Question

The adjacent diagram shows a coil wound around a soft iron bar XY. Suggest a way of increasing the strength of electromagnet so formed.

• A. By reducing resistance of circuit by rheostat.
• B. By increasing resistance of circuit by rheostat.
• C. By reducing current in circuit
• D. all the above

Answer 1

The correct option is A By reducing resistance of circuit by rheostat.

A solenoid is a long coil containing a large number of close turns of insulated copper wire wound on a conducting or non-conducting material. In this case, we consider a coil wound on a soft iron bar XY.
When the circuit is closed by pressing the key, a steady current flow through such a device, the magnetic fields produced by the solenoid are similar to the magnetic field produced by a magnet. The soft iron core helps in concentrating the magnetic lines of forces through the solenoid, so that the magnetic field is almost uniform at the end face of the core. In the absence of a core, the magnetic lines of forces start to diverge by curving sharply immediately outside the coils. The smaller the space between the coils and the core, the stronger is the magnetism induced in the core. Therefore, the best electromagnets have coils wound tightly around the core.
The strength of an electromagnet is related to factors stated below:

• Directly proportional to the number of turns in the coil.
• Directly proportional to the current flowing in the coil.
• Inversely proportional to the length of air gap between the poles.

Hence, the strength of the electromagnet can be increased through reduction of resistance in the circuit by introducing a rheostat and thereby increasing the current flowing through the coil.