Electricity: Paper 3 Guide (Form 5 SPM Physics)

Paper 3 at a glance

• Paper 3 carries 15 marks

Experiment: Comparing ohmic and non-ohmic conductors

To compare the current–voltage characteristics of an ohmic conductor (a constantan wire) and a non-ohmic component (a filament bulb).

• Manipulated: Potential difference across the component, V
• Responding: Current through the component, I
• Constant: The component under test (and, for the ohmic wire, its temperature)
• Graph: Plot I (y-axis) against V (x-axis) for both components. The ohmic wire gives a straight line through the origin (constant resistance); the filament bulb gives a curve that bends over.
• Analysis: For the ohmic conductor V / I is constant, so it obeys Ohm's law. For the filament bulb the resistance R = V / I increases as V increases, because the filament gets hotter; it is non-ohmic.

• Full experiment page

Experiment: How the length of a wire affects its resistance

To investigate the relationship between the length of a wire and its resistance.

• Manipulated: Length of the wire, l
• Responding: Resistance of the wire, R
• Constant: The cross-sectional area and material of the wire, and its temperature
• Graph: Plot R (y-axis) against l (x-axis). A straight line through the origin shows the resistance is directly proportional to the length.
• Analysis: Since R = ρl / A, the resistance is directly proportional to the length when the area and material are unchanged. The gradient of the R against l graph equals ρ / A.

• Full experiment page

Experiment: Determining the e.m.f. and internal resistance of a dry cell

To determine the electromotive force (e.m.f.) and the internal resistance of a dry cell.

• Manipulated: Current drawn from the cell, I
• Responding: Terminal potential difference, V
• Constant: The same dry cell
• Graph: Plot V (y-axis) against I (x-axis). The points give a straight line with a negative gradient; the intercept on the V-axis is the e.m.f. and the magnitude of the gradient is the internal resistance.
• Analysis: The terminal voltage obeys V = ε − Ir. The intercept of the V against I graph on the V-axis gives the e.m.f. ε, and the magnitude of the gradient gives the internal resistance r.

• Full experiment page

Paper 3 skill checklist

• Table: quantities with units in the headings, consistent decimal places, derived column calculated correctly.
• Graph: axes labelled with unit, scale using more than half the grid, points plotted with a sharp pencil, best-fit line.
• Gradient: a large triangle on the line, coordinates shown, unit of the gradient stated.
• Relationship: "y increases linearly with x" or "y is directly proportional to x" only if the line passes through the origin.
• Precaution: one that improves accuracy, stated with its reason.

More on graph skills

• Graph skills hub
• All Paper 3 experiments

More Electricity resources

• Chapter overview: Electricity
• Revision Notes
• Common Mistakes
• Practice Questions
• Paper 2 Answering Guide
• Key Terms
• Calculation practice sets

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