School Science Lessons
(UNPh32.1)
2024-07-25
Resistance, Voltage, Thermistors
Contents
32.3.0 Resistance and Ohm's law, resistivity, resistors
32.1.0 Voltage, emf sources, Voltage produced by
32.2.0 Thermistors, conductivity and temperature
32.3.0 Resistance and Ohm's law, resisitivity, resistors
See: Resistances, (Commercial).
32.3.2.0 Resistivity with metre wire bridge, (Experiment)
32.3.2.1 Heat and cool resistors, (Experiment)
32.4.6.9 Heat and light from electricity, make a model electric lamp, (Experiment)
32.4.6.7 Internal resistance of a cell, (Experiment)
32.3.4.4 Ionization by radioactivity, smoke alarms, (Experiment)
32.3.3.2 Migration of ions, speed of ions, (Experiment)
32.3.01 Ohm's law, rheostat
32.3.2.2 Put the light out with heat
32.3.2 0 Resistivity with metre wire bridge, resistivity and temperature
32.3.1.4 Megger
32.3.1.3 Ohmic and non-ohmic resistors (nonohmic resistors)
32.3.1.5 Resistance change and speed of electric motor
32.3.1.2 Resistance models
32.3.1.0 Resistor assortment, resistor colour code
32.3.1.3 Resistors, Ohmic and non-ohmic resistors, (nonohmic resistors)
32.3.1.1 Resistors, resistances, switches, resistance box
32.3.4.7 Thermionic effect in air, thermionic emission
32.1.0 Voltage, emf sources, Voltage produced by:
32.1.0 Voltage, emf sources, Voltage produced by:
4.2.11 Australian voltage
32.4.6.6 Electromotive force, emf, ε.
32.4.4.7 IR drop in a wire, potential drop along a wire
32.4.4.8 Potential drop with Wimshurst machine, static machine
TASER
Volt
32.1.5.0 Voltage produced by chemical action, battery
32.1.4 Voltage produced by light, Photoelectric cell
Experiment
32.4.6.6.1 The emf and internal resistance of a cell with an ammeter and a voltmeter
32.4.6.6.2 The emf of two cells with a potentiometer
32.1.4 Photoelectric cell, voltage produced by light
32.1.2.1 Piezoelectricity, voltage produced by mechanical stress to crystals
35.33.0 Piezoelectricity with red lead and sulfur
35.33.1 Pyroelectricity, ferroelectricity
Volt, voltage
Voltage = electrical potential difference
32.1.0 Voltage, emf sources, Voltage produced by:
32.1.2.1 Voltage produced by mechanical stress to crystals, piezoelectricity, (Experiment)
32.2.0 Thermistors, conductivity and temperature
See: Thermistors (Commercial).
32.3.4.0 Conduction in gases, Jacob's ladder, (Experiment)
32.3.2.7 Electrical conductivity of molten glass at high temperature, (Experiment)
32.3.2.1 Heat and cool resistors, (Experiment)
32.3.2.2 Put the light out with heat, (Experiment)
32.3.2.01 Resistance and temperature
32.3.2.6 Thermistors
32.1.0 Voltage, emf sources, Voltage produced by:
voltage = electrical potential difference
1. emf
The force that causes free electrons to move in a conductor may be called voltage, electromotive force, emf, difference in potential difference or even "electrical pressure".
The electromotive force, emf, is not really a "force" measured in newtons, but a potential, i.e. energy per unit charge, measured in volts, V.
Potential difference can be thought of as the concentration of charge, measured in volts, V.
The energy available to charges moving in an electrical circuit is measured using electric potential difference, which is defined as the change in
potential energy per unit charge between two defined points in the circuit.
Energy is required to separate positive and negative charge carriers; charge separation produces an electrical potential difference that can be used to drive current in circuits.
An emf of 1 volt (1V) across a component with resistance of 1 ohm drives a current of 1 A through it.
2. Volt
The volt, symbol V (Alessandro Volta 1745-1827), is the SI unit of electric potential (potential difference, emf).
The value expressed in volts is called the voltage, defined as the difference of potential between two points on a conductor carrying one ampere of current when the power dissipated is one watt.
A potential difference of 1 volt = 1 joule per coulomb, 1 J / C.
3. Power
Power is the rate at which energy is transformed by a circuit component; power enables quantitative analysis of energy transformations in the circuit.
If connected by a conductor, electrons will flow from a negatively charged body to a positively charged body until the two charges are equal and the potential difference no longer exists.
When a cell does work W to drive a charge Q around a circuit, the cell has an electromotive force E, E = W / Q = P / I.
(where W = watt, the SI unit of power, charge = Q, P = power and I = current).
P = W t = VI = IR x I = I2R.
P = power, W = work = energy transformed, t = time interval, V = potential difference, I = current.
So a source of potential difference, e.g. a cell, has electromotive force, emf in this document.
However, emf is not a "force" although it does cause charges to move around the circuit.
The emf is really energy per unit charge.
Potential difference is different from emf, because in current electricity potential difference always refers to energy loss in a circuit, e.g. conversion to heat and light in an incandescent bulb.
The unit of potential difference is the volt, V.
An electric current can flow in a conductor only if a potential difference, V, exists across it.
32.1.2.1 Piezoelectricity
Piezoelectricity is voltage produced by mechanical stress to crystals.
Ammonium dihydrogen phosphate, monobasic ammonium phosphate (piezoelectric crystal in microphones and transducers)
Earphones, crystal microphones: 38.2.05
Potassium sodium tartrate-4-water
Piezoelectricity: 32.1.2.1
Piezoelectricity with red lead and sulfur: 35.33.0
Transducer, carbon microphone: 26.9.01
Rochelle salt, potassium sodium tartrate-4-water
See diagram 31.1.1.2: Voltage produced by pressure.
See diagram 16.3.7: Potassium sodium tartrate.
Experiment
1. Show ferroelectricity hysteresis, Curie point and the direct piezoelectric effect with Rochelle salt, (potassium sodium tartrate-4-water).
Natural crystals are rare, e.g. diamond.
