Tiêu đề 23.Electromagnetic Waves.pdf ✅

NEET-2022 Ultimate Crash Course PHYSICS Electromagnetic Waves
Topic-wise analysis of NEET 2014-2021 Topic name/ year 2014 2015 2016 2017 2018 2019 2019 (Orissa) 2020 (Covid-19) 2021 T1:Electromagnetic Waves and Displacement Current 1 1 1 1 1 1 1 3 1 T2: Electro magnetic Spectrum 1 NEET 8 Years Previous Questions 1. An em wave is propagating in a medium with a velocity V Vi ˆ = . The instantaneous oscillating electric field of this em wave is along +y axis. Then the direction of oscillating magnetic field of the em wave will be along [2018] (1) –z direction (2) +z direction (3) –x direction (4) –y direction 2. In an electromagnetic wave in free space the root mean square value of the electric field is Erms = 6V/m. The peak value of the magnetic field is :- [2017] (1) 2.83 × 10–8 T (2) 0.70 × 10–8 T (3) 4.23 × 10–8 T (4) 1.41 × 10–8 T 3. Out of the following options which one can be used to produce a propagating electromagnetic wave ? [2016] (1) A charge moving at constant velocity (2) A stationary charge (3) A chargeless particle (4) An accelerating charge 4. A radiation of energy ‘E’ falls normally on a perfectly reflecting surface. The momentum transferred to the surface is (C = Velocity of light) [2015] (1) 2E C (2) 2 2E C (3) 2 E C (4) E C 5. The energy of the em waves is of the order of 15 keV. To which part of the spectrum does it belong? [2015] (1) Infra-red rays (2) Ultraviolet rays (3) g-rays (4) X-rays 6. Light with an energy flux of 25 × 104 Wm–2 falls on a perfectly reflecting surface at normal incidence. If the surface area is 15 cm2 , the average force exerted on the surface is: [2014] (1) 1.25 × 10– 6 N (2) 2.50 × 10– 6 N (3) 1.20 × 10– 6 N (4) 3.0 × 10– 6 N 7. A parallel plate capacitor of capacitance 20F is being charged by a voltage source whose potential is changing at the rate of 3 V/s. The conduction current through the connecting wires, and the displacement current through the plates of the capacitor, would be, respectively : [NEET – 2019] (1) zero, 60 A (2) 60 mA, 60 A (3) 60 A , zero (4) zero, zero 8. For a transparent medium relative permeability and permittivity, μr and  r are 1.0 and 1.44 respectively. The velocity of light in this medium would be, [NEET – 2019 (ODISSA)] (1) 2.5 × 108 m/s (2) 3 × 108 m/s (3) 2.08 × 108 m/s (4) 4.32 × 108 m/s 9. The E.M. wave with shortest wavelength among the following is [NEET – 2020 (Covid-19)] (1) Ultraviolet rays (2) X-rays (3) Gamma-rays (4) Microwaves 10. The magnetic field in a plane electromagnetic wave is given by : [NEET – 2020 (Covid-19)] ( ) 7 3 11 B 2 10 sin 10 x 3 10 t T y − =   +  Calculate the wavelength. (1) × 103 m (2) 2 × 10–3 m (3) 2 × 103 m (4) × 10–3 m
11. The ratio of contributions made by the electric field and magnetic field components to the intensity of an electromagnetic waves is : (c= speed of electromagnetic waves) [NEET – 2020 (Covid-19)] 1) 1 : c2 2) c : 1 3) 1 : 1 4) 1 : c 12. For a plane electromagnetic wave propagating in x-direction, which one of the following combination gives the correct possible direction for electric field I and magnetic field (2) respectively? [NEET-2021] 1) − + − − j k j k , 2) j k j k + − − , 3) − + − + j k j k , 4) j k j k + + , Important Points to Remember 1. Displacement current can produce magnetic field. 2. Displacement current is set up in a region where the electric field is changing with time. 3. During charging of a capacitor, the conduction current in a wire connected to a capacitor is equal to the displacement current between the plates of capacitor. 4. Electromagnetic wave carries both energy and momentum. Electromagnetic waves carry momentum and hence can exert pressure (P) on surfaces. 