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

Bin-bag capacitor

Demonstration

This introduces the construction of a capacitor and shows that a capacitor stores and releases charge.

Apparatus and materials

bin-bag (large polythene rubbish bag)

aluminium foil

EHT supply, 0-6 kV

centre-zero microammeter

3 connecting wires

2 crocodile clips

scissors

Health & Safety and Technical notes


Read our standard health & safety guidance

While the capacitor is connected to the EHT supply and the supply is switched on, do not touch any exposed metal parts such as the capacitor plates or crocodile clips.

To ensure safety, disconnect at least one lead from the supply before touching any metal parts.

A ‘bin-bag’ is a large bag, typically made of polythene, which is used to line a rubbish bin. The material chosen should be thin, uniform, and without obvious holes.

A centre-zero microammeter is desirable to show the small currents which flow. The discharge current is likely to be larger than the charging current, but will flow for a shorter time. This is because, when discharging, the current does not have to flow through the power supply which adds to the impedance of the circuit.

Procedure


The procedure for constructing a bin-bag capacitor is shown in the film below.

Bin-bag capacitor

a The capacitor is constructed using two sheets of plastic (cut from the bin-bag) and two slightly smaller sheets of aluminium foil (cooking quality is fine).

b The sheets of foil form the plates of the capacitor. They sandwich a sheet of polythene (the dielectric). The second sheet of polythene is placed below the capacitor to insulate it from the bench. The polythene should extend beyond the foil on all sides

c Connect the capacitor to the power supply via the microammeter. Students may suggest that there is not a complete circuit since there is polythene (an insulator) between the plates. However, when the power supply is switched on, there is a clear deflection of the ammeter as the capacitor charges up, showing that there has been a brief flow of charge around the circuit.

d Watch the capacitor closely as it is charged up. The foil sheets pull closely together, showing the attraction between the opposite charges they are storing.

e Disconnect from the power supply and connect the leads of the capacitor; again, a current flows briefly, in the opposite direction.

f Charge the capacitor, disconnect it from the supply, and connect the two leads to a light-emitting diode (LED). You should see a brief flash of light as the capacitor discharges.

Teaching notes


1 The voltage used to charge the capacitor is not crucial, but it should be less than breakdown voltage of the dielectric (when sparks can be heard). You might try different types of plastic bag to see if some allow higher voltages than others before breakdown.

2 The circuit used is a simple series circuit, from the upper plate of the capacitor to the positive terminal of the EHT supply, and from the negative terminal via the microammeter to the lower plate. To discharge the capacitor, remove both plugs from the supply and touch them together.

3 Although the capacitor is physically large, its capacitance is small (because polythene is a poor dieletric) and so it stores very little charge, even when charged up to 1 kV or so. This means that any current flowing is small, typically a few microamps.

 

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