Electrostatic Force
Electricity and Magnetism

Electrostatic charges

for 14-16

Historically, the first observations and experiments with electricity involved static charges. These experiments show ways of producing static charges and the effects of static charges.

Up next

Forces due to electric charges

Electrostatic Force
Electricity and Magnetism

Forces due to electric charges

Practical Activity for 14-16

Demonstration

Charged materials exert forces on each other which might be attractive or repelling. The closer the materials, the larger the forces.

Apparatus and Materials

  • Balloons, 4
  • Nylon thread, e.g. fishing line, 1 reel
  • Conducting spheres, light-weight, 2
  • Power supply, EHT, 0–5 kV (with internal safety resistor)
  • Acetate rod
  • Wire stirrup (for supporting a rod)
  • Polythene rod

Health & Safety and Technical Notes

When fixing anything to a high point, use a proper step-ladder held by a colleague.

For light-weight conducting spheres, use expanded polystyrene balls, coated with aluminium paint.

Alternatively, use an inflated balloon which is made conducting as follows: spray the balloon with an anti-static aerosol spray. (Some aluminium aerosol sprays will make a conducting coating, or the balloon can be painted with Aquadag mixed with wetting liquid; but anti-static spray is best.) Attach the conducting balloon to a long insulating handle.

Read our standard health & safety guidance


Procedure

  1. To demonstrate charging by friction, suspend two inflated balloons using long nylon threads. The balloons must be well above the table or floor, and far from any metal supports. Charge the balloons with like charges by rubbing each in turn against a woollen garment or cloth. Stand well away and let students see the effect of like charges repelling.
  2. To demonstrate attraction between unlike charges, suspend two balloons a small distance apart. Rub them against each other. In practice, this produces unlike charges on the two balloons (but the charges are likely to be unequal in size}. Hold these oppositely charged balloons apart by their threads. Gently release the threads. Students will observe the balloons attracting.
  3. Crumple a piece of plastic, such as a sandwich bag; rub it on wool and bring it near each of these balloons in turn. (There is a competition between the effect of the charge on the crumpled plastic and induced charges on one’s earthed hand, so it is better to hold the plastic on a long insulating rod.}
  4. NB Use the output via the safety resistor.
  5. In a further demonstration, one balloon is charged by friction, the other by a 5,000-volt power supply. The EHT supply will not put large enough charges on, two balloons for the forces to be noticeable. However, if one balloon is charged by rubbing on wool and a second balloon is charged from the EHT the mutual forces will be appreciable.
  6. Suspend an uncoated balloon by a long nylon thread. Rub it with wool to charge it.
  7. Attach a second balloon, with a conductive coating, to a long insulating handle. Ensure the handle is not already accidently charged by friction. (If it is wave a lighted match nearby.)
  8. To charge this second balloon, earth the positive terminal of an EHT supply and touch the balloon on the negative terminal. It is easier to do that if a small metal stud is attached to the EHT terminal.
  9. Carry the coated balloon with its charge across to the suspended balloon. Ask students to observe the forces acting on the balloons.
  10. Now earth the negative terminal of the EHT instead and charge the coated balloon from the positive terminal. Bring it near to the suspended balloon. Again, ask students to observe the forces acting on the balloons.
  11. Suspend two very light conducting spheres by very long threads, preferably from the ceiling, or from very high retort stands. Use the EHT power supply – with safety resistor – to charge them. They should first be charged with opposite charges, and then with like charges (as shown in the picture above). Rub the acetate strip and touch both balls. The balls repel.
  12. Rub the acetate strip and touch one ball; rub the polythene strip and touch the other balls. The balls attract.
  13. Repeat the above having rubbed the polythene and acetate strip together first.
  14. Hang up the wire stirrup with a nylon suspension from a single retort stand, rub the polythene rod, and fix it in the stirrup. Charged spheres brought up will attract or repel the suspended rod, as will other charged rods. This investigation will confirm in the pupils’ minds the conclusions drawn from the earlier experiments.
  15. If it is difficult to suspend the rod (because of discharging it by touching), then place it on an upturned watch-glass or convex lens. There are also supports with good rotating bearings which can be obtained.

