Demonstration

Comparisons with magnetic fields may be helpful here.

Apparatus and Materials

• Power supply, EHT (0-5kV) and/or Van de Graaff generator
• Polystyrene balls, metallised, approx 3 cm diameter
• Perspex rods, to hold balls, 2
• Retort stands and bosses, 2
• Perspex rod (about 60 cm) with paper vane
• Metal plates with insulating handles

Health & Safety and Technical Notes

Read our standard health & safety guidance

Paint the polystyrene balls with Aquadag or other conducting paint, or spray them with antistatic spray to give them a conducting surface. (Metal balls are equally good, but they should be large: diameter at least 5 cm.) Support each on a horizontal insulating rod (e.g. Perspex). Tape a small piece of aluminium foil onto each ball, to serve as an electrode. Use crocodile clips to attach leads to the supply.

Make a small paper vane, like a compass needle, about 4 cm long. Attach it to a long Perspex rod with a pin as pivot. The paper must be slightly conducting; paper is hygroscopic enough to ensure conduction in most cases. If necessary, breathe on the paper.

Procedure

1. Fix the two balls approx 10 cm apart, centre to centre. Connect the balls through the 50 MΩ. safety resistor to the 5,000-volt supply's + and - terminals, both unearthed.

2. Make sure the paper vane is free to rotate. Hold it by the long rod in the space between the balls. It will set along the field lines, because it develops induced charges in the field - rather like the behaviour of iron filings or soft iron in a magnetic field.
3. Starting with the vane near one ball, move it, steering straight ahead by the compass to map a line of force of the electric field. Show several such lines in quick succession. Ask students where they have seen a magnetic field of similar shape.
4. The device can also be used to show that the lines are not straight at the edge of a plate.

Teaching Notes

• The paper indicator acts by developing induced charges in the field. This is why it must conduct, although it need not conduct well. The charges were in the paper before, in equal amounts of positive and negative charge. They cancelled out each other's effects until the field dragged them apart, so that negative charges collect at one end and positive charges at the other. The field tugs on the charged ends of the paper indicator and pulls it round until it points along the field. In this way we can map the electric field in a similar way to mapping a magnetic field with iron filings.
• With a Van de Graaff generator you may be able to show the same field patterns, but corona discharge (from the higher potential) may mar it.

This experiment was safety-tested in December 2006

Demonstration

A shuttling ping-pong ball serves as a model of ions moving in an electric field.

Apparatus and Materials

• Power supply, EHT, 0–5 kV
• Metal plates with insulating handles, 2
• Table-tennis (ping-pong) ball, coated with Aquadag
• Nylon thread, e.g. fishing line, 1 reel
• Clamp
• Retort stands and bosses, 3

Health & Safety and Technical Notes

Read our standard health & safety guidance

Although educational EHT supplies are limited to safe output currents, the shock obtained by touching the live plate in this demonstration can make the demonstrator jump. Switch the supply off before making adjustments.

Procedure

1. Attach the two metal plates to retort stands using bosses, and set them up parallel to each other 10 cm apart.
2. Connect the plates directly to the positive and negative terminals of the EHT power supply using crocodile clips attached to the special lugs on the back of the plates.
3. Suspend the table tennis-ball, coated with Aquadag, by nylon thread from a further retort stand so that it hangs freely between the two plates.
4. Apply a potential difference of 3,000 to 4,000 volts to the plates. The ball is charged by contact with one of the plates. It will then move rapidly backwards and forwards between the two plates.
5. Vary the potential difference and the separation of the plates.

Teaching Notes

• This demonstration shows the forces on a charged object in an electric field. It also serves as a model of an ion moving in an electric field. As the ball moves to and fro between the plates, it represents a positive ion moving one way and then a negative ion moving the other.
• The electric field between parallel charged plates, E = V/d, where V is the potential difference between the plates and d is the separation of the plates.

This experiment was safety-tested in June 2007

Demonstration

A qualitative experiment to show the principle of a force on a charged body between two parallel plates.

