Physics Objective Questions and Answers: Electromagnetic Waves
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Multiple Choice Questions
(a) 36.6 m
(b) 40.5 m
(c) 42.3 m
(d) 50.9 m
Show Answer & Explanation
Answer: (a) 36.6 m
Explanation:
1. λ = c/f = 3 × 10⁸/(8.2 × 10⁶) = 36.6 m.
Teacher's Note:
1. Recall the basic fact.
(a) resistance
(b) inductance
(c) capacitance
(d) product of inductance and capacitance
Show Answer & Explanation
Answer: (a) resistance
Explanation:
1. E/H has the unit V/A, which is ohm, so E/H has the dimensions of resistance (the impedance of free space, 377 ohm).
Teacher's Note:
1. Recall the basic fact.
(a) microhertz
(b) gigahertz
(c) megahertz
(d) hertz
Show Answer & Explanation
Answer: (b) gigahertz
Explanation:
1. Microwave frequencies are of the order of 10⁹ to 10¹¹ Hz.
Teacher's Note:
1. Recall the basic fact.
(a) I and II
(b) I and III
(c) III only
(d) I, II and III
Show Answer & Explanation
Answer: (b) I and III
Explanation:
1. Total absorption gives P = I/c.
2. Total reflection reverses the momentum, giving P = 2I/c.
Teacher's Note:
1. Recall the basic fact.
(a) an emf
(b) electric displacement current
(c) a magnetic field
(d) a pressure gradient
Show Answer & Explanation
Answer: (c) a magnetic field
Explanation:
1. A changing electric field produces a magnetic field.
Teacher's Note:
1. Recall the basic fact.
(a) 100 V/s
(b) 200 V/s
(c) 300 V/s
(d) 500 V/s
Show Answer & Explanation
Answer: (d) 500 V/s
Explanation:
1. I_d = C dV/dt, so dV/dt = 10⁻³/(2 × 10⁻⁶) = 500 V/s.
Teacher's Note:
1. Recall the basic fact.
(a) sound waves and light waves
(b) water waves and radio waves
(c) light waves and X-rays
(d) sound waves and water waves
Show Answer & Explanation
Answer: (c) light waves and X-rays
Explanation:
1. Light and X-rays are e.m. waves.
Teacher's Note:
1. Recall the basic fact.
(a) the space current is in phase with the displacement current
(b) the space current lags behind the displacement current by a phase of 180°
(c) the space current lags behind the displacement current by a phase of 90°
(d) the space current leads the displacement current by a phase of 90°
Show Answer & Explanation
Answer: (b) the space current lags behind the displacement current by a phase of 180°
Explanation:
1. The displacement current leads E by 90° while the electron current lags E by 90°.
2. So the space current lags the displacement current by 180°.
Teacher's Note:
1. Recall the basic fact.
(a) n1 > n2 > n3
(b) n1 < n2 < n3
(c) n1 > n2 < n3
(d) n1 < n2 > n3
Show Answer & Explanation
Answer: (a) n1 > n2 > n3
Explanation:
1. The frequency decreases from gamma to X-rays to ultraviolet.
Teacher's Note:
1. Recall the basic fact.
(a) travel with the speed of sound
(b) travel with the same speed in all media
(c) travel in free space with the speed of light
(d) do not travel through a medium
Show Answer & Explanation
Answer: (c) travel in free space with the speed of light
Explanation:
1. In vacuum all e.m. waves travel at c = 3 × 10⁸ m/s.
Teacher's Note:
1. Recall the basic fact.
(a) p = 0, E = 0
(b) p not 0, E not 0
(c) p not 0, E = 0
(d) p = 0, E not 0
Show Answer & Explanation
Answer: (b) p not 0, E not 0
Explanation:
1. E.m. waves carry both energy and momentum, and deliver both to the surface.
Teacher's Note:
1. Recall the basic fact.
(a) energy
(b) charge
(c) momentum
(d) information
Show Answer & Explanation
Answer: (b) charge
Explanation:
1. E.m. waves carry energy, momentum and information but no charge.
Teacher's Note:
1. Recall the basic fact.
(a) is changing with time
(b) decreases
(c) does not change
(d) decreases to zero
Show Answer & Explanation
Answer: (a) is changing with time
Explanation:
1. Displacement current is ε₀ dφ_E/dt, which exists when the electric field (charge) changes with time.
Teacher's Note:
1. Recall the basic fact.
(a) microwave, infrared, ultraviolet, gamma rays
(b) infrared, microwave, ultraviolet, gamma rays
(c) gamma rays, ultraviolet, infrared, microwaves
(d) microwaves, gamma rays, infrared, ultraviolet
Show Answer & Explanation
Answer: (a) microwave, infrared, ultraviolet, gamma rays
Explanation:
1. Wavelengths decrease in the order microwave, infrared, ultraviolet, gamma rays.
Teacher's Note:
1. Recall the basic fact.
(a) \( E_0=B_0 \)
(b) \( E_0=cB_0 \)
(c) \( E_0=B_0/c \)
(d) \( E_0=c/B_0 \)
Show Answer & Explanation
Answer: (b) \( E_0=cB_0 \)
Explanation:
