Page 98 - Physics 10_Float

CURRENT ELECTRICITY

symbol Ω. If a graph is plotted between the current I and the
potential difference V, a straight line will be obtained. For your understanding
Resistance: The property of a substance which offers 1. In order to measure current
opposition to the flow of current through it is called its through a resistance, ammeter
resistance. is always connected in series
with the resistance.
This opposition comes from the collisions of moving
2. In order to measure
electrons with atoms of the substance. potential difference across a
resistance, voltmeter is always
Unit of Resistance: ohm connected in parallel with the
resistance.
The SI unit of resistance R is ohm. If we put V = 1 V, and I = 1 A,
the value of R will be 1 . Thus Ω

When a potential difference of one volt is applied across the
ends of a conductor and one ampere of current passes
through it, then its resistance will be one ohm. Physics Insights

Example 14.2: Reading on voltmeter connected across a
heating element is 60 V. The amount of current passing
through the heating element measured by an ammeter is 2 A.
What is the resistance of the heating coil of the element?
Solution: Given that, V = 60 V, I = 2 A I I
Using Ohm's law
V = RI
V 60 V
-1
or R = = = 30 V A = 30 Ω
I 2 A
14.5 V-I Characteristics of Ohmic and Non Ohmic
Conductors For your information

Ohm's law is valid only for certain materials.

Materials that obey Ohm's law, and hence have a constant Resistance
resistance over a wide range of voltages, are said to be ohmic.
Materials having resistance that changes with voltage or
current are non-ohmic.
Temperature
A thermister is a temperature
Ohmic conductors have a linear voltage- current relationship dependent resistor and its
over a large range of applied voltages (Fig. 14.11-a). The resistance decreases as
straight line shows a constant ratio between voltage and temperature rises. Thermister
current. Ohm's law is obeyed. For example, most metals is used in a circuit that senses
show ohmic behaviour. temperature change.

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