School Science Lessons
(UNPh30)
2024-07-25
Electromagnets
Contents
30.1.0 Electric motors
30.2.0 Electromagnetic induction
30.3.0 Electromagnets
30.4.0 Magnetic fields, current-carrying coils
30.5.0 Motors and generators, DC / AC
30.1.0 Electric motors
30.1.1 Make a simple electric motor, Electric motor 1
30.1.2 Make a simple electric motor, Electric motor 2
30.1.3 Make a simple electric motor, Electric motor 3
30.1.4 Magnetic effect of a current to produce movement, Electric motor 5
30.1.5 Electric motor spin
30.1.6 Principle of electric motor
30.1.7 Simple electric motor
30.2.0 Electromagnetic induction
30.2.1 Electromagnetic induction
30.2.2 Lenz's law (Electromagnets)
30.2.3 Radio waves, transmitter: amplitude modulation (a.m.) and frequency modulation (f.m.)
30.2.4 Spectrograph
Experiment
30.2.5 Cylindrical electromagnets, test the poles
30.2.6 Electricity from a magnet and a coil
30.2.7 Electromagnetic induction
30.2.8 Electromagnetic induction applications, multimeter
30.2.9 Electromagnetic induction with two solenoids
30.2.10 Direction of induced current
30.2.11 Faraday's law for induced emf
30.2.12 Horseshoe electromagnets
30.2.13 Induced emf, in conductor moving in magnetic field, open right-hand rule
30.2.14 Magnetic field from electric current in a wire
30.2.15 Magnetic field inside an open coil
30.2.16 Make a spark
30.2.17 Moving magnet induces current in a coil
30.2.18 Retentivity
30.2.19 Spark in a spark plug
30.2.20 Study an electromagnet
30.2.21 Test the strength of electromagnets
30.30 Electromagnets
30.3.1 Electromagnetism, Open right-hand rule
30.3.2 Fleming's left-hand rule
30.4.0 Magnetic fields, current-carrying coils
See: Solenoids (Commercial).
30.4.1 Attraction and repulsion of parallel coils carrying electric current
30.4.2 Attraction and repulsion of parallel electric currents
30.4.3 Bar magnet in coil-carrying current
30.4.4 Dancing spring, electric current in parallel coils, Jumping coil of wire
30.4.5 Magnetic field of electric current in a circular coil
30.4.6 Magnetic field of a solenoid
30.4.7 Magnetize inside a coil, solenoid carrying current
30.4.8 Nail becomes an electromagnet
30.4.9 Solenoid affects iron nails
30.5.0 Motors and generators, DC / AC
See: Electric Motors, (Commercial).
30.5.1 Motors and generators, DC / AC, Electric motor 4
30.5.2 Bicycle dynamo, the "missing wire"
30.5.3 Electric generator, alternator, AC generator
30.5.4 Force on current-carrying wire in a magnetic field
30.5.5 Homopolar motor, unipolar motor
30.1.1 Make a simple electric motor, Electric motor 1
See diagram 30.1.3.3: Electric motor 1.
An electric motor is like an electric generator operating in reverse, because it uses electrical energy to produce mechanical work.
This simple model uses current from a dry cell to excite the field magnets as well as the armature windings.
Experiment
Prepare a board 20 X 25 cm for the base.
Drill a small hole through the centre and drive a 15 cm spike up through it.
Wind 100 turns of insulated bell wire on to two other 15 cm spikes, leaving about 30 cm for free ends.
Drive these spikes into the base 15.5 cm apart.
Drive two small nails on the diagonal and 5 cm from the spike at the centre.
Strip the free ends of each coil and twist them several times around the nails and bend them so that they will rest in contact with the central spike.
These ends will serve as brushes.
The field coils must be wound in the proper direction.
The other ends of the coils are fastened to the screws in the corners of the base.
Construct the armature coil and commutator.
Drill a hole crosswise through the top of a 4-cm cork and force a 13-cm spike through it.
Wind 40 turns of insulated bell wire on to each end, using the direction of windings shown in the diagram.
Scrape the free ends of the bell wire.
Cut out the centre of the cork with a knife and insert the closed end of a 13-cm test-tube so that it fits tightly in the cork.
Construct the commutator.
Cut out two rectangular pieces of sheet copper about 4 cm long, and wide enough to reach around the test-tube with a gap of about 6 mm between them.
Curve the pieces of copper to fit the test-tube.
Punch small holes and into them and solder or twist one of the scraped free ends of the armature windings.
Bind these commutator plates securely into position at top and bottom with adhesive tape to complete the armature and commutator.
Set it into position on the vertical bearing and bring the brushes into contact with the commutator.
Turn the test-tube in the cork until the brushes lie across the gaps in the commutator when the armature is in line with the field magnets.
Now if your windings and connexions are all as shown, connect to one or two cells.
With a slight push of the armature the motor should start off at a lively speed.
If it does not go, examine the brushes to see whether they make a light, but certain contact.
It may also help to change the angle of the brushes.
To test this point, untwist the brushes from the nails and hold them lightly against the commutator plates with the fingers.
While holding them, always parallel, swing them around at different angles while a helper turns the armature with his hand.
Note the point at which the armature picks up most speed and set the brushes at that point.
If the rotor does not turn, check the contact of the brushes with the commutator.
30.1.2 Make a simple electric motor, Electric motor 2
See diagram 30.1.3.4: Electric motor 2.
1. Horseshoe magnet, 2. Axle, 3. Commutator, 4. Coil, 5. Brass strip, 6. Electric motor with 3 coils, A Contact (brush), Aw Wire from contact to
coil, B Contact (brush), Bw Wire from contact to coil.
Experiment
1. Fix a simple coil, mounted on an axle, between
the poles of a horseshoe magnet.
Two wires from the coil connect to the commutator.
The commutator is a cylindrical insulator revolving on the axle with
two strips of brass attached.
The commutator rotates with the coil.
Each brass strip is joined to one wire from the coil.
Two carbon contacts, brushes, touch the side of the commutator and
allow electric current to pass from the battery to the commutator.
Electric current goes from the battery to brass strip A then along wire Aw, through the coil then back through wire Bw and brass strip B then back
to the battery to complete the circuit.
When the commutator and coil make one half turn, the current enters
through brass strip B and returns through brass strip A, reversing
the current in the coil.
The electric motor runs more smoothly if more than one coil is used.
This electric motor uses a permanent magnet, but most electric motors
use a field coil that forms a more powerful electromagnet.
2. Using Fleming's left hand rule, direction of
thumb is thrust, first finger is magnetic field and second finger is current.
In the diagram, side 7 to 8 of the coil has upward force on it and
side 9 to 10 has downward force on it.
So the coil turns until it is vertical and the brushes no longer
touch the brass strips because of the gaps between them, and no current
flows.
However, due to inertia of the commutator, the coil keeps turning
so side 7 to 8 is now on the right side and side 9 to 10 is on the left
side.
The brushes touch the brass strips again and the coil keeps turning clockwise.
30.1.3 Make a simple electric motor, Electric motor 3
See diagram 30.1.3.5: Electric motor 3.
