16: Exam Questions

Question 1

[3] Explain why a magnetised rod takes considerably longer to fall through a copper tube than a unmagnetised rod. Use ideas of induced emf. in the copper tube

Answer 1

Changing magnetic field/flux in copper / tube (1)
Currents produced in copper / tube (1)
Currents set up their own magnetic fields/flux (1)
Two fields interact (1)
(Providing) an upwards force on magnet / force acting
against weight of the magnet (1)
Change in net force leads to deceleration / reduced
acceleration / reduced relative motion between
magnet and tube (1)

Question 2

In a real transformer, eddy currents in the iron core will alter the flux in the two coils.
i) [2] Explain why eddy currents are set up in the core and suggest why this alters the flux in the core

Answer 2

Eddy currents are induced within the core due to the changing magnetic
flux.
(By Lenz’s law) the eddy currents induce a magnetic flux to oppose the
flux that induced the eddy currents.

Question 3

In a real transformer, eddy currents in the iron core will alter the flux in the two coils.
ii) [2] State and explain how the core should be constructed so as to reduce eddy currents

Answer 3

The core can be laminated/built from various layers that are separated by
insulating material.
So that eddy currents across the core are reduced.

Question 4

For an ideal transformer, the magnetic flux in the secondary coil is the same as the magnetic flux in the primary coil.
Use this to explain why the quantity (emf. across the coil)/(turns of wire in the coil)
has the same value for both primary and secondary coils in an ideal transformer [2]

Answer 4

Both coils have the same flux through them at all times.

So the ratio of e.m.f. across the coils is related only to the number of turns

Question 5

The core is made from solid iron. Explain why the emf across the coil increases when the solid iron core is replaced with one made from thin sheets of iron glued together (4)

Answer 5

Reduces eddy currents
Caused by change of flux
By increasing resistance of the core
Eddy currents create a magnetic field or flux
Which reduce field or flux of rotor
Eddy currents dissipate energy in the core
Laminations increase flux/flux density/flux linkage