Page 99 - Physics 10_Float

CURRENT ELECTRICITY

Non ohmic materials have a non linear voltage-current
relationship. For example, filament lamp, and thermister. Voltage
(V)
The resistance of filament rises (current decreases) as it
Crurrent (A)
gets hotter, which is shown by the gradient getting steeper
(a)
(Fig.14.11-b). A thermister (a heat sensitive resistor)
behaves in the opposite way. Its resistance decreases
Voltage
(current increases) as it gets hotter (Fig. 14.11-c). This is (V)
because on heating, more free electrons become available
Crurrent (A)
for conduction of current.
(b)

14.6 FACTORS AFFECTING RESISTANCE Voltage
(V)
A short pipe offers less resistance to water flow than a long
pipe. Also the pipe with larger cross sectional area offers less Crurrent (A)
(c)
resistance than the pipe having smaller cross sectional area.
Same is the case for the resistance of wires that carry current. Fig.14.11: Voltage vs current
The resistance of a wire depends both on the cross sectional graph for
area and length of the wire and on the nature of the material (a) Fixed resistance
of the wire. Thick wires have less resistance than thin wires. (b) Filament lamp
(c) Thermister
Longer wires have more resistance than short wires. Copper
wire has less resistance than steel wire of the same size.
Electrical resistance also depends on temperature.
At a certain temperature and for a particular substance
1. The resistance R of the wire is directly proportional
to the length of the wire i.e.,
q
F 
q
R L ....... (14.3) Point to ponder!
1 2
It means, if we double the length of the wire, its resistance
will also be doubled, and if its length is halved, its resistance
would become one half.
2. The resistance R of the wire is inversely proportional
to the area of cross section A of the wire i.e.,
1
R ....... (14.4)
q
q
F 

1
A 2
The current versus voltage
It means that a thick wire would have smaller resistance than graph of a resistor is a straight
a thin wire. line with a constant slope. The
After combining the two equations, we get graph for light bulb is curved
L
with a decreasing slope. What
R q q can you infer from this?
F 
1
A 2
L
R = ρ ........ (14.5)
A
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