Chapter 12 · Question 6

What is an electromagnet? How is it made, and how does its magnetic strength compare with that of a solenoid alone? Give two practical reasons why electromagnets are preferred over permanent magnets in many applications.

Answer Revealed

Direct Answer:

An electromagnet is a magnet formed by placing a piece of a magnetic material, such as soft iron, inside a solenoid coil through which a current is passed. The strong, uniform magnetic field produced inside the solenoid magnetises the soft iron core, creating a much stronger magnet than the solenoid alone. Electromagnets are preferred because: (i) their magnetic strength can be varied by changing the current through the coil, and (ii) the magnetism can be switched on or off by controlling the current. This makes them ideal for devices like electric bells, cranes for lifting heavy iron loads, and relays.

Simple Explanation

An electromagnet is a solenoid with an iron heart. Take a coil of insulated copper wire (solenoid), slide a soft iron rod inside it, and pass current through the coil. The solenoid's magnetic field magnetises the iron, and the combined effect is a magnet far stronger than the solenoid alone. The beauty of an electromagnet is that it is not permanent — turn off the current and the magnetism disappears. Make the current stronger and the magnet gets stronger. This on-off and adjustable-strength feature is why electromagnets run everything from electric bells to giant scrapyard cranes that pick up and drop cars with the flick of a switch.

Exam-Ready Structure

An electromagnet is a practical device built on the solenoid principle, where a ferromagnetic core amplifies the magnetic field. 1. Construction: (a) Take a solenoid — a coil of many circular turns of insulated copper wire wrapped in the shape of a cylinder. (b) Insert a piece of soft iron (a magnetic material) inside the solenoid coil. (c) Connect the coil to a battery through a key. When current flows through the coil, a strong uniform magnetic field is produced inside the solenoid, which magnetises the soft iron core (Fig. 12.11). The magnet so formed is called an electromagnet. 2. Magnetic strength: The electromagnet is much stronger than the solenoid alone because the soft iron core, when placed in the solenoid's magnetic field, itself becomes strongly magnetised. The total magnetic field is the sum of the solenoid's field and the field of the magnetised iron core. 3. Advantages over permanent magnets: (a) Controllable strength: The magnetic strength of an electromagnet can be changed by changing the current through the solenoid. Increasing the current increases the field strength, and vice versa. (b) Switchable magnetism: The magnetism can be switched on or off by switching the current on or off. When the current is switched off, the soft iron core loses most of its magnetism quickly (soft iron has low retentivity). (c) The polarity of an electromagnet can be reversed by reversing the direction of current. 4. Applications: Cranes for lifting heavy iron and steel loads (scrapyards), electric bells, electromagnetic relays, loudspeakers, and many medical and industrial devices.

Key Points

Electromagnet = solenoid + soft iron core placed inside it
Soft iron core gets magnetised by the solenoid's field → total field much stronger than solenoid alone
Strength can be varied by changing the current through the solenoid
Magnetism can be switched on and off by turning current on and off
Polarity can be reversed by reversing current direction

What is an electromagnet? How is it made, and how does its magnetic strength compare with that of a solenoid alone? Give two practical reasons why electromagnets are preferred over permanent magnets in many applications.