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NCERT Exemplar Class 12 Physics Solutions Chapter 3 Current Electricity

NCERT Exemplar Class 12 Physics Solutions Chapter 3 Current Electricity

Edited By Safeer PP | Updated on Sep 14, 2022 12:28 PM IST | #CBSE Class 12th

NCERT Exemplar Class 12 Physics solutions chapter 3 is from the point of view of exams. It also gives students an insight on how current flows in working of different types of devices in real life such as conductors. The previous chapters dealt with charges that were considered as at rest, whereas this chapter introduces various concepts relating to steady currents. NCERT Exemplar Class 12 Physics chapter 3 solutions includes the idea of electric energy and power, electric resistivity, and conductivity, along with a detailed explanation of carbon resistors and their colour codes. The concepts discussed in the NCERT Exemplar Class 12 Physics solutions chapter 3 are important from the point of view of board exams as the JEE Main exam. The students can also make use of NCERT Exemplar Class 12 Physics solutions chapter 3 PDF download, for further learning.

Also see - NCERT Solutions for Class 12 Physics

NCERT Exemplar Class 12 Physics Solutions Chapter 3 MCQI

Question:1

Consider a current carrying wire (current I) in the shape of a circle. Note that as the current progresses along the wire, the direction of j (current density) changes in an exact manner, while the current I remain unaffected. The agent that is essentially responsible for is
A. source of emf.
B. electric field produced by charges accumulated on the surface of wire.
C. the charges just behind a given segment of wire which push them just the right way by repulsion.
D. the charges ahead.

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Answer:

b)
Current density (J) is the current per unit area and is given by \overrightarrow{J}=\sigma\overrightarrow{ E}, where conductivity \left (\sigma =\frac{l}{AR} \right ) and electric field \left ( \overrightarrow{E} \right ).

Question:2

Two batteries of emf \varepsilon _{1} and \varepsilon _{2}\left ( \varepsilon_ {2}>\varepsilon _{1} \right )and internal resistances r1 and r2 respectively are connected in parallel as shown in Figure.

capture-32
A. The equivalent emf \varepsilon _{eq} of the two cells is between \varepsilon _{1} and \varepsilon _{2}, i.e. \varepsilon _{1}<\varepsilon _{eq}<\varepsilon _{2}
B. The equivalent emf \varepsilon _{eq} is smaller than 1.
C. The \varepsilon _{eq} is given by \varepsilon _{eq}=1+2always.
D. \varepsilon _{eq} is independent of internal resistances r1 and r2.

Answer:

The correct answer is the option (a)
Explanation: Equivalent EMF in parallel is given as:
\varepsilon _{eq}=\frac{\varepsilon _{1}r_{2}+\varepsilon _{2}r_{1}}{r_{1}+r_{2}}
This value will always lie between \varepsilon _{1} and \varepsilon _{2}

Question:3

A resistance R is to be measured using a meter bridge. Student chooses the standard resistance S to be 100. He finds the null point at l_1 = 2.9 cm. He is told to attempt to improve the accuracy. Which of the following is a useful way?

A. He should measure l1 more accurately.
B. He should change S to 1000and repeat the experiment.
C. He should change S to 3 and repeat the experiment.
D. He should give up hope of a more accurate measurement with a meter bridge.

Answer:

The correct answer is the option (c)
Explanation:
R=S\left (\frac{l_1}{100-l_1} \right )=100\left (\frac{2.9}{97.1} \right )=2.98\Omega
He should aim to get the balance point near the centre (l1=50 cm)
2.98\Omega =S\left (\frac{50}{50} \right )
S\cong 3\Omega

Question:4

Two cells of emf’s approximately 5V and 10V are to be accurately compared using a potentiometer of length 400cm.
A. The battery that runs the potentiometer should have a voltage of 8V.
B. The battery of potentiometer can have a voltage of 15V and R adjusted so that the potential drop across the wire slightly exceeds 10V.
C. The first portion of 50 cm of the wire itself should have a potential drop of 10V.
D. Potentiometer is usually used for comparing resistances and not voltages.

Answer:

The correct answer is the option (b)
Explanation: The potential drop along the wires of the potentiometer should be greater than the emfs of cells. The battery of potentiometer must be in excess of 10V.

Question:5

A metal rod of length 10 cm and a rectangular cross-section of 1cm \times \frac{1}{2} cm is connected to a battery across opposite faces. The resistance will be
A. maximum when the battery is connected across 1cm \times \frac{1}{2} cm faces.
B. maximum when the battery is connected across 10cm \times 1 cm faces.
C. maximum when the battery is connected across 10cm \times \frac{1}{2} cmfaces.
D. same irrespective of the three faces.

