Additional Science-Physics Chapter 4

Some insulating materials become electrically charged when they are rubbed together. Conducting materials lose charge to the earth. Charges that are the same repel, while unlike charges attract.

Friction moves electrons from one material onto the other. Ions are atoms that have become charged.

• Rubbing a polythene rod with a dry cloth transfers electrons to the surface atoms of the rod from the cloth - rod becomes negatively charged.
• Rubbing a perspex rod with a dry cloth transfers electrons from the surface atoms of the rod onto the cloth - rod becomes positively charged.

When charge flows through a conductor there is a current in it. Electric current is the rate of flow of charge.

In a solid conductor, e.g. a metal wire, the charge carriers are electrons. Metals are good conductors of electricity because they contain free, conduction electrons that are not confined to a single atom. Insulators cannot conduct because all the electrons are held in atoms. A conductor can only hold charge if it is isolated from the ground. Otherwise, electrons will flow to or from the earth and discharge it.

The bigger the charge on an isolated object, the higher the potential difference between the object and the earth. If the potential difference becomes high enough, a spark may jump across the gap between the object and any earthed conductor brought near it.

In some situations electrostatic charge can be useful, and in some it can be dangerous. A spark from a charged object can make powder grains or certain gases explode. To eliminate static electricity, antistatic materials and earth metal objects can be used.

Objects such as car panels and bicycle frames are often painted with an electrostatic paint sprayer. The spray nozzle is connected to a positive terminal.

As the paint droplets pass through it, they pick up a positive charge. The paint drops repel each other so they spread out to form a fine cloud. The item being painted is connected to a negative terminal so the positively charged droplets are attracted to it.

The advantages of using this system are that less paint is wasted and the object receives an even coat.

In a photocopier, a copying plate is given a charge. An image of the page to be copied is projected onto the charged plate. Where light hits the plate the charge leaks away, leaving a pattern of the page. Black ink powder is attracted to the charged parts of the plate. This powder is transferred onto a piece of paper. The paper is heated so the powder melts and sticks to it, producing a copy of the original page.

Electrostatic smoke precipitators are used in chimneys to attract dust and smoke particles so that they are not released into the air. The particles pass a charged grid and pick up charge. They are then attracted to plates on the chimney walls that have the opposite charge. The particles stick to plates until they are shaken off and collected.

When powder or grain flows through a pipe, friction makes it pick up charge. This could lead to a spark igniting the powder, causing an explosion.

The filler pipes on road tankers that are used to pump fuel into storage tanks are earthed to prevent them becoming charged. A spark could cause an explosion of the fuel vapour.