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You have now met every idea in Topic P3 — static charge, electric fields, current, potential difference, resistance, series and parallel circuits, I–V characteristics, electrical power and mains safety. The challenge in the exam is rarely a single fact in isolation; it is choosing the right equation, combining several steps, and avoiding the standard traps. This final lesson, part of Topic P3 (Electricity) of OCR Gateway Science A, draws all six P3 equations together, gives a strategy for deciding which to use, recaps the two required practicals, lists the most common P3 mistakes, and works through a full multi-step problem from start to finish.
By the end of this lesson you should be able to select and use the correct P3 equation for a given problem, recall the required practicals, avoid the common P3 errors, and work confidently through multi-step electricity calculations.
Almost every P3 calculation uses one (or a combination) of these six equations. It is worth learning the table by heart, including what each symbol means and its unit.
| Equation | What it links | Symbols and units |
|---|---|---|
| Q=It | Charge, current, time | Q in C, I in A, t in s |
| V=IR | P.d., current, resistance | V in V, I in A, R in Ω |
| E=QV | Energy, charge, p.d. | E in J, Q in C, V in V |
| P=VI | Power, p.d., current | P in W, V in V, I in A |
| P=I2R | Power, current, resistance | P in W, I in A, R in Ω |
| E=Pt | Energy, power, time | E in J, P in W, t in s |
Notice how the equations chain together. The current I appears in five of them, so it is often the quantity that links one equation to the next: find I from V=IR, then feed it into P=VI or Q=It. Spotting these chains is the key skill in multi-step questions.
Exam Tip: Write out the six P3 equations from memory at the start of the exam. Many questions are simply asking "which of these six fits the quantities I have been given?" — and several need two of them used in sequence.
Faced with a calculation, work through these questions:
A quick guide to the choice:
graph TB
Start["What does the question ask for?"] --> Charge["Charge? use Q = It"]
Start --> PD["P.d. / current / resistance? use V = IR"]
Start --> Power["Power? use P = VI or P = I squared R"]
Start --> Energy["Energy? use E = QV or E = Pt"]
Exam Tip: If you are missing a quantity an equation needs, find it first with another equation. The commonest chain is: find the current (from V=IR or P=VI), then use it in the next equation. Always note your units as you go.
Topic P3 has two key required practicals. Be ready to describe the method, the apparatus and the analysis for each.
1. Investigating I–V characteristics. Connect the test component in series with an ammeter and a variable resistor, and a voltmeter in parallel across it. Vary the voltage in steps, recording I and V each time, and reverse the connections to get negative values. Plot I (vertical) against V (horizontal). The shape reveals the resistance behaviour: a straight line for an ohmic conductor, an S-curve for a filament lamp, and one-way conduction for a diode.
2. (Linked from P1/elsewhere) measuring resistance. Using the same series ammeter and parallel voltmeter arrangement, record V and I for a component and calculate R=V/I, taking repeats to improve reliability.
The golden rules for both: ammeter in series, voltmeter in parallel, use a variable resistor to change the current, take readings quickly so components do not heat up, and repeat for reliability.
Exam Tip: For any P3 circuit practical, the marks cluster around ammeter in series, voltmeter in parallel, a variable resistor to vary the current, and plotting I against V. State these clearly and you secure most of the method marks.
These are the errors that cost the most marks in P3. Learn to avoid every one.
| Common mistake | How to avoid it |
|---|---|
| Leaving time in minutes in Q=It or E=Pt | Convert to seconds (×60 for minutes, ×3600 for hours) |
| Leaving power in kilowatts in a joule calculation | Convert kW to W (×1000) before using E=Pt in joules |
| In P=I2R, multiplying before squaring the current | Square the current first, then multiply by R |
| Saying protons move in charging | Only electrons move; positive = lost electrons |
| Putting the ammeter in parallel or voltmeter in series | Ammeter in series, voltmeter in parallel |
| Thinking parallel resistance is bigger than the branches | Parallel total resistance is less than the smallest resistor |
| Confusing the thermistor and filament lamp directions | Thermistor: hotter → lower R; filament lamp: hotter → higher R |
| Mixing up live and neutral colours | Live = brown, neutral = blue, earth = green/yellow |
| Quoting "attraction proves charge" | Only repulsion proves charge (attraction can be by induction) |
Exam Tip: The single most common P3 calculation error is a unit slip — time not in seconds, or power not in watts. Before you press "=", glance at your units. An answer that is out by a factor of 60, 1000 or 3600 almost always signals a unit that was not converted.
Bringing it all together, here is a full multi-step problem of the kind that tests several P3 ideas at once. Work through it step by step, choosing the right equation at each stage.
Problem: An electric heater has a heating element of resistance 46 Ω connected to the 230 V mains. Calculate (a) the current through the element, (b) the power of the heater, (c) the charge that flows in 5 minutes, and (d) the energy transferred in that time. Then state which standard fuse — 3 A, 5 A or 13 A — should be fitted.
(a) Current — we know V and R, so use V=IR rearranged for I: I=RV=46230=5 A
(b) Power — we now know V and I, so use P=VI: P=VI=230×5=1150 W (Check with P=I2R=52×46=25×46=1150 W — the two methods agree.)
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