Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
The magnetic effect of electric current was first observed by Hans Christian Oersted, who noticed that a compass needle deflects when placed near a wire carrying electric current.
When electric current flows through a wire, the wire behaves like a magnet. This phenomenon is called the magnetic effect of electric current.
An electromagnet consists of a coil of insulated wire wrapped around a piece of magnetic material, such as a soft iron core. It acts as a magnet only when current () flows through it.
The strength of an electromagnet depends on the number of turns () in the coil and the magnitude of the current () passing through it.
Electromagnets are temporary magnets; their magnetism is lost as soon as the current is switched off.
The Electric Bell works on the principle of the magnetic effect of current, using an electromagnet to pull an iron strip that strikes a gong to produce sound.
📐Formulae
💡Examples
Problem 1:
A student winds turns of wire around an iron nail and connects it to a battery. If the student increases the number of turns to while keeping the current constant, what happens to the magnetic strength?
Solution:
The magnetic strength of the electromagnet will double.
Explanation:
The magnetic field strength of an electromagnet is directly proportional to the number of turns in the coil. Since the number of turns increased from to (a two-fold increase), the magnetic strength also increases proportionally.
Problem 2:
Why is a soft iron core used in an electromagnet instead of a steel core?
Solution:
Soft iron is used because it loses its magnetism as soon as the current stops flowing ().
Explanation:
Soft iron has low retentivity, meaning it does not retain magnetism once the magnetic field produced by the current is removed. Steel, however, can become a permanent magnet, which would prevent the electromagnet from being 'switched off'.
Problem 3:
In an electric bell, what happens to the circuit when the electromagnet pulls the iron strip?
Solution:
The circuit is broken at the contact screw.
Explanation:
When the electromagnet is energized, it attracts the iron strip. As the strip moves toward the electromagnet, it loses contact with the screw, causing the current to become . This de-energizes the electromagnet, allowing the strip to spring back and complete the circuit again.