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Ohm's Law

Ohm's Law is a relationship between potential difference applied across a conductor to the electric current flowing in it.

You must have experienced the following situations in daily life:

(1) Fluctuations in voltage which leads to dimming or brightening of electric bulb. (This is the reason why we use stabilizers for ACs - to prevent any damage due to voltage fluctuations ) 
(2)  While playing with LEDs, torch bulb, electric cells and connecting wires you must have observed that by increasing the number of cells in the circuit the LEDs glows brighter.
(3) A 12V battery provides a large amount of current than a 3V battery.

This suggests that there must be a relation between voltage (potential difference) and electric current. This relationship is known as Ohm's Law.

Ohm's law was given by George Simon Ohm in the year 1827. This is a relation between potential difference (V) applied across the ends of a conductor to the electric current (I) flowing in the conductor

According the Ohm's Law, the electric current flowing in a conductor is directly proportional to the potential difference (V) applied across the ends of a conductor provided the temperature of the wire remains the constant.

                              I    ∝    V
or it can also be written as

                            V  ∝    I
removing the proportionality sign and introducing a constant

                            V   =  R I

Where R is a constant known as the resistance of the given conductor.


The V–I graph is a straight line that passes through the origin of the graph, as shown in Figure. Thus,
is a constant ratio.


Physically, resistance is defined as a property of a conductor to resist the flow of charge through it.

The SI unit of resistance is ohm (Ω)  (Greek symbol omega)

A conductor is said to have a resistance of 1Ω when a potential difference of 1V is applied across the ends of a conductor and a current of 1A flows through it.

A fan regulator is actually a variable resistor. It changes the resistance of the circuit and thereby we can control the speed of the fan.

To test your knowledge on Ohm's Law, attempt the following test.
you will get your score immediately. 

Click the link below to attempt the test:

Some more questions on Ohm's Law:

1. Name and state the law which relates the current in a conductor to the potential difference across a conductor and the current flowing through it.

2. Let the resistance of an electrical component remains constant while the potential difference across the two ends of the component decreases to half of its former value. What change will occur in the current through it?

3. When a 12 V battery is connected across an unknown resistor, there is a current of 2.5 mA in the circuit. Find the value of the resistance of the resistor.

4. (a) How much current will an electric bulb draw from a 220 V source, if the resistance of the bulb filament is 1200 Ω? (b) How much current will an electric heater coil draw from a 220 V source, if the resistance of the heater coil is 100 Ω?

5. The potential difference between the terminals of an electric heater is 60 V when it draws a current of 
4A from the source. What current will the heater draw if the potential difference is increased to 120 V?

6. The values of current I flowing in a given resistor for the corresponding values of potential difference V across the resistor are given below –

I (amperes) 0.5    1.0     2.0      3.0       4.0
V (volts)     1.6    3.4     6.7     10.2     13.2
Plot a graph between V and I and calculate the resistance of that resistor.

7. Keeping the potential difference constant, the resistance of a circuit is doubled. By how much does the current change?

Other topics on the blog on theme Electricity:

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