Electric Charge » Dangers and Uses of Static Electricity

Static charge: a danger and a tool

This lesson is examined in Paper 2 only.

You already know how objects become charged by friction and how a large charge can make a spark. In some places a spark could be deadly, so charge must be stopped from building up. In other places charge is useful, because a charged object can be pulled or pushed exactly where it is wanted.

Key terms:

• Fuel vapour: fuel that has evaporated and mixed with the air. It catches fire very easily.
• Bonding line: a metal cable that joins two objects, such as an aircraft and a fuel tanker, so that charge can flow between them.
• Toner: the fine black powder that a photocopier uses instead of ink.

The danger: fuelling aircraft and tankers

An airliner is filled with fuel through a hose. A road tanker is filled and emptied in the same way. In both cases a liquid rushes through a pipe, and that causes a problem:

1. As the fuel flows, it rubs against the inside of the pipe. This friction transfers electrons between the fuel and the pipe.
2. The fuel is left with one type of charge and the pipe with the opposite type. Fuel is an insulator, so the charge cannot easily flow away.
3. More fuel flows, so more and more charge builds up on the fuel, the hose, the tanker and the aircraft.
4. If the charge becomes large enough, electrons jump across a gap, for example between the metal nozzle and the tank. This is a spark.
5. The air around the fuel is full of fuel vapour. The spark can ignite the vapour, causing a fire or an explosion.

Notice the order: the cable goes on before the hose is connected and comes off after the hose is removed. A road tanker delivering petrol is earthed with a cable in the same way before its hose is connected.

Use 1: the photocopier

A photocopier uses the rule that unlike charges attract. At its centre is a drum (some books call it an image plate). The drum has a special surface: it holds charge in the dark, but lets charge leak away wherever light falls on it. In the usual exam version, the drum is positive and the toner is negative.

1. The surface of the drum is given a positive charge.
2. A bright light shines on the page you want to copy. The white parts of the page reflect light onto the drum. The black parts, the words and pictures, reflect almost none.
3. Where light hits the drum, the charge leaks away. The parts that stay dark keep their positive charge. The drum now carries a pattern of positive charge in the shape of the words and pictures.
4. The toner is given a negative charge. Unlike charges attract, so the toner sticks only to the positively charged parts of the drum.
5. A sheet of paper is pressed against the drum and the toner is transferred to the paper.
6. The paper is heated. The toner melts and sticks to the paper, which is why a fresh photocopy feels warm.

So the copy is dark in the same places as the original.

Use 2: the inkjet printer

An inkjet printer uses both rules: like charges repel and unlike charges attract. It builds up letters from tiny droplets of ink, and it uses charge to aim them.

1. A nozzle fires out a stream of very small droplets of ink towards the paper.
2. As each droplet leaves the nozzle it is given a charge. Suppose the droplets are negative.
3. The droplets pass between two metal plates. One plate is positively charged and the other is negatively charged.
4. Each negative droplet is attracted towards the positive plate and repelled by the negative plate. Both forces push it the same way, so its path bends towards the positive plate.
5. The printer changes the size of the charge on each droplet. A droplet with a bigger charge feels a bigger force and is bent further. In this way each droplet is steered to exactly the right spot on the paper.

If the charges on the two plates are swapped, the droplets bend the other way.

Use 3: electrostatic paint spraying

Factories use charge to paint metal objects such as car bodies and bicycle frames.

1. The nozzle of the spray gun gives every droplet of paint the same type of charge, say positive.
2. Like charges repel, so the droplets push away from each other. They spread out into a fine, even mist.
3. The object is given the opposite charge, negative. Unlike charges attract, so the droplets are pulled onto the object.

This gives a thin, even coat and wastes less paint, because few droplets miss. The droplets are even pulled round to the back and into awkward corners that the spray gun does not point at.

Exam-style question

A road tanker delivers petrol to a filling station. The petrol flows from the tanker through a hose into a storage tank under the ground.

(a) Explain how an electrostatic charge can build up as the petrol flows through the hose. [2 marks]

(b) Explain why this charge is dangerous. [2 marks]

(c) Before the hose is connected, the driver joins the tanker to earth with a metal cable. Explain how this makes the delivery safe. [2 marks]

(d) A factory sprays metal chairs with paint. The spray gun gives each droplet of paint a negative charge, and the chairs are given a positive charge. Explain two advantages of charging the paint and the chairs in this way. [4 marks]