They manufacture most crystals used in industry.
When a crystal of quartz or Rochelle salt (Seignette salt) is compressed, some electrons move through the crystal.
This movement creates an electric potential difference between the two opposite faces of the crystal.
If an external wire is connected while the pressure and emf are present, electrons will flow until the charges are equalized.
When the force is removed, the crystal is decompressed, and immediately causes an electric force in the opposite direction.
The crystal can convert mechanical force to electrical force.
Although the power capacity of a crystal is extremely small, they are useful, because of their extreme sensitivity to changes of mechanical force or changes in temperature.
The regular piezoelectric signal from a quartz crystal in an electronic watch allow the watch to keep almost perfect time.
Piezoelectricity is the phenomenon in certain crystals anc ceramics when application of mechanical stress causes electric charge with the voltage proportional to the stress.
It occurs in crystals of cane sugar, quartz, Rochelle salt (potassium sodium tartrate-4-water), topaz, tourmaline.
2. Connect Rochelle salt to a neon lamp or electrostatic voltmeter.
Make sheets of polycrystalline Rochelle salt that show piezoelectric effects.
Measure the voltage of a Rochelle salt crystal under various stresses produced by a mass on a lever arm.
Excite a Rochelle salt crystal with an audio voltage and couple it to a sounding board.
Connect an audio oscillator to a large Rochelle salt crystal and the sound can be distinctly heard.
Apply an audio oscillator to a Rochelle salt and amplify with a wood sounding board.
3. Demonstrate piezoelectricity by dusting the cooling or warming crystal with a dust of red lead and sulfur that has passed through a silk or nylon screen.
A simple bellows can be made from a plastic nasal spray or deodorant bottle in that the aperture has been enlarged to allow a sizeable spray to be emitted.
Place in the bottle a mixture of about 2 parts red lead to 1 part sulfur.
Put a small piece of silk or nylon stocking over the mouth of the bottle.
Tighten this with a rubber band.
The dust particles receive electric charges as they pass through the screen formed by the stocking.
They settle on the end of the crystal that attracts them.
The red lead gets a positive charge and goes to the negative end of the crystal.
The sulfur gets a negative charge and settles on the positive end of the crystal.
4. Barbecue piezo-igniters
These fire lighters contain quartz crystals that have piezoelectric properties and so develop electric potential under stress.
When two crystals are struck together separation of charge in the crystal lattice can produce a very high voltage.
4. Mosquito bite clicker
This handy gadget can relieve the pain caused by bites of mosquitoes, sand flies and midges, and also the stings of small jellyfish, by injecting a small electric current into the affected skin.
It is powered by piezoelectricity and not batteries, so it is small, light weight and easy to use.
5. Piezoelectric sheets are made from ceramic lead-zirconate-titanate (PZT).
Make a ball and spring model of the piezoelectric effect.
Attach a piezoelectric sparker to a Braun electroscope.
Mount a sphere on the end of a piezoelectric gas lighter.
Use a piezoelectric gun to discharge a set of charged nylon strings.
Attach one end of a piezoelectric crystal to a needle point in the piezoelectric pistol.
The best generators of piezoelectric electric are very thin zinc oxide wires that can generate 200 milliwatts per cubic centimetres when squeezed, e.g. by the foot.
So piezoelectric crystals may be incorporated into footwear to produce people-powered energy.
A gas piezoelectric home lighter acts as a cheap source of reliable static electricity.
It is constructed by shortening the earth electrode and the attachment of an aluminized Christmas tree decoration.
32.1.4 Photoelectric cell, voltage produced by light
See: Photoelectric Planck's Constant, (Commercial).
Voltage is produced by light striking photosensitive (light sensitive) substances.
When light strikes the surface of a substance, e.g. compounds of silver oxide or copper oxide, it may dislodge electrons from the atoms at the surface, the substance becomes positively charged, and an electric force is created.
Experiment
1.See diagram 4.1.1.4a: Voltage produced by light, photoelectric cell.
This photoelectric cell has a curved light sensitive surface focussed on the central anode.
When light from the direction shown strikes the sensitive surface, it emits electrons towards the anode.
The more intense the light, the greater is the number of electrons emitted.
When you connect a wire between the filament and the back or dark side, the accumulated electrons will flow to the dark side.
These electrons will eventually pass through the metal of the reflector and replace the electrons leaving the light sensitive surface.
Thus light energy is converted to a flow of electrons, and a usable current develops.
2. Photoelectric cell construction.
This photoelectric cell is constructed in layers.
See diagram 4.1.1.4b: Photoelectric cell construction.
A base plate of pure copper is coated with light sensitive copper oxide.
An extremely thin additional layer of metal is put over the copper oxide to allow penetration of light to the copper oxide and accumulate the electrons emitted by the copper oxide.
An externally connected wire completes the electron path, the same as in the reflector type cell.
The photocell's voltage is used as needed by connecting the external wires to another device, which amplifies (enlarges) it to a usable level.
A photocell's power capacity is very small.
However, it reacts to light intensity variations in an extremely short time.
This characteristic makes the photocell very useful in detecting or accurately controlling many processes or operations.
For instance, the photoelectric cell, or some form of the photoelectric principle, is used in television cameras, automatic manufacturing process controls, door openers, burglar alarms.
32.1.5.0 Voltage produced by chemical action, battery
See diagram 3.2.84: Copper and zinc foil in a voltmeter.
Voltage is produced by chemical reaction in a battery cell.
Electrons may be removed from atoms and set in motion by energy derived from forms of energy, e.g. friction, pressure, heat, or light.
These physical actions do not alter the molecules of the substances being acted upon.
Molecules are not added, taken away, or split.
Only electrons are lost or added.
If the molecules of a substance combine with atoms of another substance, or give up atoms of its own, the action is chemical in nature.
When atoms are added to or taken from the molecules of a substance, the chemical change will cause the substance to take an electric charge.
The process of producing a voltage by chemical action is used in batteries.
32.3.01 Ohm's law, rheostat
See: AC Circuits, (Commercial).
See: Ohm's law, (Commercial).
