NCERT Class 12 Physics solution Manual

January 24, 2016 NCERT PHYSICS CLASS 12 SOLUTION MANUAL

Srikanth K S All rights reserved Page 1

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Contents NCERT Solutions for Class 12th Physics Chapter 1 – Electric Charges and Fields ................................... 2

NCERT Solutions for Class 12th Physics Chapter 2 – Electrostatic Potential and Capacitance ............. 33

NCERT Solutions for Class 12th Physics Chapter 3 – Current Electricity ............................................... 62

NCERT Solutions for Class 12th Physics Chapter 4 – Moving Charges and Magnetism ........................ 89

NCERT Solutions for Class 12th Physics Chapter 5 – Magnetism and Matter .................................... 117

NCERT Solutions for Class 12th Physics Chapter 6 – Electromagnetic Induction ............................... 140

NCERT Solutions for Class 12th Physics Chapter 7 – Alternating Current .......................................... 160

NCERT Solutions for Class 12th Physics Chapter 8 – Electromagnetic Waves .................................... 190

NCERT Solutions for Class 12th Physics Chapter 9 - Ray Optics And Optical Instruments ................. 205

NCERT Solutions for Class 12th Physics Chapter 10 – Wave Optics ................................................... 259

NCERT Solutions for Class 12th Physics Chapter 11 – Dual Nature Of Radiation And Matter ........... 277

NCERT Solutions for Class 12th Physics Chapter 12 – Atoms ............................................................. 321

NCERT Solutions for Class 12th Physics Chapter 13 – Nuclei .............................................................. 341

NCERT Solutions for Class 12th Physics Chapter 14 – Semiconductor Electronics Materials Devices

And Simple Circuits ............................................................................................................................. 378

NCERT Solutions for Class 12th Physics Chapter 15 – Communication Systems ............................... 393



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NCERT Solutions for Class 12th Physics Chapter 1 – Electric Charges

and Fields



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NCERT Solutions for Class 12th Physics Chapter 2 – Electrostatic

Potential and Capacitance



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NCERT Solutions for Class 12th Physics Chapter 3 – Current Electricity



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NCERT Solutions for Class 12th Physics Chapter 4 – Moving Charges

and Magnetism



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NCERT Solutions for Class 12th Physics Chapter 5 – Magnetism and

Matter

Question 5.1:

Answer the following questions regarding earth’s magnetism:

(a) A vector needs three quantities for its specification. Name the three independent quantities

conventionally used to specify the earth’s magnetic field.

(b) The angle of dip at a location in southern India is about 18º. Would you expect a greater or smaller

dip angle in Britain?

(c) If you made a map of magnetic field lines at Melbourne in Australia, would the lines seem to go

into the ground or come out of the ground?

(d) In which direction would a compass free to move in the vertical plane point to, if located right on

the geomagnetic north or south pole?

(e) The earth’s field, it is claimed, roughly approximates the field due to a dipole of magnetic moment

8 × 1022 J T−1 located at its centre. Check the order of magnitude of this number in some way.

(f ) Geologists claim that besides the main magnetic N-S poles, there are several local poles on the

earth’s surface oriented in different directions. How is such a thing possible at all?

Answer

(a) The three independent quantities conventionally used for specifying earth’s magnetic field are:

(i) Magnetic declination,

(ii) Angle of dip, and

(iii) Horizontal component of earth’s magnetic field

(b)The angle of dip at a point depends on how far the point is located with respect to the North Pole or

the South Pole. The angle of dip would be greater in Britain (it is about 70°) than in southern India

because the location of Britain on the globe is closer to the magnetic North Pole.

(c)It is hypothetically considered that a huge bar magnet is dipped inside earth with its north pole near

the geographic South Pole and its south pole near the geographic North Pole.

Magnetic field lines emanate from a magnetic north pole and terminate at a magnetic south pole.

Hence, in a map depicting earth’s magnetic field lines, the field lines at Melbourne, Australia would

seem to come out of the ground.



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NCERT Solutions for Class 12th Physics Chapter 6 – Electromagnetic

Induction



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Read as 2 PI B0 a2 Lambda

NCERT Solutions for Class 12th Physics Chapter 7 – Alternating

Current



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NCERT Solutions for Class 12th Physics Chapter 8 – Electromagnetic

Waves

Question 8.1: Figure 8.6 shows a capacitor made of two circular plates each of radius 12 cm, and separated by 5.0 cm. The capacitor is being charged by an external source (not shown in the figure). The charging current is constant and equal to 0.15 A. (a) Calculate the capacitance and the rate of charge of potential difference between the plates. (b) Obtain the displacement current across the plates. (c) Is Kirchhoff’s first rule (junction rule) valid at each plate of the capacitor? Explain.



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NCERT Solutions for Class 12th Physics Chapter 9 - Ray Optics And

Optical Instruments



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Question 9.1: A small candle, 2.5 cm in size is placed at

27 cm in front of a concave mirror of radius of curvature 36 cm. At what

distance from the mirror should a screen be placed in order to obtain a sharp

image? Describe the nature and size of the image.<BR>If the candle is moved

closer to the mirror, how would the screen have to be

moved?

Answer Size of the candle, h = 2.5 cm Image size = h’ Object distance, u = -27 cm Radius of curvature

of the concave mirror, R = -36 cm



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NCERT Solutions for Class 12th Physics Chapter 10 – Wave Optics

Question 10.1: Monochromatic light of wavelength 589 nm is incident from air on a water surface. What are the wavelength, frequency and speed of (a) reflected, and (b) refracted light? Refractive index of water is 1.33.



