Conductivity
Electricity and Magnetism

Electrical conduction through gases

for 14-16

These experiments assume students already have experience of currents through metal, some non-metals and some solutions. With higher potential differences (voltages), gases too will carry a current.

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Showing that a spark can pass through air

Ionising Radiation
Quantum and Nuclear | Electricity and Magnetism

Showing that a spark can pass through air

Practical Activity for 14-16

Demonstration

The Van de Graaff generator always produces excitement for students.

Apparatus and Materials

Health & Safety and Technical Notes

The makers' instructions should be followed for the care and use of your Van de Graaff generator.

Read this comprehensive safety guide:

Van de Graaff generator safety


Procedure

Sparks should be shown passing between the large sphere and the smaller sphere supplied with the generator.

Teaching Notes

  • As an introduction to the demonstration, you could say: "Can gases carry currents? Does air carry currents? Suppose the air carried electric currents as easily as copper, what would happen to the electric circuits that you have been working with? Then air cannot carry a current as easily as copper or it would spoil all these experiments. What would happen to cells? Or to the wall terminals at home? It looks as if air must be a non-conductor, or a very good insulator like paper, glass, cotton, wood or things like that. Yet it is impossible to make gases to carry currents."
  • When the charge on the insulated dome becomes high enough, a spark will pass between it and a second dome which has been connected to earth.
  • To show that it is the same kind of electricity as found in electrical circuits, connect a microammeter into the earth connection lead. Each time a spark jumps across the gap, a sharp burst of current will be indicated on the meter.
  • Once the Van de Graaff generator is set up, the kit that comes with it has lots of toys to demonstrate. Doing a series of experiments with these will delight students. Look at the collection

    Van de Graaff generator


This experiment was safety-tested in July 2007

  • A video showing how to use a Van de Graaff generator:

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Ions in a flame

Ionising Radiation
Electricity and Magnetism

Ions in a flame

Practical Activity for 14-16

Demonstration

A candle flame is electrically conductive. An EHT will visibly separate its free charges, causing a current to flow in an external circuit.

Apparatus and Materials

  • Light source, compact
  • Metal plates with insulating handles, 2
  • Power supply, EHT, 0–5 kV (with internal safety resistor)
  • Candle
  • Screen
  • Variable voltage supply, 0-12 V, 8 A

Health & Safety and Technical Notes

See guidance note on

Managing radioactive materials in schools


A school EHT supply is limited to a maximum current of 5 mA., which is regarded as safe. For use with a spark counter, the 50 MΩ. safety resistor can be left in circuit so reducing the maximum shock current to less than 0.1 mA..

Although the school EHT supply is safe, shocks can make the demonstrator jump. It is therefore wise to see that there are no bare high voltage conductors; use female 4 mm connectors where required.

Read our standard health & safety guidance


A Van de Graaff generator or a Wimshurst machine can be used instead of the EHT power supply.

A flexcam and display screen could be used instead of the light source and screen.

Procedure

  1. Fix the plates in vertical planes parallel to each other and five to ten centimetres apart by means of their handles held in retort stands.
  2. Light the candle and place it so that its flame is lit a little below the plates.
  3. Set up the small bright light source so that a shadow of the plates falls on the screen beyond. Both the light source and screen should be at least a metre from the candle, and the source preferably more.
  4. Apply a high potential from the EHT power supply to the plates. The flame divides into two parts, one towards the positive plate and one to the negative. This can be seen clearly on the screen.

Teaching Notes

  • The flame divides into two. This shows that there are two streams of particles moving in opposite directions. One stream is luminous, probably with carbon particles, and the other is not so luminous.
  • The two streams will not look equal. One is a stream of negatively-charged particles, moving to the positive plate. The other is of positively-charged particles, moving to the negative plate.

This experiment was safety-tested in August 2007

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Conduction in a gas

Conductivity
Electricity and Magnetism

Conduction in a gas

Practical Activity for 14-16

Demonstration

The spark through the air from a discharging Van de Graaff is bluish in colour, just like a more powerful lightning flash.

