P2aL15 The Van der Graaff Generator
Key Words
Earth - the ground, which carries a potential of zero volts.
Electric current - flow of charge.
Electrons - tiny particles that carry negative charge which orbit the nucleus.
Induce - electrons repelled to the far side of a charged object by a negative charge.
Ionised - Electrons knocked off (or added to) an atom to make it charged
Positive ion - charged atom that has had electron knocked off to make it positive.
Potential difference - voltage.
Van der Graaff generator - a machine that generates high static voltages.
Voltage - potential difference
Grade E
The Van der Graaff generator is a static electricity machine that can generate very high voltages. Its components are like this.
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The machine works by pumping electrons from the base to the top sphere. The electrons crowd onto the top sphere, resulting in a very large potential difference between the top sphere and the discharge sphere. Eventually the voltage gets so big that a spark jumps from the top sphere to the discharge sphere. The voltage is very high, but the current is tiny. Therefore it is quite safe to use.
If you hold the discharge sphere, you can feel it being attracted towards the machine, just before the spark jumps.
Another trick is to get someone to put their hands on the top sphere and stand in a plastic tray. Their hair stands on end as the electrons distribute around the hairs, and make them repel. It works best with medium length hair, not too short and not too long.
Grade C
The very high potential difference between the top sphere and the discharge sphere causes air particles to lose their electrons. They become ionised as positive ions. This means that they are attracted to the top sphere, helping to provide a conducting path to the discharge sphere. The dislodged electrons go to the discharge sphere.
There are some other tricks you can play. If you attach a spike to the top sphere, electrons stream off the machine. If you place an arrangement like this on the spike, it will turn.
The stream of electrons from a spike can be felt, and can blow a candle flame.
If you place a plastic canister like this onto the machine:
You will see the following happen:
The balls pick up electrons and a negative charge is applied. They are attracted to the top plate (positive) and lose their electrons there. They then drop back to the bottom and the process is repeated.
If you blow a bubble towards the machine, the electrons in the bubble are repelled around the back, leaving the face nearest the top sphere positively charged. The positive charge is induced. The bubble is attracted. As soon as it touches the top sphere, electrons rush onto the bubble and it's immediately repelled.
Grade A
Air is a good insulator. However at very high potential differences ionisation occurs and the air can conduct. This is called the breakdown voltage (strictly speaking, it's an electric field strength, which you will meet at A-level). For dry air, the breakdown voltage is 3000 V mm-1 or 3 × 106 V m-1 (volts per metre). This means that a spark 1 cm long is 30 000 V, while one of 10 cm is 300 000 V. I have experienced a spark 50 cm long (much to the amusement of my class), a voltage of about 1.5 million volts.
The Sun emits charged particles called the solar wind. The particles travel at 400 000 m/s. They are attracted by the Earth's magnetic field. When they collide with gas molecules in the atmosphere, they cause the atoms to get excited, and release energy in the form of light. So the Northern Lights (Aurora Borealis) happen. The Southern Lights (Aurora Australis) work in exactly the same way.