Teaching Notes

  • Many students will have charged a balloon by rubbing it against a sweater, particularly one made from man-made fibres, and then stuck it to the wall or ceiling. This trick can be exploited and is particularly effective if there is a glass-fronted cupboard nearby. Two balloons can be charged by friction and attached to the glass. Both balloons will stick to the glass but they will not sit closely together. Students should be encouraged to suggest reasons for these effects.
  • Both balloons receive the same charge (i.e. charges of the same sign) when they are rubbed on, for example, a sweater. The sweater and the balloon have opposite charges. The balloon sticks to the glass because it induces the opposite charge to itself onto the front face of the glass, and so the balloon is held to the glass, by the attraction of opposite charges. However the two balloons have the same charge and so they repel each other.
  • The EHT supplies found in school, maximum 5,000 V, do not supply enough charge for charging balloons, but a Van de Graaff generator could be used. A small metal ball on an insulating handle could spoon charge off the dome of the Van de Graaff and transfer the charge to the balloons. (The balloons may need to be conducting.)
  • An alternative way of charging the balloons is to coat the balloons by covering them in Aquadag (colloidal graphite), spraying them with aluminium paint, or dipping them into strong detergent, which is then allowed to dry. The balloons can then be hung up with insulating threads.
  • One balloon is charged by friction. The other uncharged balloon is brought near to the charged balloon but a thin sheet of plastic is put between them. Touch the uncharged balloon. Separate them and they will be oppositely charged.
  • Two polystyrene balls can be suspended and charged from an EHT supply. The power supply should have one terminal earthed and a metal stud in the opposite terminal (include the safety resistor). A small insulated plate should be touched onto the stud and the charge transferred to the balls. Care should be taken in using the EHT supply, although school power supplies are current-limited to less than 5 mA.. It is good practice to use only one hand and to put the other behind your back so that you cannot accidentally become part of the circuit. The balls can be charged with either the same or opposite charges.
  • If charged rods become difficult to discharge, wave them above a Bunsen burner flame.
  • Identifying charges is now much easier since the advent of coulomb meters, based on a capacitor, and so more quantitative work can be done.
  • How Science Works Extension:
  • You might like to take this opportunity to discuss with your students the difference between the observations made in these experiments and the interpretation of them, which requires a theory of charge.
  • We can observe attraction and repulsion between electrically-charged objects. The general conclusion is that there are two kinds of charge, which are in some way opposite. If two objects are charged in the same way, they show repulsion. If they are charged differently, they show attraction.
  • When two materials are rubbed together, one material gets a positive charge, by losing electrons, while the other material gets a negative charge, by gaining electrons.

This experiment was safety-tested in January 2005

A video showing static electricity and charge with an 'electric sausage' (shows a similar demonstration):

Up next

Demonstration electroscope

Electricity and Magnetism

Demonstration electroscope

Practical Activity for 14-16

Demonstration

This is a model of an historic instrument which can still be used to compare or measure charges.

Apparatus and Materials

  • Retort stand and boss
  • Perspex (insulating) rod
  • Brass or aluminium strip, about 20 cm x 1 cm
  • Metallised foil (e.g. Mylar or Melinex) strip, about 18 cm x 1 cm
  • Polythene tile
  • Cloth for rubbing
  • Electrophorus plate
  • Power supply, EHT, 0–5 kV (with internal safety resistor)

Health & Safety and Technical Notes

Read our standard health & safety guidance


Procedure

  1. Clamp a Perspex rod in a boss on a retort stand.
  2. Bend the strip of brass or aluminium sheet as illustrated and fix it over the Perspex rod.
  3. To one side of the brass, attach the strip of foil with glue.
  4. To charge the foil to a high potential, use the electrophorus plate as follows. Rub the polythene tile to give it a negative charge. Holding the plate by the insulating handle, touch the plate on the tile. While it is still in contact with the tile, touch it momentarily with a finger, and then remove it from the tile.
  5. Bring the plate into contact with the top of the model electroscope. This charging process should be repeated several times.
  6. The plate can also be charged by connecting it to the EHT power supply in series with the 50 MΩ resistor incorporated in the supply. In this case, the other terminal of the supply should be connected to the retort stand base.