Apparatus and Materials

  • Van de Graaff generator


  • Metal plates with insulating handles, 2
  • Miniature neon lamp
  • Power supply, EHT, 0–5 kV
  • Power supply, HT, 0–250 V
  • Resistor, 220 k ohms, 1 W
  • Fluorescent tube

Health & Safety and Technical Notes

Read our standard health & safety guidance


A miniature neon lamp is often supplied as an accessory with a Van de Graaff generator.

It is unwise to operate a computer close to a running Van de Graaff generator, particularly a laptop which is not earthed.

Procedure

  1. Hold the neon lamp near to the large sphere of the Van de Graaff generator and observe the glow. The illustration shows a convenient arrangement. (Alternatively, put the neon tube in a holder with leads attached. Plug one lead from the holder into the Van de Graaff generator while the lead from the other end of the holder dangles. Bring up an earthed body such as a finger near the dangling end, and the lamp glows.}
  2. Show that an ‘electric current’ supply does the same thing. Set up the two metal plates with insulated handles parallel to each other and, say, 15 cm apart. Connect the positive terminal of the EHT supply to one of the metal plates; connect the earthed negative terminal of the supply to the other metal plate. Hold the neon lamp between the plates. It will glow.
  3. Show that the neon lamp can be lit using the HT power supply. Connect a safety resistance (220 kΩ, 1 watt) in series with the lamp. Connect to the neon lamp using 4 mm leads and crocodile clips. Set the HT supply to give a d.c. voltage of, say, 200 V. The neon lamp will glow.
  4. Hold (in the hand or clamp stand) the fluorescent tube near to the dome and it will glow.

Teaching Notes

  • These demonstrations show that neon gas glows red when a potential difference from either an electrostatic generator or a power supply (a.c. or d.c.) is used as the source of a high voltage.

This experiment was safety-tested in January 2007

  • A video showing how to use a Van de Graaff generator:

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Deflecting an electron beam

Electron
Quantum and Nuclear | Electricity and Magnetism

Deflecting an electron beam

Practical Activity for 14-16

Demonstration

In this simple demonstration with a fine beam tube you can show an electron beam. You can also bend it using an electric field and a magnetic field produced by some Magnadur magnets. These are interesting in their own right, and are good preparation for other experiments.

Apparatus and Materials

  • Fine beam tube and stand
  • Power supply, low voltage, variable, 0 - 24 V, smoothed
  • Power supply, HT, 0-250 V, with special shrouded connecting leads
  • Power supply, 6.3 V, AC, for the heater filament (this is often included on the HT supply)
  • Magnadur magnets, 2
  • Connecting leads

Health & Safety and Technical Notes

The HT supply can deliver a fatal current. Use 4 mm leads with plugs that have sprung shrouds for all high-voltage connections. Ensure that the member of staff supervising the dark room is aware of the hazards and their control.

The practical work with a HT supplies should only be undertaken by teachers with a 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


Read our standard health & safety guidance


Setting up the fine beam tube: Follow the manufacturer’s instructions for setting up the fine beam tube. (This demonstration does not involve the Helmholtz coils, so remove these if this can be done simply.)

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 supply to the anode. Set this to zero. The negative terminal of the HT goes to a socket, which is often near to the heater terminals.
  • The low-voltage supply to the deflecting plates. Set this to zero.

A tube which has not been used for a while may not emit electrons. It may be possible to encourage it to do so by increasing the heater voltage slightly, to around 1 V or so. Monitor it carefully. Ensure that the heater current is only slightly exceeded.

You can use the low voltage power supply or the batteries for deflecting the electron beam.

Power supply:

  • A smoothing unit may be needed with the low voltage power supply.
  • You will need to ensure that only one point is earthed. The low voltage supply will have the negative terminal earthed. As this is connected to the anode, ensure that the positive terminal of the HT supply is earthed.
  • Do not connect the low voltage power supply to the heater. This will damage it. The heater needs a supply of about 6 V. This is usually included on the HT power supply.
  • Some power supplies have moving coil voltmeters incorporated in them. This type is helpful in this experiment.