Answer:

The correct answer is the option (a)
Explanation:R=\rho\frac{l}{A}
Resistance is directly proportional to length and inversely proportional to the cross-sectional area. Resistance will be maximum when l is maximum and A minimum. Both these conditions are met simultaneously when the battery is connected across the 1cm \times \frac{1}{2} cm faces.

Question:6

Which of the following characteristics of electrons determines the current in a conductor?
A. Drift velocity alone.
B. Thermal velocity alone.
C. Both drift velocity and thermal velocity.
D. Neither drift nor thermal velocity

Answer:

The correct answer is the option (a)
I=Anev_d

NCERT Exemplar Class 12 Physics Solutions Chapter 3 MCQII

Question:7

Kirchhoff ’s junction rule is a reflection of
A. conservation of current density vector.
B. conservation of charge.
C. the fact that the momentum with which a charged particle approaches a junction is unchanged (as a vector) as the charged particle leaves the junction.
D. the fact that there is no accumulation of charges at a junction.

Answer:

The correct answers are the options (b,d)
Explanation: According to the law of conservation of charges, the charge of a system will stay constant. From this, it can be derived that net charge flowing into a junction is equal to the net charge going out of the system.

Question:8

Consider a simple circuit shown in Figure capture-33 variable resistance R’. R’ can vary from R_0 to infinity. r is internal resistance of the battery (r<<R<<R_0).
capture-34
A. Potential drop across AB is nearly constant as R’ is varied.
B. Current through R is nearly a constant as R’ is varied.
C. Current I depends sensitively on R’.
D.I\geqslant \frac{V}{r+R} always.

Answer:

The correct answers are the options (a,d)
Explanation:
I=\frac{V}{r+R_{eq}}
As r+R<r+R_{eq}
I=\frac{V}{r+R_{eq}}\leq \frac{V}{r+R}

Question:9

Temperature dependence of resistivity \rho (T) of semiconductors, insulators and metals is significantly based on the following factors:
A. number of charge carriers can change with temperature T.
B. time interval between two successive collisions can depend on T.
C. length of material can be a function of T.
D. mass of carriers is a function of T.

Answer:

The correct answers are the options (a,b)
Explanation: Resistivity is a function of relaxation time(\tau ) and mass of charge carrier (m). Mass of charge carrier is independent of temperature, whereas length also does not fluctuate significantly over a temperature range.

Question:10

The measurement of an unknown resistance R is to be carried out using Wheat stones bridge (see Figure of NCERT Book). Two students perform an experiment in two ways. The first students take R_2 = 10\Omega and R_1 = 5\Omega. The other student takes R_2 = 1000\Omegaand R_1 = 500\Omega. In the standard arm, both take R_3= 5\Omega. Both find R=\frac{R_{2}}{R_{1}}R_{3}=10\Omega within errors.

A. The errors of measurement of the two students are the same.
B. Errors of measurement do depend on the accuracy with which R_{2} and R_{1} can be measured.
C. If the student uses large values of R_{2} and R_{1}, the currents through the arms will be feeble. This will make determination of null point accurately more difficult.
D. Wheatstone bridge is a very accurate instrument and has no errors of measurement.

Answer:

The correct answers are the options (b,c)
Explanation: Using large values of R_{1} and R_{2} , will reduce the current flowing through the system, making it difficult to get accurate readings from the galvanometer.

Question:11

In a meter bridge the point D is a neutral point (Figure).

capture-35
A. The meter bridge can have no other neutral point for this set of resistances.
B. When the jockey contacts a point on meter wire left of D, current flows to B from the wire.
C. When the jockey contacts a point on the meter wire to the right of D, current flows from B to the wire through galvanometer.
D. When R is increased, the neutral point shifts to left.

Answer:

The correct answers are the options (a,c)
Explanation: No current will flow through the Galvanometer when the jockey is placed at D. Potential Difference between B and D is zero. If the jockey is placed to the right of D, Potential at B becomes greater than the potential at D, the current flows from B to D.

NCERT Exemplar Class 12 Physics Solutions Chapter 3 Very Short Answer

Question:12

Is the momentum conserved when charge crosses a junction in an electric circuit? Why or why not?

Answer:

As drift velocity is proportional to the electric field, when charge crosses a junction the momentum is not conserved.

Question:14

What are the advantages of the null-point method in a Wheatstone bridge? What additional measurements would be required to calculate R_{unknown} by any other method?

Answer:

As no electricity will pass through the galvanometer of the Wheatstone bridge at the null point. It can be used to calculate the resistance of an unknown resistor.

Question:15

What is the advantage of using thick metallic strips to join wires in a potentiometer?

Answer:

Thick metallic strips have large cross section and thus offer less resistance

Question:16

For wiring in the home, one uses Cu wires or Al wires. What considerations are involved in this?

