The electrical resistivities of five substances A, B, C, D and E are given below:

A 5.20 × 10⁻⁸ Ω m

B 110 × 10⁻⁸ Ωm

C 2.60 × 10⁻⁸ Ωm

D 10.0 × 10⁻⁸ Ωm

E 1.70 × 10⁻⁸ Ωm

(a) Which substance is the best conductor of electricity? Why?
(b) Which one is a better conductor : A or C? Why?
(c) Which substance would you advice to be used for making heating elements of electric iron? Why?
(d) Which two substances should be used for making electric wires? Why?

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Solution

(a) E is the best conductor of electricity because its resistivity is the least among the given values.
(b) C is a better conductor than A because its resistivity is less than that of A.
(c) B should be used for making heating elements of electric iron because its resistivity is the highest among the given values.
(d) E and C should be used for making electric wires because their resistivity is less.

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Q. (a) What is meant by saying that the potential difference between two points is 1 volt? Name a device that helps to measure the potential difference across a conductor.
(b)Why does the connecting cord of an electric heater not glow hot while the heating element does?
(c) Electrical resistivities of some substances at 20°C are given below:

Silver	1.60 × 10⁻⁸ Ωm
Copper	1.62 × 10⁻⁸ Ωm
Tungsten	5.20 × 10⁻⁸ Ωm
Iron	10.0 × 10⁻⁸ Ωm
Mercury	94.0 × 10⁻⁸ Ωm
Nichrome	100 × 10⁻⁶ Ωm

Answer the following questions in relation to them:
(i) Among silver and copper, which one is a better conductor? Why?
(ii) Which material would you advise to be used in electrical heating devices? Why?

Q. The electrical resistivities of four materials A, B, C and D are given below:

A		110 × 10⁻⁸ Ω m
B		1.0 × 10¹⁰ Ω m
C		10.0 × 10⁻⁸ Ω m
D		2.3 × 10³ Ω m

Which material is : (a) good conductor (b) resistor (c) insulator, and (d) semiconductor?

Q. The electrical resistivities of four materials P, Q, R and S are given below:

P		6.84 × 10⁻⁸ Ωm
Q		1.70 × 10⁻⁸ Ωm
R		1.0 × 10¹⁵ Ωm
S		11.0 × 10⁻⁷ Ωm

Which material will you use for making: (a) heating element of electric iron (b) connecting wires of electric iron (c) covering of connecting wires? Give reason for your choice in each case.

Give reason why :

(a) copper metal is used for making electric wires.

(b) graphite is used for making electrode in a cell.

(a) Give two examples of substances which are good conductors of electricity. Why do you think they are good conductors of electricity?
(b) Calculate the resistance of a copper wire 1.0 km long and 0.50 mm diameter if the resistivity of copper is $1.7 \times 10^{- 8} Ω m .$

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A		5.20 × 10⁻⁸ Ω m
B		110 × 10⁻⁸ Ωm
C		2.60 × 10⁻⁸ Ωm
D		10.0 × 10⁻⁸ Ωm
E		1.70 × 10⁻⁸ Ωm