Batteries

  • You can use batteries instead of the low voltage supply. You will need three 6 V battery packs connected together to get a decent deflection. You can change the deflection by increasing the number of cells being used.
  • Deflect the beam one way by connecting the negative terminal to the earthed anode and the positive terminal to the deflection plate.
  • Deflect it the other way by connecting the positive terminal to the earthed anode and the negative terminal to the deflection plate.

Procedure

  1. Select the gun which gives a horizontal electron beam. (There may be a selection switch.)
  2. Always switch the heater on first, and only when it is glowing turn up the accelerating voltage on the anode.
  3. When the filament is glowing, carefully increase the anode potential difference (p.d.}. At a p.d. which may be as low as 50 volts, the fine beam should be seen. With some tubes it may take 3-4 minutes to be clearly visible. As the p.d. is slowly increased, the beam will lengthen and strike the glass envelope of the tube.
  4. Reduce the p.d. and show this transition several times. Do not increase the p.d. beyond about 200 volts.
  5. With the beam striking the wall, apply 10 to 20 volts (d.c.) to the deflecting plates and observe the movement of the beam. Reverse the connections to the deflecting plates and repeat. Increase the p.d. on the deflecting plates to the maximum available, and repeat the reversing procedure.
  6. Now deflect the beam with a magnet with face polarity (a Magnadur magnet}. Bring the magnet near to the envelope of the tube and point out the deflection of the beam.
  7. You can use two such magnets on opposite sides of the tube to produce a more uniform field. See diagram above. Take care not to bring a magnet into violent contact with the glass.
  8. If there are field coils, then a variable low voltage connected to them (up to 6 V or so) will deflect the beam into a circle, whose radius decreases with increasing voltage.

Teaching Notes

  • This may be students’ first glimpse of an electron beam. Allow them to enjoy it. This experiment is best demonstrated to the students in groups of four to five in a darkened room if full value is to be obtained.
  • Always reduce the anode to zero volts when not actually observing the beam, because the tube has a finite lifetime.
  • The electron beam is visible because there is a low-pressure gas in the tube. Electrons striking the gas molecules give them energy, which is then released as light. When they re-radiate this energy, hydrogen gas glows blue and helium gas glows green.
  • Draw the parallel with old television tubes by changing the beam to a horizontal one if possible. This has an electron gun like the fine beam tube. The electron beam is usually deflected magnetically rather than electrostatically and a different method of making it visible is used.
  • You could also draw the parallel with a cathode ray tube, as found in an oscilloscope. (See related experiments.)
  • You could try connecting a low frequency alternating supply to the deflecting plates. The beam will move from side to side. NB remove any smoothing components first! This is extended in the experiment.
  • The hot electron gun releases electrons, and a potential difference of about 180 V will accelerate electrons to about 8 x 10
  • 6 m s -1 . In a TV tube with 25 kV it is about 3 x 10 7 m s -1 .
  • Catching up on the catapult field. It is worth reminding students of the catapult effect and Fleming’s left hand motor rule. The deflection of the beam is consistent with the electrons having a negative charge. That is, to explain the direction of the deflection, the current must be flowing into the electron gun. Therefore, the charge on the particles carrying it must be negative, because they are flowing out of the electron gun.

This experiment was safety-tested in May 2007

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Electric charge and current - a short history

Charge
Electricity and Magnetism

Electric charge and current - a short history

Teaching Guidance for 14-16

Electrical phenomena result from a fundamental property of matter: electric charge. The atoms that constitute most matter we encounter contain charged particles. Protons and electrons each have one unit charge, but of opposite sign. Atoms are generally neutral because the number of electrons and protons are the same.

Electric charges at rest have been known much longer than electric currents.

The amber effect

The property now called static electricity was known to the philosophers of ancient Greece. In fact the word electricity comes from ‘elektron’, the Greek name for amber. Amber is a resinous mineral used to make jewellery. It is probable that small fibres of clothing clung to amber jewels and were quite difficult to remove. Trying to rub the fibres off made the situation worse, causing early philosophers to wonder why.

