Page 184 - NCERT Science Class 10 English Medium

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–6
have very low resistivity in the range of 10 Ω m to 10 Ω m. They are
good conductors of electricity. Insulators like rubber and glass have
resistivity of the order of 10 12 to 10 17 Ω m. Both the resistance and
resistivity of a material vary with temperature.
Table 11.2 reveals that the resistivity of an alloy is generally higher
than that of its constituent metals. Alloys do not oxidise (burn) readily
at high temperatures. For this reason, they are commonly used in
electrical heating devices, like electric iron, toasters etc. Tungsten is used
almost exclusively for filaments of electric bulbs, whereas copper and
aluminium are generally used for electrical transmission lines.
Table 11.2 Electrical resistivity* of some substances at 20°C

Material Resistivity (ΩΩ Ω Ω Ω m)
Conductors Silver 1.60 × 10 –8
Copper 1.62 × 10 –8
Aluminium 2.63 × 10 –8
Tungsten 5.20 × 10 –8
Nickel 6.84 × 10 –8
Iron 10.0 × 10 –8
Chromium 12.9 × 10 –8
Mercury 94.0 × 10 –8
Manganese 1.84 × 10 –6
Alloys Constantan 49 × 10 –6
(alloy of Cu and Ni)
Manganin 44 × 10 –6
(alloy of Cu, Mn and Ni)
Nichrome 100 × 10 –6
(alloy of Ni, Cr, Mn and Fe)
10
Insulators Glass 10 – 10 14
13
Hard rubber 10 – 10 16
15
Ebonite 10 – 10 17
12
Diamond 10 - 10 13
Paper (dry) 10 12
* You need not memorise these values. You can use these values for solving numerical
problems.

Example 11.3
(a) How much current will an electric bulb draw from a 220 V source,
if the resistance of the bulb filament is 1200 Ω? (b) How much
current will an electric heater coil draw from a 220 V source, if
the resistance of the heater coil is 100 Ω?
Solution
(a) We are given V = 220 V; R = 1200 Ω.
From Eq. (12.6), we have the current I = 220 V/1200 Ω = 0.18 A.
(b) We are given, V = 220 V, R = 100 Ω.
From Eq. (11.6), we have the current I = 220 V/100 Ω = 2.2 A.
Note the difference of current drawn by an electric bulb and electric
heater from the same 220 V source!

Electricity 179

2024-25