Secondary 4 Pure Physics Electricity Magnetism Quiz

Secondary 4 Pure Physics Quiz - Electricity Magnetism: Answer Key

Total Marks: 40

Section A: Multiple Choice and Short Answer

[Total: 20 marks]

1. Georg Simon Ohm (or "Ohm") [1]

Teaching note: The unit of resistance, the ohm (Ω), is named after the German physicist who established the fundamental relationship between voltage, current, and resistance.

2. Resistance calculation [2]

Working: Using Ohm's Law: $R = \frac{V}{I}$

$R = \frac{6.0}{0.30} = 20$ Ω [1 for formula, 1 for answer with unit]

Answer: 20 Ω

Teaching note: Ohm's Law states that for a conductor at constant temperature, the current is directly proportional to the potential difference. The resistance is the ratio $V/I$.

3. An ohmic conductor obeys Ohm's Law (current proportional to voltage, constant resistance), while a non-ohmic conductor does not obey Ohm's Law (resistance changes with voltage/current). [1]

Alternative answer: An ohmic conductor has constant resistance regardless of voltage/current; a non-ohmic conductor has resistance that varies with temperature or applied voltage.

4. Parallel resistors calculation [3]

Working: For parallel resistors: $\frac{1}{R_{total}} = \frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3}$

$\frac{1}{R_{total}} = \frac{1}{6.0} + \frac{1}{6.0} + \frac{1}{6.0} = \frac{3}{6.0} = \frac{1}{2.0}$

$R_{total} = 2.0$ Ω [1]

Using Ohm's Law: $I_{total} = \frac{V}{R_{total}} = \frac{12}{2.0} = 6.0$ A [1 for calculation, 1 for answer with unit]

Alternative method: Current through each resistor = $\frac{12}{6.0} = 2.0$ A, so total current = $3 \times 2.0 = 6.0$ A

Answer: 6.0 A

Teaching note: In parallel, the reciprocal of total resistance equals the sum of reciprocals. Alternatively, calculate current through each branch and add (same method, more intuitive).

5. Transformer question [3 total]

(a) Step-up transformer. [1] The secondary coil has more turns (400) than the primary coil (100), which increases the voltage. [1]

(b) Using the transformer equation: $\frac{V_s}{V_p} = \frac{N_s}{N_p}$

$V_s = V_p \times \frac{N_s}{N_p} = 12 \times \frac{400}{100} = 12 \times 4 = 48$ V [1 for formula, 1 for answer with unit]

Answer: 48 V

Teaching note: A step-up transformer increases voltage by having more turns on the secondary. The voltage ratio equals the turns ratio. This uses alternating current; transformers do not work with d.c.

6. Fuse explanation [2]

When too many appliances operate simultaneously, the total current drawn exceeds the fuse rating. [1] The fuse wire heats up due to its resistance, melts when the current exceeds its rating, and breaks the circuit, preventing overheating of cables and potential fire. [1]

Teaching note: Power $P = IV$. More appliances mean more total power, hence more current at fixed voltage. The fuse is a safety device that sacrifices itself to protect the circuit.

7. Kettle calculations [5 total]

(a) $P = IV$, so $I = \frac{P}{V} = \frac{2000}{230} = 8.70$ A (or 8.7 A) [1 for formula, 1 for answer]

(b) Energy = Power × time = 2000 × (5.0 × 60) = 2000 × 300 = 600 000 J [1 for conversion, 1 for formula, 1 for answer with unit]

Or: Energy = $IVt = 8.70 \times 230 \times 300 = 600$ 000 J

Answers: (a) 8.7 A (accept 8.70 A); (b) 600 000 J or $6.0 \times 10^5$ J

Teaching note: Time must be in seconds for energy calculations. 5.0 minutes = 300 s. The kettle's high power explains why kettles need thick cables and appropriate fuses.

8. Safety features in three-pin plug [4]

Any two from:

Teaching note: The earth wire is connected to the metal casing of Class I appliances. If the live wire frays and touches the casing, current flows to earth rather than through a person.