See diagram 32.3.1.1d: Circuits for demonstrating Ohm's law.
See diagram 32.3.1: Resistor colour code.
Ohm's law
(George Simon Ohm 1789-1854 Germany)
1. Introduction to Ohm's law
The law for a part of a circuit is expressed by the equation I = V / R, where I is current (also intensity of current) through a conductor, V is potential difference (also drop of voltage) between the ends of this conductor, and R is resistance of the conductor.
It could be illustrated by a diagram in (Fig. 1), which means that whatever happens outside this conductor, the numerical relations between I, V, and R remain the same.
If Ohm's law is associated with different diagram (Fig. 2), this diagram implies that the law is valid for a closed circuit with a single resistance and a battery supplying a constant potential difference V.
Students relying on this diagram may difficulty with simply measuring intensity of current I and potential difference V, because Fig. 1 b implies that other parts of the circuit have no resistance.
So, students may connect the voltmeter in a variety of ways, e.g. between A and B, or B and C, or C and D, and obtain different potential differences.
The teacher should inform students that every part of the circuit has some resistance.
For example: R is the resistance of a conductor to be measured, RA and RV are resistances of an ammeter and a voltmeter, R0 is an additional resistor necessary to transform an ammeter into a voltmeter, Rwis the resistance of conducting wires (leads), r is the internal resistance of a power source.
Such a circuit would correspond to Ohm's law for the whole circuit I = E / R + r, where R usually means the total resistance of the external circuit, E is the electromotive force of the power source, and r is its internal resistance.
Teachers usually introduce this law after the initial introduction of Ohm's law.
2. Student experience of Ohm's law
Students who investigated the effect of length and wire gauge on resistance found the need for at least half a meter of wire to measure changes of resistance.
The multimeter to measure the current had to be in the milliamp range, but the fuse on the milliammeter has a very low power rating and so it burns out easily.
Students used a 1 metre length of nichrome wire pulled tight between two clamped retort stands.
The wire ranging from 20 SWG (0.87 mm) to 32 SWG (0.25 mm).
The results were: 20 SWG 2.6 ohm, 22 SWG 3.3 ohm, 24 SWG 6.2 ohm, and 32 SWG 26.2 ohm.
The graphs were linear with a y-intercept of 0.4 ohm in all cases.
The 0.4 ohm may be the internal resistance of the DMM's battery.
The resistivity came out to 1.2 x 10^-6 ohm m, which seemed quite reasonable.
3. Ohm's law statement
Ohm's law states that the potential difference, V, needed to create electric current, I, for a wire with constant resistance, R, then V = IR, where the SI unit is the volt, V.
The electric current in a conductor is proportional to the potential difference: V = IR, I = V /R, R = V / I, where R = resistance, V = potential difference, I = current.
For ohmic resistors, resistance, R, is a constant.
4. Ohm's law application
Ohm's law applies to volts / amps relationship for electrolytes, voltage / current relationship for gases, Ohmic conductors.
Ohm's law and Kirchoff's laws in simple circuits, the connection of simple circuits and use of appropriate meters to measure current.
The emf and potential difference around the circuit, verification of Ohm's law with a simple series circuit or voltage divider network, plotting of I / V characteristic curve.
5. Equation for resistance
Ohm's law defines the equation for resistance, V = IR where V = potential difference (pd) between the ends of a resistor, I = current through the resistor, R = resistance of the resistor.
6. The ohmic and nonohmic materials
The "ohmic" materials have a linear relationship between the applied voltage and the resulting electric current.
The "nonohmic materials do not have this simple relationship, but may be very useful substances, e.g. materials to make light-emitting diodes, LEDs.
32.3.1.0 Resistor assortment, resistor colour code
See diagram 32.3.1: Resistor colour code.
See diagram 32.3.1a: Resisitor colour code.
Carbon resistors and adjustable carbon composition resistors are commonly used in electronics, because they are compact and cheap, but they are not accurate, especially at high power levels.
Wire wound resistors and adjustable wire wound resistors may be very accurate except at very high power ratings.
Examine assortment of different resistors.
32.3.1.1 Resistors, resistances, switches, resistance box
See: Resistances, (Commercial).
See diagram 32.3.1.1: Resistance circuit.
Resistors control the electric current in a circuit.
Fixed resistors containing carbon, insulating filler and binding resin vary in resistance value from 1 ohm, usually above 10 ohm.
For higher power rating, a resistance wire is wound around an insulating rod.
Simple resistors occur in filament globe lamps, toaster elements and electric heaters.
An electric current flowing through a resistor always produces heat, and sometimes heat and light.
Variable resistors, potentiometers, allow resistance to be changed, e.g. in a radio volume control.
The electric power, P, is the rate at which electrical energy is converted to another form of energy, e.g. heat.
Power = Energy / Time.
The unit of power, watt (W) is equivalent to i joule of energy consumed in 1 second.
A 60 watt electric light bulb consumes 60 joules of electricity per second.
Power of a resistor, P = voltage across a resistor, V X current through a resistor, V.
P =VI.
Power ratings of resistors may be 0.25 W, 0.5 W, 1 W.
The power rating of a resistor must be greater than the power used in it.
1. Resistance box
A resistance box contains coils of wire of known resistance connected in series by thick brass blocks.
Resistor boxes are used to show Ohm's law.
A resistance board is set up as a simple Wheatstone bridge to find the resistance.
You must remove plugs to obtain the required resistance in the circuit.
2. Relative resistance
The relative resistance of the following conductors of the same length and cross-section, with silver as a standard of "1", are arranged in an order of ascending resistance as follows:
Silver 1.0, Copper 1.08, Gold 1.4, Aluminium 1.8, Platinum 7.0, Lead 13.5.
The resistance in an electrical circuit is expressed by the symbol R and is measured in ohms.
One ohm is the resistance of a circuit element that permits a steady current of 1 ampere (1 coulomb per second) to flow when a steady emf of 1 volt is applied to the circuit.
3. Rheostats
Rheostats are used as protective resistors or voltage dividers.
Coils are rated by number of windings and resistance.
