Monday, February 22, 2010

Ohm’s Law

Ohm’s law states that the electric current, I flowing through a conductor is directly proportional to the potential difference across the ends of the conductor , if temperature and other physical conditions remain constant.

Mathematically, Ohm’s Law is written: V = IR

V = potential difference [V]
I = current [A]
R = resistance [Ω]

Ohm’s Law tells us that if a conductor is at a constant temperature, the voltage across the
ends of the conductor is proportional to the current. This means that if we plot voltage on the
y-axis of a graph and current on the x-axis of the graph, we will get a straight-line. The
gradient of the straight-line graph is then the resistance of the conductor.

The resistance, R is a term that describes the opposition experienced by the electrons as they flow in a conductor. It is also defined as the ratio of the potential difference across the conductor to the current, I flowing through the conductor.

Non-ohmic conductors

Non-ohmic conductors are conductors which do not obey Ohm’s Law.
E.g., a light bulb whose resistance increases over time due to temperature increase (heating effect of current)
Note: In this whole chapter, all conductors and light bulbs are assumed to follow Ohm’s Law. There are no non-ohmic problems at SPM level. In general non-ohmic conductors have plots of voltage against current that are curved,
indicating that the resistance is not constant over all values of voltage and current.

Worked example :
Question :
What is the value of the resistor in the figure, if the dry cells supply 2.0 V and the ammeter reading is 0.5 A?

Solution :
V = IR
2.0 = 0.5 (R)
R = 4 Ω



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