12th Physics Chapter 2 Important Questions With Answers

 CURRENT ELECTRICITY

UNIT -2

2 & 3 Marks:

1. Define electric current. Give its SI unit.

Electric current is defined as the rate of flow of charges.

SI unit: ampere

I = q/t

2. Why current is a scalar?

Current is defined as scalar product of current density and area vector.

I=j.Ā

Current can be positive or negative depending on the choice of unit vector normal to the surface area. Hence, current is a scalar.

3. Define one ampere.

One ampere of current is equivalent to 1 coulomb of charge passingthrough perpendicular cross section in 1 second.

1A= 1C/1s

4. What is meant by drift velocity? Give its unit.

Drift velocity is defined as the average velocity acquired by the electrons when it is subjected to an electric field. 

Unit: ms-1

.

5. Define mobility. Give its unit.

Mobility is defined as the drift velocity acquired per unit electric field.

SI unit: m²V-¹s-¹

6. Distinguish between drift velocity and mobility.

7. Define current density. Give its unit.

It is defined as the current per unit area of cross section of the conductor.

J = I / A

SI unit: Am-2

8. State Microscopic form of Ohm’s law.

Magnitude of current density is directly proportional to the applied electric field.

9. State Ohm’s law or macroscopic form of Ohm’s law.

At constant temperature, the steady current flowing through the conductor is directly proportional to the potential difference between the two ends of the conductor.

V=IR

10.What are ohmic and non ohmic devices?

Ohmic devices:

The devices which obey ohm’s law are called ohmic devices.

I α V holds good for ohmic devices.

V – I graph is a straight line.

Examples: Metals.

Non-Ohmic devices:

The devices which do not obey ohm’s law are called Non-ohmic devices.

I α V does not hold good. (V-I relation is complex)

V- I graph is non-linear.

Examples: Diodes, Transistors.

11.Define Resistance. Give its unit.

It is the ratio of potential difference across the conductor to the current through the conductor.

R=V/I

SI unit: Ω

12.Define electrical resistivity.

It is defined as the resistance offered to the flow of current by a conductor of unit length and unit area of cross section.

SI unit: Ωm

13. What are the factors on which electrical resistivity depends?

The resistivity of a material is

i) inversely proportional to the number density (n) of the electrons

ii) inversely proportional to the average time between the collisions(τ).

14.Define electrical conductivity.

It is the reciprocal of resistivity.

SI unit: Ω-1m-1

15.What is meant by superconductivity? What are superconductors?

The ability of certain metals, their compounds and alloys to conduct electricity with zero resistance at very low temperatures are called superconductivity. Such materials are called superconductors.

16.Define Temperature coefficient of resistance.

It is defined as the ratio of increase in resistance per degree rise in temperature to its resistance at 0⁰C.

α= 1/Ro ∆R/∆T

SI unit: per ⁰C

17.Define Temperature coefficient of resistivity.

It is defined as the ratio of increase in resistivity per degree rise in temperature to its resistivity at 0⁰C

α =∆ρ/ρo∆T

SI unit: per ⁰C

18.Define electric energy and electric power.

Electric energy: It is defined as the capacity to do electrical work.

Electric Power: It is defined as the rate at which the electrical potential energy is delivered.

19.Give the practical unit of electrical energy.

The practical unit of electrical energy is kilowatt hour (kWh)

1 kWh = 3.6 x 10⁶J

20.Define electromotive force.

Electromotive force is the amount of work done by a battery to move some charge around the circuit.

21.Define internal resistance.

Internal resistance is the resistance offered to the flow of charges within the battery.

22. State Kirchhoff’s current rule.

Algebraic sum of the currents at any junction of a circuit is zero.

23.State Kirchhoff’s voltage rule.

In a closed circuit, algebraic sum of the products of the current and resistance of each part of the circuit is equal to the total emf in the circuit.

24.Why is copper wire not suitable for a potentiometer?

Copper wire is not suitable for potentiometer because it has high temperature coefficient of resistance and low specific resistance.

25. What is meant by Seebeck effect or Thermoelectric effect?

▪ Two dissimilar metals connected to form two junctions is called thermocouple.

▪ When these junctions are maintained at different temperatures, an

emf developed in the circuit known as thermo electric emf.

▪ The current through the circuit is called thermoelectric current.

▪ This effect is called Seebeck effect or thermoelectric effect.

26.Give the applications of Seebeck effect.

i) Thermoelectric Generators,

ii) In automobiles to increase fuel efficiency,

iii) Measure temperature difference in thermocouples and thermopiles.

27.What is meant by Peltier effect?

When an electric current is passed through a circuit of a thermocouple, heat is evolved at one junction and absorbed at the other junction.

28. Define Thomson effect.

If two points in a conductor are at different temperatures, the density of electrons at these points will differ and as a result the potential difference is created between these points.