Manufactured circuit parts containing definite amounts of resistance are called resistors.
5. Resistance wire diameters
Resistance wire diameters are measured by Standard Wire Gauge, SWG (UK, Australia), OR
Brown and Sharpe (B. and S.) (American Wire Gauge).
SWG 50 is smallest gauge and SWG 70 the largest.
Cable sizes are shown as follows:
14 / 36 = 14 strands of 36 SWG wire to carry 2 amps for internal lighting in a motor car, OR
61 / 20 = 61 strands of 20 SWG wires to carry 150 amps suitable for 6 volt starter motors in a car.
Experiment
1. Connect one meter lengths of various wires in series and measure the voltage across each.
Measure voltages on a board with seven one meter lengths of various wires is series so all carry the same current.
Place 6V across a set of wires of different lengths and / or diameters and measure the currents.
2. Make a simple switch.
Fasten the end of a piece of wire to a pencil with two rubber bands.
A second wire makes a connection.
3. Examine switches in a circuit.
Put a knife switch in a circuit with a cell and a lamp and turn the light on and off by operating the switch.
Replace the lamp with a bell or buzzer and operate the switch.
Replace the knife switch with a push button switch.
Take apart some common switches such as the common household, tumbler switch, rocker switch.
See how they are constructed.
4. Find all kinds of electric switches used in daily life, e.g. pull switch, reading lamp switch, suspension wire switch, cassette switch.
Also find some electric keys, e.g. single knife switch, double knives switch, single throw switch, multithrow switch.
Disassemble some switches that can be disassembled.
Observe the composition of those switches and the connection among down leads and inner components of switches.
In the on-off operation after turning on the switch, prevent the spring from bouncing out or replace it quickly.
Discuss and summarize what kind of significance the knife number and the multithrow of switches have.
Reassemble the disassembled switches.
Connect different switches into the circuit composed of cells and bulbs.
Prepare more conducting wires and bulbs for the multiknife or multithrow experiment.
At the end of the experiment observe the values of allow voltage and allow current, and explicate their significance.
The values of allowed voltage and allowed current are labelled on the outsides of switches.
32.3.1.2 Resistance model
Experiment
Roll a ball down a board with randomly spaced nails.
Roll ball bearings simultaneously down two ramps one with pegs and one without.
32.3.1.3 Ohmic and non-ohmic resistors (nonohmic resistors)
See: Ohm's law (Commercial).
See diagram: 32.3.1.3a: I-V curve for ohmic conductors.
See diagram: 32.3.1.3: I-V curve for non-ohmic conductor.
* An electrical resistor is a device of known or variable electrical resistance used to cause a drop in voltage itself or to limit the flow of electrical current through itself.
* Resistors that obey Ohm's law are called ohmic resistors.
If you draw a graph of voltage against current, the curve is linear, the slope of the curve is constant and gives the resistance, as constant value, static resistance.
* If the resistance of a resistor changes as the voltage increases, the resistor is a non-ohmic resistor.
So if you draw a graph of voltage against current the curve is not linear, but some curvy shape.
The slope of the curve at a particular voltage shows the resistance only at that voltage.
Non-ohmic resistors include tungsten light bulb filament, diode, thermistor.
* Is the resistance of a non-ohmic conductor the ratio of the voltage to current at the applied voltage, or the inverse slope of the current voltage graph at the applied voltage?
The equation R = V / I is not a statement of Ohm's law.
It is the definition of resistance that applies equally to ohmic and non-ohmic conductors.
For the non-ohmic conductor whose I-V curve is shown in the diagram above, the resistance of the conductor, when the applied voltage is V0, is V0 / I0.
The conductor is described as non-ohmic, because its resistance, V0 / I0, varies with applied voltage.
Even pure copper is non-ohmic if current is above 1 KA per cm2.
A conductor is described as ohmic, i.e. obeying Ohm's law, if its resistance is independent of the applied voltage.
Note the slope of the I-V curve at the applied voltage, i.e. V / I.
It is called the dynamic resistance, or a-c resistance, of the conductor at this voltage.
It has significance in describing the dynamic I-V characteristics of electronic devices.
However, it is not correct to describe it as the "resistance" of the conductor.
Experiment
See diagram: 32.3.1.3b: Non-ohmic resistor experiment circuit
1. Use a 12 volt, 6 lamp and a 15 ohm, 10 watt resistor.
Apply 12 volts in the circuit and calculate the resistance of the lamp at its running temperature.
Increase the voltage and record the current.
Plot the voltage-current of the lamp.
Replace the lamp with the resistor and record the current at the same voltages.
2. The resistance of a tungsten filament in an incandescent light bulb is temperature dependent.
Turn on a 100 watt light bulb in a lamp socket, fixed vertically over the socket.
Leave it turned until too hot to touch.
Unplug the lamp and use an ohmmeter to measure the resistance of the filament every 5 seconds as it cools.
Assuming that the resistance varies linearly with temperature, plot the filament temperature as a function of time as it cooled.
In a 75 watt or 100 watt 120 volt light bulb, the filament temperature is about 2550 oC and has a resistance of about 144 ohms and draws 0.33 amps.
The "cold resistance" is about 10 ohms.
3. Ohmic and non-ohmic resistivity of "Play-Doh", a modelling compound for children.
Play-Doh contains, among other ingredients, a starch-based substance, sodium chloride, petroleum additive, borax and a colorant.
Red coloured Play-Doh exhibits ohmic resistivity up to about 1 volt potential difference, then non-ohmic resistivity.
Drill holes in the side a of a plastic tube filled with red Play-Doh and attach electrode to the ends.
As voltage and current is increased in the circuit the voltage drop across two voltmeter probes inserted in a measure distance in the Play-Doh.
Voltages from 0.1V to 1.0V produce found resistivity of about 0.2 Ωm, including a change in resistivity when it become non-ohmic.
Time may be a factor if the Play-Doh becomes dry.
32.3.1.4 Megger
Megger is a trade name for an instrument used by electricians to measure electrical insulation resistance.
