Ohm’s Law (Grade 11 NSC Matric Physical Sciences): Revision Notes

Ohm's Law

What is Ohm's law?

Ohm's Law is one of the most fundamental principles in electrical circuits. It describes the relationship between three essential electrical quantities that you need to understand:

• Current (I) - the rate at which electric charge flows through a circuit, measured in amperes (A)
• Voltage (V) - the amount of energy needed per unit charge to move that charge between two points in a circuit, measured in volts (V)
• Resistance (R) - how difficult it is to push current through a circuit element, measured in ohms (Ω)

This means that when you increase the voltage, the current increases proportionally. When you decrease the voltage, the current decreases proportionally. The resistance remains constant for ohmic conductors at constant temperature.

The relationship between current, voltage and resistance

The mathematical relationship discovered by Georg Simon Ohm can be expressed as:

$I = \frac{V}{R}$

This equation tells us that:

• Current equals voltage divided by resistance
• If voltage increases and resistance stays the same, current increases
• If resistance increases and voltage stays the same, current decreases

You can rearrange this equation to solve for different variables:

• $V = IR$ (to find voltage)
• $R = \frac{V}{I}$ (to find resistance)

Ohm's law equation and graph

When you plot voltage on the x-axis and current on the y-axis for an ohmic conductor, you get a straight line passing through the origin.

The gradient (slope) of this straight line is related to the resistance:

• Gradient = $\frac{\Delta I}{\Delta V} = \frac{1}{R}$
• Therefore: $R = \frac{1}{\text{gradient}}$

Practical investigation of Ohm's law

To verify Ohm's Law experimentally, you need to measure current and voltage values for the same resistor under different conditions.

Method:

Part 1 - Varying the voltage:

1. Set up a circuit with one cell and a resistor
2. Measure the voltage across the resistor and current through the circuit
3. Add more cells one by one, recording voltage and current each time
4. Create a data table:

Number of cells	Voltage, V (V)	Current, I (A)
1
2
3
4

Part 2 - Varying the current:

1. Use a fixed voltage but change the total resistance by adding resistors in series
2. Measure voltage across the original resistor and current through the circuit
3. Record your measurements in a similar table

Here's sample data that demonstrates Ohm's Law:

Voltage, V (V)	Current, I (A)
3.0	0.4
6.0	0.8
9.0	1.2
12.0	1.6

Notice how the current doubles when the voltage doubles - this shows the direct proportional relationship.

Ohmic and non-ohmic conductors

Not all electrical components follow Ohm's Law perfectly.

Ohmic conductors:

• Have constant resistance when temperature remains constant
• Follow Ohm's Law exactly
• Produce straight-line I-V graphs
• Examples: metal wires, carbon resistors

Non-ohmic conductors:

• Have resistance that changes with voltage or current
• Do not follow Ohm's Law
• Produce curved I-V graphs
• Examples: light bulbs, diodes, transistors

In a light bulb, the filament wire heats up as current increases. This causes the resistance to increase dramatically, which limits further current increases. The resistance can change by a factor of 10 or more between room temperature and operating temperature.

Using Ohm's law in calculations

When solving circuit problems, always start by drawing a circuit diagram. Then identify what values you know and what you need to find.

Remember!