The coercivity of a magnet is 5 × 10³ A / m. The amount of current required to be passed in a solenoid of length 30 cm and the number of turns 150, so that the magnet gets demagnetised when inside the solenoid is ________ A.

Solution

Physics sir · 3 steps

Coercivity is the reverse H that wipes out the magnetisation. Make the solenoid's H equal to it.

1. What coercivity means. Coercivity is the magnetic intensity H needed to bring the magnet's magnetisation down to zero. So the solenoid must produce H = 5 × 10³ A m⁻¹ inside it.
2. Field of a solenoid. Inside a long solenoid, H = nI, where n is the number of turns per metre. Here n = 150/(0.30 m) = 500 m⁻¹.

   I = H/n = (5 × 10³)/500

3. So. The current needed is I = 10 A, so the answer is 10.

The common slip: 1000. Leaving the length in centimetres makes n = 5 turns per cm and gives 1000 A, not 10 A. Convert 30 cm to 0.30 m first.

The idea to keep. To demagnetise a magnet, apply a magnetic intensity equal to its coercivity. Inside a solenoid H = nI, and n is always turns per metre.

Answer from the NTA final answer key. Solution by PrepWiser.