When a circuit changes, ask how much energy is transferred each second and where it goes. The link is power: P = I × V, and the energy transferred is E = P × t.
This skill sits within physical reasoning in mixed tasks because the same energy idea explains lamps, heaters and motors. All numbers below are invented for teaching.
How does an observation turn into an energy statement?
A lamp is brighter because more energy is transferred as light and heat each second. Power measures exactly that, in watts, so a brighter lamp has a higher power.
Power depends on two things you can measure: the current through the lamp in amperes (A) and the potential difference across it in volts (V). Multiply them to get watts. Then multiply by time in seconds to get joules.
How do I calculate it, step by step?
- Write the observation as a change in energy transfer: more or less energy each second.
- List the values with units: current I, potential difference V, time t.
- Calculate power using P = I × V.
- Convert time to seconds, then calculate energy using E = P × t.
- Say where the energy goes, for example electrical energy to light and thermal energy.
Worked example
A lamp is connected across a 6.0 V supply and the ammeter reads 0.50 A. Find its power and the energy it transfers in 20 s.
Step 1, observation: the lamp glows steadily, so energy is being transferred continuously.
Step 2, values: V = 6.0 V, I = 0.50 A, t = 20 s.
Step 3, power: P = I × V = 0.50 × 6.0 = 3.0 W.
Step 4, energy: E = P × t = 3.0 × 20 = 60 J.
Check by another route: charge Q = I × t = 0.50 × 20 = 10 C, and energy = Q × V = 10 × 6.0 = 60 J. Both routes agree.
Step 5, where it goes: the electrical energy is transferred to light and to thermal energy in the surroundings.
What mistake should I watch for?
A common slip is to leave the time in minutes.
Mistaken working: the same lamp runs for 2 minutes, so E = 3.0 × 2 = 6 J
The watt is a joule per second, so the time must be in seconds.
The correction is to convert first: 2 minutes = 120 s, so E = 3.0 × 120 = 360 J. A quick sense check helps too. A lamp that transfers 3 J every second cannot transfer only 6 J in two minutes.
Check yourself
Work these out, then open each answer.
1. A heater element has a current of 2.0 A at 12 V. What is its power?
Show answer
P = I × V = 2.0 × 12 = 24 W.
2. How much energy does the 24 W heater transfer in 30 s?
Show answer
E = P × t = 24 × 30 = 720 J.
3. A second cell is added and the lamp from the example gets brighter. Explain in terms of energy, without new numbers.
Show answer
The potential difference across the lamp increases, so the current increases too. Power P = I × V rises, which means more electrical energy is transferred to light and thermal energy each second.
Where this leads next
Energy transfer is also the key to heating, so continue with explaining a thermal change using two models. To see how this lesson fits the whole module, return to physical reasoning in mixed tasks.
Many students follow each formula yet hesitate when a question starts from an observation instead of numbers. That gap is something our teachers work on in online one-to-one Combined Science tuition.