Experiment
Make a simple electric motor from a wooden base, tin plate supports,
3 iron wire paper clips, seven drawing pins, 2 metres covered
copper wire and cellulose tape.
Remove insulation from the brushes and the armature contacts (commutator)
held at 90o to the coil by thin strips of adhesive tape.
For the coils, use 100 turns on each side, all in the same direction.
Bend the field electromagnet with the tips near the patch of the
armature.
Punch the holes in the tin plate with a nail.
Sketch the positions of the magnetic poles for different positions
of the armature.
Alternate attraction and repulsion between the two electromagnets
makes the armatures revolve.
Many variations, e.g. use of a permanent magnet for the field magnet,
are possible.
30.1.4 Magnetic effect of a current to produce movement, Electric motor 5
See diagram 30.1.3.8: Electric motor 5.
The electric motor uses the magnetic effect of a current to produce movement.
1. A magnetic field exists around a conductor carrying an electric current.
Reversing the direction of the current also reverses the direction of the magnetic field.
2. If electric current passes through a conductor in a magnetic field, a force acts on the conductor, which can cause it to move.
Lines of magnetic force from the two sources of magnetism interact so that a mechanical force arises.
It tends to move the conductor in the direction shown in the diagram.
3. If a conductor is arranged as a loop so that current flows in opposite directions on each side of the loop, the loop moves with rotary motion through 90o.
Then it stops, because the interaction of the two magnetic fields no longer exerts a force thrust on the conductors.
4. If the loop is connected to the current source through a commutator and brushes, and additional loops are included as in a simple generator, so the assembly
turns with continuos motion.
5. The assembly behaves like an electric motor.
30.1.5 Electric motor spin
See diagram 30.1.3.2: Spinning magnets.
Experiment
Put 3 ALNICO bar magnets in a line, south north south north south
north, such that the middle magnet is in a small glass beaker.
Turn the beaker through a small angle.
Tap the beaker.
Its magnet returns to the original line with the opposite poles nearest.
Reverse one end magnet.
Turn the beaker through a small angle.
Tap the beaker and the magnet does not return to the original line.
In an electric motor a coil carrying electric current replaces the
magnet in the beaker.
30.1.6 Principle of electric motor
See diagram 30.1.3.4a: Principle of electric motor.
Experiment
Suspend a piece of fine wire vertically between the ends of a horseshoe magnet.
(Use of open surface mercury is illegal in some school systems!)
Connect the wire to a torch cell.
Note what happens when you make the connection.
Reverse the connections on the cell and observe the result.
Note whether the magnet and the source of electrical energy both
seem necessary to produce the effect you have observed.
The electric motor uses the same principle.
30.1.7 Simple electric motor
See diagram 30.1.3: Simple electric motor.
See diagram 29.181.1: Simple electric motor 1.
See diagram 29.181: Simple electric motor 2.
1. horseshoe magnet, 2. axle, 3. commutator, 4. coil, 5. brass strip, 6. electric motor with 3 coils, A Contact (brush), Aw Wire from contact to coil,
B Contact (brush), Bw wire from contact to coil.
Experiment
1. Fix a simple coil, mounted on an axle, between the poles of a horseshoe magnet.
Two wires from the coil connect to the commutator.
The commutator is a cylindrical insulator revolving on the axle with two strips of brass attached.
The commutator rotates with the coil.
Each brass strip is joined to one wire from the coil.
Two carbon contacts, brushes, touch the side of the commutator and allow electric current to pass from the battery to the commutator.
Electric current goes from the battery to brass strip A then along wire Aw, through the coil then back through wire Bw and brass strip B then back to the
battery to complete the circuit.
When the commutator and coil make one half turn, the current enters through brass strip B and returns through brass strip A, reversing the current in the coil.
The electric motor runs more smoothly if more than one coil is used.
This electric motor uses a permanent magnet, but most electric motors use a field coil that forms a more powerful electromagnet.
2. Using Fleming's left hand rule, direction of thumb is thrust, first finger is magnetic field and second finger is current.
In the diagram, side 7 to 8 of the coil has upward force on it and side 9 to 10 has downward force on it.
So the coil turns until it is vertical and the brushes no longer touch the brass strips because of the gaps between them, and no current flows.
However, due to inertia of the commutator, the coil keeps turning so side 7 to 8 is now on the right side and side 9 to 10 is on the left side.
The brushes touch the brass strips again and the coil keeps turning clockwise.
3. Use strong bar magnets, bent pieces of tin plate, a thick piece of cardboard wound with many turns of thick 22 gauge wire.
Fray the ends of the wire so that they brush against 2 tin plate contacts connected to dry cell batteries.
To start the motor, give a little push to get the current moving.
Instead of the cells supplying the brushes with current, insert a torch globe receiving current from the brushes.
Turn the armature shaft with a pulley and belt.
4. Field magnets and brushes
Push a 15 cm nail up through the centre of a 20 × 25 cm card to act as a vertical bearing.
Wind a coil 100 turns of bell wire on 2 nails to act as field magnets.
Push the nail down into the cardboard 15.5 cm apart, each side of the spike.
Push down 2 small nails on the diagonal and 5 cm from the centre of the card.
Strip insulation from the ends of each coil so that you can twist the wire twice around the small nails then touch the central large nail.
The touching ends will act as brushes.
Check the direction of windings of the coils from the diagram.
Attach the other ends of the coils in the field magnets to screws in the corners of the card.
Make the armature coil.
Push a 15 cm nail crosswise through a 4 cm cork.
Wind 40 turns of bell wire on the nails, with the direction of windings as shown in the diagram.
Strip insulation from the ends of each coil.
Cut out part of the centre of the lower side of the cork so that the end of a test-tube fits tightly.
5. Commutator
Cut out a rectangular piece of thin sheet copper 4 cm × circumference of the test-tube.
Cut off 12 mm from the circumference length then cut the piece of copper in half.
Fit the copper pieces to curve around the test-tube with a gap of 6 mm between them.
Make a small hole in each copperplate to attach one end of an armature coil.
Fix the copper plates in position around the test-tube as a commutator.
Set up the rotor.
The armature and commutator form the rotor.
Set the rotor into position on the vertical bearing.
Adjust the brushes to contact the commutator.
Turn the test-tube within the cork until the brushes lie across the gaps in the commutator when the armature is in line with the field magnets.
Run the electric motor.
Connect the screws to the terminals of a dry cell or lead cell accumulator using a car headlight bulb to limit the current, or low voltage power supply.
Give the rotor a slight push and it should keep turning.
If the rotor does not turn check the contact of the brushes with the commutator.
30.2.1 Electromagnetic induction
Electromagnetic induction, induction, refers to the production of an electrical or magnetic state in something because of its proximity to an electrified or magnetized body without any physical contact.
See diagram: 29.2.1.5: Open right-hand rule.
Induced emf in conductor moving in magnetic field, open right-hand rule:
1. Magnetic flux measures the number of magnetic field lines crossing an area.
So if a magnetic field strength B crosses an area A at angle θ, magnetic flux Φ (phi) = (B cos θ)A = BA cos θ weber, (Wb), where 1 Wb = 1 T.m2.
1. An electromotive force, emf is produced in a circuit by a change of magnetic flux through the circuit or by relative motion of the circuit and the magnetic flux.
