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Electricity: Magnetic and Heating Effects - Electromagnets and Electric Bell

Grade 7CBSE

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

🔑Concepts

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Heating Effect of Current: When an electric current flows through a high-resistance wire (like Nichrome), the wire becomes hot and releases energy in the form of heat. This is the principle behind appliances like electric heaters, irons, and geysers.

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Factors affecting Heat: The heat produced in a wire depends on the current (II), the resistance (RR) of the material, and the time (tt) for which the current flows.

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Electric Fuse: A safety device used in electrical circuits. It is made of a wire with a low melting point that melts and breaks the circuit if the current exceeds a safe limit, preventing fires and damage to appliances.

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Magnetic Effect of Current: When electric current passes through a wire, it behaves like a magnet. This was first observed by Hans Christian Oersted when he noticed a compass needle deflecting near a current-carrying wire.

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Electromagnet: A temporary magnet consisting of a coil of insulated wire wound around a soft iron core. It loses its magnetism as soon as the current is switched off. Its strength depends on the number of turns in the coil and the magnitude of the current.

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Electric Bell: A device that uses an electromagnet to pull an iron strip (armature) attached to a hammer. The hammer hits a gong. The movement of the armature breaks the circuit, the electromagnet loses magnetism, and the spring pulls the armature back, completing the circuit again to repeat the process.

📐Formulae

H=I2RtH = I^2 R t

P=V×IP = V \times I

V=I×RV = I \times R

Strength of Electromagnet∝n×I\text{Strength of Electromagnet} \propto n \times I

💡Examples

Problem 1:

An electric heater has a resistance of 50 Ω50 \, \Omega and draws a current of 4 A4 \, A. Calculate the heat produced in 1010 seconds.

Solution:

H=(4)2×50×10=16×50×10=8000 JoulesH = (4)^2 \times 50 \times 10 = 16 \times 50 \times 10 = 8000 \, \text{Joules}

Explanation:

Using the Joule heating formula H=I2RtH = I^2 R t, where I=4 AI = 4 \, A, R=50 ΩR = 50 \, \Omega, and t=10 st = 10 \, s.

Problem 2:

What happens to the magnetic field of an electromagnet if the direction of current is reversed?

Solution:

The polarity of the electromagnet (North and South poles) will be reversed, but it will still act as a magnet.

Explanation:

According to the magnetic effect of current, the direction of the magnetic field depends on the direction of the flow of current (II).

Problem 3:

Why is a fuse wire usually thin and made of materials with low melting points?

Solution:

A thin wire has higher resistance (RR). If the current (II) becomes too high, the heat produced (H∝I2H \propto I^2) causes the wire to reach its melting point quickly, breaking the circuit.

Explanation:

This protects the circuit from 'Short-circuiting' or 'Overloading' by stopping the flow of electricity before damage occurs.