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The equation linking power, current and resistance is P = I²R, where P is power in watts, I is current in amperes and R is resistance in ohms. Free notes.
Power, Current, and Resistance is part of Energy transfers in GCSE AQA Higher Physics.
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The equation linking power, current and resistance is P = I²R, where P is power in watts, I is current in amperes and R is resistance in ohms. Free notes.
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Use this lesson to stay inside Energy transfers while connecting the detail back to the wider Electricity topic.
Read the core explanations from Power, Current, and Resistance before testing yourself from memory.
When you plug in a kettle, it heats water by converting electrical energy to thermal energy. The faster this happens, the more power the kettle uses. Power tells us how quickly energy is being transferred - and why your electricity bill depends on both what you use and for how long.
Understanding power helps explain why phone chargers stay cool but electric heaters get hot, why fuses blow when too much current flows, and how to calculate running costs for electrical appliances.
When current flows through a resistor, electrical energy converts to heat. The power depends on both how much current flows and how much resistance opposes it. This gives us our key relationship:
Notice that current is squared in this formula. This means doubling the current increases power by four times, not two. This explains why high-current appliances like electric showers need thick cables and special circuits.
Let's work through some typical exam calculations. Remember to show all your working clearly - method marks are awarded even if your final answer is wrong.
Worked example: Calculating Power from Current and Resistance
A heating element has resistance 25 Ω. When connected to a power supply, a current of 4.0 A flows through it. Calculate the power dissipated. [3 marks]
Answer: 400 W
Worked example: Finding Current from Power and Resistance
A light bulb has power rating 60 W and resistance 240 Ω. Calculate the current flowing through it. [4 marks]
Answer: 0.50 A
In the required practical, measure how current and potential difference change for a resistor or filament lamp. Set up the circuit with an ammeter in series and a voltmeter across the component, then vary the potential difference and record paired readings. Control heating as far as possible, repeat measurements where needed, and plot the data so you can see how power and resistance behave. This practical links directly to the equations P = IV and P = I²R because changing current changes the power dissipated.
The P = I²R relationship explains many everyday observations. Electric kettles use robust heating elements so they can carry the required current safely. Phone chargers convert the mains supply to a lower voltage that is safe for the device, while the charger is designed to limit current and control heating. Understanding these relationships helps explain why electrical components are designed with specific power ratings and why excess current can cause wires and resistors to overheat.
Power ratings on appliances tell you their energy consumption rate. A 2000W kettle transfers energy twice as fast as a 1000W kettle, so it boils water in half the time but uses the same total energy.
Power calculations are common in GCSE physics papers. Examiners often test your ability to rearrange the formula and apply it to unfamiliar contexts. Here's how to maximise your marks:
Common exam traps: Questions often give you current in milliamps (mA) - remember to convert to amps first. Also watch for questions asking about 'energy' when they mean 'power' - energy is measured in joules, power in watts.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Fuses blow when the current gets too high, preventing dangerous power levels that could cause fires. Excess current can cause wires and resistors to overheat because P = I²R converts electrical energy to thermal energy.
The equation linking power, current and resistance is P = I²R, where P is power in watts, I is current in amperes and R is resistance in ohms. Current is squared, so doubling the current increases power by four times.
Power dissipated is the rate at which electrical energy converts to heat when current flows through a resistor, measured in watts. It is calculated with P = I²R, so a 25 Ω element carrying 4.0 A dissipates 400 W.
Rearrange P = I²R to I² = P/R, then take the square root. A light bulb with a power rating of 60 W and resistance 240 Ω has a current of the square root of 0.25, which is 0.50 A.
LED lights are energy efficient because they use a low current, and low current means low power, since P = I²R. Power is the rate of energy transfer, so a low-power LED transfers less energy each second.
To calculate power with P = I²R, square the current, then times it by the resistance. A heating element of resistance 25 Ω with a current of 4.0 A flowing through it dissipates (4.0)² × 25 = 16 × 25 = 400 W.
Electric showers are high-current appliances, and because current is squared in P = I²R, doubling the current increases power by four times. This is why they need thick cables and special circuits to carry the current safely.
Doubling the current increases the power by four times, not two, because current is squared in P = I²R. This is why high-current appliances like electric showers need thick cables and special circuits.
A heating element with a resistance of 25 Ω and a current of 4.0 A dissipates 400 W. Using P = I²R, P = 4.0² × 25 = 16 × 25 = 400 W.
Car headlights need thick wires because their high power means a large current, and the wires must carry that current safely. Since P = I²R, excess current can cause wires to overheat.
Electric heaters get hot because current flows through a resistance wire, and P = I²R converts electrical energy to thermal energy. The power depends on both how much current flows and how much resistance opposes it.
A 2000 W kettle transfers energy twice as fast as a 1000 W kettle, so it boils water in half the time but uses the same total energy. Power ratings give an appliance's energy consumption rate.
In the I-V practical, an ammeter is placed in series and a voltmeter across a resistor or filament lamp, the potential difference is varied, and paired readings are recorded. It links to P = IV and P = I²R because changing current changes the power dissipated.
The common trap in P = I²R questions is current given in milliamps (mA), which must be converted to amps first. Another trap is mixing up energy, measured in joules, with power, measured in watts.
Phone chargers convert the mains supply to a lower voltage that is safe for the device, and they are designed to limit current and control heating. Because P = I²R, keeping the current low keeps the power converted to heat low.