Not given in the exam, recall it.
What it is for
Electrical power is used in Electricity (SPM Physics Form 5). Keep the SI units in every line of working. Electricity
Symbols and SI units
| Symbol | Meaning | SI unit |
|---|---|---|
| P | electrical power | W |
| V | potential difference | V |
| I | current | A |
| R | resistance | Ω |
Rearrangements
I = P / VR = V² / PR = P / I²
Worked example
Common trap
Understanding Electrical power
Electrical power P is the rate at which electrical energy is transferred, measured in watts (W). It links three quantities: V, the potential difference across the component in volts (V); I, the current through it in amperes (A); and R, its resistance in ohms (Ω).
The three forms P = VI, P = I²R and V²/R are all consistent because Ohm's law (V = IR) lets you substitute one quantity for another. Use P = VI when voltage and current are known; use I²R when current and resistance are known; use V²/R when voltage and resistance are known.
To rearrange, divide to isolate the unknown: I = P / V finds the current, R = V² / P finds the resistance from voltage, and R = P / I² finds it from current.
A worked example, step by step
A heating element of resistance R = 20 Ω carries a current of I = 3 A. Find the power dissipated.
Because current and resistance are known, choose the I²R form:
P = I²R = (3 A)² × 20 Ω = 9 × 20 = 180 W.
Notice that the current is squared before multiplying by the resistance. If you had wrongly used I × R you would get 60, which is the voltage across the element (V = IR = 3 × 20 = 60 V), not the power.
As a check, P = VI = 60 V × 3 A = 180 W gives the same answer to a sensible 2 significant figures. The dissipated power is 180 W.
Common mistakes and how this is tested
The main trap is choosing a form that does not match the known quantities, then forgetting to square. In I²R you square the current; in V²/R you square the voltage.
A second error is confusing power (W) with energy (J), since energy = power × time. A third is mismatched units, such as leaving power in kilowatts while current is in amperes.
Under the command word calculate, you must show the substitution with units before the final answer. Under define, state that power is energy transferred per unit time.
Under explain, you may be asked why a thicker wire dissipates less heat for the same current, which the I²R form makes clear.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English), Sijil Pelajaran Malaysia: Format Pentaksiran mulai 2021, Fizik (4531) (Bahagian Pembangunan Kurikulum (BPK), KPM)