# Electromagnetism

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## Magnetic Fields

- like poles repel, and unlike poles attract

- the magnetic field lines of a bar magnet curve around from the north pole of the bar magnet to the south pole

- induced magnetism is magnetism created in an unmagnetised magnetic material when the material is placed in the magnetic field

- steel is used instead of iron to make permanent magnets because steel does not lose its magnetism easily, but iron does

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## Magnetic Fields of Electric Currents

- the magnetic field lines around a wire are circles centered on the wire in a plane perpendicular to the wire

- the magnetic field lines in a solenoid are parallel to its axis and are all in the same direction. A uniform magentic field is one in which th magnetic field lines are parallel

- increasing the current makes the magnetic field stronger, reversing the direction of the current reverses the magnetic field lines

- an eletomagnet is a solenoid that has an iron core, it consists of an insulated wire wrapped aroudn an iron bar

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## Electromagnets in Devices

- electromagnets are used in scrapyard cranes, circuit breakers, electric bells and relays

- an electromgnet works in a circuit breaker or electric bell or a relay by attracting an iron armature which opens a switch

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## The Motor Effect

- in the motor effect, the force is:

...increased if the current or the strength of the magnetic field or the length of the conductor is increased

...reversed if the direction of the current or the magnetic field is reversed

- an electric motor has a coil that turns when a current is passed through it

- magnetic flux density is a measure of the strength of a magnetic field

- to calculate the force on a current carrying conductor at right angles to the lines of a magnetic field, use the equation:

force = magnetic flux density x current x length

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## The Generator Effect

- the generator effect is the effect of inducing a potential difference using a magnetic field

- when a conductor crosses through the lines of a magnetic field, a potetial difference is induced across the ends of the conductor

- the faster a conductor crosses through the lines of a magnetic field, the bigger is the induced potential difference

- when a direct-current electromagnet is used, it needs to be switched on or off to induce a potential difference

- the direction of an induced current always opposes the original change that caused it

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## The Alternating-Current Generator

- a simple a.c. generator is made up of a coil that spins in a uniform magentic field

- the waveform, displayed on an oscilloscope, of the a.c. generator's induced potential difference is at:

...its peak value when the sides of the coil cross directly through the magnetic field lines

...its zero value when the sides of the coil move parallel to the field lines

- a simple d.c. generator has a split-ring commutator instead of two slip rings

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## Transformers

- transformers are used to increase or decrease the size of an alternating potential difference

- the size of an alternating potential difference is increased by a step-up transformer, and decreased by a step-down transformer

- a transformer works only with a.c. because a changing magnetic field is necessary to induce a.c. in the secondary coil

- a transformer has a primary coil, a secondary coil and an iron core

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## Transformers in Action

primary p.d.    =   number of turns on primary coil

secondary p.d.      number of turns on secondary coil

- for a step-down transformer, the number of secondary turns is less than the number of primary turns, whereas for a step-up transformer, the number of secondary turns is greater than primary turns

- for a 100% efficient transformer:

primary p.d. x primary current = secondary p.d. x secondary current

- a high grid p.d. reduces the current that is needed, so it reduces power loss and makes the system more efficient

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