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# Electomagnets

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### Electomagnets

1. 1. The current direction When a current passes through a If a wire cuts through magnetic field lines, then a(from plus to minus), the copper wire a magnetic field appears voltage is induced in the wire. Or you can push amagnetic field (N to S) and around it. No current, no field. – the magnet into a coil or solenoid. This is electromagneticmovement are all at right basis of electromagnets and relays. induction.angles to each other. If the wire or coil is moved faster then the voltage is An alternating current generator uses aWe can use Fleming’s left If the wire is in a magnetic field, greater. Other ways: stronger magnetic field, more rotating coil in a magnetic field tohand rule to show this. the two fields interact to give turns on the coil. generate a current.M = thumb, motion movement of the wire – the motor If the direction of movement is reversed then the If side A of the coil goes up and side BF = finger, field effect. voltage is reversed. goes down then we have an inducedC = second finger, current. current. But when the coil rotates, side AMargate Football Club! now goes down and B goes up, so the current in the coil has reversed. Side A Motors is connected to a slip ring while side B is connected to another slip ring. The rings Generators are connected to the circuit via theThe force on the wire can be increased by: brushes.Increasing the strength of the magnetic field, If the frequency of rotation changesIncreasing the current. from 25Hz to 50 Hz the voltage will beIncreasing the number of turns on the coil. twice as much and the number of peaks will be twice too.A DC motor uses a split-ring commutator to Electricity and Magnetismreverse the direction of the current.Brushes connect the current to the split ring.Curved pole pieces produce a radial magnetic field Transformers An alternating current in the primary coil produces a changing magnetic field in the iron core. Voltage across primary coil = number of turns on primary coil The changing magnetic field cuts through the secondary coil, inducing an Voltage across secondary number of turns on secondary alternating voltage. This secondary voltage can be greater than the primary voltage if there are more turns on the secondary coil, giving a step-up In the National Grid we step up the voltage from 25000 volts to 400 000 volts so that transformer. the current is stepped down, giving less heat loss in the cables, which depends on the We are not increasing the power! The output power will be the same as the square of the current. input power if it is 100% efficient. Power in = power out VpIp = VsIs In an AC adaptor we step down from the mains voltage of 230 volts to 12 volts.