📡 Chapter 18: Electromagnetic Induction & EM Waves – Short Questions
Prepared by Muhammad Tayyab, Subject Specialist Physics, Govt Christian High School Daska. Based on PECTAA 2026 syllabus (National Curriculum 2023).
📖 What's Inside: This section covers all short questions from Chapter 18 Electromagnetic Induction & EM Waves including Faraday's Law, Lenz's Law, magnetic flux, A.C. generators, transformers, cathode rays, CRO, scattering of light, photons, and photoelectric effect. Each question is presented with a precise answer as per the official PECTAA 2026 Physics curriculum. Perfect for Punjab Boards (Lahore, Gujranwala, Multan, etc.) and all BISE boards across Pakistan.
📚 Related Resources – Chapter 18
Electromagnetic Induction covers Faraday's law, Lenz's law, A.C. generators, transformers, cathode rays, CRO, and EM waves.
📑 Quick Jump to Questions
📝 Short Questions with Answers
1 Define electromagnetic induction.
Answer: Electromagnetic induction is the process by which a changing magnetic field produces an electric current in a conductor.
✅ Electromagnetic induction: Changing magnetic field produces electric current.
2 How can induced e.m.f. be produced in a straight wire placed in a magnetic field?
Answer: Induced e.m.f. is produced when the wire moves through the magnetic field and cuts the magnetic field lines. This causes current to flow in the circuit.
✅ Wire cuts magnetic field lines → Induced e.m.f. produced.
3 How can the induced current be increased?
Answer: The induced current can be increased by: (i) Using a stronger magnetic field. (ii) Moving the loop more quickly. (iii) Using a coil with many turns instead of a single loop.
✅ Increase B, speed, or number of turns.
4 On what factors does the induced current depend?
Answer: The induced current depends on: (i) The speed of motion of the wire. (ii) The resistance of the circuit.
✅ Speed of motion & resistance of circuit.
5 Define magnetic flux.
Answer: Magnetic flux is the number of magnetic field lines passing through a certain area. The magnetic flux (\(\Phi\)) is given by \(\Phi = BA\cos\theta\), where \(B\) is magnetic field strength, \(A\) is area of the coil, and \(\theta\) is angle between the magnetic field and the normal to the coil.
✅ \(\Phi = BA\cos\theta\)
6 What is the unit of magnetic flux?
Answer: The unit of magnetic flux is weber (Wb). 1 Wb = 1 T·m². It is defined as the magnetic flux when a magnetic field of 1 tesla passes perpendicularly through an area of one square metre.
✅ Weber (Wb): 1 Wb = 1 T·m²
7 When is magnetic flux maximum and when is it zero?
Answer: Magnetic flux is maximum when the angle between the magnetic field (\(B\)) and the normal to the area of the coil (\(A\)) is \(0^\circ\). Magnetic flux is zero when the angle is \(90^\circ\).
✅ Max at 0°, Zero at 90°
8 State Faraday's Law of Electromagnetic Induction.
Answer: The average e.m.f. induced in a conducting coil of \(N\) loops is equal to the negative of the rate at which the magnetic flux is changing with time. Mathematically: \(\epsilon = -N \frac{\Delta\phi}{\Delta t}\). The negative sign indicates that the induced e.m.f. opposes the change of magnetic flux that produces it.
✅ \(\epsilon = -N \frac{\Delta\phi}{\Delta t}\)
9 State Lenz's Law.
Answer: The direction of induced current is always such that it opposes the cause which produces it.
✅ Induced current opposes the cause that produces it.
10 Why does the induced current oppose the cause producing it?
Answer: The induced current opposes the cause producing it because of the law of conservation of energy. Otherwise, energy would increase continuously, which is impossible.
✅ Conservation of energy.
11 What happens to the induced current if the magnet stops moving near the coil?
Answer: If the magnet stops moving near the coil, the induced current becomes zero because there is no change in magnetic flux.
✅ No change in flux → No induced current.
12 How does the speed of a moving magnet affect the induced e.m.f.?
Answer: The faster the magnet is moved through the coil, the greater the induced e.m.f. (and hence the induced current) produced.
✅ Faster magnet → Greater induced e.m.f.
13 What is an A.C generator?
Answer: An alternating current (A.C) generator is a device that converts mechanical energy into electrical energy using electromagnetic induction.
✅ Converts mechanical → electrical energy.
