4.2.1 Electric Charge

Question 1

What is the unit of charge

Answer 1

Coulomb

Question 2

Define Electric field and describe its direction

Answer 2

a region where a charged object will experience a force due to the electrical lines of force.

The direction of an electric field at a point is the direction of the force on a positive charge at that point.

Positive charge: go out
Negative charge: go in

Question 3

How can the presence of an electrostatic charge be detected

Answer 3

By a leaf electroscope
If a charged object is placed near the cap, charges are induced.
The metal cap gets one type of charge (positive or negative) and the metal stem and gold leaf get the other type of charge so they repel each other.

Question 4

Describe the action of a charged conducting sphere

Answer 4

• Object A has a large negative charge. When the metal sphere B is placed near it, the electrons in the sphere are repelled away. The front of the sphere that is near A has an induced positive charge.
• Now the sphere is touched either by hand or by wire connected to earth. This allows electrons to escape from the sphere.
• The connection is removed. Now the sphere has a positive charge.

Question 5

Describe inducing charge

Answer 5

Charging a body involves the addition or removal of electrons.
Example: A balloon has a + charge. It must be attracted to a – charge in the wall. The wall is neutral (equal amounts of + and – charge) but its atoms are made up of positively and negatively charged particles. When the balloon is brought close to the wall, the – charged electrons move towards the balloon because they are attracted. They may not move very far, but the effect is enough to give the surface of the wall a negative charge. The attraction is stronger than the repulsion because the attracting forces are closer than the repelling forces.

Question 6

Describe good electrical conductors and name examples

Answer 6

They are materials that let electrons pass through them. Metals are the best conductors. Some of their electrons are so loosely held to their atoms that they can pass freely between them. These free electrons also make metals good conductors.
(e.g. silver, copper aluminium)

Question 7

Name examples of semi-conductors

Answer 7

Semi-conductors: Silicon, Germanium

Question 8

Describe good electrical insulators

Answer 8

Good insulators:
They are materials that hardly conduct at all. Their electrons are tightly held to atoms and are not free to move – although they can be transferred by rubbing. Insulators are easy to charge by rubbing because any electrons that get transferred tend to stay where they are.
(e.g. plastics [PVC, polyethene, Perspex], rubber, glass)