Faraday's & Lenz's Laws (OCR A Level Physics)

Faraday's Law

• Faraday's Law connects the rate of change of flux linkage with induced e.m.f
• It is defined in words as:

The magnitude of the induced e.m.f. is directly proportional to the rate of change of magnetic flux linkage

• Faraday's Law as an equation is defined as:

• Where:
  • ε = induced e.m.f (V)
  • Δ(Nɸ) = change in flux linkage (Wb turns)
  • Δt = time interval (s)

Lenz's Law

• Lenz’s Law is used to predict the direction of an induced e.m.f. in a coil or wire
• Lenz's Law is summarised below:

The induced e.m.f. is set up in a direction to produce effects that oppose the change causing it

• Lenz's Law can be experimentally verified using:
  • A bar magnet
  • A coil of wire
  • A sensitive ammeter

Lenz’s law can be verified using a coil connected in series with a sensitive ammeter and a bar magnet

• A known pole (either north or south) of a bar magnet is pushed into the coil
  • This induces an e.m.f. in the coil
  • The induced e.m.f. drives a current (because it is a complete circuit)
• Lenz's Law dictates: 
  • The direction of the e.m.f, and hence the current, must be set up to oppose the incoming magnet
  • Since a north pole approaches the coil face, the e.m.f. must be set up to create an induced north pole
  • This is because two north poles will repel each other

• The direction of the current is therefore as shown in the image above
  • The direction of current can be verified using the right hand grip rule
  • Fingers curl around the coil in the direction of current and the thumb points along the direction of the flux lines, from north to south 
  • Therefore, the current flows in an anti-clockwise direction in the image shown
    • This induces a north pole, opposing the incoming magnet