Current Electricity

Question 1

Define Electric Current.

Answer 1

Electric Current or its strength, in a conductor, is defined as the rate of flow of charge across any cross section of the conductor.

Question 2

What are the different Units of electric current as per

(i) C.G.S. electro-static unit (esu) ,
(ii) C.G.S. electro- magnetic unit (emu),
(iii) S. I unit

Answer 2

The Units of electric current as per
(i) C.G.S. electro-static unit (esu):- 1 esu of current
(stat-ampere) = 1 esu of charge / 1 second
(ii) C.G.S. electro-magnetic unit (emu):- 1 emu of current
(ab-ampere) = 1 emu of charge / 1 second
(iii) S. I unit (ampere):- 1 ampere = 1 coulomb / 1 second

Question 3

Define Drift velocity of free electrons.

Answer 3

The velocity with which the free electrons are drifted towards the positive terminal, under the action of the applied field, is called the drift velocity of the free electrons.

Question 4

State Ohm’s Law for conductors.

Answer 4

It states that, at constant temperature current flowing through a conductor of uniform area of cross-section, is proportional to the difference of potential across its terminals.

Question 5

Define Resistance .

Answer 5

Resistance of a conductor is defined as the ratio between potential differences between the two ends of the conductor to the current flowing through it.

Question 6

What is meant by Temperature coefficient of resistance (α) ?

Answer 6

It is defined as change in resistance of the conductor per unit resistance per degree centigrade rise of temperature. i.e. α = (Rt - R0)/R0(T-T0)

Question 7

Define the terms (i) Electromotive force (ii)Potential difference.

Answer 7

(i) The electromotive force E of a cell is defined as the difference of potential between its terminals when there is no current in the external circuit, i.e., when the cell is in open circuit.
(ii) The potential difference of a cell is the difference of potential between two terminals when it is in closed circuit.
i. e. E = V+IR

Question 8

Define Internal resistance (r) of a cell .

Answer 8

It is defined as the resistance offered by the electrolyte of the cell when the electric current passes through it is known as the internal resistance of the cell.
r = R (E-V ) / V

Question 9

what are the dependent factors of Resistivity of conductor ?

Answer 9

Factors :

(i) independent of size and shape of conductor
(ii) it depends on material of conductor
(iii) temperature and pressure.

Question 10

What is the SI unit of resistivity?

Answer 10

SI Unit of resistivity is Ohm-m.

Question 11

Name the physical parameter having S I units

(Ohm-m)-1 .

Answer 11

(Ohm-m)-1 : conductivity (Inverse of resistivity) denoted by σ .

Question 12

What is meant by current density and mention its S I unit.

Answer 12

It is defined as current per unit area (taken normal to the current) J = I/A.
SI unit of current density is A/m2.

Question 13

Write I & J in terms of drift velocity(vd).

Answer 13

I = (vd)enA.

& J = (vd)en

Question 14

Define Mobility of a charge.

Answer 14

Mobility is defined as drift velocity of charge per unit electric field.

Question 15

Mention Limitations of Ohm’s law.

Answer 15

Limitations of Ohm’s Law:-

(1) V-I relations are not linear , Example: diode transistor
(2) The relation between V and I depends on the sign of V. If we change the polarity of supply voltage magnitude of current changes
(3) The relation between V and I is not unique. There may be more values of potential for same current I. Example: tunnel diode, material Ga,As

Question 16

Define series combination of resistors.

Answer 16

It is defined as series connection , if current flowing through each resistor remains same, while sum of potential drop across resistors is equal to potential drop across combination.

Question 17

State Kirchoff’s laws ?

Answer 17

(i) Junction law or rule : At any junction, the algebraic sum of all currents meeting at a junction in a circuit is zero. (In other words the sum of the currents entering the junction is equal to the sum of currents leaving the junction).
(ii) Loop law or rule or Voltage law : The algebraic sum of changes in potential around any closed loop involving resistors and emfs of cells in the loop is zero

Question 18

What is the principle of (i) Meter bridge (ii) Potentiometer

Answer 18

(i) When a constant current flowing through a wire of know material and constant area of cross section , the resistance of a part of the wire is always directly proportional to its length.
(ii) When a constant current flowing through a wire of know material and constant area of cross section , the potential difference for portion of the wire is always directly proportional to its length.