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QUESTION: 1

Materials which allow the passage of electric current are known as______

Solution:

Conductors are materials(mostly metals), which freely allow the passage of electrons through it. If electrons can flow freely through a material, it implies that even current can flow freely through that material since current is the rate of flow of electrons.

QUESTION: 2

A wire of length 2m and another wire of length 5m are made up of the same material and have the same area of cross section, which wire has higher resistance?

Solution:

Resistance is directly proportional to the length of the wire, hence as the length of the wire increases, resistance increases.

QUESTION: 3

A wire having an area of cross section = 10sqm and another wire having an area of cross section= 15sqm, have the same length and are made up of the same material. Which wire has more resistance?

Solution:

Resistance is inversely proportional to area of cross section. As area of cross section increases, resistance decreases. Hence the 10sqm wire has higher resistance than the 15sqm wire.

QUESTION: 4

Which of the following statements are true with regard to resistance?

Solution:

The expression for resistance is: Resistance=Resistivity*length of wire/ area of cross section of wire. Hence resistance is directly proportional to length.

QUESTION: 5

A wire has the same resistance as the one given in the figure. Calculate its resistivity if the length of the wire is 10m and its area of cross section is 2m.

Solution:

From the given circuit, R=V/I= 200/5 =40ohm.

Resistivity= Resistance*Area of cross section/ Length of the wire.

Resistivity= 40*2/10= 8 ohm-metre.

QUESTION: 6

Which, among the following is a unit for resistivity?

Solution:

Resistivity= RA/l= ohm*metre^{2}/metre. Hence the unit of resistivity is ohm- metre.

QUESTION: 7

What is the resistivity of Copper?

Solution:

Resistivity is a material property. Different materials have different resistivity’s. Resistivity depends on the material of the wire, hence the resistivity of copper is 1.59*10^{-8}ohm^{-metre}.

QUESTION: 8

Calculate the ratio of the resistivity of 2 wires having the same length and same resistance with area of cross section 2m^{2} and 5m^{2} respectively.

Solution:

Resistivity of first wire= RA1/l= 2R/l.

Resistivity of second wire= RA2/l = 5R/l.

Ratio of the first resistance to the second= (2A/l)/(5A/l)= 2/5.

QUESTION: 9

Which of the following statements are true with regard to resistivity?

Solution:

Resistance is a temperature dependent element. As the temperature increases, resistance also increases, hence resistance depends on temperature.

QUESTION: 10

The reciprocal of resistivity is________

Solution:

The expression for resistivity is= RA/l. The expression for conductivity= Cl/A; C=1/R => Conductivity= l/(AR) = 1/resistivity. Hence, conductivity is the reciprocal of resistivity.

QUESTION: 11

The resistance of pure metals ___________

Solution:

In a conductor, the valence band and conduction band overlap each other, there is an excess of electrons in the conduction band. When the temperature increases, there is an overcrowding of electrons in the conduction band hence reducing the mobility and hence resistance increases.

QUESTION: 12

The resistance of insulators __________

Solution:

In case of an insulator, the energy gap between the conduction band and the valence band is very large. When the temperature is increased, the electrons move from the conduction band to the valence band and hence it starts conducting. When conductance increases, resistance decreases, since C=1/R. Thus, when temperature increases, resistance decreases in insulators.

QUESTION: 13

Which of the following statements are true about metals?

Solution:

The resistance of metals increases with an increase in temperature thus, it has a positive temperature coefficient.

QUESTION: 14

Which of the following statements are true about insulators?

Solution:

Insulators have a negative temperature coefficient because as temperature increases, the resistance of insulators decreases.

QUESTION: 15

What is the unit of temperature coefficient?

Solution:

: R=Reff[1+temp. coeff(T-Teff)].

From the given expression: (R/Reff-1)/(T-Teff)= temp. coeff. Hence, the unit is the reciprocal of that of temperature= centigrade^{-1}.

QUESTION: 16

A copper coil has a resistance of 200 ohm when its mean temperature is 0 degree centigrade. Calculate the resistance of the coil when its mean temperature is 80 degree centigrade.

Solution:

The temperature coefficient of copper is 0.00428 centigrade^{-1}

R1=R0(1+temp. coeff.*T1)= 200(1+0.00428*80)= 268.5 ohm.

QUESTION: 17

The temperature of a coil cannot be measured by which of the following methods?

Solution:

Caloriemeter measures the amount of heat and not the temperature of coil coefficient. Temperature of a coil is mainly measured by thermometer.

QUESTION: 18

The rise or fall in resistance with the rise in temperature depends on________

Solution:

The rise or fall in resistance with rise in temperature depends upon the property of the material. Hence it rises with temperature in metals and falls with temperature in insulators and semi-conductors.

QUESTION: 19

If the temperature is increased in semi-conductors such that the resistance incessantly falls, it is termed as_______

Solution:

When the temperature keeps increasing, the resistance keeps falling continuously and hence the current to increase. This causes the heat in the semi-conductor to rise. This causes the temperature to increase further and the resistance to further decrease. This process continues and until there is sufficient heat to destroy the structure of the semi-conductor completely. This is known as thermal runway.

QUESTION: 20

Materials having resistance almost equal to zero is_______

Solution:

When the temperature of a material falls to absolute zero, the resistance falls to zero and hence there are no I2R losses. Since resistance is zero, conductance is almost infinity and hence these materials are known as superconductors.

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