V=IR
Electricity and Magnetism

Component characteristics

for 14-16

These experiments look at the behaviour of individual circuit components, and combinations that produce potential dividers. They assume an understanding of current, potential difference and their measurement.

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I/V characteristic of a carbon resistor

V=IR
Quantum and Nuclear | Electricity and Magnetism

I/V characteristic of a carbon resistor

Practical Activity for 14-16

Class practical

An example of the behaviour of a simple component, giving students opportunities to construct a circuit, gather data and perform some analysis.

Apparatus and Materials

  • Power supply, 0 to 12 V, DC
  • Carbon film resistor - e.g. about 100 ohms, 1 W
  • Leads, 4 mm
  • Multimeters, 2, or 1 ammeter and 1 voltmeter of suitable ranges
  • Rheostat, e.g. 200 ohms, 2 W

Health & Safety and Technical Notes

Read our standard health & safety guidance


Some components may become hot enough to burn fingers.

Procedure

  1. Set up the circuit as shown below.
  2. Use the variable power supply and the variable resistor to vary the potential difference across the resistor, from 1.0 V to 4.0 V, in intervals of 0.5 V. Record pairs of potential difference and current values in the table (see below).
  3. You can record results for currents in the opposite direction by reversing the connections on the resistor.

Analysis: Plot a graph of current/A (y-axis) against potential difference/V (x-axis). Remember to include the readings for ‘negative’ voltages.

The resistance of the resistor is equal to the ratio of potential difference to current.

Use the graph to calculate the resistance of the resistor at a number of different currents.

Describe how the resistance changes with current. Is the resistance of the resistor the same for current in both directions?

The conductance of the resistor at a particular potential difference = current/potential difference.

Use the graph to calculate the conductance of the resistor at a number of different potential differences.

Teaching Notes

  • The aim of this experiment is to develop confidence in setting up simple circuits and in taking careful measurements. If you decide that all students should attempt all the experiments in this collection, it may be sensible to start with this very simple one. The analysis is straightforward but students may well need reminding to convert mA into A where necessary. The graph should be a straight line through the origin. Many students will realize that, if the gradient of the line is constant and if it passes through the origin, all V/I values will be the same. However, there is much confusion about such ideas! See point 2 below.
  • It is often stated that the resistance of a component is the gradient of a V against I graph. Only for ohmic conductors (as in this experiment) does this happen to be true. Resistance is the ratio of V/I, so it is generally best to encourage students to take V/I ratios at specific points.
  • In this case a higher potential difference raises more electrons into the conduction band so the use of the term conductance is probably helpful.
  • Using a potential divider, as shown below, will enable students to get a full range of readings.

How Science Works extension: This experiment provides an excellent opportunity to focus on the range and number of results, as well as the analysis of them. Typically it yields an accurate set. The rheostat enables students to select their own range of results. You may want to encourage them to initially take maximum and minimum readings with the equipment and then select their range and justify it.

If they don’t think of it themselves, suggest that students take pairs of current and voltage readings as they increase the voltage from 0 V to the maximum. They then repeat these readings while reducing the voltage from the maximum to 0 V. This may help them to identify whether the resistance of the resistor remains constant when it is heated. (Turning the equipment off immediately after readings are taken and allowing the resistor to cool provides an alternative to this procedure but will considerably lengthen the time needed for the experiment. It is also possible to put the carbon resistor into a beaker of water to maintain the resistor at constant temperature.) Students could also change the direction of the current and repeat the other procedures.

You can use the fact that resistors are sold with a specified tolerance (and thus a variation in value) as the basis for a discussion about what a ‘true’ value really means in this case. Compare calculated resistance values with the manufacturer’s stated value or value range. Students can also be encouraged to identify the sources and nature of errors and uncertainties in the experimental method.

This experiment comes from AS/A2 Advancing Physics. It has been re-written for this website by Lawrence Herklots, King Edward VI School, Southampton.

This experiment was safety-tested in January 2007

Resources

Download the support sheet / student worksheet for this practical.

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I/V characteristic of a filament lamp

V=IR
Electricity and Magnetism

I/V characteristic of a filament lamp

Practical Activity for 14-16

Class practical

An example of the characteristics of a simple component, giving students opportunities to construct a circuit, gather data and perform some analysis.

Apparatus and Materials

  • Filament lamp 12 V, 24 W
  • Power supply, 0 to 12 V, DC to supply up to 4 A
  • Leads, 4 mm
  • Multimeters, 2, or 1 ammeter and 1 voltmeter of suitable ranges
  • Rheostat, e.g. 8 ohm rated at 5 A

Health & Safety and Technical Notes

Some components may become hot enough to burn fingers.

