CBSE Class 12 Physics Current Electricity Worksheet Set A

Current Electricity MCQ Questions with Answers Class 12 Physics Chapter 3

ONE MARK QUESTIONS

Question. Distinguish between emf and terminal voltage of a cell.
Answer : Emf is measured in open circuit ,p.d in closed circuit
Emf is slightly greater than p d while using.

Question. Show variation of resistivity of copper as a function of temperature in a graph.
Answer : 

Question. Two wires, one of copper and the other of manganin, have same resistance and equal thickness. Which wire is longer ? Justify your answer
Answer : R = ρl/A = ml/(ne^2 τA)
As R and A are constant, ρl is a constant, Manganin is an alloy of higher resistivity, Copper wire will be longer.

Question. 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, drift speed, electric field?
Answer : Current and electric field will remain constant
Current density will change with non-uniform area of cross section
Drift velocity will change as area changes (I = n e A V d)

Question. Two conducting wires X and Y of same diameter but different materials are joined in series acrossa battery. If the number density of electrons in X is twice that in Y, find the ratio of drift velocityof electrons in the two wires.
Answer : As (I = n e A V d),in series combination current remain same. In both X and Y,
n V d will be constant.
Therefore the ratio will be Vx/Vy=1/2

Question. A wire of resistance 8 R is bent in the form of a circle. What is theEffective resistance between the ends of a diameter AB ?
Answer : 1/R=1/4R+1/4REffective resistance will be 2R.

TWO MARKS QUESTIONS

Question. Derive an expression for drift velocity of free electrons in a conductor in terms of relaxation time.
Answer : I = n e A V d, V=- eτ/mE

Question. How does the drift velocity of free electrons in a metallic conductors change with the rise in temperature.
Answer : As the temperature of the conductor increases, average relaxation time will decrease, drift velocity will also decrease, resistivity increases.

Question. When 5V potential difference is applied across a wire of length 0.1m , the drift speed of electrons is 2.5x10-4 m/s, if the electron density in the wire is 8X 1028 m-3 . calculate the resistivity of the material of the wire.
Answer : Resitivity=1.56x10-5Ωm

Question. A potentiometer wire of length 1m has a resistance of 10 Ω .it is connected to a 6V battery in series with a resistance of 5Ω . Determine the emf of the primary cell which gives the balance point at 40 cm
Answer : E2=1.6V

Question. A wire of 15 Ω resistances is gradually stretched to double its original length. It is then cut into two equal parts. These parts are then connected in parallel across a 3 00 volt battery. Find the current drawn from the battery.
Answer : I=3/15=0.2 amp

THREE MARKS QUESTIONS

Question. With the help of the circuit diagram, explain the working Principle of meter bridge. How it is used to determine the unknown resistance of a given wire? Write the necessary precautions to minimize the error in the result.
Write the characteristics of manganin which make it suitable for making standard resistance.
Answer : S = ((100-l)/l)R
Precautions
The current should not flow for the long time
The balance point is to be taken at the middle of the bridge
No change of resistance with temperature (Image 9)

Question. In the meter bridge experiment, balance point was observed at J with AJ = l.
(i) The values of R and X were doubled and then interchanged. What would be the new position of balance point?
(ii) If the galvanometer and battery are interchanged at the balance position, how will the balance point get affected?
Answer : l’=100-l, No effect

Question. Use Kirchhoff’s rules to obtain conditions for the balance condition in a Wheatstone bridge
Answer : ( P)/Q = R/S (Image 9)

Question. In the circuit diagram shown, AB is a uniform wire of resistance 10Ω and length 1 m. It is connected to a cell E1of emf 2 V and negligible internal resistance and a resistance R. The balance point with another cell E2of emf 100 mV is found at 40 cm from end A. Calculate the value of the resistance R. 

Answer : K=40/100=0.25V/m
Balancing length=0.12 m

FIVE MARKS QUESTIONS

Question. A cell of emfE and internal resistance r is connected to two external resistances R1 and R2 and a perfect ammeter. The current in the circuit is measured in four different situations:
(i) without any external resistance in the circuit.
(ii) with resistance R1 only
(iii) with R1 and R2 in series combination
(iv) withR1 and R2 in parallel combination.
The currents measured in the four cases are 0.42 A, 1.05 A, 1.4 A and 4.2 A, but not necessarily in that order. Identify the currents corresponding to the four cases mentioned above.
Answer : 4.2 A, 1.05 A, 0.42 A, 1.4 A respectively

Question. Find the magnitude and direction of current in 1Ωresistor in the given circuit
Two students X and Y perform an experiment on potentiometer separately using the circuit diagram shown below
Keeping other things unchanged
X increases the value of resistance R,
Decreases the value of resistance S in the set up .
How will these changes affect the position of null point in each case and why?
Answer : I=132/23 A
Null point will shift towards B
No change in the null point

More question-

              Very Short Answer

Q1)       What is drift velocity?

Q2)       Define electric current?

Q3)       What is relaxation time?

Q4)       Define ampere?

Q5)       What are mobility and its unit?

              Short Answer

Q6)       What are the limitations of the Ohm’s law?

Q7)       What do you understand by electric current in conductors?

Q8)       What is nichrome and what its use?

Q9)       How mobility is different form conductivity?

Q10)     The resistance of the platinum wire of a resistance thermometer at the ice point is 4Ωand the steam point is 4.39Ω and when the thermometer is inserted in a hot bath the resistance of the platinum wire is 4.795Ω, so calculate the temperature of the bath?

              Long Answer

Q11)     Difference between resistance and resistivity?

Q12)     Write short note on the temperature dependence of resistivity?

Q13)     Explain Ohm’s law?

Q14)     Difference between capacitors and resistors?

Q15)     State the difference between electrical energy and electrical power?