Physics

Circuit diagrams

Circuit diagrams use standard symbols for components.

Circuit diagram symbols

Variable reistor:

Series and parallel

The arrangement of components in an electric circuit affects the current and potential difference.

Series circuits

• In a series circuit the same current flows through all the component
  
• The potential difference of cells arranged in series is the sum of the potential difference of each cell. This is only true if the cells are set up the same way round so that the + terminal of one cell is connected to the - terminal of the next.
• The total resistance in a series circuit is the sum of the resistances of each of the components.

Direct current

Cells and batteries supply DIRECT CURRENT (d.c.). This is an electric current that always travels in the same direction. The oscilloscope trace for d.c. from a battery is a horizontal line. The potential difference can be read off the vertical scale.

Alternating current

Mains electricity supplied to homes and businesses is an ALTERNATING CURRENT (a.c.). Alternating current is an electric current that changes direction regularly, and its potential difference is constantly changing.

Three-pin plugs

In the UK, appliances are connected to mains electricity with a three-pin plug.

You need to remember that the neutral wire is blue, the brown wire is live and the earth wire is yellow and green

Electrical safety

Electrical appliances have safety features to protect users from electric shocks and fires.

Fuses

A FUSE contains a wire made of a metal with a low melting point. It is connected to the live pin in a plug.

When a current above the rating of the fuse flows through it, the wire becomes too hot and melts. The circuit is broken so no harm can occur.

If the current rating of the fuse is too low it will melt when the normal current flows. The fuse has to be replaced. This is a nuisance.

If the current rating of the fuse is too high then it may not protect the appliance from a larger-than-normal current. This could result in overheating that could cause a fire or damage to the appliance.

Earthing

Current and power

When an electrical charge flows through a resistor, the resistor gets hot. All the components in an electric circuit, including the wires, act as resistors.

Filament lamps transfer a lot of energy to the environment as heat. This means that they are not very efficient. Compact Fluorescent lamps (CFLs) are more efficient and waste much less electrical energy heating the surroundings.

Background radiation

We are constantly receiving radiation from our surroundings. Some substances give out RADIATION from their unstable nuclei. These substances are said to be RADIOACTIVE. The process is not affected by temperature or by chemical reactions.

RADIOACTIVE DECAY is a random process. This means it is impossible to predict when a particular nucleus will give out radiation.

Nuclear reactions

The nuclei of radioactive substances are unstable and give out various types of radiation. 

There are three common types of radiation given off by radioactive nuclei.