# Induced E.M.F between Linked Coils(Edexcel International A Level Physics)

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Katie M

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Physics

## Induced E.M.F between Linked Coils

• An e.m.f can be induced in a coil when there is a change of current in another coil linked with this coil
• This is what happens in a transformer

#### Transformers

• A transformer is a device that works by the principles of electromagnetic induction
• It changes high alternating voltages at low current to low alternating voltage at high current, and vice versa

• A transformer is made up of:
• A primary coil
• A secondary coil
• An iron core

• The primary and secondary coils are wound around the soft iron core
• The soft iron core is necessary because it creates flux linkage between the primary and secondary coils
• Soft iron is used because it can easily be magnetised and demagnetised

Coils are magnetically linked, through their combined magnetic flux linkage, using a soft iron core

• In the primary coil, an alternating current producing an alternating voltage is applied
• This creates an alternating magnetic field inside the iron core and therefore a changing magnetic flux linkage

• A changing magnetic field passes through to the secondary coil through the iron core
• This results in a changing magnetic flux linkage in the secondary coil and from Faraday's Law, an e.m.f is induced

• An e.m.f produces an alternating output voltage from the secondary coil
• The output alternating voltage is at the same frequency as the input voltage

#### Worked example

When connected to a DC power supply, the primary coil of a transformer becomes an electromagnet.

Describe the changes which take place inside the secondary coil of a transformer when DC current in the primary coil is:

a) Switched on

b) Remains on

c) Switched off

Part (a)

Step 1: Describe the creation of an electromagnet

• When current is switched on a magnetic field is produced around the primary coil
• Step 2: Describe the change in magnetic flux linkage and induced e.m.f
• Magnetic flux is linked to the secondary coil
• Changing magnetic flux through the secondary coil induces an e.m.f in it
• This causes a current to flow (momentarily)

Part (b)

Step 1: Describe the non-change in magnetic flux linkage

• While the current in the primary coil remains on, there is no change in magnetic flux linkage through the secondary coil
• Step 2: Describe the effect on induced e.m.f and current
• Therefore, the induced e.m.f (and therefore, the current in it) reduces to zero

Part (c)

Step 1: Describe the electromagnet 'switching off'

• When current is switched off the primary coil is no longer an electromagnet
• Therefore, the magnetic field around it vanishes

Step 2: Describe the change in magnetic flux linkage and induced e.m.f

• Magnetic flux is no longer linked to the secondary coil
• As the current switches off, changing magnetic flux through the secondary coil (as it reduces to zero) induces an e.m.f in in the opposite direction to part (a)
• A current (momentarily) flows in in the opposite direction to part (a)

#### Exam Tip

Explaining the link in e.m.f in both sets of coil in a transformer are very common exam questions. Make sure you've mentioned every point, including the words 'change' in flux linkage and induced e.m.f. You must be specific with your terminology for full marks.

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