Electricity and magnetism
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Electrical circuits
In current electricity, electrons are pushed along a conductor by a battery or generator. When the current is switched on, any electrons free to move in the wire all move in the same direction. They flow only if they have a complete pathway of conductors. This pathway is called a circuit. All parts of a circuit must conduct electricity and must be connected to one another. A circuit may have sections or components connected in a series or in parallel. Electrons flow from the negative terminal of a battery or generator towards the positive one, even though the electric current itself is said, by convention, to travel from positive to negative.
Series or parallel
Components of an electrical circuit are connected in either a series or in parallel. In a series circuit—made up of, say, four light bulbs and a single 6-volt battery—a wire links the battery to the first bulb, from there to the second, then on to the next two in turn before returning to the battery in a continuous loop. In a parallel circuit, each bulb is wired to the battery in a separate loop. When the four light bulbs are connected in series, the same current flows through all of them but the voltage is divided into four: 1.5 V for each bulb. When the light bulbs are connected in parallel, each bulb receives a quarter of the current, but the full 6 V from the battery.
In a series circuit, every component must function for the circuit to be complete; one bulb burning out in a series would break the circuit. In a parallel circuit, each light has its own circuit, so if one were to burn out, the others would all still function. Many Christmas tree fairy lights are connected as a series circuit, so if one bulb burns out or is removed, the whole string fails.
Voltage
Electric current flowing around a circuit is a bit like water flowing through pipes. In the same way that a pump may be used to drive the water along, a battery or generator provides the pressure, or driving force, to push the electrons along the wire. This electrical pressure, known as the electro-motive force (emf), is measured in volts (V). As the voltage increases, so does the amount of electric current (I). The rate at which a current flows round a circuit is measured in amperes (A), or amps for short. An amp is the equivalent of one coulumb (the unit of electric charge) per second.
Voltage is sometimes defined as the electric potential difference between two points. Just as the water pressure in a pipe is different between one end of the pipe and the other, so there is a difference in electrical pressure between two points on a circuit. This is known as the potential difference.
Resistance
Just as the flow of water is slowed down if the pipe it runs through narrows, the electric current is reduced if the electrons have less space in which to flow: there is resistance to the flow. This may happen if the electrons are forced to squeeze along a thinner stretch of wire, for example. A thicker wire has a lower resistance to a thin one; a shorter wire has less resistance than a longer one. Resistance is measured in units called ohms (Ω). Any component in an electric circuit, such as a light bulb or a heater, will have some resistance to the flow of current.
Thin wires made of metal can be made to have a high resistance if they are coiled round and round to extend their length. The wires become hot as they resist the current flowing through them. Coils of extremely fine wire used as filaments in electric light bulbs glow white hot when the electricity is switched on, giving out light. Coils of thicker wire are used in electric fires, which glow red hot.
Ohm's Law
The amount of current in a circuit depends on both the amount of voltage (the force with which an electric current is pushed along) and the amount of resistance in the circuit (provided by, for example, a thin filament in a bulb) that reduces it. This relationship between current, voltage and resistance is known as Ohm's Law. It states that resistance equals voltage divided by current, or
R = V ÷ I
Energy consumption
The rate at which electricity (or any form of energy) is used is measured in watts. When electricity flows through a component, such as a ring on a cooker, the energy used is equal to the voltage multiplied by the current. So a 240-volt ring receiving a current of 4 amps uses 960 watts of energy. The total energy consumed by the ring can be calculated by multiplying that figure by the length of time the ring is turned on. In three hours, for example, the ring consumes 2880 watt-hours or 2.88 kilowatt-hours (kWh,1000 watts = 1 kilowatt) of electricity.
Fuses
Electrical appliances can be damaged if too large a current is allowed to flow through them, as happens when there is a "power surge" or when a fault occurs. To prevent it, appliances (and complete circuits within a house, called domestic circuits) are protected by fuses and circuit breakers. A fuse contains a small, enclosed length of thin wire that intentionally forms the weakest link in a circuit. If the current goes above a certain level ("rating"), the wire in the fuse melts, or "blows", breaking the circuit at that point. A circuit breaker is an electromagnetic switch that opens ("trips") if the current becomes too high.
Controlling resistance
The greater the resistance in an electrical circuit, the less the amount of electricity that flows through it. A device that controls the current in a circuit by adjusting the resistance up or down is called a variable resistor. The volume control in a radio and a dimmer light switch contain this device. A variable resistor has a contact, called a wiper, that slides over a resistive track when the control is adjusted, and two electrical terminals: one at the end of the track, the other on the wiper. Increasing the length of the track between the two terminals makes the resistance between them greater, so the radio's volume is reduced or the light is dimmed. Shortening the length of track lowers the resistance, so the volume increases or the light brightens.
An electric racing car set also makes use of variable resistors inside the handheld speed controllers. When you squeeze the trigger of a speed controller, a contact (wiper) slides along the resistive track of a variable resistor. This reduces the resistance, causing an increase in electric current, so the car speeds up.
Consultant: Sharon Ann Holgate