In a closed circuit an induced current will be produced.
This phenomenon is called electromagnetic induction.
The electromotive force is called inducted electromotive force.
2. Electric current is produced in a closed circuit when there is relative movement of its conductor in a magnetic field.
There is no battery or other source of power in a circuit in which an induced current appears because the energy supply is provided by the relative motion of the conductor and the magnetic field.
The magnitude of the induced current depends upon the rate at which the magnetic flux is cut by the conductor.
Its direction is given by Fleming's left hand rule.
3. If current flows through a long straight wire, the strength of the magnetic field at distance from the wire depends on the property of t he substance in which the magnetic field is measured called magnetic permeability, μ.
For a vacuum, or air ("free space") μ0 = 4π × 10-7.
4. Relative permeability compares the permeability of a substance with the permeability of free space.
Diamagnetic materials, e.g. lead, have relative permeability less than 1 so they decrease the strength of a magnetic field in a solenoid.
Paramagnetic materials, e.g. aluminium, have relative permeability slightly more than 1 less so they slightly increase the magnetism in a solenoid.
Ferromagnetic materials, e.g. iron and iron alloys, have relative permeability more than 50.
So the magnetic field of a solenoid coil is much greater if it has a "soft" iron core.
In a long coil solenoid, the right-hand grip rule on any coil shows the direction of the uniform magnetic field in the solenoid.
30.2.2 Lenz's law (Electromagnets)
Lenz's law, force opposing conductor with induced emf
See: Lenz's law (Commercial).
See diagram 30.6.02: Lenz's law.
1. The magnet approaches the coil downwards with its north pole nearest the coil so the induced current flows from right to left so that the coil behaves as if it is a magnet with its north pole nearest the magnet.
2. The magnet moves away from the coil upwards with its north pole nearest the coil so the induced current flows from left to right so that the coil
behaves as if it is a magnet with its south pole nearest the magnet.
3. Lenz's law states that an induced current flows in the direction opposing the change that caused the current.
An induced current will cause a current flow in a direction to oppose the cause of the induced current.
This law is an example of the principle of conservation of energy.
An induced emf is found in a loop of wire when there is a change in the magnetic flux around the loop of wire.
Lenz's law states that if an emf is induced in a loop of wire, it will produce a current whose magnetic flux will always oppose the original change of magnetic flux.
If a magnet is pushed into a coil, the induced emf in the coil will produce a current whose magnetic field tends to push the magnet out.
If a magnet is pulled out of a coil, the current produced by the induced emf in the coil will tend to pull the magnet back in again.
If mechanical energy is used to turn a coil in a magnetic field then a variable emf will be induced across the ends of the coil.
Lenz's law follows from the law of conservation of energy.
30.2.3 Radio waves, transmitter: amplitude modulation (a.m.) and frequency modulation (f.m.)
Radio waves are a form of electromagnetic radiation that can be controlled to produce wavelengths between one metre and a million metres.
An AC signal of a given frequency is fed onto twin conductors acting as an antenna.
An electric field that reverses each cycle is produced between the conductors.
The changing electric field produces a magnetic field perpendicular to it, which changes at the same frequency.
The changing fields join up to form loops, which move away at the speed of light.
30.2.4 Spectrograph
The force on a moving charged particle in a magnetic field causes the charged particle to move in a circular path.
The mass spectrograph can be used to measure the mass of ions for identification.
30.2.5 Cylindrical electromagnets, test the poles
See diagram 29.175: Cylindrical electromagnet.
Experiment
1. Use an iron bolt 5 cm long with a nut and two washers.
Put a washer at each end and screw the nut on to the bolt.
Leave 30 cm of wire then wind three layers of bell wire on the bolt between the washers.
Leave another 30 cm of wire then cut the wire.
Twist together the two ends of the wire.
Wind insulating tape around the ends of the bolt to prevent the wire unwinding.
Remove insulation from the two ends of the wire to link the electromagnet in a circuit with two dry cells or lead cell accumulators in series.
Use a headlight bulb in series with the electromagnet.
2. Connect the circuit and then pick up pins and nails.
Disconnect the circuit and see the iron objects fall.
The magnetic force exists only when you turn on the current.
Use a plotting compass to test the poles at each end of the electromagnet.
Reverse the connections to the source of electricity and test the poles again.
30.2.6 Electricity from a magnet and a coil
See diagram 31.83: Electricity from a magnet and a coil.
Experiment
Connect a coil of fifty turns of bell wire to a current detector.
Use long connecting wires so that the coil, and the magnet are away from the compass in the current detector.
Hold the horseshoe magnet or bar magnet in your left hand and the coil of bell wire in your right hand.
Hold the coil vertically.
Pass one pole of the magnet through the coil while observing the compass needle in the current detector.
When the coil moves through the magnetic lines of force, an electric current moves through the circuit.
30.2.7 Electromagnetic induction
Electromagnetic induction, voltage produced by electromagnetism
See diagram 30.6.1: Electromagnetic induction.
See diagram 30.3.1: Electromagnetic induction.
See diagram: 32.1.23: Voltage produced by magnetism, generator.
See diagram: 4.1.1.6: Right hand motor rule for electron flow.
Voltage is produced in a conductor when the conductor moves through a magnetic field, or, a magnetic field moves through the conductor in a
way that cuts the magnetic lines of force of the field.
Generators produce electricity by electromagnetic induction.
Voltage can be produced by magnetism by the following:
a conductor, in which the voltage will be produced,
a magnetic field in the conductor's vicinity and,
relative motion between the field and the conductor.
So all the above must be present.
The conductor must be moved to cut across the magnetic lines of force, or, the field must be moved so that the lines of force are cut by the conductor.
When a conductor moves across a magnetic field to cut the lines of force, electrons within the conductor are moved in one direction or another so
an electromotive force, emf, or voltage, is created.
In diagram 32.1.23 note the following:
1. the magnetic field existing between the poles of the C-shape magnet,
2. the copper wire conductor,
3. the relative motion as the wire is moved across the magnetic field.
In diagram 32.1.23 (A) the copper wire conductor is moving towards
you because of the magnetically induced electromotive force,
emf, acting on the electrons in the copper.
The right hand end of the conductor becomes negative and the left
hand end becomes positive.
In diagram 32.1.23 (B) the conductor is not moving, so there is no
longer an induced emf and no difference in potential between the
two ends of the copper wire.
In diagram 32.1.23 (C) the conductor is moving away from you creating
an induced emf in the reversed direction.
In diagram 32.1.23(D) it shows a path for electron flow between the
ends of the conductor.
Electrons can leave the negative end and flow to the positive end
and continue as long as the emf exists.
The induced emf can also be created by holding the conductor stationary
and moving the magnetic field.
Experiment
1. Observe the current in the secondary when:
the primary current is started or stopped,
the primary current is increased or decreased by means of the variable resistance,
a bar magnet is moved in and out of the secondary coil.
Observe no current in the secondary coil when conditions are steady.
2. Connect batteries, a galvanometer and a resistance.
Let the wire connecting the resistance touch the negative pole of
the battery instantly.
Observe the deflection direction of the galvanometer needle.