32.3.1.5 Resistance change and speed of electric motor
See diagram 32.4.06: Change of resistance.
Experiment
Connect a little electric motor to a large 1.5 volt dry cell.
Use a rheostat to make your motor start slowly, come up to full speed and then slow down.
As the copper wire is moved nearer to C, you make the electron current to flow through more of the jug element and meet more resistance.
Thus the voltage of the battery cannot push electrons around the circuit as rapidly as before and the motor slows.
32.3.2.0 Resistivity with metre wire bridge
See diagram 32.2.60: Metre wire bridge.
Constantan
Copper wire, 18 SWG, bare, 1.22 mm diameter, 0.0418 Ohm / m.
Nichrome wire, 32 SWG, bare, 0.274 mm diameter, 18.4 Ohm / m.
Resistivity symbol is ρ, (Greek ρ rho), and the unit is ohm metres.
If resistance, R, of a wire length, L, and cross-section area, a m2, R = rho × (L / a).
Resistivity
The resistance wire Constantan (Eureka wire) has a high volume resistivity and almost negligible temperature coefficient.
Resistivity depends on the material, but resistance depends on the nature of the material, its length and its cross-section area.
Resistivity in ohm metres of conductors = 10-8 to 10-6, semiconductors = 10-6 to 10-1, insulators 107 to 1023.
Resistivity is the reciprocal of conductivity.
In semiconductors, the higher the level of doping, the lower the resistivity.
An A C battery, 3 V flashlight bulb, and a copper wire coil make a hand held temperature coefficient of resistivity apparatus.
Resistance changes with temperature.
If resistance changes with temperature, a wire with resistance R0at temperature T0, then resistance R at temperature T = R0+ aR0(T - T0), where "a" = temperature coefficient of resistance.
Experiment
1. To observe resistivity with metre wire bridge set up as in diagram 32.2.60 except substitute for R1 a material, e.g. a wire, of known length and cross-section area.
Measure the resistance of length I cm of the specimen taking care that on interchanging the specimen R, and known resistance R2, the same length I cm of the specimen is exposed between the terminals in each case.
Tie small knots in the wire near each end and ensure that these knots just emerge from the terminals in each case.
Measure the length I cm of the specimen under test with a metre rule.
Use a micrometer screw gauge to measure its diameter d cm at four different places.
As in 32.2.60 for determining the resistance R, of the specimen, the resistance of a wire, is proportional to its length l, and inversely proportional to its cross-section area A, so resistance R is proportional to length l / area A.
So R = rho X (l / A), where rho is a constant called the resistivity of the material.
So rho = (RA) / l ohm cm.
2. To measure temperature coefficient of resistance of material with a metre wire bridge, insert the thermometer through the cork and wind the specimen in a coil round the stem of the thermometer keeping the turns separate.
Tie the coil to the thermometer with thread.
Connect the ends of the coiled specimen to copper wire leads.
Record this temperature t1oC after heating for ten minutes when the temperature of the coil becomes constant, and measure the balance point AB and DC.
Heat the beaker plus contents for 10 minutes, record the temperature of the coil, t2oC, measure the new balance point AB2 and B2C.
Electrical resistance of a material varies with temperature.
For metals, over small ranges of temperature, the variation is regular.
If resistance of a metal wire is R0 at 0oC, and Rt, at temperature toC, Rt = R0(1 + at) where "a" is a constant called the temperature coefficient of resistance.
So Rt1 = R0(1 + at1), and Rt2 = R0(1 + at2).
So Rt1 / Rt2 = (1 + at1) / (1 + at2).
Rt1 = R2(AB1) / (B1C) ohms.
Rt2 = R2(AB2 / (B2C) ohms.
32.3.2.01 Resistance and temperature
The resistance of short lengths of copper wire does not change much over the range of temperatures available in a laboratory.
Nichrome wire is even worse and micro-ohmmeters are very expensive.
Experiment
Use 0.25 mm diameter fine-enamelled copper armature wire, "magnet wire", wound on to cotton spools by 800 turns.
Put it in a beaker on a hotplate with a lid and a thermometer fixed into the spool.
Heated it to 70oC and let it cool back to room temperature over one hour, while measuring its resistance with a multimeter.
32.3.2.1 Heat and cool resistors
Experiment
1. Set up as in the diagram.
Place the coil into the LN2 (liquid nitrogen).
Watch the light bulb get brighter as the coil cools.
2. To observe change in the resistance of a conductor if the temperature changes, use a 6 volt lamp and adjust the variable resistance
so that you get voltmeter and ammeter readings for a range of filament temperatures, the globe changing from cool to red hot to white hot.
Tabulate your results and draw a graph plotting potential difference against current.
The resistance increases as the temperature increases.
Investigate the effect of a Bunsen flame or dry ice on the resistance of a piece of jug element.
3. To observe temperature change and resistance use 8 coils of one metre SWG 32 enamelled copper wire.
Connect long leads to the loosely wound coil of copper wire.
Adjust the rheostat to 5 amp in the coil.
Open the switch.
After 1 minute close the switch and read the ammeter and voltmeter several times during the next half minute.
During this time the coil heats and the current changes rapidly.
Repeat with the coil of copper suspended in water in the container.
Keep stirring the water.
4. Increase the current in a long U-shape of iron wire until it glows then insert half of the U into a beaker of water.
Heat and cool resistance coils with a test light bulb in series.
Put two coils of different material, but the same resistance in a Wheatstone bridge and either is heat or cool.
Heat a coil of iron wire in series with a battery and a lamp and the lamp will dim.
Heat a coil of forty turns of iron wire in a flame while connected in series with a light bulb circuit.
32.3.2.2 Put the light out with heat
Experiment
Wind a coil of iron wire on a porcelain core in series with a lamp and battery then heat until the lamp goes out.
Connect a coil of nickel wire to a battery and galvanometer then heat in a flame.
32.3.2.6 Thermistors
See: Thermistors (Commercial).
See diagram 4.1.8: Thermistor.
A thermistor is a small piece of semiconducting material the resistance of which falls with increasing temperature.
It can be used for sensitive temperature measurement and control.
