Ohm's Law: GCSE Physics Formula, Examples & Circuit Calculations

Ohm’s law is one of the fundamental relationships in GCSE Physics and forms the basis of nearly every electricity question you will face in your exam. Once you understand the formula and can rearrange it confidently, circuit calculations become straightforward.

What Is Ohm’s Law?

Ohm’s law states that the potential difference (voltage) across a conductor is directly proportional to the current flowing through it, provided the temperature remains constant. As a formula:

V = I x R

Where:

• V = potential difference, measured in volts (V)
• I = current, measured in amperes (A)
• R = resistance, measured in ohms (ohm symbol)

This means that if you increase the voltage across a component while its resistance stays the same, the current will increase proportionally. Double the voltage, double the current.

The Formula Triangle

A useful way to remember all three rearrangements is the VIR triangle. Place V at the top and I and R side by side at the bottom:

• To find V: cover V, and you are left with I x R, so V = IR
• To find I: cover I, and you are left with V over R, so I = V / R
• To find R: cover R, and you are left with V over I, so R = V / I

While the triangle is a helpful memory aid, practising algebraic rearrangement is the better long-term skill since formula triangles do not work for more complex equations.

Worked Examples

Example 1: Finding Voltage

A current of 3 A flows through a 4 ohm resistor. What is the potential difference across it?

Known values: I = 3 A, R = 4 ohm

Find: V

V = I x R = 3 x 4 = 12 V

Example 2: Finding Current

A 6 V battery is connected across a 12 ohm resistor. What current flows?

Known values: V = 6 V, R = 12 ohm

Find: I

I = V / R = 6 / 12 = 0.5 A

Example 3: Finding Resistance

A component has a potential difference of 230 V across it and draws a current of 10 A. What is its resistance?

Known values: V = 230 V, I = 10 A

Find: R

R = V / I = 230 / 10 = 23 ohm

Ohm’s Law in Series Circuits

In a series circuit, the components are connected one after another in a single loop. The key rules are:

• The current is the same at every point in the circuit
• The total voltage of the supply is shared between the components
• The total resistance is the sum of all individual resistances: R_total = R₁ + R₂ + R₃

Example: Two Resistors in Series

A 6 V battery is connected to a 4 ohm resistor and an 8 ohm resistor in series.

R_total = 4 + 8 = 12 ohm

I = V / R_total = 6 / 12 = 0.5 A

Voltage across the 4 ohm resistor: V = IR = 0.5 x 4 = 2 V

Voltage across the 8 ohm resistor: V = IR = 0.5 x 8 = 4 V

Check: 2 V + 4 V = 6 V (matches the battery voltage).

Ohm’s Law in Parallel Circuits

In a parallel circuit, the components are connected on separate branches. The key rules are:

• The voltage is the same across each branch
• The total current splits between the branches
• The total resistance is found using: 1/R_total = 1/R₁ + 1/R₂

Example: Two Resistors in Parallel

A 12 V supply is connected to a 6 ohm resistor and a 12 ohm resistor in parallel.

Current through the 6 ohm resistor: I = 12 / 6 = 2 A

Current through the 12 ohm resistor: I = 12 / 12 = 1 A

Total current from the supply: 2 + 1 = 3 A

Total resistance: 1/R = 1/6 + 1/12 = 2/12 + 1/12 = 3/12, so R = 4 ohm

Check: V = I x R = 3 x 4 = 12 V (matches the supply).

Notice that the total resistance in parallel is always less than the smallest individual resistance. This catches many students out.

The Required Practical

One of the required practicals for GCSE Physics involves investigating how the current through a component varies with potential difference. You connect the component in a circuit with a variable resistor (or use a variable power supply), an ammeter in series, and a voltmeter in parallel across the component.

By adjusting the variable resistor and recording pairs of voltage and current readings, you can plot a V-I graph. For an ohmic conductor (like a wire at constant temperature), the graph is a straight line through the origin, confirming that V and I are directly proportional. The gradient of this line equals the resistance.

For a filament lamp, the graph curves because the resistance increases as the filament gets hotter. For a diode, current only flows in one direction, giving a characteristic shape with near-zero current for negative voltages.

Rearranging Ohm’s Law

Exam questions frequently test whether you can rearrange the formula correctly. Practise these three forms until they are automatic:

• V = IR (finding voltage)
• I = V/R (finding current)
• R = V/I (finding resistance)

At Higher tier, you may need to combine Ohm’s law with the power equations (P = IV and P = I²R) to solve multi-step problems. For example, finding the current first using P = IV, then using V = IR to find resistance.

Common Mistakes to Avoid

Confusing Milliamps and Amps

Many exam questions give current in milliamps (mA). You must convert to amps before substituting into V = IR. There are 1000 mA in 1 A, so divide by 1000. For example, 250 mA = 0.25 A. Forgetting this conversion is one of the most common reasons for losing marks.

Mixing Up Series and Parallel Rules

In series, resistances add up. In parallel, you use the reciprocal formula. Students frequently apply the wrong rule. A quick sense check: parallel resistance should always be less than the smallest individual resistor.

Forgetting That Ohm’s Law Has Conditions

Ohm’s law only applies when temperature (and other physical conditions) remain constant. A filament lamp does not obey Ohm’s law because its resistance changes as it heats up. Be precise in exam answers: an ohmic conductor has a linear V-I graph through the origin.

Not Showing Working

Even simple V = IR calculations carry method marks. Write the formula, show your substitution, then give the answer with units. Never skip straight to the final number.

Exam Technique Tips

State the formula first. Before substituting, write V = IR (or whichever rearrangement you need). This earns the first mark and helps you organise your working.

Always include units. Volts, amps, and ohms must appear in your final answer. Losing a unit mark on a straightforward calculation is an avoidable error.

Check reasonableness. Household mains voltage is 230 V. A AA battery is 1.5 V. A typical LED draws about 20 mA. If your calculated current through a small circuit is 500 A, something has gone wrong.

Summary

Ohm’s law (V = IR) links voltage, current, and resistance. Learn the three rearrangements, understand how resistance combines in series and parallel circuits, and always convert milliamps to amps. With practice, electricity questions become some of the most reliable marks on the paper.

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