Observe the relationship between the direction of the current and
the deflection direction of the galvanometer needle.
Move the wire frame ABCD down through the 2 poles to A'B'.
Observe the deflection direction of the galvanometer needle and record
the direction of the induced current at the wire frame.
Move the wire frame up from A'B' to the original position.
Observe the deflection direction and size of the galvanometer needle.
Record the direction of the induced current at the wire frame and
its change in size.
3. Repeat the experiment with different speed to
move the wire frame.
Observe the deflection direction and size of the galvanometer needle.
Record the direction of the induced current at the wire frame and
its change in size.
Move the magnet, i.e. magnetic field, up and down as well while keeping
the wire frame ABCD immovable.
Observe the deflection direction of the galvanometer needle.
Move a straight connecting wire AB up and down at the magnetic field.
Observe the deflection direction of the galvanometer needle.
Fold a piece of hard connecting wire into a rectangle and connect
it to a galvanometer, but leave an end open, i.e. it is not a closed loop.
Move its long side XY up and down at the magnetic field.
Observe the deflection direction of the galvanometer needle.
30.2.8 Electromagnetic induction applications, multimeter
See: Meters, SC5000, Multimeter Digital Low cost, (Commercial).
Multimeter
Experiment
1. Usually, the batteries must be fitted.
Two test leads, one red and one black, must be plugged into the multimeter.
A multimeter with two sockets has them colour coded to show which
lead plugs into which socket.
Switch on the power switch, if it has one, select one of the OHMS ranges using the main selector switch, a large dial on the front of the multimeter,
then connect the ends of the test leads or "probes" together.
The display should read close to zero.
2. Most multimeters have four measurement options:
1. DC voltage, 2. DC current, 3. AC voltage, and 4. Resistance (ohms), with each measurement option divided into ranges.
A range is named after the maximum reading possible, e.g. a multimeter has a 10 volt DC range will be overloaded if testing more than 10 volts.
So before testing, check that the multimeter is on a suitable range and the test leads are plugged into the right sockets.
Most multimeters do not have fuse protection for higher current ranges.
3. To measure DC current, place the multimeter
in series with the circuit so that all the current flows through the multimeter.
Do this by disconnecting a wire or component then reconnecting the
whole circuit.
When measuring current, start with the multimeter on the highest range and work down with the test leads plugged into
the correct socket.
Connect to the circuit with the correct polarity.
Connect the red probe to the more positive side of the circuit.
Connect the black probe to the more negative side of the circuit.
On a digital multimeter, if connecting with the wrong polarity, you
still get the right reading, but with a "minus" sign in front of the value.
4. To measure DC voltage place the multimeter in parallel with the circuit or component being measured and measure the potential difference (voltage)
between the two points.
The test leads must be in the right multimeter sockets, observe polarity
and start with the highest range and work down.
5. To measure AC voltage, follow the same steps
as with DC voltage.
During one cycle an AC wave starts at zero, rises to a peak, falls
back to zero and then does the same thing in the other direction.
Most multimeters show the "RMS" (Root Mean Square), value of the
voltage.
This value is a type of average for the voltage that is always changing.
The RMS voltage has exactly the same "work value" as a DC voltage, so if an electric heater is supplied with 240 volts RMS, and then 240 volts
DC, the heater would give out the same heat.
Most multimeters can only indicate the correct RMS voltage when the
AC signal is a sine wave.
6. To measuring resistance, the component being
measured must not be affected by other components in the circuit.
For example, do not measure a resistor when there is another resistor
in parallel is interfering with the reading.
So remove one end of the component from the circuit to avoid any
possible influence when measuring.
The power must be turned off before making any measurement or disconnecting
any components.
Select the lowest resistance range and place the probes across the
resistor or component being tested.
If the multimeter does not give a good reading, switch up to the
next range.
Chose the range where the multimeter gives the greatest number of
digits after the decimal point.
30.29 Electromagnetic induction with two solenoids
See diagram 32.2.67: Induction with two solenoids.
When the number of magnetic lines of force from a conducting circuit changes, an induced current flows through the circuit during the change.
Experiment
1. Connect the solenoid 1, with many turns of insulated copper wire, to the centre zero galvanometer G, with full scale deflection 0.002 amps.
Its terminals should allow the direction of deflection to show the direction of the current.
Push a magnet quickly into the solenoid and observe the galvanometer.
Hold the magnet inside the solenoid and observe the galvanometer.
Withdraw the magnet quickly from the solenoid and observe the galvanometer.
2. Connect the solenoid 2, with few turns of insulated thick copper wire, to the Daniell cell, D.
3. Repeat the experiment with solenoid 2 connected to the Daniell cell, D.
Put a switch S in series with solenoid 2 and put it inside solenoid 1.
Close switch S and observe the galvanometer G.
Let the current flow through the solenoid 2, inside solenoid 1, and observe the galvanometer G.
Open switch S and observe the galvanometer G.
3. Show that the direction of the induced current
causes a magnetic field opposing the change taking place.
* Connect solenoid 1 to the galvanometer G.
Insert the north pole of the magnet quickly into end A solenoid 1.
Note the direction of the induced current, and, knowing the direction of winding on solenoid 1 deduce the polarity of end A of the solenoid.
Withdraw the magnet from end A of solenoid 1 and again deduce the polarity of end A of solenoid 1.
* Repeat the experiment using the south pole of the magnet.
* Connect solenoid 2, to the Daniell cell D.
Knowing the direction of the current in solenoid 2, and the direction of the windings of solenoid 2, deduce its polarity.
Using solenoid 2 as an electromagnet, instead of the magnet, repeat the experiment.
4. Show that the size of the induced emf and the induced current depends on the rate of change of the magnetic field associated with the circuit.
5. Repeat the experiment by varying the rate at which the magnet or solenoid 2, is pushed into, or withdrawn from, solenoid 1.
30.2.10 Direction of induced current
See diagram 30.6.01b: Direction of induced current.
Experiment
Fix a wire between the poles of an Ushered magnet.
Attach each end of the wire to a galvanometer.
Move the magnet in three directions and note any deflection of the pointer in the galvanometer.
The wire does not move.
Direction 1. AB, the magnet moves vertically up and down.
Direction 2. CD, the magnet moves horizontally, so that one of the poles becomes closer or further away from the wire.
Direction 3. EF, magnet moves horizontally, forwards and backwards in the direction of the wire.
In each case, when the magnet reverses direction, the pointer of the centre zero galvanometer changes the direction off deflection.
30.2.11 Faraday's law for induced emf
See: Faraday (Commercial).
Faraday's law of electromagnetic induction states that the induced emf (volts) of a conductor moving through a magnetic field depends on the rate of change of magnetic flux (weber).
In a coil with n loops,
(Induced emf, = - n × change of magnetic flux / time).
Use "-" because induced emf opposes the change that produces it, as in Lenz's law.
If a conductor wire length L moves with velocity v in a magnetic field, B, cutting field lines, it contains an induced emf = BLv.
(B, v and direction of conductor wire mutually at right angles).
If a magnetic flux in a coil of N turns changes by Φ, in time t, the induced emf =
-N( Φ / t), = N (Φ final - Φ initial / t final - t initial), volts.
