigate the near by magnetic field.
When demonstrating Oersted's effect using large currents, use flat-braided brass cable instead of copper wire.
2. An electric current flowing through a wire produces a magnetic field.
The field is cylindrical around the wire and obeys the right-hand grip rule:
Grip the wire with the right-hand and point the thumb in the direction of the conventional current (+ ve to -ve), I, the fingers curl around the wire in the direction of the magnetic field, B.
The magnetic field of a solenoid is a uniform magnetic field that follows a right-hand rule.
The strength of the magnetic field of a solenoid, B = 4 πknI, n = numbers of wrapped wire circles (turns), I = electric current, k = the constant of magnetic effect.
3. If a magnetic field is at an angle A to the conductor carrying a current, then the magnetic force on the conductor will be force × sin A.
The force will be a maximum when A = 90o, and will be zero when A = 0o.
Use the open right-hand rule to find the direction of the force.
If the angle A = 0o, then the current-carrying conductor is now parallel to the magnetic field, and no magnetic force is produced on the conductor.
4. Direction of a magnetic field caused by a current-carrying wire
Use the right-hand rule: (Right-hand grip rule, right-hand curl rule, magnetic field right-hand rule).
Point the thumb along the wire in the direction of the current, I.
The fingers curl around the wire in the direction of the magnetic field.
Place plotting compasses around a vertical long straight wire carrying a current, point along tangents to a circle with the wire as the centre of the circle.
5. When a current flows through a straight wire, a magnetic field occurs in concentric circles around the wire.
The right-hand grip rule states that if you grip a wire carrying a current in the right-hand, with the thumb extended in the direction of the conventional current, positive to negative, the fingers will be curled around the wire in the direction of the magnetic field.
In the centre of a current loop the magnetic field points in one direction.
3.Sprinkle iron filings on the Plexiglas on an overhead projector.
Experiment
Hold down the switch briefly connecting the bottom while tapping the Plexiglas to encourage alignment.
29.5.1.02 Magnetic field of current loop
In the centre of a current loop, all contributions to the magnetic field point in the one direction.
The Earth's magnetic field is caused by rotating loops of charge inside the Earth.
29.5.3 Direction of magnetic field using plotting compass
See diagram 30.01: Magnetic field from a straight wire.
A magnetic field surrounds a conductor carrying an electric current.
The magnetic field stops when the flow of current stops.
Experiment
Use a plotting compass to find the magnetic field about a current-carrying conductor.
Push a wire through of a piece of cardboard and connect to an electric circuit.
Move the compass around the wire and record the compass needle positions with arrows then join the arrows.
Reverse the direction of current flow through the wire so that the compass needle will point in the opposite direction as you move it around the wire.
29.5.4 Direction of magnetic field using iron filings
See diagram 30.01: Magnetic field of current-carrying wire.
Experiment
1. Remove the insulation at both ends of two copper wires.
Connect one end of each wire to the pole of a dry cell.
Put the other ends of the 2 wires in line on a piece of paper.
Sprinkle iron filings on the paper between the 2 ends of the wires.
Put the iron filings from one end of a wire to the other.
Cover the ends of the wires with iron filings.
Close the circuit and observe the motion of the iron filings.
The iron filings are lifted with the wire, because there is a magnetic field around the electric current.
Open the circuit.
The iron filings drop immediately from the wire.
2. Sprinkle iron filings around a vertical wire
running through the centre of a Plexiglas sheet.
3 Sprinkle iron filings around a current-carrying wire loop, coil and solenoid.
29.5.5 Forces between conductors carrying a current in the same direction
See diagram 30.1.3: Forces between conductors carrying a current in same direction.
Experiment
Use a flat sheet of copper, a 2 volt accumulator, two 15 cm lengths of copper wire hooked at the top, a switch and connecting wire.
The flat sheet of copper allows the bottom end of the hooked conductors to move if you apply any forces to them.
A and B are pieces of wire supported at the top and hanging vertically so that they are free to swing in any direction.
Current passes in the same direction through the conductors A and B.
Note how A and B move when current passes.
Two wires carrying current in the same direction attract each other.
29.5.6 Forces between conductors carrying a current in opposite directions
See diagram 30.1.4: Forces between conductors carrying a current in opposite directions.
Experiment
Rearrange the circuit so that current flows in opposite direction sin A and B.
Observe what force is exerted between A and B when current passes.
Two wires carrying current in the opposite direction repel each other.
29.5.7 Forces between a permanent magnet and a conductor carrying a current
See diagram 30.1.5: Forces between permanent magnet and conductor carrying a current.
See diagram Catapault effect Forces between permanent magnet and conductor carrying a current.
Experiment
N and S are the north and south poles of a horseshoe magnet.
Close the switch.
Observe how does the conductor moves.
Reverse the magnet and later the direction of the current.
Work out the directions of magnetic fields.
A catapult effect occurs when an electric current passed through a loose wire in a magnetic field.
9.4.10H">9.4.10 Bar magnet inside a coil
See diagram 9.2.6.4: Bar magnet inside coil
Use a cardboard paper tube that allows a bar magnet to be inserted and removed easily, e.g. centre of a toilet roll.
Wind wire around the tube many times to form a coil.
Leave about 50 cm at each end of the coil.
Connect the coil to a galvanometer or a compass coil as above.
Insert a bar magnet quickly into the coil and observe movement of the needle of the compass.
Remove the bar magnet from inside the coil and observe the needle again.
Rotate the compass to ensure the wire is parallel to the needle pointed to the N-S pole before experiment.
If you use a galvanometer, put it far from the coil to avoid magnetic induction.
In each case, the needle of the compass or the needle of the galvanometer will deflect due to a current being produced by moving the bar magnet in and out of the coil.
The deflection of the galvanometer needle is a measurement of the current.
The deflection of the compass needle is simply the needle aligning itself with the magnetic field produced by the current in the coil around the compass.