Read our standard health & safety guidance


Procedure

  1. Set up the circuit as shown below.
  2. Use the variable power supply and the variable resistor to vary the potential difference across the lamp, from 1.0 V to 10.0 V in intervals of 1 volt. Record pairs of potential difference and current values in the table.
  3. You can record results for currents in the opposite direction by reversing the connections on the lamp. See below for worksheet.
  4. Analysis. Plot a graph of current/A (y-axis) against potential difference/V (x-axis).
  5. The resistance of the lamp at a particular potential difference = potential difference/current.
  6. Use the graph to calculate the resistance of the lamp at a number of different potential differences.
  7. Describe how the resistance changes with potential difference.
  8. The conductance of the lamp at a particular potential difference = current/potential difference.
  9. Use the graph to calculate the conductance of the lamp at a number of different potential differences.
  10. Describe how the conductance changes with potential difference.

Teaching Notes

  • The aim of this experiment is to develop confidence in setting up simple circuits and in taking careful measurements. The analysis is fairly straightforward but students may well need reminding to convert mA into A where necessary.
  • It is often stated that the resistance of a component is the gradient of a V against I graph. This is not usually the case. Resistance is the ratio of V/I. It is therefore best to encourage students to take V/I ratios at specific points.
  • In the case of a filament lamp it is, in fact, the resistance that increases (rather than the number of charge carriers falling) due to increased lattice vibrations.
  • Using the potential divider, as shown below, will enable students to get a full range of readings.

This experiment comes from AS/A2 Advancing Physics. It has been re-written for this website by Lawrence Herklots, King Edward VI School, Southampton.

Resources

Download the support sheet / student worksheet for this practical.

Up next

I/V characteristic of a semiconductor diode

Semiconductor
Electricity and Magnetism

I/V characteristic of a semiconductor diode

Practical Activity for 14-16

Class practical

An example of the behaviour of a simple component, giving students opportunities to construct a circuit, gather data and perform some analysis.

Apparatus and Materials

  • Semiconductor diode - e.g. IN 5401
  • Protective resistor, at least 10 ohm
  • Power supply, 0 to 12 V, DC (or, better, small smooth stabilized 5 V supply)
  • Leads, 4 mm
  • Multimeters, 2, or 1 ammeter and 1 voltmeter of suitable ranges
  • Rheostat

Health & Safety and Technical Notes

Read our standard health & safety guidance


Procedure

  1. Set up the circuit as shown below.
  2. Use the variable power supply and the variable resistor to vary the potential difference across the diode from 0 V to +0.8 V in intervals of about 0.1 V. Record pairs of potential difference and current values.
  3. Repeat in the range 0 V to -4.0 V in intervals of 0.5V, by reversing the connections on the diode.
  4. Analysis. Plot a graph of current/A (y-axis) against potential difference/V (x-axis). Remember to include the readings for ‘negative’ voltages.
  5. The resistance of the diode at a particular voltage = potential difference/current reading.
  6. Use the graph to calculate the resistance of the diode at a number of different potential differences.
  7. Describe how the resistance changes with potential difference. Is the resistance of the diode the same for ‘positive’ voltages and ‘negative’ voltages?
  8. The conductance of the diode at a particular potential difference = current/potential difference.
  9. Use the graph to calculate the conductance of the diode at a number of potential differences.

Teaching Notes

  • The aim of this experiment is to develop confidence in setting up simple circuits and in taking careful measurements. The analysis is fairly straightforward but students may well need reminding to convert mA into A where necessary.
  • It is often stated that the resistance of a component is the gradient of a V against I graph. This is not usually the case. Resistance is the ratio of V/I so it is best to encourage students to takeV/I ratios at specific points.
  • The main learning point here is that the diode only allows current flow in one direction.
  • Using a potential divider, as shown below, will enable students to get a full range of readings.
  • You could discuss the miniaturization that is possible by building integrated circuits onto a wafer of semiconductor. Students may have heard of Moore’s law in which Intel’s co-founder Gordon Moore proposed the trend that the number of components on an integrated circuit would approximately double every two years. It has held from 1972 to at least 2006.
  • This experiment comes from AS/A2 Advancing Physics. It has been re-written for this website by Lawrence Herklots, King Edward VI School, Southampton.

This experiment was safety-tested in January 2007

Up next

The effect of temperature on a thermistor

Electrical Conductance
Quantum and Nuclear | Electricity and Magnetism

The effect of temperature on a thermistor

Practical Activity for 14-16

Class practical

This experiment, for advanced level students, shows that the current through a thermistor increases with temperature, as more charge carriers become available.

