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German physicist George Simon Ohm (1789-1854) published this law in 1827 after conducting studies on the relationship between the applied voltage and current flowing through various length of wire. Ohm's law holds good only for metallic conductors at moderate temperatures. At very low and very high temperatures even metallic conductors do not obey Ohm's law. This law is not obeyed by vacuum tubes, discharge tubes, semiconductors and electrolytes.

Device that obey Ohm's law are known as ohmic devices and those which do not obey Ohm's law are called non-ohmic devices. Resistors are ohmic devices whereas thermistors, diodes and transistors are non ohmic devices.

Define Ohm's Law

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Statement of this law is : At moderate constant temperature with other physical conditions remaining unaltered, the magnitude of electric current (I) through a conductor is directly proportional to the potential difference(V) applied across its ends.
I∝V

I =
$\frac{V}{R}$
where,
I is current
V is the potential difference
R is the resistance

The value of resistance depends upon the material and its dimensions.

Ohms Resistance

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From the above equation it is clear that, current, voltage and resistance are connected to each other. Among these variables R is material dependent one.

Consider a conductor of length l and cross sectional area A, then, the resistance of the wire is given by,
R = $\frac{\rho l}{A}$

where, $\rho$ is the specific resistance expressed in Ωm which is a property of the material of the wire.

As $\rho$ and dimensions of the wire change with temperature, the resistance R also changes with temperature.

Ohms Power Law

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Ohm's power law gives the idea about the power in terms resistance, current and voltage. The equations related to the power law is given below:
  • In terms of voltage and current, P = VI
  • In terms of current and resistance, P = $I^{2}$ R
  • In terms of voltage and resistance, P = $\frac{V^{2}}{R}$

Ohm's Law Experiment

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The given experiment helps to verify the Ohm's law.

Material required:
  • Voltage supply (10V)
  • Resistors (5Ω and 10Ω)
  • Conducting wire
  • Ammeter

Procedure:

Construct a small circuit using these component. First connect 5Ω resistance to the circuit. Measure the current flowing through the circuit by using the ammeter. Calculate the current using the equation also. And verify the results. Then connect the 10Ω resistance to the circuit. Check the result with ammeter as well as by manual calculation.

Result:

The measured current is high in the first case. From that we can conclude that current is inversely proportional to resistance. Hence Ohm's law is proved.

Ohm's Law Problems

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Problems related to Ohm's law are discussed here.

Solved Examples

Question 1: A potential difference of 3V causes a current of 240μA to flow in a conductor. Calculate the resistance of the conductor?

Solution:
 
From the question it is given that,
V = 3V and I = 240μA = 240×10-6A

According to Ohm's law,
I = $\frac{V}{R}$
So, R = $\frac{V}{I}$
R = $\frac{3}{240×10^{-6}}$
R = 12.5KΩ
 

Question 2: What is the voltage across an electric heater of resistance 10Ω through which passes a current of 20A?

Solution:
 
From the question it is given that,
R = 10Ω and I = 20A

According to Ohm's law,
I = $\frac{V}{R}$
So, V = IR
V = 10×20 = 200V