CIE iGCSE Co-Ordinated Science P4.5.2 The a.c. generator Exam Style Questions Paper 4
Question
A fishing boat floats on the sea.
(a) A heavy anchor is dropped from the boat and accelerates as it falls through the water to the sea bed. Name the downward force that makes the anchor accelerate.
▶️Answer/Explanation
Answer: Weight
Explanation: – Weight = mass × gravitational field strength (W=mg) – Acts vertically downward toward Earth’s center – Greater than upthrust/buoyancy force in this case – Causes net downward acceleration (Fnet = ma)
(b) The boat has a small generator to generate electricity.
Fig. 12.1 shows a simple generator.
(i) On Fig. 12.1 label a slip ring with the letter R.
▶️Answer/Explanation
Answer: R placed on circular metal rings contacting brushes
Explanation: – Slip rings maintain electrical contact during rotation – Made of conductive material (usually copper) – Different from split-ring commutator in DC motors – Allow continuous rotation without twisting wires
(ii) Describe how the use of slip rings produces an alternating voltage output from the rotating coil.
▶️Answer/Explanation
Answer (2 key points):
1. Coil cuts magnetic field lines, inducing voltage
2. As coil rotates 180°, direction of cutting reverses → voltage polarity switches
Explanation: – Faraday’s Law: changing flux induces EMF – Output follows sine wave pattern – Frequency depends on rotation speed – Each full rotation produces one AC cycle – Essential for AC generators (alternators)
(iii) On the grid in Fig. 12.2, sketch a graph of voltage output against time for the generator, when the coil is rotating at a constant speed.
▶️Answer/Explanation
Answer: Smooth sine curve with:
– Regular peaks and troughs
– Equal amplitude above/below zero
– Constant frequency
Explanation: – Mathematically: V = V0sin(2πft) – Peak voltage depends on:
• Magnetic field strength
• Number of coil turns
• Rotation speed
– Zero voltage when coil parallel to field
(c) An electric heater on the boat uses the electricity generated at 240 V. The current passing through the heater is 20 A. Calculate the charge passing through the heater in one hour.
State the unit of your answer.
▶️Answer/Explanation
Answer: 72,000 C (coulombs)
Calculation: 1. Q = It 2. Time = 1 hour = 3600 seconds 3. Q = 20 A × 3600 s = 72,000 C
Explanation: – Charge = current × time – SI unit is coulomb (C) – 1 A = 1 C/s – Real-world context: electric heating elements
(d) A fisherman on the boat is using a pair of binoculars to look at the sea. Binoculars use glass prisms to reflect light.
Fig. 12.3 shows part of a pair of binoculars. A ray of light is shown entering and leaving.
On Fig. 12.3, complete the ray diagram to show the path of the light ray through the two prisms.
▶️Answer/Explanation
Answer: Ray path showing:
1. Total internal reflection at 45° prism faces
2. Correct 90° deviation at each reflection
3. Parallel input/output rays
Explanation: – Porro prism design (double reflection) – Critical angle for glass ≈ 42° (45° > θc → TIR) – Image inversion correction – Compact optical path folding