Charge and Current

Charge

All atoms contain charged particles of two types. Electrons have a negative charge and can be easily moved between atoms. Protons have a postive charge and cannot move between atoms.

When you use a cloth to rub an insulator such as a balloon or a plastic ruler, electrons are rubbed from one to the other. A negatively charged object has gained electrons. A positively charged object has lost electrons.

If you bring two charged insulators close together, they will exert forces on each other. This is illustrate in the images below.

Two charged rods with the same charge

First of three images: a negatively charged bar moving away from another negatively charged bar with a label reading 'if the charges are the same they repel'.

Two charged rods with different charge

Second of three images: a positively charged bar moving towards a negatively charged bar with a label reading 'if the charges are opposite they attract'.

Two rods where only one is charged

Third of three images: an uncharged bar moving towards a negatively charged bar with a label reading 'if one is charged and the other is not they attract'.

An electric current is a movement of charge, so if the charge can move off an insulator, it forms a current.

Examples of charge producing a current

  • If you rub your feet on some types of carpet, you will build up a charge. When you touch something else, the charge flows from your body and you feel the shock.
  • Air masses moving past each other in clouds can build up charges. Sometimes there is enough energy to form long sparks. This is lightning.

In conductors, the electrons that carry the negative charges are free to move. The current in an electric circuit is caused by the movement of the charges through the conductors of the circuit.

The symbol for current is I.

The unit that is used to measure current is the ampere (A).

The meter used to measure current is an ammeter.

To measure the current of a circuit, we first need to break the circuit and insert the ammeter into it as shown below:

Two circuits, both with a cell, an ammeter, and a lamp. In the first, the ammeter is connected before the lamp. In the second, the ammeter is connected after the lampTwo circuits, both with a cell, an ammeter, and a lamp