William Gilbert mentioned the amber effect in his ground-breaking book On Magnetism, published in 1600. He noticed that the attraction between electrics was much weaker than magnetism and wrongly said that electrics never repelled.

Benjamin Franklin

A giant leap of understanding was required to explain observations like these in terms of positive and negative electrical charge. In the 18th century, Benjamin Franklin in America tried experiments with charges. It was Franklin who named the two kinds of electricity ‘positive’ and ‘negative’. He even collected electric charges from thunderstorm clouds through wet string from a kite.

Franklin was an advocate of a ‘single fluid’ model of electric charge. An object with an excess of fluid would have one charge; an object with a deficit of fluid would have the opposite charge. Other scientists had advocated a ‘two fluid’ theory, with separate positive and negative fluids moving around. It took over a century for the debate to come down on Franklin’s side.

It is interesting to note that Franklin coined several electrical terms which we still use today: battery, charge, conductor, plus, minus, positively, negatively, condenser (= capacitor), among others.

Electric currents

Electric currents were not fully investigated until batteries were invented in about 1800. Passing currents through salt solutions provides evidence that there are two kinds of charge carriers, positive and negative. The charge carriers that boil out of white hot metals are negative electrons, and movements of electrons produce current in a cool, metal wire.

For a time electric currents seemed so different from electric charges at rest that the two were studied separately. It seemed as if there were four kinds of electricity: positive and negative electrostatic charges, and positive and negative moving charges in currents. Now scientists know better. There are just two kinds, positive and negative, exerting the same kind of forces whether they were ‘electrostatic charges from friction’ or ‘moving charges from power supplies’.

A modern view

Electric forces are what hold together atoms and molecules, solids and liquids. In collisions between objects, electric forces push things apart.

Today we understand that electrons may be transferred when two different materials contact each other and then separate. You can list materials in order, from those “most likely to lose electrons” (gaining positive charge) to "those most likely to gain electrons” (gaining negative charge). This is called the triboelectric series.

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Classroom management in semi-darkness

Interference
Light Sound and Waves

Classroom management in semi-darkness

Teaching Guidance for 14-16

There are some experiments which must be done in semi-darkness, for example, optics experiments and ripple tanks. You need to plan carefully for such lessons. Ensure that students are clear about what they need to do during such activities and they are not given unnecessary time. Keep an eye on what is going on in the class, and act quickly to dampen down any inappropriate behaviour before it gets out of hand.

Shadows on the ceiling will reveal movements that are not in your direct line of sight.

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Van de Graaff generator - the basics

Voltage
Electricity and Magnetism

Van de Graaff generator - the basics

Teaching Guidance for 11-14 14-16

Principle of operation

Some insulating materials when separated from the surface of others, leave those surfaces electrically charged, each with the opposite sign of charge and with a high potential difference (p.d.).

A machine to make charges was invented in 1929 by a young American called Van de Graaff. Huge machines, some over 30 m high, based on his ideas have been built to produce extremely high potential differences.

Belts and rollers

A flexible belt made from an insulating material and running continuously over two rollers can, by the same process, produce a supply of charge where the surfaces separate. The two rollers have to have different surfaces (often acrylic and metal) and together with the belt-rubber, are chosen by experiment.

Combs

Charges are “sprayed” on to and removed from the moving belt by “combs” situated adjacent to the rollers. Actual contact between the combs and the belt is not essential because of the high potential differences. Combs can be simply a stretched wire, or a sharp or serrated edge: action depends on very high potential gradients due to their small radii (similar action to lightning conductors).

The lower comb is maintained at or close to earth potential and is a drain for negative charge, leaving the belt with positive charges that are carried up to the top comb.

Collecting sphere

The top comb is connected to a collecting sphere which, having inherent electrical capacity (proportional to its radius) will collect and store the charge on its outer surface until discharged either by breakdown of the surrounding air as a spark, or by conduction to an adjacent earthed object.

Charging current

So long as the belt continues to move, the process continues, the drive (motor or manual) supplying the power to overcome the electrical repulsion between the charges collected on the sphere and those arriving on the belt.

