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

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Faraday's & Lenz's Laws

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:

epsilon equals fraction numerator straight capital delta left parenthesis N capital phi right parenthesis over denominator straight capital delta t end fraction

  • 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

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Katie M

Author: Katie M

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.

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