Forces due to electric charges
Charged materials exert forces on each other which might be attractive or repelling. The closer the materials, the larger the forces.
Apparatus and materials
Nylon thread, e.g. fishing line, 1 reel
Conducting spheres, light-weight, 2
Power supply, EHT, 0-5 kV (with internal safety resistor)
Wire stirrup (for supporting a 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.
Electric sausage, a video from the National STEM Centre eLibrary, shows similar demonstrations.
a 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.
b 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.
c 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.)
NB Use the output via the safety resistor.
d 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.
Suspend an uncoated balloon by a long nylon thread. Rub it with wool to charge it.
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.)
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.
Carry the coated balloon with its charge across to the suspended balloon. Ask students to observe the forces acting on the balloons.
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.
e 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.
Rub the acetate strip and touch one ball; rub the polythene strip and touch the other balls. The balls attract.
Repeat the above having rubbed the polythene and acetate strip together first.
f 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.
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.
1 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.
2 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.)
3 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.
4 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.
5 If charged rods become difficult to discharge, wave them above a Bunsen burner flame.
6 Identifying charges is now much easier since the advent of coulomb meters, based on a capacitor, and so more quantitative work can be done.
7 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