Questions
1. Calculate: A current I = 2.0 A flows for t = 30 s. Find the unknown using Q = It.
Q = It = 2.0 × 30 = 60 C.
2. Calculate: W = 60 J of energy is transferred by Q = 10 C of charge. Find the unknown using V = W / Q.
V = W / Q = 60 / 10 = 6.0 V.
3. Calculate: A current I = 3.0 A flows through a resistor R = 4.0 Ω. Find the unknown using V = IR (R = V / I).
V = IR = 3.0 × 4.0 = 12 V.
4. Calculate: Two resistors R₁ = 3 Ω and R₂ = 5 Ω are in series. Find the unknown using R = R₁ + R₂ + ….
R = R₁ + R₂ = 3 + 5 = 8 Ω.
5. Calculate: Two resistors R₁ = 6 Ω and R₂ = 3 Ω are in parallel. Find the unknown using 1/R = 1/R₁ + 1/R₂ + ….
1/R = 1/6 + 1/3 = 1/6 + 2/6 = 3/6 = 1/2, so R = 2 Ω.
6. Calculate: A cell of ε = 1.5 V and r = 1.0 Ω drives a current I = 0.5 A. Find the unknown using ε = IR + Ir = V + Ir.
V = ε − Ir = 1.5 − (0.5 × 1.0) = 1.0 V (terminal p.d.).
7. Calculate: A device runs at V = 12 V and I = 2.0 A for t = 60 s. Find the unknown using E = VIt.
E = VIt = 12 × 2.0 × 60 = 1440 J.
8. Calculate: An appliance draws I = 5.0 A at V = 240 V. Find the unknown using P = VI = I²R = V²/R.
P = VI = 240 × 5.0 = 1200 W = 1.2 kW.
9. State the manipulated, responding and constant variables for: 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).
10. State the manipulated, responding and constant variables for: 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.
11. State the manipulated, responding and constant variables for: 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.
12. Explain: 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.
13. Explain: 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.
14. Explain: 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.
15. Define resistance.
The opposition of a component to the flow of electric current, equal to potential difference divided by current; measured in ohms (Ω).
16. Define ohm's law.
The current through a conductor is directly proportional to the potential difference across it, provided its temperature stays constant.
17. Define series circuit.
A circuit in which components are connected end to end in a single path, so the same current flows through each.
18. Define parallel circuit.
A circuit in which components are connected across common points, providing more than one path for the current, with the same potential difference across each branch.
19. Define electromotive force.
The energy supplied by a source to move one coulomb of charge around a complete circuit; measured in volts (V).
20. Define internal resistance.
The resistance within a source of electromotive force, such as a cell, which causes some energy to be lost inside the source.
Marking yourself
- Calculation: formula written, values substituted with units, final answer with unit and sensible significant figures.
- Variables: manipulated = what you change; responding = what you measure; constant = what you keep the same.
- Definition: one sentence, correct physical quantity, correct unit.
Exam tip
More Electricity resources
- Chapter overview: Electricity
- Revision Notes
- Common Mistakes
- 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)