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Current Electricity - Electrical Charges

Key Points

Certain insulating materials become charged when rubbed together

Electrons are transferred when objects become charged:

Insulating materials that become positively charged when rubbed lose electrons

Insulating materials that become negatively charged when rubbed gain electrons

Like charges repel; unlike charges attract

Página 2

Current Electricity - Resistance

Key Points

The potential difference across a component (in volts) = Work Done (in joules) / charge (in coulombs)

Resistance (in ohms) = potential difference (volts) / current (amperes)

Ohm's law states that the current through a resistor at constant temperature is directly proportional to the potential difference across the resistor

Reversing the current through a component reverses the pd across it

Página 3

Mains Electricity - Cables and Plugs

Key Points

Sockets and plug cases are made of stiff plastic materials that enclose the electrical connections. Plastic is used because it is a good electrical insulator

Mains cable consists of two or three insulated copper wires surrounded by an outer layer of flexible plastic material

In a three - pin plug or a three - core cable, the live wire is brown, the neutral wire is blue, and the earth wire is green and yellow

The earth wire is connected to the longest pin and is used to earth the metal case of a mains appliance

Página 4

Mains Electricity - Fuses

Key Points

A fuse contains a thin wire that heats up, melts, and cuts off the current if the current is too large

A fuse is always fitted in series with the live wire. This cuts the appliance off from the live wire if the fuse blows

A circuit breaker is an electromagnetic switch that opens (i.e. 'trips') and cuts off the current if too much current passes through the circuit breaker

A mains appliance with a plastic case does not need to be earthed because plastic is an insulator and cannot become live

Página 5

Radioactivity - More about alpha, beta and gamma radiation

Key Points

( a ) radiation is stopped by paper, has a range of a few centimetres in air and consists of particles, each composed of two protons and two neutrons

( B ) radiation is stopped by thin metal, has a range of about a metre in air and consists of fast-moving electrons emitted from the nucleus

( y ) radiation is stopped by thick lead, has an unlimited range in air and consists of electromagnetic radiation

A magnetic or an electric field can be used to separate a beam of alpha, beta and gamma radiation

Alpha, beta and gamma radiation ionise substances they pass through. Ionisation in a living cell can damage or kill the cell

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