Is this formula given in the exam?

No. SPM Physics papers provide no formula sheet, so this must be recalled.

What is the difference between electrical energy and electrical power?

Power is the rate of energy transfer, P = VI, measured in watts. Energy is the total transferred, E = VIt = Pt, measured in joules. Energy is power multiplied by time.

Why must time be in seconds for E = VIt?

Because the joule is defined so that 1 J = 1 W s. If you leave the time in minutes or hours, the answer will not be in joules. Convert the time to seconds first, or use kWh with time in hours instead.

How do I convert joules to kilowatt-hours?

One kilowatt-hour is the energy used by a 1 kW device in 1 hour, which is 1000 W × 3600 s = 3.6 × 10⁶ J. So divide an energy in joules by 3.6 × 10⁶ to get kilowatt-hours.

Electrical energy Formula, SPM Physics

Electrical energy

E = VIt

Not given in the exam, recall it.

What it is for

Electrical energy 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
`E`	electrical energy	J
`V`	potential difference	V
`I`	current	A
`t`	time	s

Rearrangements

V = E / (It)
I = E / (Vt)
t = E / (VI)

Worked example

A device runs at V = 12 V and I = 2.0 A for t = 60 s. E = VIt = 12 × 2.0 × 60 = 1440 J.

Common trap

Time must be in seconds; energy is in joules (1 J = 1 W s). For kWh, use E = Pt with P in kW and t in hours.

Understanding electrical energy

The equation E = VIt gives the electrical energy transferred by a component. Here E is the energy in joules (J), V is the potential difference in volts (V), I is the current in amperes (A), and t is the time in seconds (s).

The relationship builds on power: since power P = VI, the energy is just the power multiplied by the time it operates. More voltage, more current, or a longer running time all increase the energy delivered.

To find the p.d.: V = E / (It)
To find the current: I = E / (Vt)
To find the time: t = E / (VI)

Keep time in seconds so the energy comes out in joules (1 J = 1 W s).

A worked example, step by step

An appliance operates at V = 240 V drawing a current of I = 0.5 A for t = 120 s. Calculate the electrical energy transferred.

Substitute directly into the equation:

E = VIt
E = 240 V × 0.5 A × 120 s
E = 14400 J = 1.44 × 10⁴ J

The energy transferred is 1.44 × 10⁴ J (three significant figures). Because time was kept in seconds, the answer is correctly in joules.

If the time had been given as 2 minutes, it must first be converted to 120 s before substituting, or the answer would be sixty times too small.

Common mistakes and how this is tested

The most frequent slip is leaving time in minutes or hours; for joules, time must be in seconds.

Confusing energy (E, in joules) with power (P, in watts). Power is energy per second.
Forgetting that the kilowatt-hour is a unit of energy: for kWh use E = Pt with P in kW and t in hours.
Using the wrong rearrangement, for example t = VIE instead of t = E / (VI).

This is tested under calculate (find E, V, I or t) and explain (why energy depends on both power and time). Billing questions often mix joules and kilowatt-hours, so watch the units the question asks for.

Source: DSKP KSSM Physics Form 4 and 5 (Versi English), Sijil Pelajaran Malaysia: Format Pentaksiran mulai 2021, Fizik (4531) (Bahagian Pembangunan Kurikulum (BPK), KPM)

Written by the spmphysics.com.my editorial team.· Updated 5 Sept 2026