What is actually happening inside a wire carrying 1 ampere
A copper wire carrying 1 A contains roughly 8 × 10²⁸ free electrons per cubic metre — one for every copper atom. Yet each electron only shuffles forward at around 0.1 mm per second, slower than a snail. There is no single electron that races from battery to bulb; instead, the entire sea of electrons nudges in one direction simultaneously, like a tube already full of ball bearings — push one in at one end, one pops out the other instantly.
Core Content
Imagine water flowing through a pipe. The current is how much water passes a point per second. In a wire, the "water" is electric charge — mostly electrons in metals — and current is how much charge flows past a cross-section every second. The bigger the flow, the higher the current.
Conventional current flows positive → negative; electrons flow the opposite way. Both conventions describe the same physical situation.
Rearranged: ΔQ = I Δt — charge = current × time
Think of potential difference (pd) as the "push" that makes charges move. Specifically, it is the energy given to (or taken from) each coulomb of charge as it moves between two points. A 9 V battery gives 9 joules of energy to every coulomb of charge it pushes through the circuit. No pd — no current.
Potential difference is measured between two points — it is always a difference, never an absolute value (unless referenced to earth/ground).
1 volt = 1 joule per coulomb (1 V = 1 J C⁻¹)
Not all conductors use electrons. In metals, the charge carriers are delocalised electrons (negative, moving opposite to conventional current). In electrolyte solutions (e.g. salt water, batteries), both positive and negative ions carry charge. In semiconductors, both electrons and "holes" (positive vacancies) carry current. The type of carrier affects how the material behaves.
• Metals: free (delocalised) electrons — negative charge carriers
• Electrolytes: positive and negative ions
• Semiconductors: electrons and holes
• Gases (discharge tubes): positive ions and electrons
Why wrong: The ampere is defined as coulombs per second, so time must be in seconds. Using minutes gives an answer 60× too small.
Correct approach: Always convert time to seconds before substituting. 3 minutes = 180 s.
Why wrong: Voltage is energy per coulomb (V = W/Q). Total energy = VQ, which depends on the total charge the battery can deliver.
Correct approach: State "9 V battery gives 9 J per coulomb of charge it delivers."
Why wrong: Conventional current flows + to −; electrons flow − to +. These are opposite because electrons are negative.
Correct approach: Always specify which you mean. In AQA questions, "current" means conventional current unless otherwise stated.
| Command word | What it demands | For this topic |
|---|---|---|
| State | A brief factual answer. No justification needed. | "State the SI unit of charge" → coulomb (C). No justification; one word or symbol. |
| Explain | State + reason. Must include causal logic ("because…"). | "Explain why a charged particle moves in a magnetic field" → state force direction AND reason (F = BIl or F = Bqv, perpendicular to motion). |
| Describe | Say what happens. No explanation required. | "Describe how current varies with pd for a filament lamp" → say what happens to I as V increases, no explanation needed. |
| Deduce | Arrive at a conclusion from given information. Show reasoning. | "Deduce the current from the graph" → read Q from graph, identify Δt, calculate I = ΔQ/Δt. Show the working step explicitly. |
| Show that | Prove given result step-by-step. Working must be visible. | "Show that the energy transferred is approximately 900 J" → write W = VIt, substitute values, show working; do NOT just state 900 J. |
The electric current in a conductor is the rate of flow of charge. I = ΔQ / Δt.
Why each word matters: "Rate of flow" — not just "flow." Examiners check you understand current is a rate (quantity per time). "Charge" — not "electrons" — because in electrolytes ions carry charge. Missing either word typically drops one mark. V = W/Q: "work done per unit charge" — "per unit charge" (per coulomb) is mandatory. Writing "energy transferred by charge" is wrong — it must be "per unit charge."
Dimensional analysis is a powerful check: if your calculated quantity has units of A·s² instead of C, you know a factor of time has slipped in. In AQA mark schemes, a correct answer with wrong or missing units often loses 1 mark — always state units explicitly.
The definition I = ΔQ/Δt and V = W/Q can be combined directly to give the energy transferred by current flowing through a component. If charge ΔQ flows through a potential difference V, the work done is W = VΔQ. Substituting ΔQ = IΔt gives: W = VIΔt. This is the master energy formula for circuits.
As I → 0: no charge flows, no energy transferred — an open circuit. As V → 0: no potential difference means no driving force, so current → 0 (assuming an ideal component). The formula I = ΔQ/Δt breaks down for alternating current (AC) where current direction reverses — here we use RMS values. V = W/Q breaks down when Q → 0 (you cannot divide by zero charge), which is why pd is defined for continuous charge flow, not single charges.
