Electricity: Synthesis and Exam Skills

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.

The Six P3 Equations in One Place

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 $\text{C}$ , $I$ in $\text{A}$ , $t$ in $\text{s}$
$V = IR$	P.d., current, resistance	$V$ in $\text{V}$ , $I$ in $\text{A}$ , $R$ in $\Omega$
$E = QV$	Energy, charge, p.d.	$E$ in $\text{J}$ , $Q$ in $\text{C}$ , $V$ in $\text{V}$
$P = VI$	Power, p.d., current	$P$ in $\text{W}$ , $V$ in $\text{V}$ , $I$ in $\text{A}$
$P = I^2R$	Power, current, resistance	$P$ in $\text{W}$ , $I$ in $\text{A}$ , $R$ in $\Omega$
$E = Pt$	Energy, power, time	$E$ in $\text{J}$ , $P$ in $\text{W}$ , $t$ in $\text{s}$

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.

How to Choose the Right Equation

Faced with a calculation, work through these questions:

What is the question asking for? (charge, current, p.d., resistance, power, energy?) — that is your subject.
What have you been given? List the quantities and their units.
Which equation contains your unknown and your knowns? Pick the one that fits.
Do you have everything that equation needs? If not, find the missing quantity first with another equation (this is a two-step problem).
Substitute and solve, showing equation, substitution and answer with the unit.

A quick guide to the choice:

Need charge from current and time → $Q = It$ .
Relating p.d., current and resistance → $V = IR$ .
Need energy from charge and p.d. → $E = QV$ ; from power and time → $E = Pt$ .
Need power from p.d. and current → $P = VI$ ; from current and resistance → $P = I^2R$ .

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.

Required Practicals Recap

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.

Common P3 Exam Mistakes

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 $P = I^2R$ , 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.

Worked Multi-Step Problem

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 $46\ \Omega$ connected to the $230 \text{ V}$ 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 — $3 \text{ A}$ , $5 \text{ A}$ or $13 \text{ A}$ — should be fitted.

(a) Current — we know $V$ and $R$ , so use $V = IR$ rearranged for $I$ : $I = \dfrac{V}{R} = \dfrac{230}{46} = 5 \text{ A}$

(b) Power — we now know $V$ and $I$ , so use $P = VI$ : $P = VI = 230 \times 5 = 1150 \text{ W}$ (Check with $P = I^2R = 5^2 \times 46 = 25 \times 46 = 1150 \text{ W}$ — the two methods agree.)

Electricity: Synthesis and Exam Skills

Electricity: Synthesis and Exam Skills

The Six P3 Equations in One Place

How to Choose the Right Equation

Required Practicals Recap

Common P3 Exam Mistakes

Worked Multi-Step Problem

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