The dangers and uses of static charges

• A build up of electrostatic charge can result in sparks or flashes of light.
• Lightning is caused by static charges. The bottom of a dark thunder cloud picks up negative charge, while the Earth below becomes positive. The negative charge on the cloud builds up and eventually – Bang! Millions and millions of electrons rush to Earth as a flash of lightning.
• Air rushing past a moving car drags electrons off the car and leaves it positive. This can cause travel sickness for some people. Conducting tails allow the car to pick up electrons from the road surface and lose its charge.
• Electrostatic charges are important and useful in photocopying machines and in removing (extracting) dust.

Current, charge, voltage and energy
If you are studying for the higher tier, you should understand and be able to use the following equations.

Direct current and alternating current

• The electric current from a cell or battery flows in one direction. This is called direct current (d.c.).
• The electric current supplied to our homes via the National Grid flows alternately forwards and then backwards fifty times every second. This is called an alternating current (a.c.).
• Mains electricity enters our homes through electric cables which include two wires - a live wire and a neutral wire. The live wire carries the charge at a voltage of 230 V. The neutral wire stays at a voltage close to zero.

Safety in electrical circuits
Electrical circuits have fuses, earth wires, insulation and circuit breakers for safety.

• Fuses have a thin wire which melts if the current is too large.
  This prevents:
  - electrical wires getting too hot and causing a fire;
  - electric shocks when we use appliances;
  - damage to the electrical appliance itself.
• Earth wires conduct currents to Earth and prevent electric shocks.
• Insulation - Plastic (PVC) covering on the live wire, neutral wire andearth wire provides insulation for electrical wires and cables.
• Circuit breakers - When the current gets too large, contacts separate and the circuit is broken. This protects the users from electric shocks.

Paying for electricity

• power