Answer:

The following considerations are made to decide on the wiring:
  • Conductivity of metal
  • Cost
  • Availability

Question:17

Why are alloys used for making standard resistance coils?

Answer:

Because of their low temperature sensitivity and low temperature coefficient of resistance, alloys are used to make standard resistance coils. Also, they have a lower conductivity or higher resistivity, this reduces the length required to make coils.

Question:18

Power P is to be delivered to a device via transmission cables having resistance R_C. If V is the voltage across R and I the current through it, find the power wasted and how can it be reduced.

Answer:

P (Power transmitted)=VI
H (Heat Loss)=I^{2}R
High current flowing through the devices will lead to higher Energy Loss (as heat), so power should be transmitted at a high voltage and low current.

Question:19

AB is a potentiometer wire (Figure). If the value of R is increased, in which direction will the balance point J shift?
capture-36

Answer:

On increasing the applied resistance, the potential difference across AB will decrease. To balance the potential across the other battery, the length AJ' should increase. Balance point shift towards B.

Question:20

While doing an experiment with potentiometer (Figure ) it was found that the deflection is one sided and (i) the deflection decreased while moving from one end A of the wire to the end B; (ii) the deflection increased. While the jockey was moved towards the end B.

capture-37
(i) Which terminal +or –ve of the cellE_{1}, is connected at X in case (i) and how is E_{1} related to E?
(ii) Which terminal of the cell E_{1} is connected at X in case (ii)?

Answer:

i) As jockey moves closer to B, the galvanometer potential difference decreases. This requires the positive terminal of E_{1} to be at X.
ii) Exactly opposite to (i), at X E_{1}^{'}s negative terminal is connected.

Question:21

A cell of emf E and internal resistance r is connected across an external resistance R. Plot a graph showing the variation of P.D. across R, verses R.

Answer:


I=\frac{E}{R+r} and V=IR
V=\frac{ER}{R+r}=\frac{E}{1+\frac{r}{}R}
An increase in R leads to an increase in V, till the point it approaches E
Following is the graphical representation:
capture-38

NCERT Exemplar Class 12 Physics Solutions Chapter 3 Long Answer

Question:28

Two cells of voltage 10V and 2V and internal resistances 10\Omega and 5\Omega respectively, are connected in parallel with the positive end of 10V battery connected to negative pole of 2V battery (Figure). Find the effective voltage and effective resistance of the combination.

capture-46

Answer:

capture-47
Kirchoff's Junction Law at C: I_1=I_2+I_3
Kirchoff's Loop Law for efbae:10-I_1\times 10-I -3\times R=0
10-10I_2-(R+10)I_3=0
Kirchoff's Loop Law for cbadc:- 2+5I_2-I_3R=0
(10-10I_2-(R+10)I_3)+(-2+5I_2-I_3R)\times 2=0
6-I_3R+10-2I_3R=0
6=3I_3R+10I_3
I_3=\frac{6-3I_3R}{10}
Veff & Reff will fulfil the following:
V_{eff}-I_3(R+R_{eff})=0
R_{eff}=\frac{1}{\frac{1}{10}+\frac{1}{15}}=\frac{10}{}3
V_{eff}-I_3\left (R+\frac{10}{}3 \right )=0
V_{eff}=I_3R+I_3\times \frac{10}{}3
V_{eff}=I_3R+\frac{6-3I_3R}{10}\times \frac{10}{}3=I_3R+6-\frac{3I_3R}{}3=2V


Question:31

(a) Consider the circuit in Figure. How much energy is absorbed by electrons from the initial state of no current (ignore thermal motion) to the state of drift velocity?

(b) Electrons give up energy at the rate of RI_2 per second to the thermal energy. What time scale would one associate with energy in problem (a)? n = no of electron/volume = 10^{29}/m^3, length of circuit = 10 cm, cross-section = A = (1mm)^2
capture-49

Answer:

a)
A=10^{-6}m^2
R=6\Omega
n=10^{29} m^{-3}
e=1.6\times 10^{-19}C
I=\frac{V}{}R=1A
m_e=9.1\times10^{-31}Kg
L=10^{-1}m
a) I=Anev_d
v_d=\frac{I}{Ane}=6.25\times 10^{-5}m/s
Kinetic energy=
\frac{1}{}2\times m_e\times v_d^2=2\times 10^{-17}J
b) Power loss=
I^2R=6W
P=\frac{E}{}t
t=\frac{E}{}P=10^{-17}s

NCERT Exemplar Class 12 Physics Solutions Chapter 3 Current Electricity Main Subtopics Covered