5. The electric field vector E and the magnetic field vector B in an electromagnetic wave vary sinusoidally and always in the same phase. They attain their peak values E0, B0 at the same place at the same time. 6. Electromagnetic waves can be produced by oscillating charge or by transition of electron from higher energy level to lower energy level in an atom. 7. Electromagnetic waves of different frequencies travels with same velocity in vacuum (= 3 × 108 m/s). 8. The energy of electromagnetic wave is equally divided between electric and magnetic field vector. 9. Velocity of electromagnetic wave is independent of amplitude of field vectors. 10. The electromagnetic spectrum is the orderly arrangement of EM wave, the range of spectrum varies from 10–12m to 104m i.e., from g-rays to radio waves. LEVEL-1 Topic 1: Electromagnetic Waves and Displacement Current 1. An electric charge oscillating with a frequency of 1 kilo cycles/second can radiate electro-magnetic waves of wavelength (1) 100 km (2) 200 km (3) 300 km (4) 400 km 2. Displacement current goes through the gap between the plates of a capacitor when the charge of the capacitor (1) increases (2) decreases (3) is zero (4) remains unchanged 3. The speed of electromagnetic wave is same for (1) odd frequencies (2) even frequencies (3) all frequencies (4) all intensities 4. The electromagnetic waves (1) travel with the speed of sound (2) travel with the same speed in all media (3) travel in free space with the speed of light (4) do not travel through a medium 5. The figure here gives the electric field of an electromagnetic wave at a certain point and a certain instant. The wave is transporting energy in the negative z-direction. The direction of the magnetic field of the wave at that point and instant is (1) + ve x-direction (2) –ve x-direction (3) +ve z-direction
(4) –ve y-direction 6. Displacement current flows (1) in between capacitor plates and wires of circuit (2) in between capacitor plates only (3) in resistance only (4) depends on emf of cell and value of R 7. If E and B are the electric and magnetic field vectors of e.m. waves then the direction of propagation of e.m. wave is along the direction of (1) E (2) B (3) E B (4) None of these 8. The figure shows graphs of the electric field magnitude E versus time t for four uniform electric fields, all contained within identical circular regions. Which of them is according to the magnitudes of the magnetic field? (1) A (2) B (3) C (4) D 9. An electromagnetic wave of intensity I falls on a surface kept in vacuum and exerts radiation pressure P on it. Which of the following statements are true? I. Radiation pressure is I/c if the wave is totally absorbed. II. Radiation pressure is I/c if the wave is totally reflected. III. Radiation pressure is 2 I/c if the wave is totally reflected. (1) I and II (2) I and III (3) III only (4) I, II and III 10. If a source is transmitting electromagnetic wave of frequency 8.2 × 106 Hz, then wavelength of the electromagnetic waves transmitted from the source will be (1) 36.6 m (2) 40.5 m (3) 42.3 m (4) 50.9 m 11. Figure shows a parallel plate capacitor and the current in the connecting wires that is discharging the capacitor. (1) The displacement current is leftward. (2) The displacement current is rightward (3) The electric field E r is rightward (4) The magnetic field at point P is out the page. 12. A plane electromagnetic wave is incident on a material surface. If the wave delivers momentum p and energy E, then (1) p = 0, E = 0 (2) p  0, E  0 (3) p  0, E = 0 (4) p = 0, E  0 13. The electromagnetic waves do not transport (1) energy (2) charge (3) momentum (4) information 14. In electromagnetic wave, the average energy density is associated (1) with electric field only (2) with magnetic field only (3) equally with electric and magnetic fields (4) None of these 15. An electromagnetic wave passes through space and its equation is given by E = E0sin ( t – kx) where E is electric field. Energy density of electromagnetic wave in space is (1) 2 0 0 1 E 2  (2) 2 0 0 1 E 4  (3) 2 0 0 E (4) 2 0 0 2 E 