The resistance decreases when heated or when current passes through it causing heat.
So an incandescent globe may turn on at higher temperature when resistance in the circuit is less.
Thermistors are wires used in electronic circuits made of metal oxides that decrease in resistance when the temperature of the wire increases.
Experiment
1. A thermistor, thermal resistor, is a semiconductor made of Co, Ni, Mn oxides and copper powder.
Its resistance is very sensitive to temperature.
When the thermistor is cold, a current of 25 mA will not be detected by the ammeter.
Heat the thermistor very carefully with a Bunsen burner and the current rises.
Stop heating when the current reaches 0.3 amps.
2. To show that materials change their conductivity, set up a simple series circuit with a voltage supply, the rheostat, the copper coil and the ammeter.
Adjust the rheostat or the battery connectors until 0.8 amp flows.
A very low voltage is needed.
Warm the coil very gently in a low Bunsen burner flame and read the ammeter.
Also, immerse the coil in a mixture of ice and salt.
Replace the copper coil by a coil of high resistance wire and repeat the experiment with a greater emf.
Use a gently warmed thermistor to replace the copper coil.
Use a block of salt in a crucible, into which two stout pieces of copper wire dip.
Then gently heat.
Two stout pieces of copper wire are embedded into the paraffin wax so that they do not touch.
These pieces of wire are then connected by the circuit in place of the high resistance coil.
Heat the tube and observe the current.
The paraffin wax fails to pass a current even when melted.
Use a glass rod.
Wind three turns of stout copper wire round 8 cm of glass rod connected into the circuit.
Heat the rod gently.
It does not conduct electricity.
3. Use thermistors and display the differential negative resistance of a fast thermistor on a transistor curve tracer.
Show the resistance of a thermistor placed in an ice water bath.
32.3.2.7 Electrical conductivity of molten glass at high temperature
See diagram 4.1.4: Heat a glass rod.
Experiment
1. Glass can be a conductor.
Heat a glass rod until it becomes very hot and begins to soften.
Test the hot, soft part with the conductivity apparatus.
When molten, glass is a good conductor of electricity.
2. Wrap each end of two copper wires tightly on a glass tube so that the distance between the wrappings is less than one cm.
Connect the other end of the two copper wires to the lamp and storage battery to form a series circuit.
The glass tube between the wires becomes a part of the circuit.
Observe if the lamp is lights.
Heat the glass tube with an alcohol burner.
As the lamp is lights, turn off the burner immediately.
Make two closely fitting coils four turns of bare copper wire, e.g. SWG 14, on the soft soda glass rod at points 5 cm apart.
Bend the lengths of wire at right angles to the rod and terminate them in two well insulated 4 mm plugs.
Put a tray of sand under the glass rod.
Support this assembly so that the electrodes are insulated from the iron retort stands.
Connect the two electrodes into a series circuit consisting of a rheostat 05 AC ammeter and the 240 volt AC mains supply.
Retort stands and the Bunsen burner should be connected to earth.
Close the switch.
No current flows.
Heat the glass rod.
Watch closely the contacts between the copper coils and the glass.
When the glass starts to become self-luminous, it will conduct electricity.
Remove the flame and watch the rod slowly redden, and melt.
Open the switch.
3. Heat a capillary tube in a Bunsen burner until it is hot enough to sustain a current that maintains a bright glow.
Heat a glass tube with a flame until it is hot enough to sustain conduction then vary the current by changing the ballast resistance.
Heat a Nerst glower with a flame until the resistance is low enough to sustain electrical heating, negative temperature coefficient of resistance.
The glower is a solid body radiator that is made up of a filament of rare earth oxides.
Heating the filament by continuous ohmic heating results in conduction.
The glower operates best in wavelengths from 2 to 14 m.
32.3.3.2 Migration of ions, speed of ions
Show KMnO4 migrating with current towards the positive electrode in KNO3.
Permanganate ions migrate in an electric field.
Experiment
Dip two platinum electrodes into an ammoniated copper (II) sulfate solution containing some phenolphthalein.
Blue moves from the anode of in a potassium chloride gel when 120 volts is applied.
32.3.4.0 Conduction in gases, Jacob's ladder
See: Jacob's Ladder (Commercial).
See diagram 32.3.4.0: Conduction in neon lamp.
See diagram 32.3.4.0.1: Jacob's ladder.
A climbing arc show the power of static electricity.
Voltage / current relationship for a gas
Experiment
1. Set up a series circuit consisting of the supply, the neon lamp and the 100 mA meter.
Connect the voltmeter across the lamp and a 100 mA meter.
Apply increasing voltages from 0 to 240 volts and record both the current and the voltage.
The striking potential for a neon lamp is about 170 volts.
The glow will be extinguished when they reduce the voltage to about 150 volts.
To prevent excessive currents, neon lamps have ballast resistors of about 2 000 ohms in the bases.
An arc rises between rabbit ear electrodes attached to a 15 KV transformer.
A "mini" Jacob's ladder can be made with four AA batteries.
2. Jacob's ladder demonstration
See: Jacob's Ladder (Commercial).
A Jacob's Ladder, climbing arc, can be made with a 15, 000 VRMS (current limited to 20 to 30 mA), neon sign transformer, (luminous tube transformer) and two copper tubing insulators or old metal coat hangers.
WARNING: ELECTROCUTION AND/OR BURN HAZARD!.
Be very cautious while performing this demonstration.
Never move or lift the apparatus by the insulators, but only use the handles attached to the base of the transformer.
Turn on the power and the discharge will travel up the wires.
The distance between the wires is the critical adjustment.
The wires should not be moving.
This demonstration does not work well on humid days.
Unplug after use so that students don't inadvertently play with it.
This demonstration shows an electric arc that rises between two vertical conducting bars separated wider at the top.
Once the arc reaches the top of the "ladder" and vanishes, another is generated at the bottom to repeat the cycle.
To show how high the temperature of the arc is, place a piece of paper between the rods so the rising arc will burn the paper.
3. Jacob's ladder
See diagram 32.3.4.0.1, Jacob's Ladder.
