Static electricity and electric fields

Physics · Electricity. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

When two insulating materials are rubbed together, electrons are rubbed off one and move onto the other. The material that gains electrons becomes negatively charged. The one that loses electrons is left with an equal positive charge. Only electrons move; positive charges stay where they are.

Two charged objects push or pull each other without touching. Like charges repel and unlike charges attract.

A charged object has an electric field around it. Any other charged object placed in the field feels a force. The field is strongest close to the object and gets weaker further away. A very strong field can make charge jump through the air as a spark.

Rules to remember

Gaining electrons gives a negative charge; losing electrons gives an equal positive charge
Like charges repel; unlike charges attract
The electric force is a non-contact force, and it gets weaker as the distance increases
Field lines show the direction of the force on a positive charge: away from a positive sphere, towards a negative sphere
Around an isolated charged sphere the field is radial; lines closer together mean a stronger field
A spark happens when the p.d. between a charged object and an earthed conductor is large enough for charge to flow through the air
Connecting an object to earth lets the charge flow away, so no large p.d. builds up

Common mistake

Students write that positive charges or protons move onto the object. Protons are fixed in the nuclei; an object becomes positive only by losing electrons.

Worked example

An acetate rod is rubbed with a cloth and becomes positively charged. It is then held near a negatively charged polythene rod that hangs on a thread. What has happened to the acetate rod, and what does the polythene rod do?

The acetate rod is positive, so it has lost electrons to the cloth.
The polythene rod is negative, so the two rods carry unlike charges.
Unlike charges attract, and the force acts without the rods touching.

Answer: The acetate rod lost electrons, and the hanging polythene rod swings towards it.

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