# Physics Class 12 Chapter 3 Subjective Questions

## Current Electricity

1. Define steady current.

It is defined as the current whose magnitude does not change with time.

1. Define varying current.

It is defined as the current whose magnitude changes with time.

1. Define current density. Tell whether it is a scalar or a vector quantity. Also give its S.I. unit.

It is defined as the current flowing per unit area of the conductor. It is a vector quantity and its SI unit is Ampere-metre2 (Am2). .

1. How does the relaxation time of electrons in the good conductor varies with temperature?

The relaxation time of electrons in the good conductors is inversely proportional to the temperature. As the temperature decreases, relaxation
time increases and vice-versa.

1. What is the direction of current density?

It flows in the same direction as that of the flow of current.

1. Define resistance of a conductor. What is a resistor?

It is defined as the opposition offered by the conductor to the flow of electrons. A resistor is defined as a conducting substance which offers resistance to the flow of current in the circuit.

1. Define the resistivity/specific resistance of the material of a conductor?

It is defined to be numerically equal to the resistance of a conductor of unit length and unit area of cross-section.

1. How the resistivity of a material changes when its length and area of cross-section are trippled?

We know that the resistivity of the material a conductor is independent of the dimensions of the conductor, hence it will remain same.

1. How does the conductivity of a semi conductor vary with temperature?

The conductivity of a semiconductor increases with the increase in its temperature.

1. State the condition in which terminal voltage across a secondary cell is equal to its e.m,f.

When the cell is in the open circuit i.e., when no current is drawn from the cell, then terminal voltage is equal to its e.m.f.

1. On what factors does (he internal resistance of a cell depend?

It depends upon the following factors:

nature of the electrolyte of the cell.

nature of the electrodes of the cell.

distance between the electrodes of the cell.

Area of the electrodes inside the electrolyte.

1. Is a wire carrying current charged? Why?

No. At any instant, the number of protons is equal to the f number of electrons, hence wire is uncharged. Simply there is a continuous flow of electrons.

1. Two wire A and B of the same metal have the same area of cross-section and have their lengths in the ratio 3:2. What will be the raitio of currents flowing through them when the same potential difference is applied across length of each of them?

We know that R where l is the length of the conductor. Now as the lengths of the two wires are in the ratio 3 :2, so the ratio of their resistances is also 3:2. Also we know that I ∝ 1/R, so the currents in the two wires will be in the ratio 2:3 when the same potential difference is applied across the two wires.

1. A piece of metal and another of silicon are cooled from room temperature to 100 K. What will happen to their conductivities? Explain.

We know that the temperature coefficient of resistivity of metals is positive while that of semiconductor is negative. Th&s when their temperatures are lowered from room termperature to 100 K, the resistivity of metal decreases and that of silicon increases, hence the conductivity of metal increases and that of the silicon decreases.

1. Can the potential difference across a battery be greater than its e.m.f.?

Normally not. But during the charging when the battery is connected to the external source, the terminal potential difference V is greater than E i.e.,
Vapplied = E + 1/R.

1. Can Kirchhoff s laws be applied both to D. C (direct current) and A.C. Circuits?

Yes, Kirchhoff’s laws can be applied both to D.C. and A.C. circuits. .

1. Define a wheat stone bridge.

It is defined as an arrangement of four resistances used to measure one of them in terms of other three.

1. Define a slide wire bridge.

It is defined as a device used to measure the unknown resistance.

1. Define a potentiometer.

It is defined as a device used to measure the internal difference across the terminals of a cell.

1. State the principle of potentiometer?

It states that when a constant current is passed through a wire of uniform area of cross-section, the potential drop across any part of the wire is directly proportional to its length, i.e., V ∝ l.

1. Why is a potentiometer named so?

It is named so because it is used to measure the potential difference across the terminals of a cell.

1. Define electric energy.

It is defined as the heat energy produced in a conductor of resistance R on passing a current I through it for a given time t.
i.e„ U = I2Rt = V2t/R = VIt.

1. Define KWh. Derive the relation between 1 KWh and 1J.

Electric energy consumed or dissipated in an electrical circuit is said to be 1 KWh if a device of 1 KW power is used for one hour.

We know that
1J = IV x 1A x 1s
Also we know that
1 KWh = 1 KW x 1 hour
= 103 watt x 1600 s
= 103 x 1 V x 1 A x 3.6 x 103 s
or 1 KWh = 3.6 x 106 J.

1. Why does a potentiometer act like an ideal voltmeter?

An ideal voltmeter has infinite resistance. Potentiometer draws no current from the circuit and hence its resistance is infinite as R = V/I = V/O = ∞ So it acts like an ideal voltmeter.

1. If a wire is stretched to four times its original length without the loss of mass, how will the resistivity of the wire be influenced?

As the resistivity depends upon the nature of the conductor and not upon its dimensions, so it remain unchanged.

