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

Electric fields

Demonstration

This shows the shape of electric fields, in much the same way that magnetic fields are demonstrated with iron filings.

Apparatus and materials

Power supply, EHT, 0-5 kV

Electric fields apparatus

Semolina

Castor oil

Health & Safety and Technical notes


Do not be tempted to try an HT (0 – 350 V) power supply. The current which can be delivered by such a unit is too high to be used with bare electrodes.

Elect fields apparatusA Van de Graaff generator can be used in place of the EHT power supply. If both these supplies are used in turn, students will see that electrostatic charges make the same field patterns as the charges provided by a power supply which can drive currents.

The ‘electric fields apparatus’ consists of two electrodes mounted in a glass dish – see the illustration. The electrodes can be made from aluminium sheet or can be purchased complete with dish. Apart from the care which needs to be taken with the insulation, this unit is readily improvised.


Procedure


a Fill the electrode unit with a layer of castor oil to a depth of about 0.5 cm. Sprinkle a thin layer of semolina over the surface. (A thin piece of glass tubing drawn out to give a fine pointed stirrer is helpful so that the semolina is evenly distributed.) It is better to start with too little semolina than to start with too much. You can always increase the quantity later. 

b Place the electrodes in the castor oil. Connect the positive and negative terminals of the EHT power supply to the electrodes. Adjust the supply to give 3,000 to 4,000 volts. When the voltage is switched on, the field lines will be clearly visible.

c Try electrodes of different shapes. For example, one can be a 'point' electrode whilst the other is a plate, or two point electrodes can be used. A wire circular electrode with a point electrode at the centre will show a radial field. The field with two plates quite close together should also be shown.

Plates
 

Teaching notes


Electric field patterns1 Just as scientists talk of a magnetic field in the space around a magnet, they talk of electric fields in the space around an electric charge. The grains of semolina behave like electric compass needles (electric dipoles), and line up to show the direction of the electric field. 

2 There is an electric field spreading out from any electric charge, ready to grip on any other charge and exert a force on it. This is similar to the Earth’s readiness to grip another mass such as the Moon, or a student, or a mug on the edge of a table, with a gravitational force. However, the force that an electric field exerts is not there until there is a charge for the field to push or pull on. You could say ‘charged’ just means ‘ready to make forces’ in the same way as soldiers say that a ‘gun is charged with explosive ready to make forces on a bullet’.

3 The illustrations show some electric field patterns which can be modelled in this demonstration.

This experiment was safety-tested in December 2006 
 

Related guidance


Electric charge and current - a short history

Electric fields