Teaching Notes

  • This is intended to be a very simple, large-scale introduction to the electroscope. It illustrates how charge can be transferred from one object to another, and how the presence of charge can be identified by the deflection of the metallized foil.
  • The gold leaf electroscope is a very important piece of apparatus for use in electrostatics experiments. The design is so simple that it is absolutely clear how it works, so there can be no mystery about how it provides the results you are looking for (unlike modern electronic meters).

This experiment was safety-tested in january 2007

Up next

Gold leaf electroscope

Electrostatic Force
Electricity and Magnetism

Gold leaf electroscope

Practical Activity for 14-16

The device is used for detecting electric charge and can also identify its polarity, if compared with a known charge.

Up next

Charging by electrostatic induction

Electrostatic Force
Electricity and Magnetism

Charging by electrostatic induction

Practical Activity for 14-16

Class practical

Electrostatic induction is a quick way of using a charged object to give something a charge, of the opposite sign, without losing any of the original charge.

Apparatus and Materials

  • Gold leaf electroscope


  • Acetate rod or strip
  • Polythene rod or strip
  • Cloth for rubbing
  • Small calorimeters (or similar metal canisters), 2
  • Polythene tile
  • Electrophorus plate
  • Proof plane
  • Tin can

Health & Safety and Technical Notes

Check the tin cans before use for a sharp edge where it was opened. Remove it if necessary by pressing it outwards with a steel rod.

Read our standard health & safety guidance


Procedure

  1. Charge the polythene rod by rubbing it with the cloth. Rub the charged rod on the top plate of the electroscope. Observe the movement of the gold leaf. Discharge the electroscope by touching it.
  2. Bring a charged polythene strip near to the top plate of the uncharged electroscope, but without touching it. Then touch the electroscope plate with a finger so that the leaf falls. Take away the finger, then take away the charged polythene strip and watch what happens. The electroscope has been charged by induction.
  3. Charge the electroscope by rubbing a charged polythene strip along the plate. The leaf should hang at about 45°.
  4. Invert the metal canisters and place each on a polythene tile. Lift the tile to bring each canister in turn close to the electroscope, to show they are uncharged.
  5. Place the tiles on the bench, with the two canisters touching each other.
  6. Rub the polythene rod with the cloth and bring it up close to one of the canisters; while the rod is still there, separate the canisters. Bring each up to the gold leaf electroscope.
  7. Repeat the above, but this time, when the charged strip is brought up near one of the two canisters, earth the second canister by touching with a finger. Test each canister.
  8. With the gold leaf electroscope discharged, bring up a rubbed polythene strip, close to the plate. Touch the plate momentarily so that the leaf falls, then remove the polythene strip.
  9. Rub one of the tiles with the cloth. Place the electrophorus plate on the tile and then remove it and test it on the electroscope. Repeat this, this time earthing the plate with your finger whilst it is in contact with the tile, before testing on the electroscope.
  10. Put a tin can on top of the electroscope. Give a charge to the end of the proof plane and bring that charge, held by the insulator, into the tin on the electroscope. Watch what the electroscope does when the charge is moved about inside it, without touching it, and then when the charge is removed. Repeat the whole experiment, this time allowing the charged object to touch the inside of the can.
  11. Put a tin can on top of the electroscope. The electroscope should be discharged with the leaf down. Insert a cloth into the tin, if necessary holding it down with a mass. Insert the polythene rod into the cloth (having previously discharged the strip by passing it over a Bunsen flame). Then pull out the rod, rubbing it against the cloth in the process. Observe how the leaf moves. Then re-insert the rod and observe the leaf.
  12. The metal washer on the end of the Perspex rod, which forms the proof plane, can be used to investigate the distribution of charge over a charged conductor. Place one of the canisters on a polythene tile and charge it by repeated contact with the electrophorus plate. Touch different parts of the canister with the proof plane, each time testing it on the charged electroscope. Compare the sides, the edges and the bottom of the inside.

