Electrical Circuits

Ohm’s Law

Ohm’s Law is the named after the German scientist Georg Ohm who in the 1820s conducted experiments investigating the relationship between potential difference and current on various lengths and types of metal conductors.

The findings of his experiments led to Ohm’s Law which states:

In other words, if the current doubles the potential difference doubles or if the current triples the potential difference triples.

Ohm’s Law can also be stated for the current through a resistor as:

The relationship between potential difference, current and resistance is given by the equation:

 

Current – Potential Difference Graphs

Current – potential difference graphs are used to show how the current through a component varies with the potential difference across it.

Current – Potential Difference Graph for a resistor at constant temperature

Current – Potential Difference graph for a Filament Lamp

Current – Potential Difference graph for a Diode

Current – Potential Difference graph for a Thermistor

Current – Potential Difference graph for a Light Dependant Resistor (LDR)

Using the Equation: V = I x R

A simple way of using the Ohm’s Law relationship is to use the triangle method. This involves taking the equation in the form where there is only multiplication and no division. In the case of Ohm’s Law in the form V = I x R and arranging it in a triangle so that V is at the top and I and R at the bottom.

Example

In the circuit below the voltmeter reading across the lamp is 3V and the ammeter reads 2A of electrical current. What is the resistance across the lamp?

Potential Difference across the lamp = V = 3V
Electrical current through the lamp = I = 2A
Electrical resistance across the lamp = R = ?

Using V = I x R

Require resistance therefore cover R in the triangle.

Thus,
R = V / I
R = 3 / 2 = 1.5Ω