NCERT Solutions for Class 12 Physics Chapter 3 – Current Electricity

Current Electricity is a core and high-weightage chapter in Class 12 Physics that forms the foundation for understanding electric circuits. This chapter covers essential topics such as electric current, drift velocity, Ohm’s law, resistance and resistivity, temperature dependence of resistance, Kirchhoff’s laws, Wheatstone bridge, meter bridge, potentiometer, and electrical energy & power. These concepts are frequently tested in CBSE board exams and competitive exams like JEE and NEET.

NCERT Solutions for Class 12 Physics Chapter 3 – Current Electricity are prepared strictly according to the latest CBSE syllabus and exam pattern. The solutions are explained in simple, step-by-step language with clear derivations, circuit diagrams, and solved numericals, helping students strengthen conceptual clarity, practise effectively, and score well in board examinations.

NCERT Solutions for Class 12 Physics Chapter 3 – Current Electricity

Current Electricity
Medium
Q.
The storage battery of a car has an emf of 12 V. If the internal resistance of the battery is 0.4 Ω, what is the maximum current that can be drawn from the battery?
Current Electricity
Medium
Q.
In a potentiometer arrangement, a cell of emf 1.25 V gives a balance point at 35.0 cm length of the wire. If the cell is replaced by another cell and the balance point shifts to 63.0 cm, what is the emf of the second cell?
Current Electricity
Medium
Q.
Choose the correct alternative:

(a) Alloys of metals usually have (greater/less) resistivity than that of their constituent metals.
(b) Alloys usually have much (lower/higher) temperature coefficients of resistance than pure metals.
(c) The resistivity of the alloy manganin (is nearly independent of/increases rapidly) with increase of temperature.
(d) The resistivity of a typical insulator (e.g., amber) is greater than that of a metal by a factor of the order of (1022/1023).
Current Electricity
Medium
Q.
Answer the following questions:

(a) A steady current flows in a metallic conductor of non-uniform cross- section. Which of these quantities is constant along the conductor: current, current density, electric field, drift speed?
(b) Is Ohm’s law universally applicable for all conducting elements? If not, give examples of elements which do not obey Ohm’s law.
(c) A low voltage supply from which one needs high currents must have very low internal resistance. Why?
(d) A high tension (HT) supply of, say, 6 kV must have a very large internal resistance. Why?
Current Electricity
Medium
Q.
What conclusion can you draw from the following observations on a resistor made of alloy manganin?
Current Electricity
Medium
Q.
Two wires of equal length, one of aluminium and the other of copper have the same resistance. Which of the two wires is lighter? Hence explain why aluminium wires are preferred for overhead power cables. (ρAl = 2.63 × 10−8 Ω m, ρCu = 1.72 × 10−8 Ω m, Relative density of Al = 2.7, of Cu = 8.9.)
Current Electricity
Medium
Q.
(a) Six lead-acid type of secondary cells each of emf 2.0 V and internal resistance 0.015 Ω are joined in series to provide a supply to a resistance of 8.5 Ω. What are the current drawn from the supply and its terminal voltage?
(b) A secondary cell after long use has an emf of 1.9 V and a large internal resistance of 380 Ω. What maximum current can be drawn from the cell? Could the cell drive the starting motor of a car?
Current Electricity
Medium
Q.
The earth’s surface has a negative surface charge density of 10−9 Cm−2. The potential difference of 400 kV between the top of the atmosphere and the surface results (due to the low conductivity of the lower atmosphere) in a current of only 1800 A over the entire globe. If there were no mechanism of sustaining atmospheric electric field, how much time (roughly) would be required to neutralise the earth’s surface? (This never happens in practice because there is a mechanism to replenish electric charges, namely the continual thunderstorms and lightning in different parts of the globe). (Radius of earth = 6.37 × 106 m.)
Current Electricity
Medium
Q.
The number density of free electrons in a copper conductor estimated in Example 3.1 is 8.5 × 1028 m−3. How long does an electron take to drift from one end of a wire 3.0 m long to its other end? The area of cross- section of the wire is 2.0 × 10−6 m2 and it is carrying a current of 3.0 A.
Current Electricity
Medium
Q.
A storage battery of emf 8.0 V and internal resistance 0.5 Ω is being charged by a 120 V dc supply using a series resistor of 15.5 Ω. What is the terminal voltage of the battery during charging? What is the purpose of having a series resistor in the charging circuit?
Current Electricity
Medium
Q.
A battery of emf 10 V and internal resistance 3 Ω is connected to a resistor. If the current in the circuit is 0.5 A, what is the resistance of the resistor? What is the terminal voltage of the battery when the circuit is closed?
Current Electricity
Medium
Q.
(a) In a metre bridge [Fig. 3.27], the balance point is found to be at 39.5 cm from the end A, when the resistor Y is of 12.5 Ω. Determine the resistance of X. Why are the connections between resistors in a Wheatstone or meter bridge made of thick copper strips?
(b) Determine the balance point of the bridge above if X and Y are interchanged.
(c) What happens if the galvanometer and cell are interchanged at the balance point of the bridge? Would the galvanometer show any current?
Current Electricity
Medium
Q.
Determine the current in each branch of the network shown in fig 3.30:
Current Electricity
Medium
Q.
A heating element using nichrome connected to a 230 V supply draws an initial current of 3.2 A which settles after a few seconds to a steady value of 2.8 A. What is the steady temperature of the heating element if the room temperature is 27.0°C? Temperature coefficient of resistance of nichrome averaged over the temperature range involved is 1.70 × 10−4°C−1.
Current Electricity
Medium
Q.
A silver wire has a resistance of 2.1 Ω at 27.5°C, and a resistance of 2.7 Ω at 100°C. Determine the temperature coefficient of resistivity of silver.
Current Electricity
Medium
Q.
A negligibly small current is passed through a wire of length 15 m and uniform cross section 6.0 × 10−7 m2, and its resistance is measured to be 5.0 Ω.What is the resistivity of the material at the temperature of the experiment?
Current Electricity
Medium
Q.
At room temperature (27.0°C) the resistance of a heating element is 100 Ω. What is the temperature of the element if the resistance is found to be 117 Ω, given that the temperature coefficient of the material of the resistor is 1.70 × 10-4°C-1.
Current Electricity
Medium
Q.
(a) Three resistors 2 Ω, 4 Ω and 5 Ω are combined in parallel. What is the total resistance of the combination?
(b) If the combination is connected to a battery of emf 20 V and negligible internal resistance, determine the current through each resistor, and the total current drawn from the battery.
Current Electricity
Medium
Q.
(a) Three resistors 1 Ω, 2 Ω, and 3 Ω are combined in series. What is the total resistance of the combination?
(b) If the combination is connected to a battery of emf 12 V and negligible internal resistance, obtain the potential drop across each resistor.
Current Electricity
Medium
Q.
(a) Given n resistors each of resistance R, how will you combine them to get the (i) maximum (ii) minimum effective resistance? What is the ratio of the maximum to minimum resistance?
(b) Given the resistances of 1 Ω, 2 Ω, 3 Ω, how will be combine them to get an equivalent resistance of (i) (11/3) Ω (ii) (11/5) Ω, (iii) 6 Ω, (iv) (6/11) Ω?
(c) Determine the equivalent resistance of networks shown in Fig. 3.31.
 