9. Factors affecting solenoid magnetic field strength [2]

Any two from:

• Number of turns per unit length (more turns = stronger field) [1]
• Magnitude of current (larger current = stronger field) [1]
• Presence of soft iron core (greatly increases field strength) [1]

10. Force direction using Fleming's left-hand rule [2]

Direction: Upwards (or "out of page/up") [1]

Explanation using Fleming's left-hand rule: First finger points in direction of magnetic field (N to S, left to right), second finger points in direction of current (into page), thumb points in direction of force (upwards). [1]

Teaching note: For conventional current (positive to negative), use left hand. The force is perpendicular to both field and current. The ⊗ symbol means current going into the page (arrow moving away).

Section B: Structured Response

[Total: 12 marks]

11. Thermistor experiment [5 total]

(a) Submerging in water ensures even heating of the thermistor and allows accurate temperature measurement with the thermometer. [1] Direct heating would causeuneven temperature distribution and possible damage. [1] Accept: water bath provides controlled, uniform temperature.

(b) $R = \frac{V}{I} = \frac{4.8}{0.12} = 40$ Ω [1 for formula, 1 for answer with unit] [2]

(c) In a thermistor (semiconductor), increasing temperature releases more charge carriers (electrons) from the crystal lattice. [1] More charge carriers available for conduction means lower resistance and higher current for the same voltage. [1]

Teaching note: This is opposite to metallic conductors where resistance increases with temperature. In semiconductors, thermal energy frees electrons from bonds, increasing carrier density dramatically.

12. D.C. motor [4 total]

(a) Current flows through the coil in a magnetic field. [1] By Fleming's left-hand rule, one side of the coil experiences force upwards, the other side experiences force downwards, creating a couple/torque that causes rotation. [1] The split-ring commutator reverses current every half turn to maintain rotation in one direction. [1] (Any 2 points for 2 marks)

(b) Any two from:

• Increase the current through the coil [1]
• Increase the strength of the magnetic field (stronger magnets/more turns) [1]
• Increase the number of turns on the coil [1]
• Insert a soft iron core in the coil [1]

13. A.C. generator [3 total]

(a) $V_{r.m.s.} = \frac{V_{peak}}{\sqrt{2}} = \frac{340}{\sqrt{2}} = \frac{340}{1.414} = 240$ V [1 for formula, 1 for calculation, 1 for answer] [2]

Accept 240 V or 241 V.

(b) The r.m.s. value represents the equivalent d.c. voltage that would deliver the same power to a resistive load. [1] It allows direct comparison of heating effect and power transfer between a.c. and d.c. supplies. [1]

Teaching note: 240 V is standard mains voltage in Singapore/UK. The peak is about 340 V. The √2 factor comes from averaging the squared sine wave over a cycle.

14. Transformer calculations [5 total]

(a) $\frac{V_s}{V_p} = \frac{N_s}{N_p}$

$N_s = N_p \times \frac{V_s}{V_p} = 1150 \times \frac{12}{230} = 1150 \times 0.0522 = 60$ turns [1 for formula, 1 for answer]

(b) $\eta = \frac{P_{output}}{P_{input}} \times 100\% = \frac{V_s I_s}{V_p I_p} \times 100\%$

$0.80 = \frac{12 \times 2.0}{230 \times I_p}$

$I_p = \frac{12 \times 2.0}{0.80 \times 230} = \frac{24}{184} = 0.13$ A [1 for efficiency formula, 1 for substitution, 1 for answer]

Answers: (a) 60 turns; (b) 0.13 A (accept 0.130 A)

Teaching note: For efficiency, output power is always less than input power. Don't forget to convert 80% to 0.80 in calculations. Real transformers have energy losses due to eddy currents, hysteresis, and copper resistance.

