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DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET PDF Download

Introductory Exercise 20.1

Q.1. In the Bohr model of the hydrogen atom, the electron is pictured to rotate in a circular orbit of radius 5 × 10-11 m, at a speed of 2.2 × 106 m/s. What is the current associated with electron motion?
Ans.

DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET
DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET
= 1.12 × 10-3A = 1.12mA

Q.2. Copper has one conduction electron per atom. Its density is 8.89 g/cm3 and its atomic mass is 63.54 g/mol. If a copper wire of diameter 1.0 mm carries a current of 2.0 A. What is the drift speed of the electrons in the wire?
Ans.

ρ = 8.89 × 103 kg/m3
Mass of 1 m3 = 8.89 × 103 kg= 8.89 × 103 kg
∴ Number of gram moles
DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET
Number of atoms = 1.4 × 105 × 6.02 × 1023
= 8.42 × 1028
One atom emits one conduction electron. Therefore number of free electrons in unit volume (or lm3 volume)
n = 8.42 × 1028 per m3
Now i = neA vd
DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET
DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET
= 1.9 × 10-4m/s

Q.3. When a steady current passes through a cylindrical conductor, is there an electric field inside the conductor?

Ans. Under electrostatic conditions (when no current flows), E = 0. When current is non-zero, then electric field is also non-zero.

Q.4. Electrons in a conductor have no motion in the absence of a potential difference across it. Is this statement true or false?

Ans. False, there is random or thermal motion of free electrons in the absence of potential difference.

Q.5. In an electrolyte, the positive ions move from left to right and the negative ions from right to left. Is there a net current? If yes, in what direction?

Ans. Current due to both is from left to right. So, the two current are additive.

Q.6. The current through a wire depends on time as, i = (10 + 4t)
Here, i is in ampere and t in seconds. Find the charge crossed through a section in time interval between t = 0 to t = 10 s.

Ans. The resistance of a copper wire and an iron wire at 20°C are 4.1 Ω. and 3.9 Ω respectively. Neglecting any thermal expansion, find the temperature at which resistances of both are equal.
DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET 

Introductory Exercise

 20.2

Q.1. In household wiring, copper wire 2.05 mm in diameter is often used. Find the resistance of a 35.0 m long wire. Specific resistance of copper is 1.72 ×10-8 Ω-m.

Ans. DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET
= 0.18 Ω

Q.2. An aluminium wire carrying a current has diameter 0.84 mm. The electric field in the wire is 0.49 V/m. What is,
(a) the current carried by the wire?
(b) the potential difference between two points in the wire 12.0 m apart?
(c) the resistance of a 12.0 m length of this wire? Specific resistance of aluminium is 2.75 × 10-8 Ω-m.

Ans. 

(a) In 1 m, potentials difference,
V = 0.49
V = iR
DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET
DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET
= 9.9 A
(b) PD between two points, 12 m apart = (0.49 V/m) (12m) = 5.88 V
DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET
DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET

Q.3. A conductor of length l has a non-uniform cross-section. The radius of the cross-section varies linearly from a to b. The resistivity of the material is ρ. Find the resistance of the conductor across its ends.
DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET

Ans.

Radius at distance x from end P,
DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET
DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET
Resistance of element of thickness dx is
DC Pandey Solutions: Current Electricity | Physics Class 12 - NEETDC Pandey Solutions: Current Electricity | Physics Class 12 - NEET
DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET

Q.4. The product of resistivity and conductivity of a conductor is constant. Is this statement true or false?

Ans. DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET
ρσ = constant

Q.5. The resistance of a copper wire and an iron wire at 20°C are 4.1 Ω. and 3.9 Ω respectively. Neglecting any thermal expansion, find the temperature at which resistances of both are equal.
αCu = 4.0 × 10-3K-1. and αFe= 5.0 × 10-3 K-1.

Ans. 4.1[1 + 4.0 × 10-3(θ - 20)]
= 3.9[1 + 5.0 × 10-3(θ - 20)]
Solving we get,
θ = 85°C

Introductory Exercise

 20.3
Q.1. Find the current through 2Ω and 4Ω resistance.
DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET

Ans. PD across each resistance is 10V.
DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET

Q.2. In the circuit shown in figure, find the potentials of A, B, C and D and the current through 1Ω and 2Ω resistance.
DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET

Ans. VA = 0V (as it is earthed)
V- VA = 5W
KC = 5V V- VA = 2V
VB = 2V V- VC = 10V
V= 10 + VC = 15 V
DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET= 3A from C to B as K> K
DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET = 7.5A from D to A as VD > VA

Q.3. For what value of E the potential of A is equal to the potential of B?
DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET

Ans. FA = FB
VAB = 0 or E - ir = 0
DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET
Solving this equation we get E = 5V

Q.4. Ten cells each of emf 1V and internal resistance 1Ω. are connected in series. In this arrangement polarity of two cells is reversed and the system is connected to an external resistance of 2Ω . Find the current in the circuit.

Ans. Net emf = (n - 2m)E
= (10 - 2 × 2) (1) = 6 V
DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET

Q.5. In the circuit shown in figure, R1 = R2 = R3 = 10Ω. Find the currents through R1 and R2
DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET

Ans.
DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET 

Q.6. Draw:
(a) current versus load, and
(b) current versus potential difference graph for a cell.

Ans. 


