From electrons to electronics

Quantum and Nuclear

The "electron gun" or valve diode

Practical Activity for 14-16

Demonstration

The main purpose of this experiment is to explain the principle of an electron gun. You can also use the apparatus to demonstrate a valve diode – a device that lets the current flow in just one direction.

Apparatus and Materials

Hot filament diode tube and stand
Power supply, HT
Power supply, 6.3 V, AC, for the heater filament (this is often included on the HT supply)
Demonstration meter with a centre zero dial, -2.5 mA. to +2.5 mA

Health & Safety and Technical Notes

HT (high tension) power supplies (generally supplying voltages up to 400 V) can cause fatal electric shock.

It is essential that all HT connectors and cables are rated at the voltage to be used. The HT connectors should be the shrouded type so that accidental contact is highly unlikely. Any meter used in the HT circuit should be a type rated for the voltage used, and with shrouded connectors. All HT connections should be made with the HT switched off, and no adjustments made to the HT connections or wires once the HT is switched on.

The practical work with HT supplies should only be undertaken by teachers with good knowledge of HT electricity and the dangers.

Students should observe well away from the apparatus when it is being used.

Post-16 students may undertake the practical with supervision. See Topics in Safety (ASE 2001), Chapter 17...

CLEAPSS

The tubes are fragile (and expensive!) and should be handled carefully. They will implode if broken. Use the stands specifically designed for holding them.

Read our standard health & safety guidance

Follow the manufacturer’s instructions for setting up the diode.

Ensure that you can identify the following:

The 6.3 V supply to the cathode heater. (If you connect the wrong voltage to the heater you can easily damage the tube beyond repair.)
The HT (High Tension) supply to the electrode. Set this to zero.
The collection plate and its connection terminal in the diode tube.

With no potential difference (p.d.) across the tube, a small current of about 50 mA. flows, owing to the energy with which electrons are emitted from the filament (Edison effect). But this will probably not be noticed in the experiment described here.

Procedure

Set up the diode in its stand, and connect the heater filament to the 6.3 V supply.
Connect the plate in the tube, through the milliammeter, to the HT supply.

Connect the other terminal of the HT supply to earth and to one of the filament terminals as shown in the diagram below. The supply enables the plate to be at 400 volts either positive or negative relative to the filament.

With the filament heater switched off, try a big positive potential difference (p.d.) and then a big negative p.d. You could try a bit of drama here by building up the possibility of getting a big current to flow through the vacuum with a big enough p.d.; then feign concern when there is no current.
In reality, with the filament not glowing, there will be no current for any p.d. (positive or negative).
Now switch on the filament with a positive voltage on the collection plate. You will get a current.
Try a negative p.d. on the collection plate. There will be no current.

Teaching Notes

Electron gun

This experiment shows that charges can flow through the vacuum - as long as one terminal is heated and that this heated terminal is a cathode.
It is reasonable to infer that the charges originate from the heated element (because with the heater switched off, there is no current).
Given that charges only flow through the vacuum when the heated electrode is a cathode, it is also reasonable to infer that the charges are negative. Positive charge would not flow from a cathode to an anode, whereas negative charges will (being attracted to the positive anode).
You can explain the results of the experiment using the idea of electrons. These tiny negative particles are free to move in the metal. As the metal is heated up, some of them ‘evaporate’ from the surface. They form a ‘gas’ of electrons above the surface of the hot plate. If the heated plate is put in a circuit and made negative with respect to another plate, the electrons are pulled through the vacuum and so a current flows between the plates. If the heated electrode is positive, the negative electrons are pulled back to the electrode’s positive surface.

Valve diode

The essential story is that the diode can carry a current only one way. So you should let the class take the measurements 'both ways' with the valve diode. With the milliameter in the circuit, you can measure the current for different voltages in each direction. You can point out that there is a current when the diode is connected one way round (with the heated element connected to the negative terminal of the supply) and not when it is connected the other way round. In other words, the ‘valve’ lets current through one way, but not the other.
You could mention that this is the basis of early valve diodes. The birth of the diode marked the beginning of electronics. However, diodes have now been replaced with components made of semi-conducting materials like silicon. Semi-conductor devices are often called ‘solid state’ because they do not rely on the ‘gas’ of electrons passing between contacts in a vacuum tube.

This experiment was safety-tested in May 2007

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

Set up the circuit as shown below.

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.
Repeat in the range 0 V to -4.0 V in intervals of 0.5V, by reversing the connections on the diode.
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 diode at a particular voltage = potential difference/current reading.
Use the graph to calculate the resistance of the diode at a number of different potential differences.
Describe how the resistance changes with potential difference. Is the resistance of the diode the same for ‘positive’ voltages and ‘negative’ voltages?
The conductance of the diode at a particular potential difference = current/potential difference.
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

Electron

Electricity and Magnetism

Using a CRO to show rectification by a diode

Practical Activity for 14-16

Demonstration

The CRO (cathode ray oscilloscope) gives a visual display of the rectifying action of a diode.

