IGCSE Physics 0625 Paper 4: Electricity, Circuits and Electromagnetic Induction

Level: Year 10-11 (IGCSE) | Subject: Chemistry 0620 | Time: 75 minutes | Total: 100 marks | Parts: 33

Paper at a glance

Paper	Time	Marks	Calculator
Paper 4 (Extended, structured)	1 h 15 min	75	Not allowed

Questions

Question 1 (16 marks) – Circuits, current and voltage

(a) State the rule for current in a series circuit and explain why it holds. (3 marks)

(b) State the rule for voltage across parallel branches and explain why it holds. (3 marks)

(c) Use R = V / I to define resistance and give its unit. (3 marks)

(d) State how an ammeter and a voltmeter must be connected to a circuit. (3 marks)

(e) Explain why terminals must be clean and tight in an experiment. (2 marks)

(f) A lamp carries a current of 1.2 A for 300 s. Calculate the charge that flows through it. (2 marks)

Question 2 (18 marks) – Resistance, Ohm law and series and parallel calculations

(a) State the conditions for Ohm law to hold and give one case where it fails. (3 marks)

(b) Two resistors of 4 ohm and 6 ohm are in series with a 12 V supply. Calculate the total resistance and the current. (4 marks)

(c) For the series circuit above, calculate the voltage across each resistor and check that the two add up to the supply voltage. (4 marks)

(d) The same two resistors are now connected in parallel across the 12 V supply. Calculate the equivalent resistance and the total current. (4 marks)

(e) Compare the total power of the two connections and explain the difference. (3 marks)

Question 3 (17 marks) – Electrical power, energy and running cost

(a) Explain the meaning of P = V I and give another common power formula. (3 marks)

(b) A kettle is marked 2000 W, 240 V. Calculate its working current and its resistance. (4 marks)

(c) The kettle is used for 180 s. Calculate the energy used and express it in kWh. (4 marks)

(d) If electricity costs 1.20 dollars per kWh, calculate the cost of that heating. (3 marks)

(e) A 60 W filament lamp is replaced by a 12 W LED used 5 hours a day. Calculate the energy saved each day. (3 marks)

Question 4 (17 marks) – Electromagnetic induction and transformers

(a) State the conditions needed to produce an induced electromotive force. (3 marks)

(b) List three ways to increase the induced electromotive force, with one sentence of explanation each. (4 marks)

(c) A transformer has 1200 turns on the primary and 60 on the secondary. The primary voltage is 240 V. Calculate the secondary voltage. (4 marks)

(d) If the primary current is 0.5 A and the transformer is 100 percent efficient, calculate the secondary current. (3 marks)

(e) Explain why a transformer works only with alternating current. (3 marks)

Question 5 (16 marks) – Mains electricity and safety

(a) Describe the function of a fuse and state which wire it is fitted in. (3 marks)

(b) Appliances of 1500 W, 900 W and 600 W are used together on a 240 V circuit. Calculate the total current. (3 marks)

(c) Which fuse should be used, 3 A, 13 A or 30 A? Explain your choice. (3 marks)

(d) Explain the purpose of the earth wire. (3 marks)

(e) Explain why a copper wire must not be used instead of a fuse. (2 marks)

(f) Explain how double insulation improves electrical safety. (2 marks)

Question 6 (16 marks) – Magnetism, electromagnets and applications

(a) Describe the shape and direction of the field around a straight current-carrying wire. (3 marks)

(b) List three ways to make an electromagnet stronger. (3 marks)

(c) An electromagnet has 200 turns carrying 0.5 A. Calculate the ampere-turns; if the turns are doubled at the same current, how does the field change? (3 marks)

(d) Explain the purpose and the working principle of a relay. (3 marks)

(e) Explain why soft iron is better than steel for the core of an electromagnet. (2 marks)

(f) Give one practical use of an electromagnet and say why it is used instead of a permanent magnet. (2 marks)

Answers and marking notes

Question 1 – Circuits, current and voltage

(a) The current is the same through every component in a series circuit (one mark); charge does not accumulate anywhere in the circuit (one mark); so the same amount of charge passes every cross-section in the same time (one mark). [3 marks]

(b) Each parallel branch has the same voltage across it (one mark); because the ends of every branch are connected to the same two points (one mark); so the potential difference is the same (one mark). [3 marks]

(c) Resistance is the voltage across a component divided by the current through it (one mark); a resistance of one ohm gives a current of one ampere for a voltage of one volt (one mark); the unit is the ohm (one mark). [3 marks]

(d) An ammeter is connected in series (one mark); a voltmeter is connected in parallel across the component being tested (one mark); the ammeter has a very low resistance and the voltmeter a very high resistance so the circuit is barely changed (one mark). [3 marks]

(e) Dirt or a loose connection raises the contact resistance (one mark); this contact resistance heats up and makes the measured current and voltage lower or unsteady (one mark). [2 marks]

(f) Q = I t (one mark); = 1.2 x 300 = 360 C (one mark). [2 marks]

Question 2 – Resistance, Ohm law and series and parallel calculations

(a) At constant temperature the current through a conductor is proportional to the voltage (one mark); a metal wire roughly obeys this at steady temperature (one mark); a lamp filament heats up as the current rises so its resistance changes and the law fails (one mark). [3 marks]