Experiment
Test Faraday's laws
See diagram 30.5.01a: Test Faraday's laws.
The induced emf increases with the speed of the magnet or the coil relative to each other, the number of turns in the coil, and the strength of the magnet.
Test Faraday's laws by varying the speed of moving the magnet inside the coil,
decrease or expand the number of turns in the coil, while moving the magnet at constant speed.
Use a stronger magnet and compare the deflection in the galvanometer.
30.2.12 Horseshoe electromagnets
See diagram 31.79: Horseshoe electromagnets.
Do NOT use a lead cell accumulator, car battery, for this experiment, because the resistance of these coils is low and the current will be too large
with a significant fire risk.
If you use horseshoe magnets or C-shape magnets, wind the coil in opposite directions on each arm of the magnet.
Experiment
Use a U-shape piece of iron.
Wind a coil of three layers of bell wire on each straight arm of the iron, but not on the curving part.
Leave 30 cm of wire before you start winding the coil from the end of one arm.
Cross to the other arm.
Wind a coil of three layers and leave 30 cm of wire at the end.
Wind three layers of wire on this pole then wind insulating tape around the wires so they cannot unwind.
Remove the insulation from the ends of the coil, connect the horseshoe magnet in series with a car headlight bulb, connect to two dry cells or lead cell accumulators, and
test the poles of the electromagnet.
One pole should be a north pole and the other pole should be a south pole.
If each pole has the same polarity, you have wound the second coil in the wrong direction so you must unwind the coil and rewind it in the opposite direction.
Use the magnet to attract different things.
Compare the strength of this electromagnet with the cylindrical electromagnet.
Use the magnet to attract different things.
Reverse the connections to the source of electricity and test the poles again.
30.25.13 Induced emf, in conductor moving in magnetic field, open right-hand rule
See diagram 29.2.1.5: Open right-hand rule.
If current flows perpendicular to a uniform external magnetic field, then a magnetic force will be produced on the current at right angles to the
directions of both the current and the magnetic field.
Experiment
To observe electromagnetic induction, wrap 50 turns of insulated wire diameter >5 cm around a cylinder with a smooth surface and leave connecting wires.
Put the coil on the cylinder and fix every turn of the coil with adhesive plastic.
Connect the 2 ends of this coil to a galvanometer to form a closed circuit.
Insert a pole of a horseshoe-shaped electromagnet into the coil and observe whether the compass of the galvanometer moves.
After the compass settles down, quickly take the coil off the magnet while keeping the magnet immovable.
Observe whether the compass of the galvanometer moves.
Cover the coil into another pole of the magnet and slowly move it around.
Observe whether the compass of the galvanometer moves again.
30.2.14 Magnetic field from 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.
30.2.15 Magnetic field inside an open coil
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.
30.2.16 Make a spark
Connect the primary coil to a potential difference of four volts and adjust the contact points to open until you can hear a buzzing sound.
Observe the points of the adjustable rods connected to the ends of the secondary coil when you move the secondary coil backwards and forwards over the primary coil.
A spark forms.
The length of this spark indicates how much potential difference.
It is greatest when the secondary coil completely surrounds the primary coil.
30.2.17 Moving magnet induces current in a coil
See diagram 29.180: Electricity from magnet and coil.
See diagram 30.6.2.3: Crude galvanometer with large coil and magnet.
See diagram 30.6.2.3.1: Induction coil and magnet.
Move a magnet in and out of a coil connected to the galvanometer.
Experiment
1. Construct a simple current detector by fixing a compass needle inside a coil connected to bell wire.
Connect a coil of fifty turns of bell wire to the simple current detector.
Use long connecting wires to ensure that the coil and the magnet are a long way from the simple current detector.
Hold the horseshoe magnet or bar magnet in your left hand and the coil of bell wire in your right-hand.
Hold the coil vertically.
While observing the simple current detector, pass one pole of the magnet (a) towards the coil (b) away from the coil (c) stationary within the coil.
During (a) and (b) the compass needle in the simple current detector deflects in opposite directions.
During (c) the compass needle in the simple current detector does not deflect.
The moving magnet induces a current when it is moving towards or away from a coil.
The same magnet held stationary within the coil does not induce any current.
2. Connect a large coil to a galvanometer or ammeter.
Insert a permanent magnet's north pole into the coil and observe the deflection direction of the galvanometer needle.
Take the magnet away from the coil and observe the deflection direction of the galvanometer needle again.
Repeat the experiment with different speeds to insert the magnet into the coil or remove the magnet from the coil.
Observe the deflection direction of the galvanometer needle.
3. Repeat the experiment, but with the magnet's south pole.
Observe the deflection direction of the galvanometer needle.
Place a small coil into the large coil and connect the small coil to a battery and a switch with connecting wire to form a closed loop.
Turn the switch on to make the small coil carry current.
Take it away from the large coil and observe the deflection direction of the galvanometer needle.
Insert the small coil into the large coil again and observe the deflection direction of the galvanometer needle.
Change the direction of the current at the small coil by inverting the lead connecting the battery.
30.2.18 Retentivity
Retentivity means ability to respond after stimulus is removed, e.g. in ferromagnetic substances measured as residual flux density.
In a magnetic hysteresis loop, retentivity is the value of B at zero magnetic field and is called remanence.
A soft iron bar will cling to a U-shaped electromagnet when the current is turned off, but no longer attract after it is pulled away.
30.2.19 Spark in a spark plug.
See 32.5.5.0: Motor vehicle ignition system.
Electromagnetic induction
Connect the primary lead of a car ignition coil to a 6 volt or a 12 volt battery.
Join the secondary lead from the coil to the central electrode of a spark plug.
Connect the outer electrode to the other secondary wire.
They joined to the outer metal case.
Use a switch to start and stop the primary current and observe the electrodes of the spark plug.
30.2.20 Study an electromagnet
See diagram 32.2.3: Electric kettle heating efficiency.
Experiment
To study the magnetism and polarity of an electromagnet, wrap about 20 turns of wire around a large nail.
Use the connecting wires to connect the nail to a power supply via a touch bulb.
Here the role of the bulb is to show if the circuit is on or off.
Set the power supply to 2, 4, 6 volts DC, and turn it on in turn.
Each time, use a pocket compass to test which is the north pole and south pole of the electromagnet.
Reverse the connections to the power supply under the condition of the same voltages.
Observe what will happens.
Finally, use the head of the nail attracted pins and observe the number of the pins being attracted roughly.
Record the phenomenon under each voltage.
Increase the number of the wire turns of the nail to 40 turns.
Repeat the steps of the above experiment and take a detail record.
Let every student observe and analyse the record seriously and independently.
Think how to conclude the record into several aspects, each one can be described only by one or two sentences.
To study the magnetism and polarity of an electromagnet, wrap about 20 turns of wire around a large nail.
Use the connecting wires to connect the nail to a power supply via a touch bulb.
Here the role of the bulb is to show if the circuit is on or off.
Set the power supply to 2, 4, 6 volts DC, and turn it on in turn.
Each time, use a pocket compass to test which is the north pole and south pole of the electromagnet.
Reverse the connections to the power supply under the condition of the same voltages.