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NCERT Solutions for Class 12th Physics Chapter 11 – Dual Nature Of

Radiation And Matter Question 11.1: Find the (a) maximum frequency, and (b) minimum wavelength of X-rays produced by 30 kV electrons.



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NCERT Solutions for Class 12th Physics Chapter 12 – Atoms Question 12.1: Choose the correct alternative from the clues given at the end of the each statement: (a) The size of the atom in Thomson’s model is ………. the atomic size in Rutherford’s model. (much greater than/no different from/much less than.) (b) In the ground state of ………. electrons are in stable equilibrium, while in ………. electrons always experience a net force. (Thomson’s model/ Rutherford’s model.) (c) A classical atom based on ………. is doomed to collapse. (Thomson’s model/ Rutherford’s model.) (d) An atom has a nearly continuous mass distribution in a ………. but has a highly nonuniform mass distribution in ………. (Thomson’s model/ Rutherford’s model.) (e) The positively charged part of the atom possesses most of the mass in ………. (Rutherford’s model/both the models.) (a) The sizes of the atoms taken in Thomson’s model and Rutherford’s model have the same order of magnitude. (b) In the ground state of Thomson’s model, the electrons are in stable equilibrium. However, in Rutherford’s model, the electrons always experience a net force. (c) A classical atom based on Rutherford’s model is doomed to collapse. (d) An atom has a nearly continuous mass distribution in Thomson’s model, but has a highly non-uniform mass distribution in Rutherford’s model. (e) The positively charged part of the atom possesses most of the mass in both the models



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NCERT Solutions for Class 12th Physics Chapter 13 – Nuclei



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NCERT Solutions for Class 12th Physics Chapter 14 – Semiconductor

Electronics Materials Devices And Simple Circuits

Question 14.1: In an n-type silicon, which of the following statement is true: (a) Electrons are majority carriers and trivalent atoms are the dopants. (b) Electrons are minority carriers and pentavalent atoms are the dopants. (c) Holes are minority carriers and pentavalent atoms are the dopants. (d) Holes are majority carriers and trivalent atoms are the dopants. Answer The correct statement is (c). In an n-type silicon, the electrons are the majority carriers, while the holes are the minority carriers. An n-type semiconductor is obtained when pentavalent atoms, such as phosphorus, are doped in silicon atoms. Question 14.2: Which of the statements given in Exercise 14.1 is true for p-type semiconductors. Answer



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The correct statement is (d). In a p-type semiconductor, the holes are the majority carriers, while the electrons are the minority carriers. A p-type semiconductor is obtained when trivalent atoms, such as aluminium, are doped in silicon atoms.



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NCERT Solutions for Class 12th Physics Chapter 15 – Communication

Systems

Question 15.1:

Which of the following frequencies will be suitable for beyond-the-horizon communication

using sky waves?

(a) 10 kHz

(b) 10 MHz

(c) 1 GHz

(d) 1000 GHz

Answer

(b) Answer:

10 MHz

For beyond-the-horizon communication, it is necessary for the signal waves to travel a large distance.

10 KHz signals cannot be radiated efficiently because of the antenna size.

The high energy signal waves (1GHz − 1000 GHz) penetrate the ionosphere. 10 MHz frequencies get

reflected easily from the ionosphere. Hence, signal waves of such frequencies are suitable for beyond-

the-horizon communication.



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Question 15.2: Frequencies in the UHF range normally propagate by means of:

(a) Ground waves.

(b) Sky waves.

(c) Surface waves.

(d) Space waves.

Answer (d) Answer:

Space waves

Owing to its high frequency, an ultra high frequency (UHF) wave can neither travel along the

trajectory of the ground nor can it get reflected by the ionosphere. The signals having UHF

are propagated through line-of-sight communication, which is nothing but space wave

propagation.

Question 15.3: Digital signals

(i) Do not provide a continuous set of values,

(ii) Represent values as discrete steps,

(iii) Can utilize binary system, and

(iv) Can utilize decimal as well as binary systems.

Which of the above statements are true?

(a) (i) and (ii) only

(b) (ii) and (iii) only

(c) (i), (ii) and (iii) but not (iv)

(d) All of (i), (ii), (iii) and (iv).

Answer (c) Answer:

A digital signal uses the binary (0 and 1) system for transferring message signals. Such a

system cannot utilise the decimal system (which corresponds to analogue signals).

Digital signals represent discontinuous values.

Question 15.4:

Is it necessary for a transmitting antenna to be at the same height as that of the receiving antenna for

line-of-sight communication? A TV transmitting antenna is 81m tall. How much service area can it

cover if the receiving antenna is at the ground level?

Answer

Line-of-sight communication means that there is no physical obstruction between the transmitter and

the receiver. In such communications it is not necessary for the transmitting and receiving antennas to

be at the same height.

Height of the given antenna, h = 81 m

Radius of earth, R = 6.4 × 10^6 m

For range, d = 2Rh, the service area of the antenna is given by the relation:

A = πd2

= π (2Rh)

= 3.14 × 2 × 6.4 × 10^6 × 81

= 3255.55 × 10^6 m^2

= 3255.55



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Education

NCERT Class 12 Physics solution Manual