The charging current is usually a few mA and potential difference achieved by “junior generators” will be 100-150 kV and by “senior” generators up to about 300 kV.

The whole apparatus

The mechanical arrangement of the belt/roller system is very simple. The lower roller is driven either manually or by motor. The former usually involves a hand wheel and pulley with belt-drive; this pulley can be mounted directly on the motor spindle. In “junior” models, fixed speed, shaded-pole induction motors are usual; “senior” models often incorporate small H.P. variable-speed (sewing-machine) motors, with carbon brushes, control being by either a simple rotary rheostat or a solid state circuit. The motors, control switches and mains input socket are housed in a metal or plastic enclosure, although some junior models have used a transparent plastic cake-cover.

The support column for the collecting sphere can be a simple PVC plastic rod or acrylic tube or a pair of acrylic strips with separators. In some models the belt is enclosed within a plastic pipe with “windows” along its length. Not all generators have means of adjusting the separation of the upper and lower rollers i.e. the belts have to be tailored for a particular machine.

Since the diameter of the collecting sphere determines the maximum p.d. (voltage) achievable, large spheres are mounted on taller columns to be more remote from the earth motor and control box.

Machines are usually supplied with a “discharger", often another, smaller, sphere mounted on a metal rod that has to be earthed to draw sparks from the collecting sphere.

Demonstrations and accessories

Certainly the Van de Graaff generator can produce striking demonstrations. The usual experiments are:

Faraday’s cylinder to show electric charge resides on the outer surface of a charged hollow conductor.

Bouncing ball. Suspend a conducting ball a non-conducting thread. When the ball touches the charging sphere, it will become charged and be repelled away from the sphere. If the ball is then allowed to discharge (touching an earthed surface, or leaking charge to the air) it will be attracted once more to the sphere, to be recharged ... and so the process continues.

The head of hair is another demonstration of repulsion. Real hair or shredded paper strips bunched at one end are used and provide a sensitive means of detecting charge.

The electric wind is produced by release of ions at the end of a pointed conductor and is enough to deflect a candle’s flame.

Hamilton’s mill utilizes the electric wind at the pointed ends of four arms to cause rotation about a pivot. This is similar to the action of a lightning conductor, which allows charge transfer at sharp points.

Kinetic theory model You can show random motion of metallic balls continuously affected by repulsion and loss of charge within a transparent vessel.

Neon indicator shows luminous discharge from the gaseous excitation by the high electric fields near the generator.

An apparatus note on the Van de Graaff generator gives information about good housekeeping and repairs:

Van de Graaff generator


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Van de Graaff generator safety

Voltage
Electricity and Magnetism

Van de Graaff generator safety

Teaching Guidance for 14-16

Van de Graaff generator demonstrations can provide useful insights into electrical phenomena, which are at the same time memorable.

  • It is essential the Van de Graaff generators for school science are obtained through reputable school science equipment suppliers. The sphere has a capacitance, and will store energy electrostatically as a result of the separation of charge. The energy stored electrostatically by the sphere should not exceed 0.5 J.
  • Do not add devices to the sphere that increase the capacitance.
  • Van de Graaff generators with mains powered pulleys must be electrically inspected and tested in the same way as other mains powered equipment.
  • When carrying out the hair-standing-on-end demonstration, do it with one person at a time. After the demonstration, to avoid a sudden discharge, the person should take their hand off the sphere and touch the surface of a wood bench top (avoiding metal fittings such as gas taps). Alternatively, hand the charged person a wooden metre rule. After a few moments, they will be discharged.
  • It is not advisable for people to participate in practical work with Van de Graaff generators if they have heart conditions, or a pacemaker, or other electronic medical equipment fitted.
  • The electrical discharge from a Van de Graaff generator can wreck electronic circuits, so equipment such as computers and instrumentation with electronic circuits should be kept well away.