Observe what will happens.
Finally, use the head of the nail attracted pins and observe the number of the pins being attracted roughly.
Record the phenomenon under each voltage.
Increase the number of the wire turns of the nail to 40 turns.
Repeat the steps of the above experiment and take a detail record.
Let every student observe and analyse the record seriously and independently.
Think how to conclude the record into several aspects, each one can be described only by one or two sentences.
30.2.21 Test the strength of electromagnets
Do not use lead cell accumulators for this experiment because the resistance of these coils is low and the current will be large with a significant fire risk.
1. Wind 25 turns of bell wire on a straight iron bolt and connect one dry cell or lead cell accumulator to the ends of the wire.
Record the number of pins or paper clips you can pick up with the electromagnet.
2. Repeat the experiment with two dry cells or lead cell accumulators connected in series.
3. Wind on 25 more turns of wire in the same direction.
Join them to the first 25 turns.
Repeat the experiment.
4. Repeat the experiment with two dry cells or lead cell accumulators connected in series.
5. Wind on another 50 turns.
Join them to the first 50 turns.
Repeat the experiment.
6. Repeat the experiment with two dry cells or lead cell accumulators connected in series.
Remove 50 turns and rewind them on the bolt in the opposite direction.
7. With 100 turns so wound, repeat the experiment with two dry cells or lead cell accumulators connected in series.
30.3.1 Electromagnetism, Open right-hand rule
See diagram: 29.2.1.5: Open right-hand rule.
1. By the open right-hand rule, the extended thumb points in the direction of the conventional current, I, the fingers point in the direction of the magnetic
field, B, the pushing palm points in the direction of magnetic force, F.
2. Induced emf, dynamo, electric generator
See: Dynamo/generator (Commercial).
A changing magnetic field can induce a current in a circuit.
The force driving the current is the induced electromotive force.
The induced current occurs only when the magnetic field changes and the size of the induced current depends on the rate of change of the magnetic field.
A conductor moving to the right, at a right angle to a uniform magnetic field, will have an induced emf, V, across its ends.
he thumb points in the direction of the movement of the conductor, because positive charges move in that direction, i.e. the direction of the conventional
current, the fingers point in the direction of the magnetic field, B, the pushing palm shows the end of the conductor that becomes positive.
The other end becomes negative as electrons move to that end.
The resulting potential difference across the ends of the rod is called the induced emf, in volts.
If you connect the ends of the moving conductor through an external circuit, a conventional current will flow externally from the positive end to the
negative end, and inside the conductor from the negative end to the positive end.
The current through the conductor moving perpendicular to magnetic field will cause a magnetic force to oppose further movement of the conductor.
The conductor can keep moving only if an applied force is used.
The size of the induced emf depends on the velocity of the conductor.
3. A motor turns a coil in magnetic field, B.
The coil with N loops, each area A, rotates with frequency F for time t, cutting the magnetic field lines to induce an emf between its terminals =
2π X NABF X cos 2pft.
The mechanical energy input from the motor that turns the coil =
electrical energy output + (energy lost to heat and friction).
30.3.2 Fleming's left-hand rule
See diagram 30.6.0:Fleming's left-hand rule.
A current carrying conductor at a right angle to a uniform magnetic field experiences a force.
Use Fleming's left-hand rule: the first finger points in the direction of the magnetic field, the middle finger points in the direction of the conventional current,
the thumb points in the direction of thrust, movement of the conductor.
If the current carrying conductor is at less than a right angle to the uniform magnetic field, the thrust is less.
If the current carrying conductor is parallel to the uniform magnetic field, the thrust is zero.
30.4.1 Attraction and repulsion of parallel electric currents
(Use of open surface mercury is illegal in some school systems!)
See diagram 30.3.11.0: Parallel electric currents.
See diagram 30.3.11.01: Parallel wires.
See diagram 30.3.11.1: Electric compass.
See diagram 30.3.11.2: Magnetic effects of currents in a coil.
André-Marie Ampère (1775-1836, France), showed that parallel electric currents attract each other if they move in the same direction and repel
if they move in opposite direction because of the magnetic fields they generate.
However, the force of attraction is negligible at low current values.
An electric current passing through a coil acts like a magnet.
Experiment
1. Two insulated wires hang from a frame.
Turn on the switch.
Wires will attract or repel each other.
Turn off the switch and reverse the leads in the binding posts.
Turn the switch on.
Wires will repel or attract each other.
2. Use two copper wires 50 cm long on copper loops, so that their free ends are 1 cm apart and hang in mercury.
* Connect one terminal of a 6 volt battery to the upper ends of both copper wires and connect the other terminal to the mercury.
Turn the switch on and observe any movement of the copper wires.
In this experiment the electrons were flowing in the same direction in the 2 wires.
* Change the circuit to make the current flow in opposite directions in the wires.
Turn the switch on and observe any movement in the wires.
Notice what happens.
* To show that the forces between wires carrying electric current are not electrostatic, hold a strip of charged Perspex then a permanent magnet near the wire.
The magnet does not attract small pieces of paper or wood shavings.
Charged Perspex does attract small pieces of paper or wood shavings.
3. To avoid the use of mercury on this experiment, informants reported that they replaced the mercury pool with a saturated solution of sodium chloride.
The informants observed significant hydrolysis and was able to make coils that interacted with magnetic fields from bar magnets brought into proximity.
However, they were unable to observe anything more than a very tiny movement of the coil by itself in one instance, certainly no "dancing".
They tried various sources of copper wire of different gauges with a large six volt battery to no avail.
4. Wind a coil around a perspex rod.
Attach each end of the coil to a battery and use a hook to raise the wires and so suspend the coil around the perspex rod.
Close the circuit and the coil behave like a suspended bar magnet affected by a magnetic field.
5. Use a bar magnet to determine the magnetic effects of current in a coil.
30.4.2 Attraction and repulsion of parallel coils carrying electric current
See diagram 30.3.12: Attraction and repulsion of parallel coils.
See diagram 30.3.12.1: Interacting coils.
Experiment
1. Hang two coils from a support rod.
Connect the DC power supply so that the current is in the same direction in both coils and watch them attract.
Connect the DC power supply in opposite directions in the coils and watch them repel.
A good storage battery can be used instead of the power supply.
2. Wind 2 coils each of 20 turns of insulated copper wire around a plastic drink bottle.
Fix the coils so that they are standing upright and 2 cm apart.
Connect both coils to a 6 volt battery so that the currents flow around the 2 coils in the same direction.
3. Change the connecting wires to make the current flow in the opposite direction.
When the currents flow in the same direction, the coils move together.
When the currents flow in opposite direction, the coils move apart.
30.4.3 Bar magnet in coil-carrying current
See diagram 30.3.12: Bar magnet in coil, carrying current.
Experiment
1. Hold one end, then the other end, of a bar magnet near one coil carrying current.
Note the attraction or repulsion depending on the end of the magnet and the direction of current through the coil.
The circular coil carrying current behaves like a bar magnet because of electric currents within the iron.
2. Repeat the experiment by substituting a balanced compass needle for the bar magnet.
30.4.4 Dancing spring, electric current in parallel coils
(Use of open surface mercury is illegal in some school systems!)