  • Introduction
  • Electric Current
  • Electric Currents in Conductors
  • OHM’s Law
  • Drift of Electrons and the Origin of Resistivity
  • Limitations of OHM’s Law
  • Resistivity of Various Materials
  • Temperature Dependence of Resistivity
  • Electrical Energy, Power
  • Combination of Resistors- Series and Parallel
  • Cells, EMF, Internal Resistance
  • Cells in Series and in Parallel
  • Kirchhoff’s Rules
  • Wheatstone Bridge
  • Meter Bridge
  • Potentiometer
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NCERT Exemplar Class 12 Physics Chapter Wise Links

Important Topics To Cover For Exams From NCERT Exemplar Class 12 Physics Solutions Chapter 3 Current Electricity

· NCERT Exemplar solutions for Class 12 Physics chapter 3 introduces new principles/laws as OHM’s law, Kirchhoff’s rule, etc., which display and govern the flow of current in various devices, and how it can be determined with the help of other factors.

· Students will study different circuits in which a number of resistors and cells are interconnected for which different calculations need to be done to obtain different values of current, resistance, etc., with the help of Class 12 Physics NCERT Exemplar solutions chapter 3

· NCERT Exemplar Class 12 Physics solutions chapter 3 also introduces different devices such as potentiometer, meter bridge and exhibits the methods, and formulas for understanding current, resistance, etc., through these devices.

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A block of mass 0.50 kg is moving with a speed of 2.00 ms-1 on a smooth surface. It strikes another mass of 1.00 kg and then they move together as a single body. The energy loss during the collision is

Option 1)

0.34\; J

Option 2)

0.16\; J

Option 3)

1.00\; J

Option 4)

0.67\; J

A person trying to lose weight by burning fat lifts a mass of 10 kg upto a height of 1 m 1000 times.  Assume that the potential energy lost each time he lowers the mass is dissipated.  How much fat will he use up considering the work done only when the weight is lifted up ?  Fat supplies 3.8×107 J of energy per kg which is converted to mechanical energy with a 20% efficiency rate.  Take g = 9.8 ms−2 :

Option 1)

2.45×10−3 kg

Option 2)

 6.45×10−3 kg

Option 3)

 9.89×10−3 kg

Option 4)

12.89×10−3 kg

 

An athlete in the olympic games covers a distance of 100 m in 10 s. His kinetic energy can be estimated to be in the range

Option 1)

2,000 \; J - 5,000\; J

Option 2)

200 \, \, J - 500 \, \, J

Option 3)

2\times 10^{5}J-3\times 10^{5}J

Option 4)

20,000 \, \, J - 50,000 \, \, J

A particle is projected at 600   to the horizontal with a kinetic energy K. The kinetic energy at the highest point

Option 1)

K/2\,

Option 2)

\; K\;

Option 3)

zero\;

Option 4)

K/4

In the reaction,

2Al_{(s)}+6HCL_{(aq)}\rightarrow 2Al^{3+}\, _{(aq)}+6Cl^{-}\, _{(aq)}+3H_{2(g)}

Option 1)

11.2\, L\, H_{2(g)}  at STP  is produced for every mole HCL_{(aq)}  consumed

Option 2)

6L\, HCl_{(aq)}  is consumed for ever 3L\, H_{2(g)}      produced

Option 3)

33.6 L\, H_{2(g)} is produced regardless of temperature and pressure for every mole Al that reacts

Option 4)

67.2\, L\, H_{2(g)} at STP is produced for every mole Al that reacts .

How many moles of magnesium phosphate, Mg_{3}(PO_{4})_{2} will contain 0.25 mole of oxygen atoms?

Option 1)

0.02

Option 2)

3.125 × 10-2

Option 3)

1.25 × 10-2

Option 4)

2.5 × 10-2

If we consider that 1/6, in place of 1/12, mass of carbon atom is taken to be the relative atomic mass unit, the mass of one mole of a substance will

Option 1)

decrease twice

Option 2)

increase two fold

Option 3)

remain unchanged

Option 4)

be a function of the molecular mass of the substance.

With increase of temperature, which of these changes?

Option 1)

Molality

Option 2)

Weight fraction of solute

Option 3)

Fraction of solute present in water

Option 4)

Mole fraction.

Number of atoms in 558.5 gram Fe (at. wt.of Fe = 55.85 g mol-1) is

Option 1)

twice that in 60 g carbon

Option 2)

6.023 × 1022

Option 3)

half that in 8 g He

Option 4)

558.5 × 6.023 × 1023

A pulley of radius 2 m is rotated about its axis by a force F = (20t - 5t2) newton (where t is measured in seconds) applied tangentially. If the moment of inertia of the pulley about its axis of rotation is 10 kg m2 , the number of rotations made by the pulley before its direction of motion if reversed, is

Option 1)

less than 3

Option 2)

more than 3 but less than 6

Option 3)

more than 6 but less than 9

Option 4)

more than 9

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