14 What are the main parts of a simple A.C. generator?
Answer: A simple A.C. generator consists of: (i) A rectangular coil. (ii) A permanent magnet. (iii) Two slip rings. (iv) Carbon brushes.
✅ Coil, magnet, slip rings, carbon brushes.
15 State the principle of an A.C. generator.
Answer: An A.C. generator works on the principle of electromagnetic induction. When a coil rotates in a magnetic field, a changing magnetic flux induces an e.m.f. in the coil.
✅ Electromagnetic induction.
16 What is the function of slip rings in an A.C. generator?
Answer: Slip rings help to maintain continuous electrical contact with the rotating coil while allowing current to alternate naturally.
✅ Maintain continuous contact, allow alternating current.
17 What is a transformer?
Answer: A transformer is a device used to increase or decrease the voltage in an alternating current (A.C.) circuit. It works without any electrical connection between its input and output coils.
✅ Increases/decreases A.C. voltage.
18 On what principle does a transformer work?
Answer: A transformer works on the principle of electromagnetic induction. A changing magnetic field produced by the primary coil induces an e.m.f. in the secondary coil.
✅ Electromagnetic induction.
19 What are the main parts of a simple transformer?
Answer: A simple transformer has: (i) Primary coil with \(N_p\) turns. (ii) Secondary coil with \(N_s\) turns. (iii) Soft iron core. Both coils are wound on a soft iron core, which helps carry the magnetic field from one coil to the other.
✅ Primary coil, Secondary coil, Soft iron core.
20 What causes the buzzing sound in transformers?
Answer: The buzzing sound in transformers is due to the iron core vibrating at 50 or 60 Hz. This vibration is caused by varying magnetic field and not by the current directly.
✅ Iron core vibrating at 50/60 Hz.
21 How does a transformer work?
Answer: When an alternating voltage is applied to the primary coil, it produces an alternating current that creates a changing magnetic field in the iron core. This magnetic field passes through the secondary coil, where it induces an alternating voltage. There is no direct electrical connection between the two coils. Energy is transferred through the magnetic field in the core.
✅ Changing magnetic field transfers energy from primary to secondary.
22 Describe the types of transformer.
Answer: There are two types: (i) Step Up Transformer: Secondary coil has more turns than primary, increases output voltage. (ii) Step Down Transformer: Secondary coil has fewer turns than primary, reduces output voltage.
✅ Step-up: \(N_s > N_p\), Step-down: \(N_s < N_p\)
23 Write the turns-ratio equation for a transformer.
Answer: The turns-ratio equation is: \(\frac{V_s}{V_p} = \frac{N_s}{N_p}\), where \(V_p\) is primary voltage, \(V_s\) is secondary voltage, \(N_p\) is primary turns, and \(N_s\) is secondary turns. The equation shows that voltage ratio is directly proportional to the turns ratio.
✅ \(\frac{V_s}{V_p} = \frac{N_s}{N_p}\)
24 What are step-up and step-down transformers used for?
Answer: Step-up transformers increase voltage for long-distance transmission, while step-down transformers reduce it to safe levels in our home.
✅ Step-up: Transmission, Step-down: Home use.
25 Why does a transformer work only with alternating current (A.C.)?
Answer: A transformer works only with alternating current (A.C.) because only a changing current produces a changing magnetic field, which is necessary to induce voltage in the secondary coil.
✅ A.C. provides changing magnetic field.
26 What are cathode rays?
Answer: Cathode rays are a stream of negatively charged electrons moving with high speed. The direction of electron beam can be changed by electric and magnetic fields.
✅ Stream of negatively charged electrons.
27 How are cathode rays deflected by an electric field?
Answer: Cathode rays are negatively charged electrons. Each electron is attracted by the positive electrode and repelled by the negative electrode. Therefore, cathode rays are deflected toward the positive electrode in the presence of an electric field.
✅ Deflected toward positive electrode.
28 How are cathode rays deflected by a magnetic field?
Answer: A magnetic field exerts a force on electrically charged particles that are in motion. Therefore, electrons passing through a magnetic field are deflected, and Fleming's Left-Hand Rule is used to determine the direction of the force and the deflection of the electron beam.
✅ Deflected by magnetic force, Fleming's Left-Hand Rule.
29 What is a Cathode Ray Oscilloscope (CRO)?
Answer: A Cathode Ray Oscilloscope (CRO) is a widely used electronic instrument designed to display, observe, and measure how electrical signals change with time.
