7.18 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

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)