Physics Revision: Electricity

Electrical Charges can move easily through some substances, such as metals

Electrical current is a form of this charge

CURRENT= CHARGE/TIME

POTENTIAL DIFFRERENCE OR VOLTAGE= ENERGY TRANSFERRED/CHARGE

Current-Potential Difference graphs show how a current varies depending on the potential difference across the component.

The resistance of a component can be found by measuring the current through it and the p.d. across it

VOLTAGE=CURRENT X RESISTANCE

The potential difference of cells connected in series is the sum of the p.d. of each cell

For series circuits:

• The total resistance is the sum of the resistance of each component
• The same current is through each component
• The total p.d. of the supply is shared between the components

For parallel circuits:

• The p.d. is the same across each component
• The total current through the circuit is the sum of the currents through the seperate components

An LED will emit light when current is flowing through it in the forward direction, the use of these is increasing as they require less current than alternative forms of lighting.

When an electrical charge flows through a resistor, its temperature will increase. A lot of energy is lost through heat in filament bulbs, and less so in power saving lamps like CFLs. There is now more choice in terms of energy-saving bulbs than in recent years.

Cells and batteries supply direct current, i.e. passes in the same direction. An alternating current is one that constantly changes direction.

Mains electricity is an a.c. supply.In the UK it has a frequency of 50Hz and is about 230V

A diode can be used for half-wave rectification of a.c. 

Most electrical appliances are connected to the mains through a cable and 3-pin plug. 

If the current becomes too great, the circuit is disconnected through a circuit breaker or fuse in the live wire.

When the current in a fuse exceeds the rating of the fuse the fuse melts, breaking the circuit.

Some circuits are protected by Residual Current Circuit Breakers (RCCBs) which are significantly quicker than fuses. They detect a difference in the current between the live and neutral wires.

Appliances with metal cases are usually earthed, however some appliances are doubled insulated and so do not have an earth wire connection.

The rate an appliance transfers energy is called its power.

POWER=ENERGY TRANSFERRED/TIME

POWER=CURRENT X POTENTIAL DIFFERENCE

ENERGY TRANSFERRED= POTENTIAL DIFFERENCE X CHARGE

ENERGY TRANSFERRED FROM THE MAINS= POWER X TIME 

The National Grid distributes electricity from power stations to consumers.

For a given power, increasing the voltage reduces the current required, and reduces energy loss in the cables.

Step-up and step-down transformers are what the National Grid use to change voltages, and these are often essential