1. Define temperature coefficient of resistance of a given material.

It is defined as the change in resistance per unit resistance per degree rise or fall in the temperature of a given material.

1. Comment on the direction of conventional and electronic currents.

The direction of conventional current is in the direction of motion of positive charges while the direction of electronic current is opposite to the
conventional current and is in the direction of motion of electrons.

1. What is the effect of increase in temperature on the drift speed of free electrons in a metallic conductor?

As the temperature of conductors is increased, average relation time decreases due to the increase in the collision frequency of electrons and thus their resistance increases. We know that Vd ∝ I∝ I/R, thus drift velocity of electrons will decrease in a metallic conductor.

1. What is the effect of temperature on the conductivity of electrolytes?

The conductivity of electrolytes will increase because of the large number of ions and greater mobility of ions.

1. Bends in a rubber pipe reduce the flow of water through it. How would the bends in wire affect its electric resistance?

The bends in a wire will not affect its resistance. This is because the electrons are extremely small in size as compared to the bends. Thus electrons can easily change their direction of motion. Hence the resistance will remain unchanged.

1. Why effective resistance increases when the resistances are connected in series?

When resistances are connected in series, then there is an increase in the effective length. Since resistance is directly proportional to the length, so the effective resistance is increased.

1. When the resistances are connected in parallel, the effective resistance decreases. Why?

When the resistances are connected in parallel, then there is an effective increase in the area of cross-section. As the resistance is inversely proportional to the area of cross-section, hence the effective resistance decreases due to the increase in area.

1. Why is it not advisable to use copper wire in a potentio¬meter?

It is not advisable to use copper wire in a potentiometer because the copper has small resistivity and large temperature coefficient of resistance.

1. Can the internal resistance of a cell may be called a defect?

Yes, this is because a part of the electrical energy is used up in overcoming the internal resistance of the cell.

1. How are the coils wound in a resistance box?

The resistance coil is doubly wound on a bobbin to avoid electromagnetic induction. The two ends of the resistance coil are connected to the two brass studs having a small gap between them. A plug is fitted into this gap. When this plug is taken out of the gap, the resistance is included in the circuit.

1. Why do we prefer a potentiometer of longer bridge wire?

We prefer a potentiometer of longer bridge wire so as to increase its sensitivity. When the bridge wire is longer, the potential gradient is smaller. Smaller the potential gradient, more is the sensitivity of the potentiometer wire.

1. On what factors does the e.m.f. of a cell depend?

The e.m.f. of a cell depends upon the following factors :

Nature of the electrolyte.

Temperature of the electrolyte.

Concentration of the electrolyte.

Nature of the electrodes.

1. Why the light of a motor car becomes slightly dim when it is started?

The starter initially draw a larger current from the battery that causes a large voltage drop across the internal resistance of the battery. Hence the potential difference across the terminals of the battery is reduced leading the light to get dim.

1. Does the e.m.f. represents force? Does e.m.f. has an electrostatic origin?

No, the e.m.f. is not a force but it is the work done to move a unit charge from one terminal to the another terminal of the cell. The e.m.f. does not have the simple electrostatic origin.

1. What are the differences between e.m.f. and terminal potential difference?

e.m.f.

(1) It is defined as the potential difference between the two terminals of a cell when it is in the open circuit.

(2) It is independent of the resistance of the external circuit.

(3) It is a cause of electric current.

(4) e.m.f. of a cell is greater than the potential difference between the two terminals of the cell.

(5) It is used for source.

Terminal Potential difference :

(1) It is defined as the potential difference between the two terminals of a cell when it is in the closed circuit.

(2) Potential difference between any two points of a circuit is proportional to the resistane between these two points.

(3) It is an effect of electric current.

(4) It is lesser than the e.m.f. of the cell.

(5) It can be measured between any two points of the circuit.

12th Physics Subjective Questions in English,12th physics, Class 12th physics ncert solutions chapter 3, class 12th physics subjective questions in english, physics kaksha 12 chapter 3, physics kaksha 12 model paper, physics kaksha 12 ncert, physics kaksha 12 english medium, physics kaksha 12 important questions, physics kaksha 12 lesson 3, kaksha 12 ka physics ncert, kaksha 12 physics adhyay 3, Current Electricity, chapter 3 questions and answers class 12 physics, current electricity objectives questions in english, physics chapter 3 objectives questions in english, physics mcqs for class 12 chapter wise with answers pdf, class 12 physics chapter 1 subjectives questions in english, class 12 physics chapter 3, chapter 3 physics class 12 important questions, 12th physics, physics, 12th physics chapter 3, science sangrah official, bhautiki kaksha 12 subjective questions in english, bhautiki kaksha 12 chapter 3, physics class 12 chapter 3 in english, current electricity important questions, chapter 3 physics class 12, science sangrah physics class 12 chapter 3, physics class 12 chapter 3 english medium,