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 potential difference and e.m.f.?

Potential difference is the energy transferred per coulomb between two points in a circuit (V = W / Q). E.m.f. is the total energy supplied per coulomb by a source such as a cell. When a current flows, the terminal p.d. is a little less than the e.m.f. because some energy is lost inside the source.

Why is the volt equal to a joule per coulomb?

Because V = W / Q. When W is in joules and Q is in coulombs, dividing them gives joules per coulomb, which is the definition of the volt: 1 V = 1 J C⁻¹.

How do I find the energy if I only know V and the current?

First find the charge using Q = It (current in amperes, time in seconds). Then use W = VQ. For example, V = 6 V, I = 2 A, t = 10 s gives Q = 20 C and W = 6 × 20 = 120 J.

Potential difference Formula, SPM Physics

Potential difference

V = W / Q

Not given in the exam, recall it.

What it is for

Potential difference 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
`V`	potential difference	V
`W`	work done (energy)	J
`Q`	charge	C

Rearrangements

W = VQ
Q = W / V

Worked example

W = 60 J of energy is transferred by Q = 10 C of charge. V = W / Q = 60 / 10 = 6.0 V.

Common trap

W is energy in joules, Q is charge in coulombs. Recall that 1 V = 1 J C⁻¹.

Understanding potential difference

The equation V = W / Q defines potential difference across two points in a circuit. Here V is the potential difference in volts (V), W is the work done, or energy transferred, in joules (J), and Q is the charge that flows in coulombs (C).

The relationship says that potential difference is the energy given to each coulomb of charge as it passes between the two points. If more energy is delivered per unit charge, the potential difference is larger.

This is why 1 V is defined as 1 J C⁻¹.

To find the energy transferred: W = VQ
To find the charge: Q = W / V

Decide which quantity is unknown, rearrange first, then substitute the known values with their units.

A worked example, step by step

A lamp transfers W = 96 J of energy while a charge of Q = 8 C flows through it. Calculate the potential difference across the lamp.

Start from the defining equation:

V = W / Q
V = 96 J / 8 C
V = 12 V

The potential difference across the lamp is 12 V. Notice the unit falls out naturally: joules divided by coulombs gives volts, because 1 V = 1 J C⁻¹.

Quoting the answer to two significant figures, V = 12 V. Keeping the units attached to every line makes it easy to check that the final unit is correct.

Common mistakes and how this is tested

The most common slip is confusing W (energy in joules) with power or with charge. W is energy, not the number of coulombs.

Forgetting that V, not W, is measured across the two points, the p.d. is energy per unit charge, not total energy.
Using minutes for time in a follow-up part where Q = It is needed; charge needs current in amperes and time in seconds.
Writing the wrong rearrangement, for example Q = VW instead of Q = W / V.

This appears under the command word define (state what potential difference means) and calculate (find V, W or Q). Always show the rearranged formula before substituting numbers.

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