Question

Assertion (A): A transformer can't work on dc.
Reason (R): dc changes neither in magnitude nor in direction.
A. A, R are true and R is the correct reason for A
B. A, R are true and R is not the correct reason for A
C. A is true, R is false
D. A is false, R is true

Answer 1

Hint:The assertion and reason are based in the working principle of a transformer and the theory of alternating or ac current and direct or dc current. AC current is alternating current and it keeps changing its direction but the DC which is direct current does not change direction and stays constant.

Complete step by step solution:
Step 1:
The working principle of transformer:-
A transformer can be defined as a static device which helps in the transformation of electric power in one circuit to electric power of the same frequency in another circuit. The voltage can be raised or lowered in a circuit, but with a proportional increase or decrease in the current ratings.
The main principle of operation of a transformer is mutual inductance between two circuits which is linked by a common magnetic flux. A basic transformer consists of two coils that are electrically separate and inductive, but are magnetically linked through a path of reluctance.
Step 2: Direct current (DC) is the flow of electric charge in only one direction. Alternating current (AC) is the flow of electric charge that periodically reverses direction.
Step 3: DC is not used in case of transformers because dc current neither changes magnitude nor change direction. So in the case of dc there is no time varying field generated. Therefore no e.m.f induced in the secondary coil of the transformer.
Now for ac current a time varying field will generate to the secondary coil due to mutual inductance between primary and secondary coil.

$\therefore $Transformer does not work in dc current.
$\because $Assertion and reason both are true and reason is the correct explanation of the assertion,
$\therefore $Option (A) is the correct option.
Note:Mutual inductance is the phenomenon when two coils are brought in proximity with each other the magnetic field in one of the coils tends to link with the other. Only ac current can introduce magnetic fields in a coil, so only ac current can generate mutual inductance to the secondary coil in case of transformers.