Forces and Motion | Electricity and Magnetism

Electric forces

Physics Narrative for 5-11 11-14 Supporting Physics Teaching

Electric forces acting on a pair of charged objects

You can demonstrate the effect of an electric force by first rubbing a pair of party balloons on your jumper, then separating them from the jumper: the balloons will then repel each other.

This force is often called an electrostatic force or a force due to static electricity. First you charge the balloons, then you isolate them.

The force arises because electrically charged particles called electrons are transferred through the rubbing action – from the balloon to the jumper or vice versa. These are the same charged particles (or charge carriers) that drift around the bench-top and domestic circuits and so constitute the electric current in a metal wire.

However, because the balloon or sweater do not conduct electricity, the electrons are unable to move around and are therefore stationary on the balloon or the sweater (hence the term static electricity). The balloon is isolated if there is no route for the charged particles to move onto or off of the balloon.

As with magnetic and gravitational forces, scientists describe the space around the balloon as containing an electric field. The electric field is set up by the charge on the balloon and marks out the space throughout which the charged balloon is able to exert an electrical force.

Electrical forces acting on uncharged objects, by charged objects

You can also demonstrate the effect of an electric force by rubbing a single party balloon on your jumper and then sticking it to a wall. The balloon is pulled towards the wall before the charged balloon comes into contact with the uncharged wall. If the balloon is rubbed and held over some small pieces of paper (without touching them) the uncharged pieces of paper will be attracted to the charged balloon.

In the first case you can re-describe the pulling as an electrical force acting on the balloon, and in the second case as an electrical force acting on the paper. (If you've chosen to isolate the balloon from its environment in the first case, and the paper in the second.)

This is somewhat different from the first example of the force between two charged objects, as one of the objects is not charged. The force on either object is always attractive. A different mechanism is in play, and it is rather subtle. As the objects (balloon, wall, paper) are isolated, after the initial charging, electrons cannot flow onto or off of the objects, so their charge cannot change: the wall and paper remain neutral, yet an electrical force is acting on them.

The charged balloon still has an electrical field around it, and is able to exert an electrical force on things in that volume. But there are no charged things: the paper and the wall are neutral: they do not have an excess or deficit of electrons.

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