OCR GCSE Physics B (Twenty First Century)
OCR GCSE Physics B P3 Electric Circuits Exam Questions
Free OCR GCSE Physics B revision for P3 Electric Circuits: what you need to know, common mistakes and exam-style questions with answers. Covers charge, current, potential difference, resistance, series and parallel circuits, electrical power, magnetic fields, the motor effect, motors, generators and transformers.
Practise P3 Electric Circuits with past papers
Once you have learnt this topic, find the P3 Electric Circuits questions in real OCR GCSE Physics B past papers and mark them with the mark scheme. Doing lots of real questions is the quickest way to learn how examiners ask about each idea.
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About OCR GCSE Physics B P3 Electric Circuits
Chapter P3 is the largest chapter in OCR Physics B and covers how electric circuits work and how electricity and magnetism are linked. You use a model of charge flow to explain current, potential difference and resistance, investigate I-V characteristics, and apply rules for series and parallel circuits. You then calculate electrical power and energy transfer, describe magnetic fields around magnets and wires, and explain how motors work. Separate physics adds electrostatics, electromagnetic induction, generators and transformers. P3 links to the National Grid in P2. Expect many equation questions, circuit diagrams and explanations of the motor effect.
What you need to know
- P3.1 Electric charge Separate physics only: objects become charged by friction when electrons are transferred. Like charges repel and unlike charges attract, and charged objects create electric fields.
- P3.2 Current and resistance Charge (C) = current (A) x time (s) and potential difference (V) = current x resistance. A fixed resistor at constant temperature is ohmic; a filament lamp's resistance rises as it heats; a diode lets current flow one way; LDR resistance falls with more light and thermistor resistance falls as temperature rises.
- P3.3 Series and parallel In series the current is the same everywhere, potential differences add up and total resistance is the sum of the resistances. In parallel the potential difference is the same across each branch and branch currents add to the total; adding resistors in parallel lowers total resistance.
- P3.4 Power and energy Power = potential difference x current = current squared x resistance, and energy transferred = charge x potential difference. Potential difference = work done / charge.
- P3.5 and P3.6 Magnetism and motors Field lines go from north to south; a current in a wire produces a circular magnetic field and a coil makes an electromagnet. A current-carrying wire in a magnetic field feels a force F = BIL, which makes a motor coil turn.
- P3.7 Generators and transformers Separate physics only: moving a wire or magnet so the field changes induces a potential difference. Generators use this, and transformers obey Vp x Ip = Vs x Is and Vs / Vp = Ns / Np.
Common mistakes in P3 Electric Circuits
- Saying current is used up in a circuit. Current is the same before and after a component in series; energy is transferred, not charge.
- Connecting a voltmeter in series. Ammeters go in series and voltmeters go in parallel across the component.
- Adding resistances for parallel branches. Total resistance in parallel is less than the smallest branch resistance.
- Forgetting to convert minutes to seconds in Q = It. Time must be in seconds.
- Saying a thermistor's resistance rises with temperature. For the thermistors studied at GCSE, resistance falls as temperature rises.
Try three P3 Electric Circuits exam-style questions
1. A resistor has a potential difference of 6.0 V across it and a current of 0.30 A through it. Calculate its resistance. (2 marks)
Show the mark scheme answer
R = V / I = 6.0 / 0.30; = 20 ohms.
2. A current of 0.50 A flows for 2 minutes through a lamp with a potential difference of 12 V across it. Calculate the charge and the energy transferred. (3 marks)
Show the mark scheme answer
Q = I x t = 0.50 x 120 = 60 C; energy = Q x V = 60 x 12; = 720 J.
3. A 4 ohm resistor and a 6 ohm resistor are connected in series to a 12 V supply. Calculate the current in the circuit. (3 marks)
Show the mark scheme answer
Total resistance = 4 + 6 = 10 ohms; I = V / R = 12 / 10; = 1.2 A.
P3 Electric Circuits FAQs
What does OCR Physics B P3 Electric Circuits cover?
It covers current, potential difference and resistance, component characteristics, series and parallel circuits, electrical power, magnetic fields and motors. Separate physics adds electrostatics, generators and transformers.
Which paper is it examined in?
OCR Physics B has two papers at each tier, Breadth in physics and Depth in physics, and both can assess content from every chapter, so P3 can appear on either paper.
What equations do I need for P3?
Key ones are Q = It, V = IR, P = VI, P = I^2 R, E = QV and F = BIL; separate physics adds the transformer equations. Practise rearranging each and converting units such as mA to A.