✅ Displays and measures electrical signals.
30 What are the main parts of a Cathode Ray Oscilloscope (CRO)?
Answer: The main component of the CRO is a cathode ray tube (CRT), which includes: (i) Electron gun (ii) Deflecting system (iii) Fluorescent screen.
✅ Electron gun, Deflecting system, Fluorescent screen.
31 What is a saw-tooth waveform?
Answer: A saw-tooth waveform is a pattern of voltage that increases steadily over time and then rapidly drops to its starting value, repeating this cycle continuously. It causes the electron beam to sweep from left to right, creating a time axis on the screen.
✅ Voltage increases steadily, drops rapidly.
32 What can a CRO measure?
Answer: A CRO can: (i) Show the waveform of a signal. (ii) Measure its amplitude. (iii) Determine its time period and frequency. (iv) Compare two signals to measure their phase difference.
✅ Waveform, amplitude, time period, frequency, phase difference.
33 What is scattering of light?
Answer: Scattering of light is the phenomenon in which sunlight is scattered by the tiny gas molecules present in the Earth's atmosphere.
✅ Scattering by gas molecules in atmosphere.
34 Why is the sky blue during the day?
Answer: As sunlight passes through the atmosphere, it interacts with tiny gas molecules. Blue light, having a shorter wavelength, scatters more easily than red or green light. During daytime, this scattered blue light reaches us from all directions, making the sky appear blue.
✅ Blue light scatters more due to shorter wavelength.
35 Why does the sky appear red during sunrise and sunset?
Answer: During sunrise or sunset, sunlight travels through a longer path in the atmosphere. Much of the blue light is scattered out of the path, while the longer wavelengths, such as red and orange, scatter less. These colours dominate the sky near the horizon, giving it a reddish appearance. Dust, pollution, and water droplets enhance this effect.
✅ Longer path → Blue light scattered away → Red/orange remain.
36 What is the particle nature of light?
Answer: According to modern physics, light can be viewed as a stream of massless particles known as photons. These photons carry energy and momentum, even though they have no rest mass.
✅ Light as photons: massless particles carrying energy and momentum.
37 What are photons?
Answer: Photons are massless particles of light that carry energy and momentum, although they have no rest mass.
✅ Massless particles of light.
38 What is the photoelectric effect?
Answer: When light shines on a metal surface, it can transfer energy to electrons. If the energy is high enough, electrons are ejected from the surface. These are called photoelectrons. This effect proves that light energy comes in packets (photons), and not as a continuous wave.
✅ Light ejects electrons from metal surface → proves particle nature.
39 What is radiation pressure?
Answer: Radiation pressure is the tiny force exerted by light when it strikes an object. Though the force is very small, it has practical applications, such as solar sails on spacecraft.
✅ Force exerted by light on objects.
40 What are solar sails?
Answer: Solar sails are designed to reflect sunlight. The momentum of the photons gradually pushes the sail forward, allowing the spacecraft to move through space without using any fuel.
✅ Use photon momentum for spacecraft propulsion.
41 What is the dual nature of light?
Answer: This concept demonstrates the dual nature of light: it behaves like both a wave and a particle, depending on how it interacts with materials.
✅ Light behaves as both wave and particle.
📐 Key Formulas – Electromagnetic Induction
\[ \epsilon = -N \frac{\Delta \phi}{\Delta t} \]
Faraday's Law (Induced e.m.f.)
Faraday's Law (Induced e.m.f.)
\[ \phi = BA \cos \theta \]
Magnetic Flux
Magnetic Flux
\[ \frac{V_s}{V_p} = \frac{N_s}{N_p} \]
Transformer Turns Ratio
Transformer Turns Ratio
\[ 1 Wb = 1 T m^2 \]
Weber (Unit of Magnetic Flux)
Weber (Unit of Magnetic Flux)
Unit 10Thermal Physics
Unit 11Transfer of Thermal Energy
Unit 12Waves
Unit 13Sound
Unit 14Light
Unit 15Electrostatics
Unit 16Electricity
Unit 17Electromagnetism
Unit 18✨ Electromagnetic Induction & EM Waves
Unit 19Electronics
Unit 20Atomic and Nuclear Physics
Unit 21Space and Environment
📖 Complete syllabus coverage for Class 10 Physics (PECTAA 2026) – Units 10 to 21
Created by Hira Science Academy | Aligned with PECTAA 2026 Syllabus