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Electrical appliances cannot work using static electricity. They need a constant flow of moving charges: this is called electric current or current electricity. Moving charges need an unbroken path to flow along: scientists call this an electric circuit. No current will flow if the circuit has a break in it. A circuit needs four things: • a source of electrical energy, such as a battery cell, power pack or power point • a switch to turn the circuit on and off • something to use up the electrical energy, such as a globe, motor or heating element • wires to connect all the parts.
Current measures the amount of charge flowing around a circuit every second. A large current has more charge flowing every second than a small current.
Current is measured in a unit called ampere (A), which is sometimes shortened to ‘amps’. Milliamps (mA) are used to measure small currents and microamps(μA) measure even smaller currents. A milliamp is one-thousandth the size of an ampere. A microamp is one-millionth the size of an amp. An instrument called an ammeter measures current. It must be placed within the path of the current to be measured. This involves ‘breaking’ the circuit and inserting the ammeter.
Voltage is a measure of the amount of energy available to push charges around a circuit. Voltage is supplied by batteries, power packs or power points. Using the water analogy, voltage is like the pressure that pushes water through pipes. The more pressure it has, the more energy it has to use. Voltage is measured in volts (unit symbol V). An instrument called a voltmeter measures the energy used in a particular section of a circuit. A battery, power pack or power point provides all the electrical energy and voltage to a circuit. This energy source acts as a charge pump. It creates an electric field that pushes electrons around the circuit, just like the way a water pump creates pressure that pushes water through pipes. Electricity in the home typically comes from a power point or batteries. Power points and batteries supply energy to the electrons so that they can travel around their circuit. In the water analogy, power points and batteries are our pumps. Nothing would get moving without them.
Power points supply most of the electricity in your home and should be treated with extreme care. The 240 volts they supply can be deadly, so always make sure that the switch is off before connecting or disconnecting appliances. Batteries are used when you need a portable source of electricity. A typical small battery, such as an AA battery, provides 1.5 volts, while a car battery supplies12 volts.
A conductor is a substance that allows current to flow through it easily. Metals are conductors of electricity. Copper wire is a low cost and widely available conductor commonly used in electric circuits around the house, in factories and in the car. Aluminium is more expensive but is used where copper would be too heavy, such as high-voltage transmission lines that need to be strung between tall and distant pylons. Materials that do not normally allow current to pass through them are called insulators. Plastic and rubber are two very effective insulators.
Electrical circuits A circuit needs four things: • a source of electrical energy • a switch to turn the circuit on and off • something to use up the electrical energy, • wires to connect all the parts. Current Current measures the amount of charge flowing around a circuit every second. Current is measured in a unit called ampere (A) An instrument called an ammeter measures current. Voltage Voltage is a measure of the amount of energy available to push charges around a circuit. An instrument called a voltmeter measures the energy used in a particular section of a circuit. Conductors A conductor is a substance that allows current to flow through it easily.
