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Electricity: Magnetic and Heating Effects - Electromagnets: Construction and Uses

Grade 8CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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The Heating Effect of Electric Current occurs when an electric current passes through a high-resistance wire (like Nichrome), causing it to become hot. The heat produced HH depends on current II, resistance RR, and time tt.

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The Magnetic Effect of Electric Current was discovered by Hans Christian Oersted, who observed that a compass needle deflects when placed near a current-carrying wire, proving the wire acts like a magnet.

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An Electromagnet is a temporary magnet consisting of a coil of insulated copper wire wound around a soft iron core. It only behaves as a magnet when current flows through the circuit.

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The strength of an electromagnet can be increased by: (i) Increasing the number of turns nn in the coil, or (ii) Increasing the magnitude of current II flowing through the wire.

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Soft Iron is used as the core of an electromagnet because it gets magnetized easily and loses its magnetism almost completely when the current is switched off, making it a temporary magnet.

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Uses of Electromagnets: They are used in electric bells, telecommunication equipment, cranes to lift heavy iron scrap, and by doctors to remove small iron splinters from the eye.

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The Electric Bell works on the principle of electromagnetism. It consists of an electromagnet, an armature, a contact screw, a hammer, and a gong. When the circuit is closed, the electromagnet pulls the armature, causing the hammer to strike the gong.

📐Formulae

H=I2RtH = I^2 R t

V=IimesRV = I imes R

P=VimesIP = V imes I

B∝n×IB \propto n \times I

💡Examples

Problem 1:

An electric heater of resistance R=50 ΩR = 50 \, \Omega is connected to a power supply such that a current of I=2 AI = 2 \, A flows through it. Calculate the heat energy HH produced in 1010 minutes.

Solution:

Given: R=50 ΩR = 50 \, \Omega, I=2 AI = 2 \, A, and t=10 minutes=10×60=600 st = 10 \, \text{minutes} = 10 \times 60 = 600 \, s. Using the formula H=I2RtH = I^2 R t: H=(2)2×50×600H = (2)^2 \times 50 \times 600 H=4×50×600=120,000 JH = 4 \times 50 \times 600 = 120,000 \, J (or 120 kJ120 \, kJ)

Explanation:

The heat generated is calculated using Joule's Law of Heating, ensuring time is converted to the SI unit (seconds) before calculation.

Problem 2:

Why is a steel core not used in making an electromagnet for an electric bell?

Solution:

Steel is a hard magnetic material. Unlike soft iron, steel does not lose its magnetism immediately when the current is switched off. If used in an electric bell, the hammer would stay stuck to the core, and the bell would not ring continuously.

Explanation:

Electromagnets require temporary magnetism. Steel forms a permanent magnet, whereas soft iron provides the necessary temporary magnetic property required for the make-and-break mechanism of an electric bell.

Electromagnets: Construction and Uses Revision - Class 8 Science CBSE