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AS Level · Physics 2 (IAL) WPH12

Current, Potential Difference, Resistance & Power

Revise Current, Potential Difference, Resistance & Power for Physics 2 (IAL) WPH12 (AS Level) — revision notes, 2 practice questions and instant AI marking.

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Edexcel IAL Physics — Unit 4

Current, Potential Difference,
Resistance & Power

💡 Big Idea: Electricity is just charge on the move — current tells you how much charge flows per second, potential difference tells you how much energy each bit of charge carries, and resistance tells you how hard it is for that charge to get through. Everything else in this chapter — Ohm's law, series/parallel circuits, power — is just these three ideas combining in different ways.

📋 Chapter Summary — The Whole Thing in 8 Bullets

Current (I) is the rate of flow of charge: Q = It. Measured in amps, using an ammeter in series.
Charge (Q) is measured in coulombs. One electron carries 1.6×10⁻¹⁹ C, so Q = ne.
Conventional current flows + to −. Electrons actually flow − to + — opposite directions!
Potential difference (V) is work done per unit charge: V = W/Q. Measured in volts, using a voltmeter in parallel.
Resistance (R) opposes current: R = V/I (Ohm's Law), measured in ohms.
Kirchhoff's 1st Law (charge conservation): current into a junction = current out. Kirchhoff's 2nd Law (energy conservation): sum of e.m.f.s = sum of p.d.s around a loop.
Series: R = R₁+R₂+R₃… Parallel: 1/R = 1/R₁+1/R₂+1/R₃…
Power: P = VI = I²R = V²/R. And I–V graphs reveal a component's identity: straight line = resistor, S-curve = filament lamp, hockey-stick = diode.

1. Electric Current & Charge

1.1What is current, really?

Picture a hosepipe. Water doesn't teleport from one end to the other — it's a continuous stream of water molecules moving past any given point. Current is exactly the same idea, but with electric charge instead of water. It's the amount of charge that flows past a point in a circuit every second.

Formally: Electric current is the rate of flow of charge. It's measured in amperes (A), or amps for short.

Core Equation

Q = I t

charge (C) = current (A) × time (s) | rearranged: I = ΔQ / Δt

The little "Δ" symbol (delta) just means "change in". So ΔQ means "the amount of charge that has flowed" and Δt means "the time that has passed". You'll see this notation a lot in physics — don't let it intimidate you, it's just shorthand.

1.2Electric charge — the stuff that's actually moving

Charge is a property that some particles have. Protons are positively charged, electrons are negatively charged. In a solid metal wire, it's the electrons that are free to move around — they're the charge carriers.

Elementary charge (e): the charge on a single electron = 1.6 × 10⁻¹⁹ C. This is one of the smallest possible "packets" of charge — charge always comes in whole-number multiples of this value.

Charge from number of electrons

Q = n e

Q = total charge (C) | n = number of electrons | e = charge on one electron (1.6×10⁻¹⁹ C)

Since one electron is 1.6×10⁻¹⁹ C, it takes a huge number of electrons to make just 1 coulomb — specifically 6.25 × 10¹⁸ electrons. That's why current in a wire looks "smooth" and continuous even though it's really billions of discrete tiny charges bumping along.

1.3Conventional current vs. electron flow — the classic trap

Here's something that trips up almost every student at least once: conventional current and electron flow point in opposite directions.

1.4Measuring current

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Also in the full note

2. Potential Difference
3. Resistance & Ohm's Law
4. Charge & Energy Conservation in Circuits
5. Resistance in Series & Parallel
6. Electrical Power
7. Current–Potential Difference (I–V) Graphs
📌 What to Memorise
✅ Concepts Checklist

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