Electricity: Revision Notes (Form 5 SPM Physics)

What this chapter covers

• 3.1 Current and Potential Difference
• 3.2 Resistance
• 3.3 Electromotive Force (e.m.f.) and Internal Resistance
• 3.4 Electrical Energy and Power

Key ideas explained

• What potential difference really is, Potential difference is the energy given to each unit of charge as it moves between two points. It is what pushes current through a component and is measured in volts.
• Why resistors in parallel lower the total resistance, Adding a resistor in parallel opens another path for current to flow. More paths let more current through for the same voltage, so the combined resistance is lower than any single resistor.
• What e.m.f. and internal resistance mean, E.m.f. is the total energy a cell gives each unit of charge. Internal resistance is the cell's own resistance, which uses up some of that energy, so the voltage available to the circuit drops when current flows.

Formulas to memorise (none are given in the exam)

• Q = It, Electric charge, Q = electric charge (C); I = current (A); t = time (s)
• V = W / Q, Potential difference, V = potential difference (V); W = work done (energy) (J); Q = charge (C)
• V = IR (R = V / I), Ohm's law, V = potential difference (V); I = current (A); R = resistance (Ω)
• R = R₁ + R₂ + …, Resistors in series, R = total resistance (Ω); R₁ = first resistance (Ω); R₂ = second resistance (Ω)
• 1/R = 1/R₁ + 1/R₂ + …, Resistors in parallel, R = total resistance (Ω); R₁ = first resistance (Ω); R₂ = second resistance (Ω)
• ε = IR + Ir = V + Ir, Electromotive force, ε = electromotive force (e.m.f.) (V); I = current (A); R = external resistance (Ω); r = internal resistance (Ω); V = terminal potential difference (V)
• E = VIt, Electrical energy, E = electrical energy (J); V = potential difference (V); I = current (A); t = time (s)
• P = VI = I²R = V²/R, Electrical power, P = electrical power (W); V = potential difference (V); I = current (A); R = resistance (Ω)

Experiments the DSKP names

• 3.2.1 · Compare and contrast ohmic and non-ohmic conductors
• 3.2.4 · Investigate the factors affecting the resistance of a wire
• 3.3.3 · Determine the e.m.f. and internal resistance of a dry cell

Terms you must define precisely

• Electromotive force, The energy supplied by a source to move one coulomb of charge around a complete circuit; measured in volts (V).
• Internal resistance, The resistance within a source of electromotive force, such as a cell, which causes some energy to be lost inside the source.
• Electrical energy, The energy transferred by an electric current, equal to power multiplied by time; measured in joules (J).
• Electrical power, The rate at which electrical energy is transferred or used (power = potential difference × current); measured in watts (W).
• Resistivity, A property of a material that measures how strongly it opposes the flow of current, independent of the size or shape of the sample; measured in ohm metres (Ω m).
• Electric charge, A basic property of matter that causes it to experience a force in an electric field; measured in coulombs (C).
• Electric current, The rate of flow of electric charge through a conductor (current = charge ÷ time); measured in amperes (A).
• Potential difference, The work done to move one coulomb of charge between two points in a circuit; measured in volts (V).

A revision order that works

1. Derive each formula once from its meaning, then practise rearranging it before substituting numbers.
2. For each experiment, write the manipulated, responding and constant variables from memory, then the graph you would plot.
3. Finish with a timed set of calculations with units on every line.
4. Read the 4 standards above and tick the ones you can already explain aloud without notes.
5. Rewrite every definition in one sentence with the correct unit; definitions are the cheapest marks in Paper 2.

More Electricity resources

• Chapter overview: Electricity
• Common Mistakes
• Practice Questions
• Paper 2 Answering Guide
• Paper 3 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)