The Van de Graaff generator

The Van de Graaff generator is a staple experiment in almost every school and the basis of every static electricity introduction. From sparking in air to raising the hair, the Van de Graaff is always a winner with students.

SO, WHAT IS A VAN DE GRAAFF?

It’s a way of generating electrostatic charge.

HOW DOES IT WORK?

• The motor is switched on and the lower (negative) roller turns, moving the insulating belt.

• The roller builds up a negative charge, leaving a positive charge on the belt (the excess negative charge on the roller escapes to earth via the metal brush at the bottom).

• The negative charge on the roller also ionises the air between the roller and the bottom (metal) brush, causing more electrons to flow to earth from the brush ends, and allowing the positive ions in the air to be attracted to the negative roller (but as the belt is in the way they add to this charge on the belt).

• As the positively charged belt moves up to the top roller and metal brush (which is in contact with the hollow metal sphere), electrons from the top brush are attracted to the positive belt and move to the tips of the brush.

• The air is again ionised by the motion of the roller and belt, allowing electrons to move to the belt and positive ions to move to the brush.

• The charged belt is in contact with the hollow metal sphere (via the ionised air and the brush) and will discharge to the sphere, leaving it with positive charge.

Warning! At this point in the process the sphere has a large charge on it and anyone touching it will receive an electric shock, as the rush of negative charge moves to neutralise the heavily positively charged sphere.

• Any experiments using a Van de Graaff generator should be undertaken following a risk assessment and training on the machine.

• No one with a pacemaker should be allowed to take part in these experiments.

• Always follow health and safety guidelines (CLEAPSS/ SSERC where appropriate).

1. Pushing the discharge sphere near to the main (charged) sphere without touching it will allow electrons to ‘jump’ across to the charged sphere as they rush to neutralise the charge – you can both see and hear these lightning strikes.

2. When a fluorescent light tube approaches the negatively charged generator, the electrons on the generator flow through the tube and the person holding it. Flowing electrons result in an electrical current, lighting up the light tube. It doesn’t take very much current to light a fluorescent tube!

3. When a student puts a hand on the sphere, the electrons will spread out onto that person as they repel from the other electrons. They are most obvious in a person’s hair because the like charges of the electrons repel each other and cause the hairs to stand up and spread away from each other. As long as the person is standing on an insulated platform, the electrons will not be able to travel down to the ground and their hair will remain standing up.

4. Blow some soap bubbles at the generator. You’ll see a flurry of bubbles rush toward the machine’s sphere, then make an abrupt about-face in the opposite direction. At first, carrying the charge of the person who blew them, the bubbles are attracted to the charged sphere, which carries the opposite charge. But when that first bubble or two hits the sphere, they absorb its charge and are immediately repelled, popping in the process. Encountering the spray from these burst bubbles, the incoming bubbles will be repelled.

5. Electrostatic induction is how to induce a charge onto an uncharged object: bring an uncharged (insulated) metal ball close to the charged sphere of the Van de Graaff. This forces the electrons in the ball to be attracted to the sphere leaving the further side positively charged. If you momentarily touch the ball, electrons will rush to the ball to neutralise the positive side. Now pull the ball away and it will be negatively charged. You can show it is now charged by demonstrating the effect on small pieces of paper/polystyrene.

6. Conductors and non-conductors: A string connected between an electrostatic generator and an electroscope will not conduct charge, but a metal wire will.

Top tips when using a Van de Graaff.

• A damp environment affects the ability to ionise air molecules, so avoid using the Van de Graaff on a humid or very damp day (the charge collection will not be as good).

• You can use a hairdryer to dry the air around the Van de Graaff before you start to charge the dome.

• Keep the brushes clean and free from dust and debris, as this will affect the air gap between the rollers and the brushes, which is needed to help charge flow.

• Make sure that anyone touching the Van de Graaff is standing on an insulated stool, and if they remove their hand from the machine, they should not replace it as they will get a shock.

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