Teaching Notes

  • Note the sequence when charging a gold leaf electroscope. A charged object is brought close to the top plate (so that the leaf deflects). With the charged object still close by, the top plate is touched to earth it (the leaf falls). The earthing finger is removed. The charged object is removed (the gold leaf rises again).
  • The proof plane consists of a metal washer on the end of an insulating handle. By touching it on a charged object, a fraction of the object’s charge is removed.
  • If a charged proof plane is brought near a charged electroscope, the deflection of the gold leaf will change. If the charges are like, the deflection will increase; if they are unlike, it will decrease.
  • Step 11 shows that charge is conserved; the cloth gains a certain amount of charge; the rod gains an equal but opposite charge.
  • Step 12 shows that charge is not evenly distributed over a conductor; it is more concentrated at points which are strongly curved, e.g. edges.

This experiment was safety-tested in January 2007

Up next

Further electrostatic experiments

Electrostatic Force
Electricity and Magnetism

Further electrostatic experiments

Practical Activity for 14-16

Demonstration

These experiments could be used at the same time as elementary experiments with charged rods and polystyrene balls. Or, they could be used as revision experiments before a more advanced look at electrostatics.

Apparatus and Materials

  • EHT power supply (5000 volts)
  • Megohm resistor
  • Demonstration meter
  • DC dial, 2.5–0–2.5 mA.
  • Melinex, metal-coated, 2 strips
  • Melinex, plain, un-coated, 2 strips
  • Perspex rods, 2
  • Retort stands and bosses, 2
  • Sellotape
  • Leads (shrouded), 3

Health & Safety and Technical Notes

Read our standard health & safety guidance


Melinex is an aluminium-coated polyester film. It can be obtained from Papersafe, 2 Green Bank, Adderley, Market Drayton, Shropshire, TF9 3TH. Telephone: 01424 871556, Fax: 0870 054 8747.

A fairly large roll, 50 micron thick, costs £8.00 (January 2007).

The strips should be about 12 cm x 2 cm. They must be flexible. 25-guage Melinex is suitable. For steps b and c it must be metal coated on both sides.

For step 4 the strips must be of plain flexible non-conductor (Melinex, or strips of polythene film cut from a sandwich bag).

Procedure

  1. Current, driven by power supply: connect the EHT supply to 2 megohms and the demonstration milliameter. Simply point to the current.
  2. Charges at rest, provided by power supply: hang two strips of metal-coated Melinex from horizontal Perspex rods on stands. Place stands just far enough apart to prevent the strips touching when they attract each other. Carry leads from the EHT's + and - terminals (neither of them are earthed) to the strips, and touch them momentarily.
  3. Charges of the same kind, provided by power supply: earth one terminal of the EHT supply. Connect the other terminal momentarily to each strip in turn. Move the stands nearer, so that the repulsion is more marked.
  4. Charges made by 'friction': tape two strips of uncoated flexible plastic to a Perspex rod in a stand. Run dry fingers down the pair of strips, with one finger between them.
  5. Note: If the strips are already charged, they are easily discharged by waving a lighted match nearby.

Teaching Notes

  • Step 1 above: A power supply can continue to drive a current through an external or an internal resistance. Students might be worried when you say that you are going to connect a delicate milliameter across the terminals of a 5,000-volt supply. This is part of the showmanship of the teacher. The internal resistance of the supply will ensure that the current is small.
  • Step 2 above: An EHT power supply pumps charges onto the Melinex. The strips should be far enough apart so that they do not touch when they attract, otherwise a spark will travel up to them and their aluminium surface will evaporate.
  • Step 3 above: This will show the strip repelling.
  • Step 4 above: When you rub your finger between the two strips electrical forces will pull negative particles off your skin. Both strips will be charged with the same charge and so repel.

