Electrical Revision

Electromotive force or e.m.f is the name given to the force which pushes the electricity around a circuit. Current can only flow when there is something to push it around. A battery or cell has an e.m.f so do alternators and dynamos.

Effects of an electric current are Chemical, Magnetic and Heating

Chemical Effect - When an electric current passes through an electrolyte, a liquid that allows ions (electrically charged atoms) to move freely across it, it breaks down and hydrogen ions (positive) are collected at the cathode. Sulphate ions (negative) are collected at the anode.

The anode and the cathode are just two dissimilar metals. The passing of the current through the electrolyte is called charging. The density of the electrolyte is reduced when a cell gives out a current. The density of the electrolyte is restored when the cell or battery is re-charged. The instrument for measuring the level of charge is a hydrometer and it measures the specific gravity of the electrolyte.

Magnetic Effect - When an electric current passes through a conductor a magnetic field appears around it. The magnetic field increases when the wire is formed into a coil. If the coil is wound on an iron core the iron will become magnetised.

Heating Effect - When current passes through a conductor, there is some resistance to that flow of current. The combination of resistance and current flow produces heat

Types of Cells

Simple Cell

If a load is connected between the plates then the current will flow from the Anode to the Cathode. The delivery of current to the load is called charging.

Batteries are a collection of cells. There are two types of cell in general use.

Primary which is only used once and cannot be recharged.
Secondary which can be recharged.

There are two types of secondary cells lead-acid and alkaline.

Secondary Cells

The energy supplied from a cell is sufficient only for a limited time as the chemical qualities of electrolyte deteriorate with use. When batteries are being charged hydrogen is given off and care must be taken to avoid explosions.