The electrical resistivities of five substances A, B, C, D and E are given below
A) $5.20 \times 10^{- 8} Ω m$
B) $110 \times 10^{- 8} Ω m$
C) $2.60 \times 10^{- 8} Ω m$
D) $10.0 \times 10^{- 8} Ω m$
E) $1.70 \times 10^{- 8} Ω m$

(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 advise to be used for making heating elements of electric irons? Why?
(d) Which two substances should be used for making electric wires ? Why?

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Solution

(a) E is the best conductor as it has the least resistivity.
(b) C is a better conductor of electricity as it has lower resistivity.
(c) For making heating elements we can use B as it has higher resistivity than any other material and can dissipate more heat.
(d) For making electrical wires we can use E or C because they have the least resistivities in the whole group.

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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?
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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

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

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A		110 × 10⁻⁸ Ω m
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C		10.0 × 10⁻⁸ Ω m
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(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?
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