NCERT Solutions for Class 12 Physics Chapter 3 – Current Electricity

Q. 1) What conclusion can you draw from the following observations on a resistor made of alloy manganin?

Current (A) Voltage (V) Current (A) Voltage (V)
0.2 3.94 3.0 59.2
0.4 7.87 4.0 78.8
0.6 11.8 5.0 98.6
0.8 15.7 6.0 118.5
1.0 19.7 7.0 138.2
2.0 39.4 8.0 158.0

Ans: From the given table it can be concluded that the ratio of voltage with current is a constant, which is equal to 19.7. Therefore, manganin is an ohmic conductor as it follows Ohm’s law. According to Ohm’s law, the ratio of voltage with current is equal to the resistance of the conductor. Therefore, the resistance of manganin is 19.7 Ω.

Q. 2) Choose the correct alternative:

(a) Alloys of metals usually have (greater/less) resistivity than that of their constituent metals.

(b) Alloys usually have much (lower/higher) temperature coefficients of resistance than pure metals.

(c) The resistivity of the alloy manganin (is nearly independent of/increases rapidly) with increase of temperature.

(d) The resistivity of a typical insulator (e.g., amber) is greater than that of a metal by a factor of the order of (1022/1023).

Ans:

(a) greater

(b) lower

(c) is nearly independent of

(d) 1022

Note: Q&A containing MathML or Latex or Katex code cannot be rendered in pdf document.

FAQs: Class 12 Physics Chapter 3 – Current Electricity

Q1. Is Current Electricity important for exams?
Yes, it is a high-weightage chapter for CBSE, JEE, and NEET.

Q2. Which topics are most important in this chapter?
Ohm’s law, Kirchhoff’s laws, Wheatstone bridge, and potentiometer.

Q3. Are numericals asked from this chapter?
Yes, circuit-based and potentiometer numericals are very common.

Q4. Are derivations important in this chapter?
Yes, derivations related to drift velocity and Kirchhoff’s laws are frequently asked.

Q5. How do NCERT Solutions help?
They provide NCERT-aligned, exam-ready explanations with solved numericals and diagrams.