Apparatus and Materials

• Macro-Millikan apparatus
• Power supply, 0–5 kV (Extra High Tension, EHT)
• Proof plane
• Polythene tile
• Charging cloth

Health & Safety and Technical Notes

Use an EHT supply of no more than 5 kV, which is current-limited to less than 5 mA.. The power supply for the heater MUST have adequate insulation. Leads used MUST have shrouded connectors and insulation capable of withstanding 5 kV.

Make all connections with the power supply turned off. Do not adjust connections while the EHT is switched on.

Electron beam tubes are fragile. Because they are evacuated, they will implode if they break. The tubes are also expensive, so handle them with great care. Use the purpose-designed holders during practical work.

Note that when switching the EHT supply off, it can take a little while for the voltage output from the EHT to fall to zero. Allow sufficient time before disconnecting.

Be Careful not to touch the exposed plates.

Read our standard health & safety guidance

The illustrations show the ‘macro-Millikan’ apparatus. It consists of a pair of metal plates, one of which has a central hole through which a small polystyrene ball can pass. The ball has a length of nylon thread attached, and at the other end of the thread is a Pyrex glass spring.

The best arrangement for securing the upper end of the spring is to attach it to another loop in a nylon thread. This is taken up over a pulley connected to the ceiling, and then to an eyelet on a block of wood. Alternatively, a support from a long retort stand rod can be used, as shown below, and brought up over the plates, forming a hooked support.

Procedure

1. With the EHT switched off, set the plates up horizontally, 7 to 10 cm apart, one above the other.
2. Connect the earth terminal of the power supply to its negative terminal. Connect the lower plate to the earth terminal of the EHT power supply. Connect the upper plate to the positive terminal. To reverse the field, change the leads to the supply, but in each case the lower plate should be earthed.
3. Lower the small conducting sphere with its nylon suspension through the hole in the upper plate. The upper end of the nylon suspension is looped and connected to the Pyrex glass spring.
4. Rub the polythene tile and put the proof plane on it, touching to charge the proof plane by induction. Without making contact with the plates, bring the proof plane up to the conducting sphere to charge it by contact. Adjust the suspension so that the sphere is almost exactly half-way between the two plates. This is most conveniently done in the first arrangement described above. By moving the block of wood nearer to or away from the apparatus, the sphere can be lowered or raised.
5. Switch on the EHT supply, set at 2 - 4 kV. The sphere will move as the extra force stretches the spring. The sphere can be brought back to the central position by moving the block of wood. Students will see the movement quite clearly if their eyes are in line with the plates.

Teaching Notes

• There are no measurements to be made in this demonstration: it is a qualitative experiment to show the principle of a force on a charged body between two parallel plates. Quantitative experiments do not lead to very satisfactory results unless considerable trouble is taken. There is a tendency for the charge to leak away along the suspension.
• If the ball is near one plate, the charge on it induces an opposite charge on that plate, so there is attraction. You do not want that ‘image-force’ to appear in the demonstration. The ball must be almost exactly half-way between the two plates, so that the image forces cancel out.
• Students' eyes need to be in line with the plates. It is easier to see the movement of the ball if a plane mirror is placed behind the suspension with a horizontal line ruled across it, whilst a small cardboard disc is attached to the nylon suspension to act as a pointer. This pointer is aligned with the mark on the mirror before the field is switched on, but after the sphere has been positioned at the centre of the plate. However, this special arrangement may divert attention from the general idea and need not be used in this qualitative demonstration.

• You may like to experiment with a realistic model of the Millikan experiment. Place a very light metal-coated ball in the field between the plates. Adjust the field to make the ball float upwards, fall slowly downwards, or even remain poised at rest for a short time. The ball is charged by contact with one of the plates, which then repels it. A very light ball is needed, or perhaps a scrap of aluminium leaf. When the ball is poised, its equilibrium is made unstable by image forces. To minimize that disadvantage, the charge on the ball should be made as small as possible and the electric field as large as possible.

This experiment was safety-tested in March 2008