Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

Electric charge using capacitors

Demonstration

Shows that electrostatic charges are the same as the charges passing through wires and components in electric circuits.

Apparatus and materials

Capacitors and resistors as follows

500 μF electrolytic capacitor (50 V working)

50 μF electrolytic capacitor (350 V working)

0.001 μF capacitor (15 kV working) See technical note 3

4.7 kΩ resistor (2 W)

100 kΩ resistor (2 W)

Ammeters (preferably 2.5-0-2.5 mA), 2

EHT power supply (0-5 kV), output current limited to 2 mA or 5 mA on old models

HT power supply (0-300 V), VERY HAZARDOUS BECAUSE OUTPUT CURRENT IS HIGH

Low voltage power supply (0-12 V)

Electrostatic generator, Van de Graaff or Wimshurst

Lamp holder (BC) on base

Lamp, 240 V, 15 W, in holder

Safety screen

Leads, including shrouded leads 

Health & Safety and Technical notes


Safety screens should also be set up in case the capacitor explodes.

1 With electrolytic capacitors, correct polarity should always be observed, and, if they have not been used recently, they should be re-formed (see CLEAPSS Lab. Handbook). 

2 It is useful to have two identical demonstration meters fitted with d.c. dials (2.5–0–2.5 mA).

3 A 0.001 μF capacitor (15 kV working) is probably suitable. These are available from RS Components Ltd (117-473 or 119-097).
Safety
HT power supplies must always be connected with shrouded plug leads and never with bare crocodile clips.

When high voltages are used, high-voltage leads should be used and all bare terminals securely insulated. EHT supplies are limited to 2 mA, but when the capacitor is charged to 5,000 V it is ready to discharge a very high initial current. The charge stored is only about 5 μC and the current would die out in a few microseconds, yet the shock would be unpleasant. So well-insulated flying leads are essential.


Procedure


Charging a 500 μF capacitor (no resistance in circuit) 
a Set up the series circuit shown, setting the low voltage supply to 4 V and using the 500 μF electrolytic capacitor (50 V working). 

series circuit

b On completing the circuit, the transient nature of the current will be revealed by the two galvanometers. Students will see the momentary pulses of current. Allow the capacitor to discharge by disconnecting the lead N from the supply and connecting it to M.

Charging a 500 μF capacitor through a large resistor
c Modify the circuit above by including a 4.7 kΩ resistor in series with the capacitor. Use 12 V from the supply.

include a 4.7kOmega resistor

d Charge the 500 μF capacitor as before. In this experiment, the slow charging process will be apparent. Students will see the current dying exponentially as the charge rises to the full value.

e Again connect lead N to lead M to show the discharge through the resistor.

Charging a 0.001 μF capacitor to a high voltage
f Set the EHT power supply to provide 5 kV and then use it to charge the 0.001 μF capacitor. This is best done by holding the capacitor horizontally in a clamp and connecting the stud mounting end to the earthed negative terminal of the power supply.

Connect the positive terminal to the capacitor through a 100 kΩ resistor. (If the power supply has a built-in 50 MΩ safety resistance, you could use this in place of the 100 kΩ resistor. But, as the safety resistor is less obviously part of the circuit, it is better to use a separate 100 kW resistor.) Care should, of course, be taken when working at these voltages.

Connection between the end of the resistor and the top of the capacitor is made with a well-insulated flexible lead, held by hand.

g After a moment or two, remove this lead. Use another insulated lead to short-circuit the capacitor.

Charging a 0.001 μF capacitor using an electrostatic generator
h Again it is best to clamp the capacitor in position.

Where a Wimshurst is used, the connection can be made directly. Where a Van de Graaff is employed, flying leads must be used. In the latter case, the well-insulated flying lead is best held by hand against the sphere, so that it can readily be removed from contact and used to short circuit the capacitor. Care must, of course, be exercised: 1- or 2-cm sparks can be obtained from a capacitor charged in this way.

The circuit should have a high resistance included in it, such as a piece of wet string, in order to slow down the charging and avoid overstraining the capacitor.

Once the capacitor is charged, then bring another well-insulated lead from one of the terminals round to the other terminal of the capacitor. There will be a small spark.

You could also connect the terminals of the charged capacitor to a gold leaf electroscope.

Immediately afterwards, set up the Van de Graaff with its discharging ball connected to earth through a sensitive galvanometer. Each time a spark leaps across to the ball then a deflection will be shown on the galvanometer. Take care - the capacitor stores more charge than the dome of the Van de Graaff, and so the initial current will be larger and the spark fatter.

Note: The capacitors used are not intended for use at these voltages and may break down. If the capacitor is damaged replacement may be impossible.

Van de Graaff generator set-up

500 μF capacitor discharged through a lamp
i Charge a 50 μF electrolytic capacitor (350 V working) from an HT power supply set to give 240 V. A safety resistor of l00 kΩ should be included in the charging circuit. Allow thirty seconds for charging.

Disconnect the capacitor and allow it to discharge through a 240 V, 15 W mains lamp. (The safety resistor should not be included when discharging.)
 

Teaching notes


1 The electric charge measured by current x time is the same kind of thing as the electric charge that you gather by rubbing plastic with wool, or pile up on the dome of a Van de Graaff machine. 

These demonstrations show that charges can be obtained from:

  • current sources such as batteries, power packs and dynamos; 
  • electrostatic sources such as friction - charged insulators or a Van de Graaff. 

They also show two kinds of behaviour from charges:

  • electric current behaviour, such as lighting a lamp or moving the pointer on a sensitive meter; 
  • electrostatic behaviour, such as sparks or attracting small pieces of paper. 

2 The capacitor is simply a pair of metal plates, separated by an insulator, rolled up and housed in a box. The demonstrations show charges running to those plates, and from them, and the charges accumulated on those plates producing sparks or making an electroscope leaf rise.

In step a when the current is turned on, there will be a momentary pulse. Positive charge will be piled up on one plate and negative charge on the other. When the battery is removed from the circuit and the charged capacitor is shorted through the meters, the pulse will be in the opposite direction.

In step c the charging currents are smaller, the charging process is slower, and you can see the currents dying away exponentially as the charges on the capacitor rise to their full value.

Unfortunately the charges stored on the plates by a low voltage battery will not create sparks, and so the charging voltage must be increased.

In step c the high resistance will reduce the currents and slow the charging process down so that the higher voltage can be used.

In step f, you can show current effects by removing the EHT supply and letting the capacitor discharge through the high resistance and the meters.

You can show electrostatic effects by taking one terminal of the charged capacitor round to the other terminal and producing a spark. The energy dissipated by the spark is 1/2 CV2, where C is the capacitance and V is the potential difference.

Step h shows that the charges which are now resting on the plates can produce an electrostatic effect.

In step i, lighting the lamp reinforces the idea that the charges piled up on the plates of a capacitor behave just like the charged plates of a battery, albeit a battery which can only supply a limited amount of charge.

This experiment was safety-tested in January 2007 
 

Related experiments


Experiments with a Van de Graaff generator
 

Weblinks


RS Components Ltd