The Van de Graaff generators designed for schools are usually the triboelectric type; these are the most suitable. The transfer of charge is achieved by a rubber belt driven by a plastic pulley, with an arrangement of metal combs at either end of the belt. Charge is transferred to a metal sphere (a capacitor) and very high voltages are achieved between the sphere and ground, typically in the range 200 kV to 300 kV.

Using a Van de Graaff generator, one is quite likely to receive a short shock by accidental or intentional contact with the charged dome. An enquiry to CLEAPSS has revealed no recorded incident of direct injury caused by shocks from the correct use of school Van de Graaff generators. However, some people are more sensitive than others and can find the shocks very unpleasant and painful. For this reason, only volunteers should take direct part in the practical work.

The shock is a single unidirectional pulse of short duration; a capacitor discharge through the resistance of the body and contacts. Using rough values of sphere capacitance, 10 pF, and body resistance, 1000 ohms, the discharge time through a person will be of the order of nanoseconds and certainly less than a millisecond. The capacitance of the sphere needs to be low enough that at maximum voltage, the energy transferred by the discharge current through the body causes at most no more than an unpleasant or painful sensation. The current flowing and energy transferred to the body should be well below that which could cause any risk of ventricular fibrillation.

For an impulse current I Amps of short duration t seconds ( t < 10 ms) through the body, the principal factor for the initiation of ventricular fibrillation is the value of I  ×  t or I 2  ×  t (IEC 2007). At high applied voltages, the resistance of the adult body (left hand to right hand) is at least 575 ohms for 95% of the population (IEC 2005). The total body resistance for children is expected to be higher but of the same order of magnitude. The IEC gives a threshold value of 'Specific Fibrillation Energy', for a 1 ms current impulse, of ValueExponent{2}{-3}A 2s. Below this threshold there is no evidence of fibrillation. The Specific Fibrillation 'Energy' can be regarded as the energy dissipated per unit resistance of the body through which the current flows. Note that ‘specific’ here means ‘per unit resistance’ rather than ‘per unit mass’.

At 575 ohms and discharge time not exceeding 1 ms, the energy stored electrostatically by the capacitor would need to be at least 1.1 J to reach the Specific Fibrillation Energy. Although this is a very conservative estimate because the discharge time is likely to be much less than 1 ms, Van de Graaff generators that can store more than 1 J of energy electrostatically by the sphere should be avoided. A discharge of 1 J affects everybody severely (BSI 1991).

An estimate of the energy stored electrostatically by the sphere can be made by calculating the sphere capacitance, C = 4 π ε0 r (where C is in farads, r is the sphere radius in metres and ε0 is the permittivity of free space), estimating the voltage, V , using the length of spark gap, and calculating the energy E , in joules, from E = 0.5 CV 2.

Generally speaking, sphere diameters of Van de Graaff generators should be about 20 cm or less. Using data from one manufacturer’s specification, the sphere diameter is given as 20 cm and the maximum voltage 250 kV, the energy stored electrostatically would be 0.35 J, well below 1 J. If this is compared to a sphere diameter of, say, 25 cm and a maximum voltage of 350 kV, then energy stored electrostatically would be 0.85 J. This would still be below 1 J, but the shocks would be correspondingly more unpleasant and painful, and this may put off some people from using the generator.

If you wish to estimate the voltage across the sphere and ground, you can do this by finding the maximum spark length. Wait until the sphere is fully charged, then bring up a grounded sphere slowly until you obtain a spark discharge. This technique has limitations and you should do it carefully several times to find the maximum spark length. Do the test on a dry day with low relative humidity so the Van de Graaff generator is working at its best.

Note that the rule-of-thumb 3 kV/mm is only a reasonable rule for voltages below 100 kV.

References

IEC 2007. IEC/TS 60479-2:2007. Effects of human beings and livestock – Part 2: Special aspects.

IEC 2005. IEC/TS 60479-1:2005. Effects of human beings and livestock – Part 1: General aspects.

BSI 2002. BS EN 60052:2002. Voltage measurement by means of standard air gaps.

BSI 1991. BS5958-1:1991. Code of practice for control of undesirable static electricity Part 1: General considerations. [Replaced by PD CLC/TR 50404:2003 but remains current.]

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