See diagram 30.3.10: Dancing spring.
This experiment shows the effect adjacent coils have on one another if the current is running in the same direction in each coil.
Experiment
1. Current is passed through a limp copper spring dangling in a pool of mercury causing it to dance.
A helix of fine wire hanging vertically into a pool of mercury contracts and breaks contact repeatedly.
2. A large heavy wire clip rests in pools of mercury between the poles of a strong magnet.
3. An aluminium bar in a magnet has its ends in mercury.
Short the mercury pools to a storage battery and the aluminium bar hits the ceiling.
4. A wire hangs into a pool of mercury and between the poles of a U-shaped magnet.
Current is passed through the wire it deflects out of the mercury and breaks the circuit.
5. Jumping coil of wire
See diagram 30.3.10.1: Jumping coil of wire.
A long wire is coiled around one pole of a permanent magnet.
Close the switch.
The wire collapses into the magnet.
Turn the coil over and repeat.
The wire "jumps" out of the magnet.
6. A coil of wire wound around one pole of a horseshoe magnet jumps off when energized.
Run twenty amps through a wire in a horseshoe magnet.
30.4.5 Magnetic field of electric current in a circular coil
Experiment
Wind some turns of insulated copper wire around an iron nail.
Switch on power supply and watch the iron flings on the piece of paper.
Switch off the current and watch the iron filings.
More filings are attracted to the nail when the current is on.
The nail becomes a magnet when the current flows around it.
Most of the iron filings fall off when you switch the current off, which shows that you have not permanently magnetized the iron.
However, some filings still cling to the nail showing that the nail must retain some magnetism produced by the electric current.
30.4.6 Magnetic fields of a solenoidzzz
See: Magnetic fields, Solenoid, (Commercial).
See diagram 30.3.9a: Magnetic field of a solenoid.
See diagram 30.3.3: Magnetic field of solenoid.
See diagram 29.179: Open solenoid.
Experiments
1. To map the magnetic fields of a solenoid and a bar magnet, draw 2 parallel lines 2 cm apart on a 10 cm2 sheet of cardboard.
Punch two rows of holes through the cardboard, 0.5 cm apart.
Using DC C22 gauge copper wire, wind a solenoid onto the cardboard by threading the wire through the holes.
Support the cardboard horizontally, sprinkle iron filings over it and connect your solenoid to a 6 volt battery.
Tap the cardboard and notice what happens to the iron filings.
The little pieces of iron have themselves become magnetized by induction and, when jostled by tapping, set themselves in the direction of the fields at the various positions.
2. Place a card through a solenoid.
The solenoid could consist of many turns of a wire spring or a thin wire twisted round a suitable former, e.g. a test-tube.
Connect the solenoid to a switch and battery and sprinkle iron filings on the cardboard.
Note if you see a similarity in the patterns produced by the bar magnet and the solenoid.
3. Use a magnetic field about a conductor wind a coil of wire around it.
The magnetic fields created by the current in the several loops of the coil combine into one composite field as to create a solenoid.
The strength of the magnetic field of a solenoid is proportional to the number of turns of wire in the coil and the current flowing through the wire.
You can measure the strength of the magnetic field of a solenoid in ampere turns, i.e. current in amperes × number of turns.
If a soft iron core is inside a solenoid, the lines of magnetic force will take the easier path through the iron, and the magnetic field produced at the ends of the iron core will be stronger.
To make a coil, wrap an insulated connecting wire around a small test-tube turn by turn and leave connecting wires.
Peel off their end insulation layers then connect them to a DC low voltage electrical source.
If you use a dry cell battery, do not turn it on for too long a time.
Place several large needles into the test-tube.
Turn on the electrical source.
Note the needles' directions such as their needle points pointing to the bottom or mouth of the test-tube.
Take the needles out of the test-tube and let them to attract fine iron filings or pins to observe every needle's magnetism intensity.
4. 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 length ways.
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.
5. To observe the distribution of magnetic lines of force inside an open solenoid, wrap 5 turns of single electric connecting wire around a cylinder
with a smooth surface and leave connecting wires.
Put the coil off the cylinder and separate theses turns at certain distance.
Cut off 2 grooves from a cardboard cylinder, with each groove wider than the diameter of the connecting wire and the distance between the 2 grooves is narrower than the inner diameter of the coil.
The bar of the cardboard between the two grooves can support the coil when you insert the coil into the grooves.
Connect the 2 ends of this coil to a DC source.
Connect a slip rheostat in series to prevent the electrical source overloading.
Scatter fine iron filings on the cardboard.
Gently tap the cardboard then observe the distribution of the iron filings.
Usually you call such a magnetic field a magnetic field inside an open solenoid.
If there is no space between turns of a coil, you call it a closed solenoid.
This method is also used to observe magnetic field inside a closed solenoid.
Another method is to substitute sheets of plastic or Plexiglas for the cardboard.
Cut holes in the sheets corresponding to turns of the coil.
30.4.7 Magnetize inside a coil, solenoid carrying current
Experiment
Use an electrical source, which may provide 5A current.
Connect it to a solenoid and a switch.
Use a 4 cm × 5 cm piece of sheet iron as a movable gauge.
At one end solder a piece of wire to move it.
At the other end solder a metal ball to make the gauge keep in a vertical plane.
Attach a piece of wire to the solenoid with adhesive tape.
Place the removable gauge into the solenoid then turn on the switch to make the solenoid current.
Here the solenoid carrying current produces magnetic field inside the solenoid.
If you magnetize the gauge and wire at the same time, they repel each other so that the gauge is moved.
The distance moved depends on the size of the magnetic force between the gauge and the wire.
The magnetic force depends on the magnetization characters of the materials of the gauge and wire and the size of the current at the solenoid.
So the size of the current may be estimated according to the distance.
30.4.8Nail becomes an electromagnet
An electromagnet is a piece of soft iron surrounded by a coil of wire so that it can become temporarily magnetic by passing electric current through the wire.
Use a big nail that will not pick up iron filings.
Hold it inside a coil and turn the current on and off.
The nail picks up iron filings when the current is turned on, but the iron filings drop off when the current is turned off.
30.4.9 Solenoid affects iron nails
Experiment
Wind fifty turns of cotton covered copper wire on an 8 cm length of glass tubing, 0.6 cm in diameter.
Such a coil is called a solenoid.
Connect this to your power supply with a switch.
Make a better switch with a piece of springy brass from an old torch battery and several thumbtacks.
Place a nail halfway into one end of the glass tube and switch the current on for a second.
The electron current in the coil produces a strong magnetic effect around it and this pulls the nail into the coil.
30.5.1 Motors and generators, DC / AC, Electric motor 4
30.5.1 Motors and generators, DC / AC, Electric motor 4
See: Dynamo/generator (Commercial).
See diagram 30.1.3.7: Electric motor 4.
1. Electric generators use mechanical work to produce electrical energy with emf, where = NBAω sinωt.
where N = number of turns of the coil, B = magnetic field, A = area of the coil, and ω = the angular speed of the coil.
2. An electric motor converts electrical energy into mechanical energy, like an electric generator operating in reverse.
The coil in an electric motor moves in the same way as the moving coil in a meter.