Apparatus and Materials

Diode (see technical note)
Power supply, low-voltage, AC
Crocodile clips, 2
Leads, 4 mm, 3
Oscilloscope

Health & Safety and Technical Notes

A cathode-ray tube requires voltages classified as hazardous live. The casing nearly always has ventilation holes, some of which may give access to these voltages. Classes should be warned not to poke anything through the holes.

Read our standard health & safety guidance

1N4001 diodes are very cheap and suitable.

Procedure

Set the power supply to 12 V AC.

Connect the power supply, as shown, to the oscilloscope to display the waveform. The gain control should be set to 2 V/cm and the time-base to 10 ms/cm (10 ms/div).

Switch off the power supply. Connect the diode into the circuit as shown above. Switch on and observe the waveform.
If you have a double beam CRO, then you can show the original signal as well as the rectified one.
Repeat with the diode reversed.

Teaching Notes

One way round, the diode will remove the positive peaks of the trace. The other way round, the negative troughs will be removed.
Full-wave rectification by a diode bridge, made from 4 diodes as shown below, could also be shown.

Think about the way to arrange the four diodes so that in the course of a cycle of the AC supply current will go through in each half cycle and make humps in the same direction. The sketch does not show you which way each diode must point. You need to decide that.
If you are using a real CRO (rather than a datalogger), you could mention that the CRO itself is using an electron beam to display the changing voltage at its input.
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 was safety-tested in January 2007

A video showing how to use an oscilloscope:

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

Set up the circuit as shown below.

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).
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.

IV characteristic of a carbon resistor results table - worksheet.pdf

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

Set up the circuit as shown below.

Pour boiling water into the beaker and take readings of the current through the thermistor as the temperature falls. Record the results.

Analysis

Plot a graph of current/ mA (y-axis) against temperature/ °C (x-axis).
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.

Electrical Circuit

Electricity and Magnetism

Working with simple electrical components

Teaching Guidance for 14-16

It is often up to a teacher and a particular class to decide what equipment to use to introduce electric circuits. There are two general types of equipment used in schools for experimenting with electric circuits:

Circuit boards (such as the Worcester Circuit Board) are designed with simple components so that the shape of the circuit which is constructed looks like a circuit diagram. This helps students to work from a circuit diagram or draw one themselves as a record of the work they have done. Some teachers find circuit boards can confuse less able students - they don’t realize that parts of the board without anything connected are not part of the circuit. Circuit boards have an advantage in that the connection of the cells in parallel is discouraged.
Separate components connected by wires. This can be a cheaper solution, but it can also produce a tangle of wires so that the circuit becomes confusing.

Give students simple instructions on how to use the kit. As work progresses, make simple testing devices available, to test whether a cell is flat, a lamp is broken, or a lead not providing a good connection. These are easy to assemble with the item to be tested being the missing component in a simple series circuit consisting of lamp, cell and connecting wires. Learning how to trouble-shoot a circuit probably teaches more than circuits which give the predicted result the first time.

Good maintenance is essential

Time spent in checking the equipment before a lesson will pay dividends in the students’ understanding.

Some agreement must be established within the class so that the brightness of one lamp used with one cell is ‘normal’ brightness. In more complex circuits the brightness of the lamps can then be compared to this standard.

For this to be clear, students need to be given cells which have the same voltage (checked when they are driving a current through a lamp and not on open circuit), and all the lamps in a student’s collection need to produce the same brightness with the same cell. This is quick to do if three cells are connected in series to three rows, each consisting of three lamps, so that all lamps glow with normal brightness. If possible, new cells should be used at the beginning of each year and the old cells used up doing other jobs. The quality control, during production, on simple lamps is not good and even new lamps from the same packet can vary widely.

The difference in brightness of the lamps might be difficult to see in bright sunlight or with laboratory lighting and so the laboratory should be dimmed a little.

What type of cell is best?

The cost of cells has led some teachers to try rechargeable cells, which have their own problems. They have low internal resistance, so, if shorted, allow a large current. And they need to be completely flat before they are recharged. Cheap zinc-chloride cells are best for elementary work. Alkaline-manganese cells may be used where shorted cells are unlikely.

Some teachers even use power supplies. However, power supplies suffer from their internal resistance, just like cells. They may give unexpected, but entirely correct, results when the simple story about electric circuits is being told and internal resistance is being neglected. In order to avoid running down some of the cells and not others during experimenting, students should be issued with switches, or asked to disconnect the circuit when they are doing other things.

Terminology

The language may vary in different teaching programmes with an insistence on cell for the simple 1.5 volt (approximately) simple cell and battery being reserved for several cells in series. Bulb may also be used instead of lamp.

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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