In 1827, Georg Simon ohm, a German scientist discovered the most fundamental law of electricity. He discovered that the current flowing through a conductor is directly proportional to the potential difference or voltage and inversely proportional to the resistance of conductor. He state it ‘Ohm Law‘.
Ohm law deals with the relationship between current, voltage and resistance. Ohm law state that the current flowing through a conductor is directly proportional to the potential difference across its end, provided the physical conditions (temperature, pressure, etc.) remains constant.
Mathematic expressions of ohm law
Let us consider a resistive circuits is connected across a battery of V volts. Let I is the current flow in the circuit. Then according to ohm law I is directly proportional to V i.e.
I ∝ V
V/I = R = constant
Where R is a constant of proportionality and is called resistance of the circuit.
From the above equations ohm law can also be defined that the ratio of voltage provided across a conductor current to the current flowing through conductor is always constant or equal to resistance of the conductor.
i.e. V/I = constant = R
V represent voltage and measured in volt.
I represent current and measured in Ampere.
R represent resistance and measured in ohm.
The above equation can be written as
I = V/R (amp)
V = IR (volt)
R = V/I (ohm)
Ohm’s law is not applicable for the circuits containing electronic tubes or transistors, diode etc. because such elements are not bilateral i.e. they behave in a different manner when the direction of flow of current is reversed. Ohm’s law is also not valid for the circuits consisting non-linear elements such as powered carbon, thyrite, electric arc etc.
Limitations of Ohm laws
- It is not applicable to the nonlinear devices such as diodes, zener diode, voltage regulators etc.
2.It cannot be applied if circuit consist unilateral elements like transistor, diode etc.
3. It is not good for non metallic conductor.
Graphical representation of ohm law
Let us consider a resistor of resistance R is connected across a variable voltage source. the voltage value of variable voltage source varies 0V from is 20V.
Let an ammeter is connected in series with resistor. When the potential difference or voltage across resistor is zero volt, the ammeter reads zero ampere.
As the voltage increases the current in the circuit also increases simultaneously.
When voltage is increases to 5 volt, let us the ammeter reads 1 ampere. Then the resistance of circuit is calculated as 5 ohm.
R = V/I = 5/1 = 5 ohm
When voltage is increased to 10 volt, the ammeter must read 2 ampere. because we never change the resistance of circuit. Resistance is still 5 ohm.
R = V/I = 10/2 = 5 ohm
When voltage is increases to 20 Volt, the ammeter must read 4 ampere.
R = V/I = 20/4 = 5 ohm
We observed that the ratio of voltage to the current is same for each situations. Thus, ohm law is verified.
If we draw a graph between potential difference and current it will a straight line passing through the origin.
Example: Find the current flowing through a circuit consisting a bulb having constant resistance of 400 Ω and supply voltage is 220 V.
Potential difference across the bulb, V = 220 volts
Resistance of the circuit, R = 400 Ω
Current flowing through the circuit = ?
According to the ohm law, I = V/R
Current, I = V/R = 220/400 = 0.55 Amp
Therefore current flowing through the circuit is 0.55 Ampere.