Physics · Grades 9–12
Electrical energy and power
Combine voltage, current, resistance and time without mixing units.
Learn the idea
Power
Electrical power in a steady DC element is potential difference times current.
Ohmic substitutions
For a resistor, substitute V = IR to obtain P = I²R or V²/R under the same assumptions.
Energy
Energy equals power times time when power is constant. Use compatible units such as W and s, or kW and h.
No hidden claims
A nameplate value is not necessarily the actual power under every operating condition; supplied values define each model.
Worked examples
A component has 6 V across it and current 2 A. Find electrical power.
P = VI = 6 × 2 = 12 W.
12 W
A 20 W device operates for 30 s. Find energy transferred.
E = Pt = 20 × 30 = 600 J.
600 J
Check your understanding
A 500 W device runs for 4 h at that constant power. Find energy in kWh.
Which expression gives power in an ohmic resistor?
- I²R
- IR² in all cases
- I/R
Why is writing kW per hour instead of kWh a different quantity?
Answers and marking notes
Answers and marking notes
- 2 kWh
0.5 kW × 4 h = 2 kWh.
- I²R
Substituting V = IR into P = VI gives I²R.
- kWh multiplies power by time to measure energy; kW/h describes a rate of change of power.
Credit multiplication versus division and their dimensions.
Further reading and review references
These are reference links, not copied exercises, institutional endorsement or proof of independent review.
Independent educational review required. Grade bands are approximate; these are not officially aligned curricula.