A Jacob's Ladder, climbing arc, can be made with a 15, 000 VRMS (current limited to 20 to 30 mA), neon sign transformer, (luminous tube transformer) and two copper tubing insulators or old metal coat hangers.
WARNING: ELECTROCUTION AND / OR BURN HAZARD!
Be very cautious while performing this demonstration.
Never move or lift the apparatus by the insulators, but only use the handles attached to the base of the transformer.
Turn on the power and the discharge will travel up the wires.
The distance between the wires is the critical adjustment.
The wires should not be moving.
This demonstration does not work well on humid days.
Unplug after use so that students don't inadvertently play with it.
This demonstration shows an electric arc that rises between two vertical conducting bars separated wider at the top.
Once the arc reaches the top of the "ladder" and vanishes, another is generated at the bottom to repeat the cycle.
To show how high the temperature of the arc is, place a piece of paper between the rods so the rising arc will burn the paper.
TASER
A taser (conducted electrical weapon, used by police), steps the voltage up to around 10, 000 V, but the high voltage is oscillated at a frequency that is not supposed to affect life functions, only localized muscle.
32.3.4.4 Ionization by radioactivity, smoke alarms
Conduction in air by ions
1. All ionization smoke alarms use an extremely small amount of a radioactive element in their ionization chambers, e.g. 37 Bq of Americium 241, in compliance with US NRC safety criteria in 10CFR 32.27.
In Australia, Queensland (10 year phase out of Ionisation Smoke Alarms beginning 1 January 2017) and the Northern Territory only allow the use of photoelectric smoke alarms, while fire departments in those and other states also recommend the use of photoelectric smoke alarms for home use.
Experiment
Charge an electroscope with a radioactive source.
Bring various sources of ionization near parallel wires attached to a 100 V battery and a Zeleny electroscope.
Increase the voltage across a plate close to a wire mesh with a radioactive source nearby and observe the current with a Zeleny electroscope.
Use an electrometer to measure the current between parallel plates as a flame is burned between them or an α-source is held nearby.
In a Cerberus smoke detector combustion products decrease conductivity in a chamber with an α-source.
32.3.4.7 Thermionic effect in air, thermionic emission
Experiment
Use a neon tube to apply 90 V forward and reverse and monitor the current.
A neon lamp lights at about 80 V and shuts off at about 60 V.
32.4.4.7 IR drop in a wire, potential drop along a wire
See diagram 32.4.4.7: Potential drop along a wire.
Experiment
1. To observe change in voltage as a current flows through a wire, use a straightened electric jug element.
Attach one metre of it to a board.
Observe any voltage drop between any two points in the circuit by pressing the bared ends of the voltmeter connecting wires to the points.
The potential difference between two points along a uniform conductor is proportional to the distance between the points.
2. To measure the fall in potential along a wire carrying current note that the shorter the length of the wire the smaller the fall in potential.
If a wire has uniform cross-section, the potential difference V between two places along the wire should be proportional to the distance between them.
If potential falls uniformly along the wire, a graph of distance potential against distance should be a straight line.
Adjust the rheostat so that when the sliding contact B is near C and the switch is closed, the voltmeter V shows full scale deflection, e.g. 3 V.
Close switch S and make contact the resistance wire AC so that AB = 10 cm and record the potential difference V volts between A and B.
Repeat for AB = up to 100 cm.
Plot a graph of AB cm (x-axis) against V volts (y-axis).
3. Clip wires from the terminals of flashlight lamps at various points along a stretched wire carrying 2 to 5 amps.
Use a voltmeter and ammeter to measure current and voltage on several samples of wire of the same length or use a slide clip to vary length.
Measure the voltage at six points on a long resistance wire.
32.4.4.8 Potential drop with Wimshurst machine, static machine
See: Wimshurst Machine, (Commercial).
31.6.1 Wimshurst machine, induction generator
Experiment
Attach a 3 m long wood bar at one end to one terminal of a static machine, with the other end grounded or insulated, then attach electroscopes along the bar to show flow of charge and potential drop.
Attach two ends of a dry stick to a static machine then measure with an electrostatic voltmeter and micro-ammeter.
32.4.6.6 Electromotive force, emf, ε.
See: Voltmeters (Commercial).
See diagram 29.03: Open right hand rule.
See diagram 32.2.58: Two accumulator cells and another cell.
In electromagnetism and electronics, electromotive force, emf, measured in volts, is an electrical action caused by any non-electrical source.
The volt is the standard unit of electromotive force, emf, (voltage).
An emf of 1 volt (1 V) across a component of a circuit with resistance 1 ohm drives a current of 1A through that component.
An electric current flows through a conductor, only if there is a potential difference, V, across it.
The source of the potential difference is the electromotive force, emf, measured in volts.
1. Electromotive force, emf, measured in volts, provides a potential difference across a conductor and causes an electric current to flow through the conductor.
Sources of emf include batteries, generators, photocells and thermocouples.
When a potential difference across a conductor produces an electric field that pushes on charges that force them to move and cause current flow, the direction of the electric field is from higher potential to lower potential.
Show direction of current as the direction of the electric field in the conductor.
By convention, current goes from higher potential to lower potential.
In liquids and gases that conduct electricity, positive charges move in the direction of the electric field and negative charges move in the opposite direction to the electric field.
In metals and vacuum tubes only electrons (negative charges) move, and they move in the opposite direction to the electric field.
Although the current starts moving around a circuit almost instantaneously, the charges move slowly, e.g. electrons in a current of five amps through a copper wire move at about 0.5 mm per second, yet in the vacuum of a cathode ray tube the electrons can move at 3 × 107 metres per second.
32.4.6.6.1 The emf and internal resistance of a cell with an ammeter and a voltmeter
See diagram 32.2.56: The emf of a cell.
Experiment
The emf, E volts, of a cell is the potential difference between its terminals, when the circuit is open.
The resistance of the voltmeter is high so little current passes through it.
When the switch is closed, the voltmeter reads V volts, i.e. less than E volts.
