Chapter 3: Current Electricity

Consider a current-carrying wire in the shape of a circle. Note that as the current progresses along the wire, the direction of j changes in an exact manner, while the current I remain unaffected. The agent that is essentially responsible for this is

a) source of emf

b) electric field produced by charges accumulated on the surface of the wire

c) the charges just behind a given segment of wire which push them just the right way by repulsion

d) the charges ahead

Two batteries of emf ε1 and ε2 and internal resistances r1 and r2, respectively, are connected in parallel, as shown in the figure.

a) the equivalent emf εeq of the two cells is between ε1 and ε2 that is ε1 < εeq < ε2

b) the equivalent emf εeq is smaller than ε1

c) the εeq is given by εeq = ε1 + ε2 always

d) εeq is independent of internal resistances r1 and r2

A resistance R is to be measured using a meter bridge. The student chooses the standard resistance S to be 100Ω. He finds the null point at l1 = 2.9 cm. He is told to attempt to improve accuracy. Which of the following is a useful way?

Two cells of emf’s, approximately 5V and 10V, are to be accurately compared using a potentiometer of length 400 cm.

a) the battery that runs the potentiometer should have 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 wire itself should have a potential drop of 10V

d) potentiometer is usually used for comparing resistances and not voltages

A metal rod of length 10 cm and a rectangular cross-section of 1 cm × 1/2 cm is connected to battery across opposite faces. The resistance will be

a) maximum when the battery is connected across 1 cm × 1/2 cm faces

b) maximum when the battery is connected across 10 cm × 1 cm faces

c) maximum when the battery is connected across 10 cm × 1/2 cm faces

d) same irrespective of the three faces

Which of the following characteristics of electrons determines the current in a conductor?

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 the charged particle leaves the junction

d) the fact that there is no accumulation of charges at a junction

Consider a simple circuit in the figure.

stands for a variable resistance R’. R’ can vary from R0 to infinity. r is internal resistance of the battery

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 ≥V/r+R always

Temperature dependence of resistivity ρ(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

The measurement of an unknown resistance R is to be carried out using a Wheatstone bridge. Two students perform an experiment in two ways. The first student takes R2 = 10Ω and R1 = 5Ω. The other student takes R2 = 1000 Ω and R1 = 500 Ω. In the standard arm, both take R3 = 5 Ω. Both find R = R2/R1 R3 = 10 Ω 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 R2 and R1 can be measured

c) if the student uses large values of R2 and R1, 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