Electromagnetism is one of the most fascinating and practical chapters in Class 10 Physics. It explores how electric currents produce magnetic effects, how magnetic fields can influence current-carrying conductors, and how these principles are applied in everyday devices like D.C. motors, A.C. generators, and transformers.
In this chapter, you will learn:
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The magnetic effects of a steady current and how currents create magnetic fields.
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Force on a current-carrying conductor in a magnetic field and the turning effect on a current-carrying coil, which forms the basis of electric motors.
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The working principle of a D.C. motor and the concept of electromagnetic induction, including Lenz’s Law and the direction of induced e.m.f.
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How A.C. generators produce alternating current and the concept of mutual induction.
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The role of transformers in high-voltage transmission of electricity and their importance in reducing energy loss over long distances.
Important MCQs – Unit 15 Electromagnetism
1. Which statement is true about the magnetic poles?
(a) Unlike poles repel
(b) Like poles attract
(c) Magnetic poles do not affect each other
(d) A single magnetic pole does not exist ✅
2. The direction of induced e.m.f. in a circuit is in accordance with conservation of:
(a) Mass
(b) Charge
(c) Momentum
(d) Energy ✅
3. The turn ratio of a transformer is 10. It means:
(a) Is = 10 Ip
(b) Ns = Np / 10
(c) Ns = 10 Np ✅
(d) Vs = Vp / 10
4. Which device converts mechanical energy into electrical energy?
(a) Motor
(b) Battery
(c) A.C Generator ✅
(d) Transformer
5. According to Faraday’s law, induced e.m.f. is proportional to:
(a) Coil temperature
(b) Magnetic field strength only
(c) Rate of change of magnetic lines ✅
(d) Resistance of coil
6. Which law explains the mutual induction phenomenon?
(a) Newton’s Law
(b) Faraday’s Law of Electromagnetic Induction ✅
(c) Ohm’s Law
(d) Coulomb’s Law
Short Questions – Unit 15 Electromagnetism
1. What happens when electric current passes through a conductor?
Answer:
A magnetic field is produced around the conductor.
Explanation:
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When an electric current flows through a conductor, circular magnetic field lines are formed around it.
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The direction of the magnetic field can be determined using the Right Hand Thumb Rule:
2. State the Right Hand Grip Rule.
Answer:
Hold the wire with your right hand such that the thumb points in the direction of current. The curled fingers show the direction of the magnetic field.
Explanation:
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This rule is used to determine the direction of the magnetic field around a current-carrying conductor.
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Thumb → direction of current
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Curled fingers → direction of magnetic field lines
3. State Fleming’s Left Hand Rule.
Answer:
If the forefinger, middle finger, and thumb are placed at right angles, then:
Explanation:
This rule is used to determine the direction of force on a current-carrying conductor in a magnetic field, such as in a D.C. motor.
4. State Lenz’s Law.
Answer:
The direction of the induced current is such that it opposes the cause that produces it.
Explanation:
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Lenz’s Law ensures that energy is conserved.
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When the magnetic flux changes, the induced current flows in a direction that opposes this change.
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This law is used to predict the direction of current in electromagnetic induction.
Long Questions – Unit 15 Electromagnetism
Q1: State and explain Faraday’s Law of Electromagnetic Induction with examples and diagrams.
Introduction:
In 1831, Michael Faraday discovered the phenomenon of electromagnetic induction. He observed that whenever there is relative motion between a magnetic field and a coil of wire, an electric current is induced in the coil. This discovery is the foundation of many electrical devices like generators, transformers, and induction motors.
Faraday’s Law of Electromagnetic Induction:
The magnitude of induced e.m.f. in a circuit is directly proportional to the rate of change of magnetic flux through the circuit.
Mathematically:
Induced e.m.f. ∝ ΔΦ / Δt
Where:
Φ = magnetic flux = B × A × cos(theta)
B = magnetic field strength
A = area of the coil
theta = angle between magnetic field and normal to coil
Key Points:
Examples of Faraday’s Law:
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Moving a magnet in and out of a coil.
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Rotating coil in a magnetic field (AC generator).
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Transformers work on electromagnetic induction to step up or step down voltage.
Applications:
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Electric Generators: Convert mechanical energy into electrical energy.
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Transformers: Change voltage levels for efficient power transmission.
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Induction Cookers: Heat utensils using induced currents.
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Electric Bells and Relays: Operate using electromagnetic induction.
Explain with Digram:
1. Magnet moving into a coil → Induced current clockwise
Explanation:
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As the north pole of the magnet moves into the coil, the magnetic flux increases.
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According to Faraday’s Law, an induced current flows in a direction that opposes the increase (Lenz’s Law).
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Here, current flows clockwise as seen from the magnet side.
2. Magnet moving out of a coil → Induced current anticlockwise
Explanation:
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As the magnet is withdrawn, the magnetic flux decreases.
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The induced current flows in a direction to oppose the decrease, i.e., opposite to the previous direction.
3. Rotating coil in a magnetic field (AC Generator)
Explanation:
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When a coil rotates in a magnetic field, the magnetic flux through the coil changes continuously.
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This produces alternating current (AC) in the coil.
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The faster the rotation, the greater the induced e.m.f.
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