This experiment was safety-tested in January 2007

Up next

Flowing fluids can become charged

Electrostatic Force
Electricity and Magnetism

Flowing fluids can become charged

Practical Activity for 14-16

Demonstration

To illustrate the dangers associated with transferring fuel.

Apparatus and Materials

  • Small polythene funnel
  • 50 cm polythene tubing
  • 50 cm of copper tubing
  • polystyrene beads (e.g. as used in bean bags)
  • Small beaker
  • Small calorimeter can
  • Coulombmeter or gold leaf electroscope
  • Clamp stand

Health & Safety and Technical Notes

Read our standard health & safety guidance


There is a risk of slipping if the spheres fall onto the floor.

If possible stand the whole apparatus on a tray to contain any spilled polystyrene spheres.

Procedure

  1. Assemble the apparatus as in the diagram:
  2. Place the calorimeter on top of the coulombmeter or electroscope
  3. Clamp the tube and funnel above the calorimeter with the end of the tube just at the centre of the calorimeter.
  4. Pour polystyrene spheres into the funnel so they flow down into the calorimeter.
  5. The charge on the spheres will cause the gold leaf to rise, or produce a reading on the coulombmeter.

Teaching Notes

  • The polystyrene spheres, which are insulators, become charged by friction as they tumble down the insulating tubing. It is important that this tubing is polythene and not PVC. This illustrates how insulating liquids such as aviation fuel can become charged as they flow through pipes.
  • Repeat the demonstration with copper tubing connected to the calorimeter, to show that any charge developed is now safely discharged.

This experiment was submitted by Sylvia Bell, Head of Physics at Nottingham High School for Girls.

Up next

Electrostatics

Electrostatic Force
Electricity and Magnetism

Electrostatics

Practical Activity for 14-16

You need to avoid planning an electrostatics lesson and then finding that the atmosphere and the equipment are so damp that they get no effect at all, or else the results you do get appear to give the ‘wrong’ result so are confusing. There are precautions that can be taken which will usually ensure success.

All dusters used to charge objects should be freshly laundered and fluffy, and kept in a clean bag. When laundering, do not put fabric conditioner in the water as it's an anti-static agent. Students should have clean hands. Polystyrene balls, balloons, acetate and polythene rods should all be cleaned regularly. It is helpful to store all electrostatic equipment, including the Van de Graaff generator, in a cupboard which is kept warm and dry with a low wattage lamp burning. However even on a wet day, putting all the equipment near to an electric heater for some time before the lesson ensures that it is dry enough.

Today’s synthetic materials are well-known for becoming charged very easily, so that cars and carpets can give quite a nasty shock. Try separating bed-clothes in the dark of night and you will really see sheet lightning!

In modern laboratories with water fed through plastic pipes, it may be very difficult to find any point electrically bonded to earth. In such cases, an earth for electrostatics experiments can be provided by burying a substantial metal rod in the ground with a wire running through the wall to a terminal in the laboratory.

It helps to be familiar with the electrostatic series:

  • Perspex (acrylic) ELECTROPOSITIVE
  • Glass
  • Silk
  • Wood
  • Sulphur
  • Cotton
  • Ebonite
  • Indian rubber
  • Polythene ELECTRONEGATIVE

If two materials are rubbed together, the material higher in the list will gain a positive charge, while the lower material will gain a negative charge.

Good luck: these lessons can be fun!

Up next

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.

Up next

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


Up next

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.]

IOP DOMAINS Physics CPD programme

The IOP is proud to present our new Forces CPD Domain

This is the first of a collection of videos for you to view ahead of our live support sessions for teachers and coaches of physics.

Find out more