Is this formula given in the exam?

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

Does Ohm's law apply to every component?

No. It applies only to ohmic conductors kept at constant temperature, where current is proportional to potential difference. A filament lamp and a diode are non-ohmic, so their V–I graphs are not straight lines through the origin.

How do I read resistance from a V–I graph?

For a straight-line V–I graph through the origin, the resistance equals the gradient, R = V / I. Take a large pair of readings from the line rather than a single close-together point to reduce error.

What happens to the current if I double the resistance?

For a fixed potential difference, doubling R halves the current, because I = V / R. For example, at V = 12 V, R = 6 Ω gives I = 2 A, while R = 12 Ω gives I = 1 A.

Ohm's law Formula, SPM Physics

Ohm's law

V = IR (R = V / I)

Not given in the exam, recall it.

What it is for

Ohm's law 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
`I`	current	A
`R`	resistance	Ω

Rearrangements

R = V / I
I = V / R

Worked example

A current I = 3.0 A flows through a resistor R = 4.0 Ω. V = IR = 3.0 × 4.0 = 12 V.

Common trap

Ohm's law holds only for ohmic conductors at constant temperature. Resistance R = V / I.

Understanding Ohm's law

The equation V = IR relates the potential difference across a component to the current through it. Here V is the potential difference in volts (V), I is the current in amperes (A), and R is the resistance in ohms (Ω).

The relationship says that, for an ohmic conductor at constant temperature, the current is directly proportional to the potential difference. The resistance R is the constant of proportionality: a larger R means less current for the same voltage.

To find resistance: R = V / I
To find current: I = V / R

Choose the unknown, rearrange the formula, then substitute the values with units so the answer comes out in the correct unit.

A worked example, step by step

A potential difference of V = 6.0 V is applied across a resistor of R = 12 Ω. Calculate the current through it.

Rearrange Ohm's law to make current the subject:

I = V / R
I = 6.0 V / 12 Ω
I = 0.50 A

The current through the resistor is 0.50 A. The unit works out correctly because V / Ω = A. The answer is given to two significant figures, matching the data.

Writing the rearranged formula first, then substituting with units, keeps the working clear and helps avoid dividing the wrong way round.

Common mistakes and how this is tested

The main trap is applying Ohm's law to a non-ohmic component. It holds only for an ohmic conductor at constant temperature; a filament lamp does not obey it because its resistance rises as it heats.

Dividing the wrong way, for example I = R / V instead of I = V / R.
Reading a graph incorrectly: for a V–I graph the resistance is the gradient, not a single point value.
Forgetting to convert mA to A or kΩ to Ω before substituting.

This is tested under calculate (find V, I or R) and explain (why a filament lamp is non-ohmic). Reference the V–I graph carefully when the question gives one.

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