(a) DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET
i versus R graph is shown in answer.
DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET
(b) V =E - ir
V versus i graph is shown in answer

DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET



Introductory Exercise 20.4

Ques 1: In the circuit shown in figure, a 12 V power supply with unknown internal resistance r is connected to a battery with unknown emf. E and internal resistance 1Ω  and to a resistance of 3Ω . carrying a current of 2 A. The current through the rechargeable battery is 1 A in the direction shown. Find the unknown current i, internal resistance r and the emf E.
DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET
Ans: 
Applying loop law equation in upper loop we have,
E + 12 - ir - 1 = 0 ...(i)
Applying loop law equation in lower loop we have where i = 1 + 2 = 3A
E + 6 - 1 = 0 ...(ii)
Solving these two equations we get, E = -5Vand r = 2Ω

Ques 2: In the above example, find the power delivered by the 12 V power supply and the power dissipated in 3 W resistor.
Ans: 
Power delivered by a battery = Ei
= 12 × 3 = 36 W
Power dissipated in resistance = i2R
= (3) (2)2 = 12W

Introductory Exercise 20.5

Ques 1: Find the equivalent emf and internal resistance of the arrangement shown in Fig.
DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET
Ans: 

DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET
r = 0.5 W

Ques 2: If a battery of emf E and internal resistance r is connected across a load of resistance R. Show that the rate at which energy is dissipated in R is maximum when R = r and this maximum power is P = E2/4r.
Ans:

DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET
P = power across R = i2R
DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET
For power to be maximum,
DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET
By putting weDC Pandey Solutions: Current Electricity | Physics Class 12 - NEET get, R = r
Further, by putting R = r in Eq. (i)
we get,DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET

Ques 3: Two identical batteries each of emf E = 2 volt and internal resistance r =1 ohm are available to produce heat in an external resistance by passing a current through it. What is the maximum power that can be developed across an external resistance R using these batteries.
Ans: 
As derived in the above question,
DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET
Here, E = net emf = 2 + 2 = 4 V
and r= net internal resistance
= 1 + 1 = 2Ω
DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET

Ques 4: The full scale deflection current of a galvanometer of resistance 1Ω is 5 mA. How will you convert it into a voltmeter of range 5 V ?
Ans: 
V = ig(G + R)
DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET = series resistance connected with galvanometer
DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET

Ques 5: A micrometer has a resistance of 100Ω and full scale deflection current of 50μA. How can it be made to work as an ammeter of range 5 mA ?
Ans:

DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET
DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET

Ques 6: A voltmeter has a resistance G and range V. Calculate the resistance to be used in series with it to extend its range to nV.
Ans: 
V = igG
DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET
Now, nV = ig(G + R) = V/G (G + R) R = (n - 1)G

Ques 7: The potentiometer wire AB is 600 cm long.
(a) At what distance from A should the jockey J touch the wire to get zero deflection in the galvanometer.
(b) If the jockey touches the wire at a distance 560 cm from A, what will be the current through the galvanometer.
DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET
Ans: 
(a)
DC Pandey Solutions: Current Electricity | Physics Class 12 - NEETor emf of lower battery
DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET
Solving this equation we get,
l = 320 cm

(b) Resistance of 560 cm =DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET
= 14r Now the circuit is as under,
DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET
Applying loop law in upper loop we have,
E - 14r(i1 - i2) - i1r - i1r = 0 ...(i)
Applying loop law in lower law loop we have,
-E/2 - i2r + (i:- i2)(14r) = 0 ...(ii)
Solving these two equations we getDC Pandey Solutions: Current Electricity | Physics Class 12 - NEET

Introductory Exercise 20.6

Ques 1: For the given carbon resistor, let the first strip be yellow, second strip be red, third strip be orange and fourth be gold. What is its resistance?
Ans: 
Yellow → 4
Red → 2
Orange → 10
Gold → 5
R = (4.2 × 103 + 5%) Ω

Ques 2: The resistance of the given carbon resistor is (2.4 ×106Ω ± 5%)Ω . What is the sequence of colours on the strips provided on resistor?
Ans: 2 → Red
4 → Yellow
106 → Blue
5% → Gold

The document DC Pandey Solutions: Current Electricity | Physics Class 12 - NEET is a part of the NEET Course Physics Class 12.
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FAQs on DC Pandey Solutions: Current Electricity - Physics Class 12 - NEET

1. What are the important topics covered in DC Pandey Solutions: Current Electricity?
Ans. The important topics covered in DC Pandey Solutions: Current Electricity include Ohm's Law, Kirchhoff's Laws, Resistors in Series and Parallel, Wheatstone Bridge, Potentiometer, and Electric Power and Energy.
2. How can DC Pandey Solutions: Current Electricity help in exam preparation?
Ans. DC Pandey Solutions: Current Electricity provides detailed step-by-step solutions to the questions from the textbook. These solutions help in understanding the concepts and solving similar problems in the exams.
3. Are the solutions in DC Pandey Solutions: Current Electricity easy to understand?
Ans. Yes, the solutions in DC Pandey Solutions: Current Electricity are written in a simple and easy-to-understand language. They are designed to help students grasp the concepts easily and apply them effectively in problem-solving.
4. Are there any solved examples in DC Pandey Solutions: Current Electricity?
Ans. Yes, DC Pandey Solutions: Current Electricity includes numerous solved examples that illustrate the application of concepts and techniques. These examples provide a clear understanding of the topics and help in strengthening problem-solving skills.
5. Can DC Pandey Solutions: Current Electricity be used as a reference for competitive exams?
Ans. Yes, DC Pandey Solutions: Current Electricity is a comprehensive resource that covers all the important topics related to current electricity. It can be used as a reference for competitive exams like JEE, NEET, and other engineering entrance exams.
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