Question

a) The switch in the primary is kept In the ON position. Will the bulb in the secondary glow? justify your answer. b) When the current in the primary is switched off, what happens to the magnetic field in the secondary coil? How will this change influence the current in the secondary and the glow in the electric bulb?

Answer 1

Hint: Current induced in a coil is directly proportional to the rate of change of magnetic flux linked with the coil. So check if the flux is changing in any case and check the possibility of induced current in the secondary coil.

Complete step by step solution:
The power supply connected to the primary coil is a DC supply. When the switch in the primary is kept on, the current will become constant at steady-state and the magnetic field linked with the primary coil will be time-invariant and will be a constant.
Now the same magnetic flux is linked with the primary coil passing through the secondary coil.

The current induced in the secondary coil will depend on the rate of change of magnetic flux linked with the coil. As we know here the magnetic flux is not changing with the time we get the current induced in the secondary coil is zero.

Hence the current in the secondary coil will become 0 and the torch bulb will not glow.

Now consider the second case in which the current in the primary is switched off.

In this case, we are switching off the supply in the primary coil, this causes the current in the primary coil to become zero and as a result, due to this change in current from a constant value to a zero value, the magnetic flux linked with the coil will also change for a short period of time. Now as a result of the change in the flux short period of time, the current will be induced in the secondary coil and the bulb in the secondary coil will glow for a short period of time and finally goes off.

In conclusion, we have used a concept of electromagnetic induction which says that Current induced in the coil is directly proportional to the rate of change of magnetic flux linked with the coil. in the first case due to a DC supply the current in the primary coil is a constant and hence the magnetic field linked with the coil is also a constant. As a result, the voltage induced in the secondary coil becomes zero and the bulb doesn't glow. but in the second case due to a change in the current value, the magnetic flux will also change for a short period of time and hence this change in the magnetic field induces a current in the secondary coil and lets the bulb glow for a short period of time.

Note: The possible mistake that can be done in this kind of problem is the student may assume the current in the secondary coil as zero in the first case even when the source given in the primary is a sinusoidal supply. In the case of sinusoidal supply, the current changes its direction continuously and as a result, the magnetic field is not constant and hence induces a current in the secondary coil and lets the bulb glow. So the bulb glows when it supplies sinusoidal voltage.