Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
The magnetic effect of electric current was first discovered by the scientist Hans Christian Oersted, who noticed that a compass needle deflects when placed near a wire carrying an electric current.
When an electric current flows through a wire, it behaves like a magnet. This phenomenon is known as the Magnetic Effect of Electric Current.
An Electromagnet is a coil of insulated wire wound around a soft iron core that acts as a magnet only when an electric current passes through the coil.
The magnetism of an electromagnet is temporary; it loses its magnetic property as soon as the current is switched off.
The strength of an electromagnet depends on two main factors: the number of turns in the coil () and the amount of current () flowing through it.
Electromagnets are used in various devices such as electric bells, loudspeakers, cranes for lifting heavy iron scrap, and in medical science to remove small pieces of magnetic material from the eye.
📐Formulae
(Where is the strength of the electromagnet, is the number of turns, and is the current)
(Joule's Law of Heating: where is heat, is current, is resistance, and is time)
💡Examples
Problem 1:
Identify the core material usually used to make a strong electromagnet and explain why steel is not preferred over soft iron.
Solution:
Soft iron is used as the core material. Steel is not preferred because it retains its magnetism even after the current is switched off (becoming a permanent magnet), whereas an electromagnet must lose its magnetism instantly when the circuit is broken.
Explanation:
Electromagnets are temporary magnets. Soft iron has high susceptibility but low retentivity, making it ideal for applications like electric bells.
Problem 2:
An electromagnet is used in a crane to lift iron scrap. If the operator wants to lift a heavier load, what two changes can be made to the electrical setup?
Solution:
- Increase the electric current () flowing through the coil. 2. Increase the number of turns () of the wire around the iron core.
Explanation:
Since the magnetic field strength is directly proportional to the product of current and number of turns (), increasing either will strengthen the magnet.
Problem 3:
Explain the role of the electromagnet in an electric bell.
Solution:
When the switch is pressed, current flows through the coil, turning the iron core into an electromagnet. This attracts the iron armature, causing the hammer to hit the gong. This movement also breaks the circuit, turning off the electromagnet, and the spring pulls the armature back, completing the circuit again.
Explanation:
This repetitive 'make and break' of the circuit allows the bell to ring continuously as long as the switch is pressed.