Wind the coil on the core so that both move round.
This is called an armature.
As the coil in the electric motor turns from one side of the magnetic field to the other, the current through it must be reversed, so that forces on it will keep it
rotating in one direction.
Make the electrical contacts with the coil in the motor through a split ring commutator, so that the current reverses at the right position.
3. The current flowing through a coil in a magnetic field produces a torque on the coil = NIAB sin A, where A = angle between the magnetic field
lines and a line at right angles to the plane of the coil.
A split ring commutator reverses the direction of current, I, each time "sin A" changes sign, so the torque always rotates the coil in the same direction.
However, the rotating coil also acts as a generator to produce a back emf in the coil that opposes the source of voltage that drives the motor.
(Potential difference across the terminals of the coil = voltage supplied to the coil - a back emf).
(Current through the coil = voltage supplied to the coil - back emf / resistance of the coil).
30.5.2 Bicycle dynamo, the "missing wire"
See: Dynamo/generator (Commercial).
See diagram 30.1.3.1: Bicycle dynamo.
1. A dynamo is a machine that converts mechanical energy into electric energy by electric induction, usually by a rotation conductor in a magnetic
field, e.g. the bicycle dynamo.
People who ride bicycles with lights powered by bicycle dynamos are aware that the faster they ride the brighter the headlight.
To demonstrate the effect of speed in the laboratory, mount the dynamo on a spindle that can be turned with a crank.
Complete the circuit by connecting the electrical output leads to 1.5 V lamp in a lamp holder to a galvanometer, 2.0 - 2.5 MA DC.
The faster you turn the crank the brighter the light or the greater the reading on the galvanometer.
A dynamo converts mechanical energy into ac or dc electrical energy usually by rotating a conductor in a magnetic field.
2. The four ways to get a dynamo to give a higher current are as follows:
* Use stronger magnets.
This generates a higher voltage that forces a higher current in the circuit.
* Reduce the load, i.e. the resistance of the external circuit of lamps and appliances.
* Rewind the dynamo with heavier wires because its own internal resistance of windings may not carry a bigger current without overheating.
* Run it faster.
In a bicycle dynamo the rotor is a permanent magnet and the emf is induced in the coil that remains at rest.
3. Bicycle dynamo, the "missing wire" in a bicycle generator circuit
In a bicycle dynamo, the rotor is a permanent magnet, usually an 8 pole circular magnet and the emf is induced in two coils that are stationary in reference to the rotor.
Only one wire connects a bicycle generator to the bicycle light.
Where is the other wire to complete the circuit?
When the bicycle is in motion, a permanent magnet inside the generator rotates in the centre of a coil of tightly wound copper wire.
The spinning magnet creates electricity in the copper coil.
Electricity flows through the wire to the light, through the filament of the bulb, through the light casing, the bicycle fork, and the metal casing of the generator, back to the coil in the generator.
A screw in the generator casing pierces the layer of paint on the fork to complete the circuit.
30.5.3 Electric generator, alternator, AC generator
See: Dynamo/generator (Commercial).
An electric generator is a machine that converts mechanical energy into electrical power.
Electromagnetic generators work by either:
* rotating a magnet inside a coil, e.g. bicycle "dynamo", or
* rotating a coil inside a magnet, e.g. motor car generator, power station.
An electromagnetic generator that produces alternating current it may be called an alternator, but if it produces direct current it may be called a dynamo.
A bicycle dynamo, e.g. the popular sidewall-running bottle-shaped "dynamo", produces alternating current, so it really is a bicycle "generator" if this distinction is made.
See diagram 32.5.6.4: Voltage of a generator.
In a motor vehicle, a generator gets mechanical energy from the fan belt and, in turn, produces electrical energy that charges the battery by being
converted into chemical energy converts some electrical energy into heat in the armature and the connecting wires.
Some chemical energy from the battery may be converted back to electrical energy that may be converted into light, or sound when the horn is pressed.
The generator has an inner set of coils that become an electromagnet that turn inside an outer set of coils.
30.5.4 Force on a current-carrying wire in a magnetic field
See diagram: 29.2.1.5: Open right-hand rule.
1. A wire carrying an electrical current through in a magnetic field, has each of the moving charges of the current experiencing the Lorentz force,
which together create a force on the wire, the Laplace force.
So F = I × × B, where I is the conventional current flow I, is the length of wire, and B is the magnetic field.
2. Right-hand rule for a current-carrying wire in a magnetic field B
Apply the right-hand rule with the hand flat on the table with the fingers pointing to the left and the thumb pointing away from you, the fingers point
in the direction of the magnetic field, the thumb points in the direction of conventional current and the direction of the force is from the palm upward.
The angle between the fingers and the thumb, α, is 90o.
3. If you think of conventional electric current as a flow of positive electric charges, the electric current will experience a force if it flows through a
magnetic field in the same way that charges moving through a magnetic field experiences a force predicted by the open right-hand rule.
The force F (newton) on a straight uniform wire length in a uniform magnetic field = I (amperes) × (metres) × B (tesla) sin α, where α =
angle between the direction of the current, I, and the direction of the magnetic field.
If the wire is parallel to the magnetic field lines, the force is zero.
If the wire is at right angles to the magnetic field lines, the force is maximum.
Experiment
See diagram 30.1.3.0.1: Force on a current-carrying wire in a magnetic field.
1. Use two retort stands to hang a thin strip of aluminium strip between the poles of the magnet, e.g. Rola permanent magnet.
Connect an AC / DC Power supply (10 amps).
Close the switch, S1, and the aluminium strip experiences a force caused by the interaction between the current I and the magnetic field B.
Use a rotary switch, S2, to reverse the direction of current flow to deflect the strip in the opposite direction.
2. A rectangular loop of wire aligns perpendicular to a magnetic field.
30.5.5 Homopolar motor, unipolar motor
See diagram 30.1.3.6: Homopolar motor.
Experiment
1. A homopolar motor does not have components to reverse the polarity during operation, there is only one current-carrying path.
Experiment
Wear safety glasses.
Put a flat head ferromagnetic screw on a circular neodymium disk magnet with conductive sides, with the screw flat head down.
Attach the magnet and screw to one end of a 1.5 V battery or an alkaline C-cell, so that the contact between the point of the screw and the battery can serve as a bearing.
With the left hand, hold the upper stripped end of the wire down on the negative end of the battery.
With the right hand, lightly touch the lower stripped end of the wire to the side of the circular magnet.
The magnet and screw start to spin.
The battery is short circuited by the copper wire, so run the motor for a very short before the battery is used up and the copper wire gets very hot!
2. Use the Lorentz force law to determine the direction of spin.
Conventional current flows from the + end of the battery to the screw, sideways through the magnet to the wire, through the wire to the negative end of the battery.
The magnetic field, B, is parallel to the axis of symmetry of the magnet, i.e. upward.
Electric current, i, flows through the magnet from the centre of the magnet to the edge, i.e. in ae radial direction, perpendicular to axis of symmetry of the magnet.
So there is a force, i × B, forwards and perpendicular both to the moving electric charges and the direction of the magnetic field.
The force is in the tangential direction, and the magnet begins to spin, forwards, towards you.