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What happens to the magnetic field in the solenoid when the length of the solenoid increases?
Increases
Decreases
Remains constant
Becomes zero
Decreases
When the length of a solenoid increases while keeping the total number of turns and current constant, the magnetic field strength decreases. This happens because the magnetic field density inside a long solenoid is inversely proportional to its axial length.
When the length of a solenoid increases while keeping the total number of turns and current constant, the magnetic field strength decreases. This happens because the magnetic field density inside a long solenoid is inversely proportional to its axial length.
B=╬╝nI тАФ Magnetic field strength inside a solenoid
n=LNтАЛ тАФ Number of turns per unit length
The magnetic field inside an ideal long solenoid is given by B=╬╝0тАЛnI=╬╝0тАЛ(LNтАЛ)I, where N is the total number of turns, L is the length, and I is the current. Increasing the length L reduces the turn density n=LNтАЛ, thereby weakening the overall magnetic field inside the core.
Magnetic field strength B is directly proportional to current I and turns per unit length n.
Magnetic field strength B is inversely proportional to the solenoid length L for a fixed number of turns N.
Design of electromagnets and inductors
Relays and solenoid actuators in automated machinery
Option A is incorrect because increasing length decreases turn density, reducing field strength.
Option C is incorrect because length directly affects turn density and field intensity.
Option D is incorrect because the magnetic field becomes zero only if the current is zero.
B is correct тАФ The magnetic field decreases because it is inversely proportional to the length of the solenoid for a fixed number of turns.
Always verify if the question assumes fixed total turns N or fixed turn density n; if turn density n remains constant while extending length, field strength B remains constant.