Page 100 - Physics 10_Float

CURRENT ELECTRICITY

where ‘ρ’ is the constant of proportionality, known as specific
resistance. Its value depends upon the nature of conductor
i.e., copper, iron, tin, and silver would each have a different
Interesting Information
values of ‘ρ’. Diamond does not conduct
2
If we put L = 1 m, and A = 1 m in Eq. (14.5), then R = ρ, i.e., the electricity, because it has no
resistance of one metre cube of a substance is equal to its free electrons. However, it is
specific resistance. The unit of ‘ρ’ is ohm-metre (Ω m). very good at conducting heat
Example 14.3: If the length of copper wire is 1 m and its because its particles are very
firmly bonded together.
diameter is 2 mm, then find the resistance of this copper wire.
Jewellers can tell if a diamond is
Solution: Given that, length of the wire L = 1 m, diameter a real diamond or a fake one
-3
of the wire d = 2 mm = 2× 10 m made from glass, by holding it
Cross sectional area of the wire to their lips. A real diamond
feels very cold due to good
. 3 14 2 (  10  3 ) m m m 2 2 2 ability of transferring heat four
2
2
A = πd /4 =
ﻰ or five times better than copper.
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A = 3.14 ×10 m 2
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Specific resistance of copper ρ = 1.69 × 10 Ωm
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Now we have R = ρ × L/A = 1.69 × 10 Ωm × 1 m/3.14 × 10 m 2
-2
R = 0.54 ×10 Ω
14.7 CONDUCTORS
Why do we always use metal wires for conduction of electricity?
Because, they are good conductors of electricity and offer less For your information
resistance to the flow of current. But how can they conduct
Specific
electricity with much ease? Metals like silver and copper have
Metal resistance
excess of free electrons which are not held strongly with any (10 Ω m)
-8
particular atom of metals. These free electrons move randomly
in all directions inside metals. When we apply an external electric Silver 1.7
field these electrons can easily move in a specific direction. This Copper
movement of free electrons in a particular direction under the
Aluminium
influence of an external field causes the flow of current in metal
Tungsten
wires. The resistance of conductors increases with increase in
temperature. This is due to increase in the number of collisions of Platinum
electrons with themselves and with the atoms of the metals. Iron 9.8

14.8 INSULATORS Nichrome 100

Graphite 3500
All materials contain electrons. The electrons in insulators,
like rubber, however, are not free to move. They are tightly

100 Not For Sale – PESRP