(b) R = 4 + 6 = 10 ohm (one mark); I = V / R (one mark); = 12 / 10 (one mark); = 1.2 A (one mark). [4 marks]

(c) V1 = I R1 = 1.2 x 4 = 4.8 V (two marks); V2 = I R2 = 1.2 x 6 = 7.2 V (one mark); 4.8 + 7.2 = 12 V which equals the supply voltage (one mark). [4 marks]

(d) 1/R = 1/4 + 1/6 (one mark); R = 2.4 ohm (one mark); I = V / R = 12 / 2.4 (one mark); = 5 A (one mark). [4 marks]

(e) Series power = 12 x 1.2 = 14.4 W (one mark); parallel power = 12 x 5 = 60 W, about 4.17 times larger (one mark); the parallel equivalent resistance is smaller so the total current is much larger (one mark). [3 marks]

Question 3 – Electrical power, energy and running cost

(a) Power is the energy transferred each second (one mark); P = V I means power equals voltage times current (one mark); another common form is P = I squared R (or P = V squared / R) (one mark). [3 marks]

(b) I = P / V = 2000 / 240 (one mark); = 8.33 A (one mark); R = V / I = 240 / 8.33 (one mark); = 28.8 ohm (one mark). [4 marks]

(c) E = P t = 2000 x 180 (one mark); = 360000 J (one mark); 1 kWh = 3.6 x 10^6 J (one mark); = 0.10 kWh (one mark). [4 marks]

(d) Energy used 0.10 kWh (one mark); cost = 0.10 x 1.20 (one mark); = 0.12 dollars (one mark). [3 marks]

(e) Power difference = 60 – 12 = 48 W (one mark); energy = 48 x 5 = 240 Wh (one mark); = 0.24 kWh (one mark). [3 marks]

Question 4 – Electromagnetic induction and transformers

(a) A conductor must be in a magnetic field (one mark); there must be relative motion between conductor and field, or the flux through a coil must change (one mark); the circuit must be closed for an induced current to flow (one mark). [3 marks]

(b) Move the magnet or coil faster so the flux changes more quickly (one mark); use a stronger magnet so the flux change is larger (one mark); use more turns so the emf of each turn adds up (one mark); wind the coil on a soft iron core to concentrate the flux (one mark). [4 marks]

(c) Turns ratio = 1200 / 60 = 20 (one mark); Vs / Vp = Ns / Np (one mark); Vs = 240 / 20 (one mark); = 12 V (one mark). [4 marks]

(d) Input power = 240 x 0.5 = 120 W (one mark); output power is the same, so Is = 120 / 12 (one mark); = 10 A (one mark). [3 marks]

(e) A transformer relies on a changing flux to induce an emf (one mark); alternating current makes the flux in the core keep changing (one mark); steady direct current gives a steady flux and no induced emf (one mark). [3 marks]

Question 5 – Mains electricity and safety

(a) The fuse melts when the current exceeds its rating and breaks the circuit (one mark); it is fitted in the live wire (one mark); it protects the whole circuit between the switch and the appliance (one mark). [3 marks]

(b) Total power = 1500 + 900 + 600 = 3000 W (one mark); I = P / V = 3000 / 240 (one mark); = 12.5 A (one mark). [3 marks]

(c) A 13 A fuse (one mark); the total current of 12.5 A is above 3 A so a 3 A fuse would blow in normal use (one mark); 13 A is just above the working current so it carries the load continuously (one mark). [3 marks]

(d) The earth wire connects the metal case to the ground (one mark); if the case becomes live the current flows safely to earth (one mark); the large current blows the fuse so the case does not stay live (one mark). [3 marks]

(e) Copper has a high melting point and low resistance so it does not melt at an excessive current (one mark); the cable then overheats and can start a fire, so the protection is lost (one mark). [2 marks]

(f) Live parts are enclosed by two layers of insulation (one mark); if the outer layer is damaged the inner layer still keeps the case from becoming live, so no earth wire is needed (one mark). [2 marks]

Question 6 – Magnetism, electromagnets and applications

(a) The field forms concentric circles centred on the wire (one mark); the direction is given by the right-hand grip rule (one mark); the field becomes weaker further from the wire (one mark). [3 marks]

(b) Use more turns on the coil (one mark); increase the current (one mark); use a soft iron core (one mark). [3 marks]

(c) Ampere-turns = 200 x 0.5 = 100 A turns (two marks); doubling the turns gives 200 ampere-turns so the field roughly doubles (one mark). [3 marks]

(d) A relay uses a small current to switch a separate circuit carrying a large current (one mark); the energised coil attracts the armature and closes the contacts (one mark); a low-voltage circuit can therefore control high-voltage or high-power equipment safely (one mark). [3 marks]

(e) Soft iron is easily magnetised and easily demagnetised (one mark); its magnetism disappears quickly when the current is switched off, which suits switching applications (one mark). [2 marks]

(f) For example an electromagnetic crane in a scrapyard (one mark); its magnetism can be switched on to lift and off to drop the load, which a permanent magnet cannot do (one mark). [2 marks]

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