Magnetic Fields

Question 1

What is the standard answer for how a transformer works?

Answer 1

• An A.C. induces a magnetic field/flux through the primary coil.
• The iron core links the magnetic field/flux to the secondary coil.
• The change in magnetic flux linkage through the coil induces and e.m.f in the secondary wire.
• E.m.f = - N x rate of change of magnetic flux = - rate of change of flux linkage, so the second emf is greater in magnitude, as N is greater, for the same change in flux
• Mention eddy currents if appropriate

Question 2

What is the equation for magnetic flux? And magnetic flux linkage?

Answer 2

φ = ABcosθ
ψ = NABcosθ

Question 3

What is Faraday’s Law?

Answer 3

The induced e.m.f. is directly proportional to the change in magnetic flux linkage.

Question 4

What is Lenz’s law?

Answer 4

The direction of an induced current is always such as to oppose the change in the circuit or the magnetic field that produces it.

Question 5

What is the force on a moving charge in a magnetic field?

Answer 5

F = Bqvsinθ

Question 6

What is the force on a wire in a magnetic field?

Answer 6

F = Bllsinθ

Question 7

Why does putting a rotating coil of wire in a magnetic field slow it down?

Answer 7

• The change in flux linkage will induce an e.m.f. due to Faraday’s law.
• This will produce a current.
• A current-carrying conductor in a magnetic field experiences a force.
• According to Lenz’s law, this force will always oppose the motion that produced the current.
• This will therefore slow down the rotating coil.