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

Revision Note

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Katie M

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Physics

• 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

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