CURRENT ELECTRICITY AND MAGNETISM (1)

1. Current electricity

The existence of an electric field in a conductor produces a CURRENT in the conductor. Current is a flow of charge, measured by the rate at which positive charge passes through any specified cross-sectional area. Current is a scalar quantity denoted by the symbol I. A CONVENTIONAL CURRENT flows from a positive terminal to a negative terminal, i.e. in the same direction as the field.

In metals, the current comprises ELECTRONS which move in the field in the direction OPPOSITE to the conventional current. We will be concerned mainly with metallic conductors.

In solution, the current comprises CATIONS (positive ions) flowing in the direction of the field and ANIONS (negative ions) flowing in the opposite direction to the conventional current.

Gases at low pressures can also conduct an electric current comprising both ions and electrons.

2. Direct current

A current which is set up in response to a constant field is called a DIRECT CURRENT. This is produced by all electrochemical cells.

3. Early experiments

In 1820, Oersted showed that an electric current was able to cause a magnetic needle, placed close to a current-carrying conductor, to be deflected. This experiment is simulated below.

Consider the diagram on the left. When the switch is closed, thus causing an electric current to flow in the conductor, a magnetic needle placed above the conductor is deflected. As soon as the current stops flowing, the needle returns to its original position.

If the direction of the current is reversed (note the polarity of the battery!), as in the diagram on the right, the needle is deflected in the opposite direction. Again, as soon as the current stops flowing, the needle returns to its original position.

This simple experiment, soon repeated and extended by Ampère, established that electric currents produce magnetic fields. It led to the realisation that electricity and magnetism were in fact a single fundamental force of nature (as gravity is), and was named the ELECTROMAGNETIC FORCE.

3. Additional questions