V is the potential difference needed to cause the current I amps to flow through the resistance external to the cell, mainly the rheostat Rh.
(E - V) volts = potential difference required for the current I amps to flow through the internal resistance r of the cell.
So I = (E - V) / r or r = (E - V) / I.
With the switch S open, record the reading E volts on the voltmeter across the cell C, e.g. Daniell cell.
Close the switch and adjust the rheostat to give a small current I amps and V volts on the voltmeter.
Adjust the rheostat Rh to get five pairs of current I amps, and potential difference V volts.
Calculate R = V / I for each pair of readings of readings.
Calculate the internal resistance of the cell r = (E -V) / I.
32.4.6.6.2 emf of two cells with a potentiometer
See: Potentiometers, (Commercial).
A potentiometer is a length of resistance wire AC of uniform cross-section with a terminal at each end, and a graduated scale.
A potentiometer is a three-terminal resistor with a sliding or rotating contact that forms an adjustable voltage divider.
If only two terminals are used, one end and the wiper, it acts as a variable resistor or rheostat.
See diagram 32.2.56: Potentiometer.
Experiment
1. When a current flows through the resistance wire there is a steady fall in potential from A to C.
So the difference in potential between two places on the resistance wire is proportional to the distance between them.
See diagram 32.2.58: Two accumulator cells and another cell.
2. Use two accumulator cells, and a Leclanché cell, carbon electrode positive (dry cell) at L.
Put a resistor as a protective shunt across a sensitive centre zero galvanometer G.
Close switch S.
Touch the potentiometer wire with the sliding contact near A then touch near C to check that the galvanometer G deflections are be in opposite directions.
If not, adjust the rheostat Rh to increase the current through the circuit.
Move the sliding contact to a point B1 on the resistance wire where the galvanometer shows no deflection.
Disconnect the shunt across the galvanometer to make it more sensitive and measure the distance AB1 more accurately.
3. Use two accumulator cells, and a Daniell cell (copper electrode positive) at D.
Replace the shunt across the galvanometer.
Move the sliding contact to a point B2 on the resistance wire where the galvanometer shows no deflection. Measure AB2.
When the galvanometer shows no deflection, no current is supplied by the cell at C2, that circuit is an open circuit and the potential difference between A and B is equal to the emf of the cell.
The emf E1 of the Leclanché cell is proportional to AB1.
The emf E2 of the Daniell cell is proportional to AB2.
So (emf E1) / (emf E2) = AB1 / AB2.
32.4.6.7 Internal resistance of a cell
See: Voltmeters (Commercial).
The terminal potential difference, voltage, of a cell when it causes current I to flow is related to its electromotive force, emf, and its internal resistance r, so the potential difference across each cell in series = (emf - Ir).
Total emf = (emf1 + emf2 + emf3) - (Ir1 + Ir2 + Ir3).
Terminal voltage (terminal potential difference)
When a battery produces current, i.e. discharges, terminal voltage, V = (emf) - (voltage drop in internal resistance), V = emf - Ir
When a battery is receiving current, charging, terminal voltage V = (emf) + (voltage drop in internal resistance), V = emf + r.
Experiment
1.See diagram 32.2.58: Two accumulator cells and another cell.
To measure the internal resistance of a cell with a potentiometer, put a resistor as a protective shunt across a galvanometer G.
Close switch S1.
With switch S2 open, measure the balance point B1 on the potentiometer wire AC,
* with the protective shunt,
* without the protective shunt.
Record AB1.
Put a resistor as a protective shunt across a galvanometer G.
Close switch S1.
With R = 5 ohms and S2 closed, measure the new balance point AB2.
Record AB2.
With R = 4 ohms and with S2 closed, measure the balance point AB3. 2. Repeat with R = 3 ohms.
Repeat with R = 2 ohms.
E = I(R + r), E is the emf of the cell D and r the internal resistance of cell D.
V = IR, where V is the PD between the terminals of D when it is sending current through R.
So E / V = + r) / R, r = (E - V) / V × R.
AB1 is proportional to E and AB2 is proportional to V,
So r = (AB1 - AB2) / AB2 × R ohms.
32.4.6.9 Heat and light from electricity, make a model electric lamp
See diagram 32.4.6.9: Model electric lamp.
Experiment
1. Push the ends of two pieces of copper wire, 16 swg, through a cork in a small bottle.
Connect the ends of the copper wire inside the bottle with a stand of steel wool.
Connect this model electric lamp model in a circuit with one or more dry cells, or lead cell accumulators, and a switch.
Close the switch until the fine wire filament begins to glow.
At first the heated iron wire produces light, but soon the iron combines with the oxygen of the air inside the bottle and burns.
Examine a manufactured lamp bulb.
It contains no oxygen.
It has a tungsten carbide wire filament that may be heated to a high temperature so that it glows without burning.
35.33.0 Piezoelectricity with red lead and sulfur
Demonstrate piezoelectricity by dusting then cooling or warming crystal with a dust of red lead and sulfur that has passed through a silk or nylon screen.
A simple bellows can be made from a plastic nasal spray or deodorant bottle in that the aperture has been enlarged to allow a sizeable spray to be emitted.
Place in the bottle a mixture of about 2 parts red lead to 1 part sulfur.
Put a small piece of silk or nylon stocking over the mouth of the bottle.
Tighten this with a rubber band.
The dust particles receive electric charges as they pass through the screen formed by the stocking.
They settle on the end of the crystal that attracts them.
The red lead gets a positive charge and goes to the negative end of the crystal.
The sulfur gets a negative charge and settles on the positive end of the crystal.
35.33.1 Pyroelectricity, ferroelectricity
The minerals tourmaline and quartz have pyroelectric properties, i.e. electricity is generated when the crystal is heated and opposite charges gather
on the opposite faces of asymmetric crystals.
So the heated crystals generate electricity.
Temperature or pressure changes cause such minerals to get an electric charge when they are warmed or cooled or pressed.
Other substances are pyroelectric, but are termed ferroelectric, because the direction of charge changes if external electric fields are applied, e.g. gallium nitrate (GaN) and lithium tantalate (LiTaO3).