Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
An electromagnet is a temporary magnet formed by winding a coil of insulated copper wire around a soft iron core. It behaves as a magnet only as long as an electric current passes through the coil.
The magnetic effect of electric current was first discovered by Hans Christian Oersted, who observed that a compass needle deflects when placed near a current-carrying wire.
The strength of the magnetic field () of an electromagnet is directly proportional to the magnitude of current () flowing through it: .
The strength also depends on the number of turns () in the coil: . More turns result in a stronger magnetic field.
The material of the core plays a vital role. Soft iron is used because it is easily magnetized and demagnetized (low retentivity), whereas steel is not used for electromagnets because it retains magnetism even after the current is switched off.
Polarity of an Electromagnet: According to the Clock Rule, if the current flows in a clockwise direction at one face of the coil, that face becomes the South () pole. If it flows anticlockwise, it becomes the North () pole.
Uses of Electromagnets: They are used in electric bells, cranes for lifting heavy iron scrap, telephone diaphragms, and MRI machines.
Electric Bell: It works on the principle of electromagnetism. When the circuit is closed, the electromagnet attracts the iron armature, causing the hammer to hit the gong. This movement breaks the circuit at the contact screw, demagnetizing the core and allowing the spring to pull the armature back, completing the circuit again (make-and-break mechanism).
📐Formulae
(where is permeability, is turns per unit length, and is current)
💡Examples
Problem 1:
An electromagnet has turns. If the number of turns is increased to while keeping the current constant, how many times stronger will the magnetic field become?
Solution:
The strength of an electromagnet is directly proportional to the number of turns (). If the turns increase from to , the factor of increase is . Therefore, the magnetic field will become times stronger.
Explanation:
Since the magnetic field is additive for every loop of wire, tripling the loops triples the total field produced.
Problem 2:
Why is soft iron preferred over steel for making the core of an electromagnet used in an electric bell?
Solution:
Soft iron has high magnetic permeability but low retentivity. This means it becomes a strong magnet quickly when current flows and loses its magnetism immediately when current stops. Steel has high retentivity and would become a permanent magnet, preventing the 'make-and-break' mechanism of the bell.
Explanation:
The electric bell requires the magnet to turn off so the armature can return to its original position; steel would keep the armature stuck to the magnet.
Problem 3:
If the current in a solenoid is and it is doubled to , what happens to the magnetic field strength?
Solution:
Given . If the new current , then .
Explanation:
The magnetic field strength is directly proportional to the current; doubling the current results in doubling the magnetic field strength.