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Transformers are simple devices that convert voltage from high to low and vice-versa. They are used to increase the voltage from power stations to the high voltages needed to transmit electricity around the country efficiently. Transformers are also found in electricity substations where they reduce high voltage to lower ones suitable for homes and factories. Many home appliances do not require the 230 V provided by mains electricity. To overcome this, transformers are used to convert the voltage supplied to the required level for the appliance. Transformers that increase the voltage are referred to as step-up transformers whilst those that reduce the voltage are known as step-down transformers.
Transformers are also used in chargers for electronic appliances such as smartphones and laptops. These appliances require direct current, rather than the alternating current of the mains. Transformers work only with AC and their output is also AC, so for chargers and small electronic devices like computers, they are fitted with a rectifier circuit. This is a system that uses diodes to ensure that the output current from the transformer always flows in the same direction i.e. is direct current.
Transformers are very simple electrical devices. They consist of two separate coils of wire wrapped round opposite sides of a core. The core is made of iron which is why transformers are usually quite heavy for their size. The core is laminated to make it as efficient as possible. They work because of the principle of electromagnetic induction. When a magnet is moved next to a wire, a potential difference is created (more scientifically we say the current has been induced) between the ends of the wire. This potential difference can be used to drive a current - think about a generator. The opposite is true, if a current passes through a wire, a magnetic field can be detected around the wire. The important thing to remember is that the magnet needs to be moving or the current needs to be changing for electromagnetic induction to work.
So back to the coils of wire on the same iron core. If you pass a direct current through the first coil, in the fraction of a second that the current builds up from zero to the full flow, there will be a magnetic field, then the field will die away because there is no change. If you then swap the DC for AC, the current is constantly changing and so the magnetic field will last for as long as the current flows, changing direction as the current changes direction. The iron core concentrates and focusses the magnetic field and so it passes through the second coil. Because the current is constantly changing direction, electromagnetic induction creates a potential difference in the second coil. This can be used to push current round a circuit. The crucial thing to remember here is that no electricity passes directly between the two coils.
If the two transformer coils have the same number of turns around the core, the output voltage would be the same as the input voltage which would be pointless, so transformers all have different coils on the input and output sides. The voltage produced by electromagnetic induction depends on several factors, including the number of turns on the coil - more turns will give a greater potential difference if all other factors are constant. If the input (primary) coil of a transformer has a greater number of turns than the output (secondary) coil, the potential difference induced in the secondary coil will be lower than that of the primary coil and vise versa. The relationship between the potential difference across the ends of each coil is simple; if one coil has twice as many turns as the other, its potential difference will be twice that of the other one and so on, the two are directly proportional.