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Electromagnets

What you need to know

Reflections and Exam tips

 

Electromagnets

Electromagnetism - a magnetic field is created when an electric current flows through a wire.

The coiling of wire around a soft iron core creates a strong magnetic force which is used in electromagnets.

The current magnetises the iron core and when the current is turned off the iron loses its magnetism.

Electric bells and relay switches for example employ electromagnets which can be switched on and off.

The strength of an electromagnet can be increased by:

  1. Increasing the number of coils on an electromagnet will increase the strength of an electromagnet
  2. Certain cores will make an electromagnet stronger than others
  3. Increasing the voltage of the connecting battery/current passing through the wire will increase the strength of an electromagnet

Applications of electromagnets

The ability of electromagnets to attract magnetic metals (iron, steel, nickel and cobalt) makes them useful in cranes to lift and drop iron and steel in scrapyards or recycling depots.

Outline of how an electric bell works

  1. As current flows through the circuit, a magnetic field is created by the electromagnet
  2. The springy metal arm is attracted by electromagnet
  3. The gong makes a sound when hitted by the arm
  4. This breaks the circuit as the arm is out of position
  5. The electromagnet is turned off and the springy metal arm moves back
  6. The circuit becomes complete again
  7. The cycle is repeated as long as the switch is closed

Motor Effect

An electric motor uses the motor effect of electromagnets to create motion.

The force on one side of the wire causes it to move up

The force on the other side of the wire causes it to move down

The motor rotates.

Graphite ‘brushes’ are used to connect the split-ring to the battery because:

  1. Graphite is an excellent conductor
  2. It causes very little friction on the conducting ring

Electromagnetic induction

Electromagnetic induction is the term used to describe how electricity is created in a wire.

This is one of the ways that humans know how to generate electricity.

By moving the magnet within the coil of wire, current can be induced within the current.

However, the current is only induced when the magnet or coil is moving.

The current induced can be increased by:

  1. Increase the number of coils
  2. Increase the speed at which the magnet/coil moves
  3. Increase the strength of the magnet

Transformers

Transformers are an integral component of the national grid.

Transformers increase the voltage after the power station and decrease the voltage when it gets to homes.

Transformers consist of the following:

  1. Only work with an alternating current (a.c)
  2. Need an iron core
  3. Use electromagnetic induction to work

Outline stages of how transformers work:

  1. Alternating current in the primary coil changes iron core into an electromagnetic
  2. As the current is alternating, the magnetic field moves changing direction
  3. The moving magnetic field causes a current to be induced in the secondary coil
  4. When the number of coils on the secondary is higher, the potential difference will increase and it is a step-up transformer
  5. When the number of coils on the secondary is lower, the potential difference will decrease and it is a step-down transformer

The following equation links together the voltage and number of coils in a transformer:

Vp/Vs = np/ns

Where:

Vp - Voltage on primary coil (v)

Vs - Voltage on secondary coil (v)

np - number of turns on primary coil

ns - number of turns on secondary coil

Transformers are usually about 98% efficient which means Power in = Power out.

 


You should be able to explain how electromagnetic appliances work by interpreting diagrams

You are required to know how four different components that use electromagnets work for your exam

As part of the GCSE course, you are required to know which way a wire placed into a magnetic field moves