Induction of emf

Hi !

The diagram represents two identical coils X and Y. The planes of both coils are parallel
and their centres lie on a common axis.
Coil Y is connected to a cell, a variable resistor and a closed switch.
Under which of the following circumstances would a current be induced in coil X in the
same direction as the current shown in coil Y?
A The coils are moved closer together.
B The switch is opened.
C The resistance of the variable resistor is decreased.
D No change is made to the arrangement.

The mark scheme says that the answer is B.

I understand that the magnetic field across X is in the same direction as the magnetic field in Y ( because both are the same magnetic field) and the change in magnetic flux due to opening of the switch.

Is this the correct reason to why the induced emf in X is in the ''same direction'' as in Y?

I would highly appreciate any help :-)

Reply 1

uberteknik

Original post by tahmidbro

Lenz's law: any change in the magnetic field will create a current attempting to maintain the the original flux.

i.e. opening the switch collapses the magnetic field surrounding/linking both coils. The induced current in the second coil will therefore be in the same direction as the original current as it attempts to compensate for the collapsing field.

a) moving the coils closer together increases the flux coupling in the second coil. The induced current would be in the opposite direction as it attempts to prevent the increase in flux.
c) same for reducing the resistance of the variable resistor which increase the current and strengthens the flux in both coils which must therefore induce an opposing current in the second coil as it attempts to prevent the increase in flux.
d) violates Lenz's law - there must be a change of flux hence this answer cannot be true.

b) is correct.

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/farlaw.html#c2

(edited 3 years ago)