Current Electricity

Question 1

Define the term current and state it’s S.I. unit.

Answer 1

Current is defined as the rate of flow of charge. The S.I. unit of charge is coulomb

Question 2

Define the term electric potential. State it’s S.I. unit.

Answer 2

the amount of work done per unit charge in bringing a positive test charge from infinity to that point. The S.I. unit of electric potential is volt.

Question 3

How is the electric potential difference between the two points defined? State it’s S.I. unit.

Answer 3

The potential difference (p.d.) between two points is equal to the work done per unit charge in moving a positive test charge from one point to the other. It’s S.I. unit is volt. It is a scalar quantity.

Question 4

Explain the statement ‘the potential difference between two points is 1 volt’

Answer 4

if the work done in moving 1 coulomb charge from one point to other is 1 joule

Question 5

State whether the current is a scalar or vector?

Answer 5

Current is a scalar quantity.

Question 6

What does the direction of current convey?

Answer 6

mean that thedirection of motion of electrons is opposite to it.

Question 7

State whether the potential is a scalar or vector?

Answer 7

Potential is a scalar quantity.

Question 8

What does the positive and negative sign of potential convey?

Answer 8

work has to be done on the positive test charge against the repulsive force due to the positive charge in bringing it from infinity

theattractive force on test charge due to the negative charge does work by itself.

Question 9

Define the term resistance. State it’s S.I. unit.

Answer 9

Theobstruction offered to the flow of current by the conductor (or wire)is called it’s resistance.

The S.I. unit of resistance isOhm.

Question 10

Name the particles which are responsible for the flow of current in a metallic wire.

Answer 10

free electrons.

Question 11

Explain the flow of current in a metallic wire on the basis of movement of the particles

Answer 11

In a metallic wire, the moving charges are the free electrons which constitute the current.
Q (charge) = n × e

and the current in conductor is

​I=Q/t=ne/t

Question 12

State Ohm’s law

Answer 12

According to Ohm’s law,the current flowing in a conductor is directly proportional to the potential difference applied across it’s ends provided that the physical conditions and the temperature of the conductor remain constant.

Question 13

Name and state the law which relates the potential difference and current in a conductor.

Answer 13

Ohm’s law

Ohm’s law states thatthe current flowing through the conductor is directly proportional to the potential difference across it’s ends provided that the physical conditions and the temperature of the conductor remain constant.

Question 14

What is the necessary condition for a conductor to obey the ohm’s law

Answer 14

the physical conditions and the temperature should remain constant.

Question 15

What is an ohmic resistor? Give one example of an ohmic resistor.

Answer 15

The conductors which obey the Ohm’s law are called the ohmic resistors or linear resistances. Examples are all metallic conductors (such as Silver, Aluminium, Copper, Iron etc.)

Question 16

What are non-ohmic resistors? Give one example

Answer 16

The conductors which do not obey the Ohm’s law are known as the non-ohmic resistors (or non-linear resistances). Examples — LED, solar cell, junction diode, etc.

Question 17

Give two differences between an ohmic and non-ohmic resistor.

Answer 17

Refer book

Question 18

How does the resistance of a wire depend on it’s radius?

Answer 18

resistance of a wire is inversely proportional to the square of the radius of the wire.

Question 19

How does the resistance of a wire depend on it’s length ?

Answer 19

Resistance of a wire is directly proportional to the length of the wire.

R ∝ l

Question 20

How does the resistance of a metallic wire depend on it’s temperature?

Answer 20

resistance of a conductor increases with an increase in it’s temperature.

Question 21

Two wires, one of copper and other of iron, are of the same length and same radius. Which will have more resistance? Give reason.

Answer 21

The iron wire will have more resistance.

Iron wire has more resistance as compared to copper because specific resistance of iron is more than that of copper.

Question 22

Name three factors on which resistance of a given wire depends and state how it is affected by the factors stated by you?

Answer 22

affected by the factors stated by you?

Answer

The three factors on which the resistance of wire depends are —

Dependence on length of the wire — the resistance of a wire is directly proportional to the length of the wire.
R ∝ l

Dependence on the thickness of the wire — the resistance of a wire is inversely proportional to it’s area of cross section (a) normal to the direction of flow of current.
R ∝ 1/a
​

Dependence on the temperature of the wire — the resistance of conductor increases with an increase in it’s temperature.

Question 23

Define the term specific resistance and state it’s S.I. unit.

Answer 23

Specific resistance of a material is the resistance of a wire of that material of unit length and unit area of cross section.

The S.I. unit of specific resistance is ohm × metre or (Ω m)

Question 24

State the order of specific resistance of (i) a metal, (ii) a semiconductor and (iii) an insulator.

Answer 24

The specific resistance is very low for metals, because it allows most of current to pass through it.

The specific resistance is low for semiconductor.

The specific resistance is very high for insulators, as the current won’t pass through it.

Question 25

Name two factors on which the specific resistance of a wire depends?

Answer 25

Material of the substance — It is a characteristic property of the substance. It is different for different substances.
Temperature of the substance — It increases with the increase in temperature for metals, but it decreases with the increase in temperature for the semiconductor.

Question 26

Name a substance of which the specific resistance remains almost unchanged by the increase in temperature.

Answer 26

manganin

Question 27

How does the specific resistance of a semi-conductor change with the increase in temperature?

Answer 27

Decrease

Question 28

How does (a) resistance a wire depend on it’s (i) length, and (ii) radius?

Answer 28

Resistance of a wire is directly proportional to length of the wire.

Resistance of a wire is inversely proportional to the square of radius of the wire.

Question 29

How does specific resistance of a wire depend on it’s (i) length, and (ii) radius?

Answer 29

Specific resistance of a wire does not depend on length of the wire as it is a characteristic property of the material of wire.

Specific resistance of a wire does not depend on radius of the wire as it is a characteristic property of the material of wire.

Question 30

Name the material used for making connection wires. Give reason for your answer.

Answer 30

Copper or aluminium

materials are used for making connection wires because their specific resistance is very low and hence they possess least possible resistance.

Question 31

Why should a connection wire be thick?

Answer 31

resistance becomes low

Question 32

Name a material which is used for making the standard resistor. Give a reason.

Answer 32

Standard resistors are made of magnanin, constantan etc. for which the specific resistance is high and the effect of change in temperature on their resistance is negligible.

Question 33

Name the material used for making fuse wire. Give a reason.

Answer 33

The material used for making fuse wire is an alloy of lead and tin because it’s melting point is low and it’s specific resistance is more than that of copper or aluminium

Question 34

Name the material used for (i) filament of an electric bulb

Answer 34

a tungsten wire because it has a high melting point

Question 35

Name the material used for heating element of a room heater.

Answer 35

nichrome because the specific resistance of nichrome is high and it’s resistance increases to a great extent with the increase in temperature.

Question 36

What is a superconductor ?

Answer 36

A superconductor is a substance of zero resistance (or infinite conductance) at a very low temperature.

Example — Mercury below 4.2 K, lead below 7.25 K and niobium below 9.2 K

Question 37

Which of the following is an ohmic resistance?

Answer 37

nichrome wire

Question 38

For which of the following substances, resistance decreases with increase in temperature?

Answer 38

carbon