Apparatus and Materials

  • timer or clock
  • Leads, 4 mm
  • Crocodile clip holder
  • Thermometer -10°C to 110°C
  • Thermistor - negative temperature, coefficient, e.g. 100 ohm at 25°C (available from Rapid Electronics).
  • Power supply, 5 V, DC or four 1.5 V cells
  • Beaker, 250 ml
  • Kettle to provide hot water
  • Digital multimeter, used as a milliammeter
  • Heat-resistant mat
  • Power supply, low voltage, DC, continuously variable or stepped supply with rheostat (>1 A)

Health & Safety and Technical Notes

Read our standard health & safety guidance


A thermistor may be described as:

  • ntc negative temperature coefficient: its resistance decreases as the temperature increases
  • ptc positive temperature coefficient: its resistance increases as the temperature increases

If you have both types available, students may be interested in comparing them.

Procedure

  1. Set up the circuit as shown below.
  2. Pour boiling water into the beaker and take readings of the current through the thermistor as the temperature falls. Record the results.
  3. Analysis
  4. Plot a graph of current/ mA (y-axis) against temperature/ °C (x-axis).
  5. Assuming that the voltage is constant, describe how the conductance or resistance varies with temperature.

Teaching Notes

  • The thermistor is made from a mixture of metal oxides such as copper, manganese and nickel; it is a semiconductor. As the temperature of the thermistor rises, so does the conductance.
  • The increase in conductance is governed by the Boltzmann factor. Whether or not your students need to understand Boltzmann, they should be able to grasp that
  • as the temperature goes up, the resistance goes down
  • in this case, it happens because more charge carriers are released to engage in conduction.

This experiment comes from AS/A2 Advancing Physics. It has been re-written for this website by Lawrence Herklots, King Edward VI School, Southampton.

Up next

Switching on a lamp

V=IR
Electricity and Magnetism

Switching on a lamp

Practical Activity for 14-16

Class practical

To measure the current through and potential difference across a lamp as it is switched on.

Apparatus and Materials

For each student group

  • 6 V battery
  • Switch
  • Wires
  • 6 V miniature lamp (pea lamp) in holder
  • Datalogging current sensor
  • Voltage sensor
  • If possible a light sensor should also be incorporated into the experiment. You would then also need a black cloth.

Health & Safety and Technical Notes

Read our standard health & safety guidance


The lamp switches on in about 0.1 s so you will need fairly fast data collection.

Procedure

  1. Connect a simple circuit for measuring the current through the lamp and potential difference across it. If you have a light sensor, place it next to the lamp and cover both with a black cloth.
  2. Set the datalogging equipment to record at least 100 measurements per second.
  3. Start the datalogger, close the switch, wait about 2 seconds and open the switch. Stop the datalogger.
  4. If possible, use the datalogger to calculate the power output (V x I) and resistance (V/I) of the lamp as it switches on.

Teaching Notes

There are a number of points to come out of this:

  • Sensible use of dataloggers - here to capture data which changes too fast for humans to record. Students may not think that lamps take any time to switch on but this experiment shows otherwise. They may have seen the new LED traffic lights which have been placed in parallel with normal lamps at some junctions - the LED lights clearly switch on faster.
  • The potential difference will increase instantly - this is controlled by the speed at which the switch is closed. The current will be high initially and then drop to a constant value. This shows the resistance of the lamp increasing as it warms up.
  • The resistance graph, if plotted, will tell this story more clearly. Be careful - the initial current, being zero, will give nonsense values for resistance before the switch is closed, the graph would need to be cut off so that it starts just after the current starts to rise.
  • The power output can be found and compared with the light level. The light output steadily increases (as the lamp gets hotter) but the power peaks with the current and then falls. This can be explained by energy transfers. Initially energy is being transferred to the lamp by an electric current. The filament of the lamp warms up and emits light (and infrared) radiation. Once the lamp reaches constant temperature, work done electrically balances the radiation transfer.
  • This experiment can be presented at a very low level, simply as an introduction to resistance (current drops as lamp heats up). Or it can be used right through to the highest level with power calculations. Data can be gathered in a few seconds. The experiment is readily repeated if there is a glitch.

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Further note on component characteristics

Electrical Conductance
Quantum and Nuclear

Further note on component characteristics

Teaching Guidance for 14-16

Conductance and resistance

In some circumstances it is useful to consider the conductance of a component rather than its resistance. An increase in conductance suggests an increase in charge carriers – which is often the case when the ‘resistance’ is said to decrease. Using both terms can help students get a feel for the microscopic changes that are altering measurable quantities.

Why use a variable resistor?

Students are occasionally confused when the independent variable does not change in a regular manner. Including the variable resistor allows this to happen, but confident students can simply leave it out and use the power supply to produce their own figures for the voltage values.

Voltage or potential difference?

Voltage is an everyday term which may suit students at an introductory level, but they should later be encouraged to use the correct, descriptive term ‘potential difference’.

Using a potential divider

More able or experienced students can be encouraged to construct a potential divider circuit as shown in these experiments. This will allow them to gain a full range of readings.

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