# P3 Revision - Electromagnetism

• Created by: Théa
• Created on: 21-05-13 15:31

## Basic Principle

-When electricity flows through a wire, a magnetic field is formed around the wire

-The stronger the current, the stronger the magnetic field will be

-When a current stops flowing there is no magnetic field

Electromagnets:

-If we made a coil of wire called a solenoid the combined effect of the magnetic field around each wire results in a bar magnet being created

-The more coild the stronger the combined magnetic field

-If we put a soft iron core inside the solenoid when a current flows an electromagnet is formed

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## Uses of Electromagnets

-Loud speakers

-Generators

-Motors

-Relays

-In scrapyard equipment

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## Electric Bell

When you put the switch on, the current flows through the circuit, causing the iron core to become magnetic. This attracts the iron hammer, which hits the bell, but when this happens, the contacts are no longer touching. This breaks the circuit, so the electromagnet turns off, and the hammer goes back to its original position. This makes the contacts tough again and the elecromagnet turns on again. It repeats.

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## The Motor Effect

-When a wire carrying an electric current is placed in a magnetic field, it experiences a force. The magnetic field generated around the wire by the current interacts with the field from the magnet and the two fields will push or pull on each other.

As the current flows round the coil:

-One side of the coil feels an UPWARD push, whilst the other side feels a DOWNWARD push.

-Together, these two forces make the coil turn on its axis.

-The current then switches direction, due to the split ring commuter so the coil keeps turning.

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## Electromagnetic Induction

-A potential difference can be induced if a wire cuts through a magnetic field. This can be achieved by either the wire moving or the magnet moving

-If the circuit is complete then the induced potential differnce will cause a current to flow

-The induced potential difference can be increased by:

-Number of coils

-Strength of magnetic field

-Speed of movement

-The direction of the induced current can be changed by:

-Switch north/south poles

-Direction of movement

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## Transformers

When electrical current flows you get resistance in the conductor. With resistance you always get heat. This heat is wasted energy, making the system less efficient. You get more wasted energy at lower potential differences than high potential differences. So the potential difference of the supply is increased before it is transferred to the National Grid. In this way less energy is wasted. Then, when it ggets to your home, a school or a factory, the potential difference is stepped down so that it is safe to be used. Transformers are used to change the potential difference of an energy supply.

Primary (input) p. d. / Secondary (output) p. d.  =   no. of turns on primary coil / no. of turns on                                                                                                                             secondary coil

Vp / Vs  =  Np / Ns

Step-up transformer = increases voltage

Step-down transformer = decreases voltage

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## Transformers

A transformer works by:

-Alternating current passing through the primary coil produces an alternating magnetic field

-The lines of the alternating magnetic field pass through the secondary coil and induce an alternating p. d. in it

Transformers are very efficient (we assume 100%), so no energy is wasted

This means power in = power out

Power = current x potential difference

Current (p) x p.d (p) = current (s) x p. d. (s)

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## Switch Mode Transformers

-They operate at high frequency, between 50kHz and 200kHz

-Used for charging mobile phones

-Greater efficiency

-Smaller in size and weight, plus are cheaper

-Use only a small amount of power when they are switched on, but no load is applied (they are plugged in but you are not charging your phone, etc)

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