15. Potential divider [4 total]

(a) Using potential divider formula: $V_{out} = V_{total} \times \frac{R_2}{R_1 + R_2} = 12 \times \frac{8.0}{4.0 + 8.0} = 12 \times \frac{8.0}{12.0} = 12 \times \frac{2}{3} = 8.0$ V [1 for formula, 1 for answer]

Alternatively: Total current $I = \frac{12}{12000} = 0.001$ A; $V_{out} = IR_2 = 0.001 \times 8000 = 8.0$ V

(b) The output voltage decreases. [1] Adding a 4.0 kΩ resistor in parallel with R2 reduces the effective resistance of the lower part of the divider. [1] The parallel combination: $\frac{1}{R_{parallel}} = \frac{1}{8.0} + \frac{1}{4.0} = \frac{3}{8.0}$, so $R_{parallel} = 2.67$ kΩ. [1] Since this is less than the original 8.0 kΩ, the output voltage falls.

Teaching note: The potential divider splits voltage in proportion to resistances. Loading the output (adding a parallel resistor) changes the division ratio. This is why voltage followers (high input impedance) are used in electronics.

Section C: Extended Response

[Total: 8 marks]

16. High voltage transmission [2]

Power loss in cables is $I^2R$, where $R$ is the resistance of the transmission cables. [1] To transmit the same power ($P = IV$), using higher voltage means lower current, which dramatically reduces $I^2R$ heat losses in the cables. [1] This improves efficiency and allows thinner, cheaper cables to be used.

17. Electromagnetic induction experiment [3]

Apparatus: Bar magnet, coil of wire, galvanometer (sensitive ammeter) or center-zero microammeter. [1]

Procedure: Move the magnet into the coil. Observe deflection on galvanometer. Move magnet out of coil; observe opposite deflection. [1]

Direction dependence: When the N pole enters, current flows one direction; when N pole exits, current reverses. [1] Or: entering vs exiting produces opposite deflections; inverting the magnet (S pole first) reverses the deflection again.

Alternative: Use two coils (primary and secondary) with changing current in primary.

Teaching note: Faraday's Law states that induced e.m.f. is proportional to rate of change of magnetic flux linkage. Lenz's Law gives the direction: the induced current opposes the change causing it.

18. Lightning conductor [2]

The lightning conductor is a metal rod with a pointed tip at the top of the building, connected by a thick copper strip to a metal plate buried in the earth. [1] When a charged cloud approaches, the sharp point creates a strong electric field that ionizes air molecules, allowing charge to leak to the atmosphere gradually (corona discharge), preventing sudden discharge. [1] If lightning does strike, the low-resistance path safely conducts the enormous current to earth, preventing damage to the building.

19. Filament lamp resistance [3]

As voltage increases, current increases, and the filament temperature rises significantly. [1] Lattice ions in the metal filament vibrate more vigorously at higher temperatures. [1] This increases collisions between conduction electrons and lattice ions, impeding electron flow. [1] Therefore resistance increases, shown by the decreasing gradient (I/V ratio falls) on the graph.

Teaching note: The I-V curve becoming less steep means resistance is increasing (as $R = V/I$, or use $R = \frac{\Delta V}{\Delta I}$ from gradients). For metals, resistance increases with temperature; this is why filaments glow white-hot.

20. Circuit breaker vs fuse evaluation [3]

Advantage of circuit breaker: Can be reset after tripping, more convenient; responds faster to some faults; can detect earth leakage (RCCB/RCD). [1]

Disadvantage of circuit breaker: More expensive to install initially; more complex mechanism can fail. [1]

Advantage of fuse: Simple, cheap, reliable; fails-safe (destroys itself to break circuit). [1]

Evaluation: The claim is too absolute. For domestic situations, circuit breakers offer convenience and additional protection types, but fuses remain appropriate for simple, cost-sensitive applications or where absolute simplicity is valued. [1] The best choice depends on application, cost, and required protection features.

(Any valid advantage/disadvantage pair with balanced evaluation for full marks)

END OF ANSWER KEY