The equation ΔQ = IΔt is linear in time — if you plot Q against t for constant current, you get a straight line through the origin with gradient = I. This is experimentally useful: measure the gradient of a Q–t graph to find current without needing to measure current directly. For variable current, the charge is the area under an I–t graph: Q = ∫I dt. This integral interpretation becomes essential for AC circuits and capacitor charging.
• A mobile phone charger draws ≈ 1–3 A at 5 V — about 15 W.
• A human hair is ~70 μm wide; at 1 A, ~10²⁰ electrons per second squeeze past that cross-section.
• A lightning bolt carries ≈ 30 000 A for about 30 μs — total charge ≈ 1 C.
• The human body's nerve signals work at milliampere (10⁻³ A) currents across cell membranes.
• The electron charge is 1.60×10⁻¹⁹ C — so 1 coulomb contains 6.25 × 10¹⁸ electrons.
Where it holds: Qualitative behaviour — higher pressure → more flow; narrower pipe → less flow; flow rate is "used up" by doing work (turning a water wheel). Conservation of flow in series/parallel is exactly analogous to Kirchhoff's current law.
Where it breaks: Water is compressible at high pressure; electrons are not. Water stores energy in its pressure AND its kinetic energy; in circuits energy is in the electromagnetic field (Poynting vector), not the electron's motion. Electrons don't "run out" — they circulate. Water analogy fails completely for AC circuits.
Ammeter: Connected in series with the component being measured. Internal resistance must be very low (ideally zero) so it doesn't significantly reduce the circuit current. Digital ammeters typically read to ±0.01 A; analogue have ±0.5 of the smallest division.
Voltmeter: Connected in parallel. Internal resistance must be very high (ideally infinite) so negligible current flows through it. A typical multimeter on the 20 V DC range has input resistance ~10 MΩ.
Multimeter setup: Select DC volts/amps; insert probes in correct sockets; set range above expected value; zero before use. Record: reading ± half the last digit resolution.
Battery capacity (amp-hours): a 3000 mAh phone battery stores 3.0 A × 3600 s = 10 800 C of charge — multiply by voltage (~3.7 V) to get ≈ 40 000 J ≈ 40 kJ. Smart electricity meters measure charge flow in coulombs and multiply by voltage to calculate energy in kilowatt-hours (1 kWh = 3.6 MJ). Electric vehicles are specified by battery capacity in kWh, directly applying W = VQ. Electrolysis in industry uses Q = It to calculate how much material is deposited (Faraday's laws).
The light does not need the electrons that entered the switch to reach the bulb — all electrons in the circuit start moving simultaneously the moment the switch closes. The electric field propagates at close to the speed of light. Think of a bicycle chain: when you push one link forward, the link at the other end moves immediately — the "signal" (tension wave) travels at the speed of sound in the chain, much faster than any individual link.
The "current uses up" paradox: Many students think current is "used up" by components — more current enters a resistor than leaves it. This is wrong. By Kirchhoff's current law (conservation of charge), the current entering any component must equal the current leaving it. What is consumed is energy (potential difference × charge), not current. The electrons that go in must come out; they just leave with less energy per electron (lower potential).
Why does the electron carry exactly the charge it does (1.60×10⁻¹⁹ C) and not some other value? The Standard Model gives no prediction — the electron charge is a measured input, not a derived result. Why is charge quantised at all? Dirac showed that if a single magnetic monopole existed anywhere in the universe, charge quantisation would follow automatically — but no monopole has ever been observed. Whether dark matter particles carry any electric charge remains an open experimental question.
James Turrell's immersive light installations at the Guggenheim Bilbao use precisely programmed currents through LED arrays — he describes himself as "sculpting with current." Early electrochemist Humphry Davy staged public electric-arc demonstrations in 1802, the first time Londoners ever saw light brighter than a candle that was not fire. The electric eel (Electrophorus electricus) generates up to 860 V and roughly 1 A by stacking electrocytes — biological cells acting as batteries in series — a living demonstration that V = W/Q applies equally to chemistry and physics.
Checkpoint
TEACH IT
A confused student is waiting. They have been reading but they do not quite get it. They will ask you questions, offer wrong explanations and ask if they are right, and push back when your explanation is vague. Explain current, charge and potential difference until they understand. This is the deepest test in the lesson — you do not really know something until you can teach it.
Virtual Lab
Using the PhET Circuit Construction Kit (DC), you will build a simple series circuit and use an ammeter and clock to measure how charge accumulates over time at different current settings. You must record all data manually — the simulation does not auto-record. Plot charge Q (y-axis) against time t (x-axis) for three different current settings, draw best-fit straight lines through the origin, and calculate the gradient (= current I) from each.
Open virtual experiment