Induced E.M.F between Linked Coils (Edexcel International A Level Physics)
Revision Note
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)
Examiner Tips and Tricks
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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