1. E0/B0 = c.
Teacher's Note:
1. Recall the basic fact.
(a) in mutually perpendicular planes but vibrating with a phase difference of π
(b) in mutually perpendicular planes but vibrating with a phase difference of π/2
(c) in randomly oriented planes but vibrating in phase
(d) in mutually perpendicular planes but vibrating in phase
Show Answer & Explanation
Answer: (d) in mutually perpendicular planes but vibrating in phase
Explanation:
1. E and B are perpendicular to each other and oscillate in phase.
Teacher's Note:
1. Recall the basic fact.
| Column I | Column II |
|---|---|
| (A) Radio waves | (1) 1 × 10¹⁶ to 3 × 10²¹ Hz |
| (B) Gamma rays | (2) 1 × 10⁹ to 3 × 10¹¹ Hz |
| (C) Microwaves | (3) 3 × 10¹⁸ to 5 × 10²² Hz |
| (D) X-rays | (4) 5 × 10⁵ to 10⁹ Hz |
(a) A-4, B-2, C-3, D-1
(b) A-4, B-3, C-2, D-1
(c) A-1, B-3, C-2, D-4
(d) A-3, B-4, C-2, D-1
Show Answer & Explanation
Answer: (b) A-4, B-3, C-2, D-1
Explanation:
1. Radio waves are the lowest, 5 × 10⁵ to 10⁹ Hz (A-4).
2. Gamma rays are the highest (B-3).
3. Microwaves lie between 10⁹ and 3 × 10¹¹ Hz (C-2).
4. X-rays are 10¹⁶ to 3 × 10²¹ Hz (D-1).
Teacher's Note:
1. Arrange the spectrum by frequency.
Assertion (A): The basic difference between various types of electromagnetic waves lies in their wavelengths or frequencies.
Reason (R): Electromagnetic waves travel through vacuum with the same speed.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation
Answer: (b) Both A and R are true, but R is not the correct explanation of A.
Explanation:
1. All e.m. waves have the same speed in vacuum so they differ only in wavelength or frequency.
2. The statement of equal speed is true but is a related fact rather than a full explanation.
Teacher's Note:
1. Same speed, different frequency.
(a) \( \frac1{\sqrt{\mu_0\varepsilon_0}} \)
(b) \( \frac1{\sqrt{\mu\varepsilon}} \)
(c) \( \sqrt{\mu/\varepsilon} \)
(d) \( \sqrt{\mu_0/\varepsilon} \)
Show Answer & Explanation
Answer: (b) \( \frac1{\sqrt{\mu\varepsilon}} \)
Explanation:
1. v = 1/√(με) in a medium.
Teacher's Note:
1. Recall the basic fact.
(a) the frequency of the microwaves has no relation with the natural frequency of water molecules
(b) microwaves are heat waves, so always produce heating
(c) infrared waves produce heating in a microwave oven
(d) the frequency of the microwaves must match the resonant frequency of the water molecules
Show Answer & Explanation
Answer: (d) the frequency of the microwaves must match the resonant frequency of the water molecules
Explanation:
1. Resonant absorption by water molecules gives the most efficient heating.
Teacher's Note:
1. Recall the basic fact.
(a) their velocities
(b) their frequencies
(c) their distance from the transmitter
(d) none of these
Show Answer & Explanation
Answer: (b) their frequencies
Explanation:
1. Absorption and reflection by the atmosphere depend on frequency.
Teacher's Note:
1. Recall the basic fact.
(a) electricity
(b) magnetism
(c) mechanics
(d) both electricity and magnetism
Show Answer & Explanation
Answer: (d) both electricity and magnetism
Explanation:
1. They unify electricity and magnetism.
Teacher's Note:
1. Recall the basic fact.
(a) X-rays
(b) gamma rays
(c) microwaves
(d) radio waves
Show Answer & Explanation
Answer: (b) gamma rays
Explanation:
1. Gamma rays have the shortest wavelength.
Teacher's Note:
1. Recall the basic fact.
(a) 3 V/m
(b) 6 V/m
(c) 9 V/m
(d) 12 V/m
Show Answer & Explanation
Answer: (b) 6 V/m
Explanation:
1. E0 = cB0 = 3 × 10⁸ × 20 × 10⁻⁹ = 6 V/m.
Teacher's Note:
1. Recall the basic fact.
(a) \( I=\tfrac12c\varepsilon_0E_0^2 \)
(b) \( I=c\varepsilon_0E_0^2 \)
(c) \( I=\frac{E_0^2}{2c\varepsilon_0} \)
(d) \( I=\frac{cB_0^2}{\mu_0} \)
Show Answer & Explanation
Answer: (a) \( I=\tfrac12c\varepsilon_0E_0^2 \)
Explanation:
1. The average energy density is (1/2)ε₀E0² (electric and magnetic parts are equal).
2. I = u c = (1/2) c ε₀ E0² = c B0²/(2μ₀).
Teacher's Note:
1. I = u c.
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Multiple Choice Questions (MCQs) for JEE Physics Electromagnetic Waves
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