Lenz's Law (Edexcel A Level Physics)

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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

Experimental Evidence for Lenz’s Law

  • To verify Lenz’s Law, the only apparatus needed is:
    • A bar magnet
    • A coil of wire
    • A sensitive ammeter
  • Note, a cell is not required

20-2-lenzs-law-experiment-1

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, in order to induce a north pole opposing the incoming magnet

  • Reversing the magnet direction would give an opposite deflection on the voltmeter
    • Lenz's Law now predicts a south pole induced at the coil entrance
    • This would attract the north pole attempting to leave 
    • Therefore, the induced e.m.f always produces effects to oppose the changes causing it

  • Lenz's Law is a direct consequence of the principle of conservation of energy
    • Electromagnetic effects will not create electrical energy out of nothing
    • In order to induce and sustain an e.m.f, for instance, work must be done in order to overcome the repulsive effect due to Lenz's Law

Exam Tip

A typical exam question may ask you to explain the presence of the negative sign in Faraday's Law, which is the equation that tells you the size of the induced e.m.f ε as: 

epsilon space equals space minus fraction numerator d left parenthesis N capital phi right parenthesis over denominator d t end fraction

You should remember that the negative sign is representative of Lenz's Law, which says that the induced e.m.f ε is set up to oppose the change causing it. The 'change' causing an induced e.m.f, in this case, is the changing flux linkage (represented by the quantity fraction numerator d left parenthesis N capital phi right parenthesis over denominator d t end fraction). 

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Ashika

Author: Ashika

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