Pearson Edexcel International GCSE in Physics · 4PH1

Charge and Current

What is actually moving in a wire, how fast it is really going, and why the arrows on every circuit diagram point the wrong way.

Topic 2 · Electricity — one of 9 lessons in this topic, and one of 65 in Physics.

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

  • 2.1 — Use the units ampere, coulomb, joule, ohm, second, volt and watt
  • 2.14 — Current as the rate of flow of charge
  • 2.15 — Use Q = I × t
  • 2.16 — Current in solid metallic conductors is a flow of negatively charged electrons

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

A metal is unusual among solids. Its atoms are packed in a regular lattice, and each one has released one or more of its outer electrons, which are no longer attached to any particular atom and can move freely through the whole structure. Physicists call them the sea of DELOCALISED electrons, and they are the reason a metal conducts. Nothing else in the wire moves: the positive metal ions stay exactly where they are, vibrating on the spot.

Connect that wire to a cell and the free electrons, which had been moving in every direction at random, acquire a slow overall drift in one direction. A CURRENT is that flow, and it is measured as the rate at which charge passes a point. One AMPERE means one coulomb of charge passing every second.

Think of it like a bicycle chain, not a queue of buses

It is tempting to imagine electrons setting off from the battery and arriving at the lamp, and to wonder why the lamp lights instantly if they move so slowly. The better picture is a bicycle chain: push it anywhere and the whole loop moves at once, because it was already full and every link pushes the next. The wire is already packed with free electrons everywhere along it, including inside the lamp. When the switch closes, they all begin to drift together, so the lamp lights immediately even though any individual electron is crawling — typically less than a millimetre per second. Nothing had to travel from the battery to the lamp. The electrons were already there.

Why current is the same all the way round a series circuit

  1. CHARGE IS CONSERVED — electrons are not created in a wire and not destroyed in a lamp.
  2. The circuit is a complete loop with nowhere for charge to leave it, and the wire is already full of free electrons.
  3. So the number of electrons passing one point each second must equal the number passing every other point each second.
  4. Therefore the current is the same everywhere in a series circuit, whatever the components are and in whatever order they are placed.
  5. A lamp does not 'use up' current. It converts energy carried by the charge; the charge itself carries straight on.

Now the awkward point, and it is worth facing directly. Electrons are NEGATIVELY charged, so in a metal wire they drift from the negative terminal of the cell towards the positive one. But every circuit diagram you will ever meet marks the current going the other way, from positive to negative. That direction is called CONVENTIONAL CURRENT, and it is a historical accident: the convention was fixed before anyone knew what was moving, and by the time the electron was discovered in 1897 every book, instrument and equation already assumed the other direction. Changing it would have been worse than living with it. So both statements are true at once — conventional current runs from + to −, and the electrons in a metal run from − to +.

Charge, current and time

A current of 0.25 A flows through a lamp for 4.0 minutes. How much charge passes through it?

  1. Convert the time to seconds: 4.0 minutes = 4.0 × 60 = 240 s. (The ampere is defined per second, so a time in minutes will always give a wrong answer.)
  2. Q = I × t = 0.25 × 240.
  3. = 60 C.

Answer: 60 coulombs — which is about 3.7 × 10²⁰ electrons, and a small torch bulb does this every four minutes.

The units of this topic are worth collecting in one place, because statement 2.1 asks for them and because each one is really a sentence in disguise. An AMPERE is a coulomb per second. A COULOMB is the quantity of charge. A VOLT is a joule per coulomb — the energy given to each unit of charge. An OHM is a volt per ampere. A WATT is a joule per second. Read that list again and notice that four of the seven are defined as one thing per another thing, which is why the equations of this topic are almost all divisions in disguise.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Q = I × t

Q is charge in coulombs (C), I is current in amperes (A), t is time in SECONDS (s)

Units: C, A, s. Rearranged: I = Q ÷ t and t = Q ÷ I. Convert minutes to seconds before substituting.

Learn this definition · Electric current

The rate of flow of electric charge. A current of one ampere means one coulomb of charge passes a point every second.

Statement 2.1 — every unit in this topic, and what it means

Ampere (A)
The unit of current: one coulomb of charge passing a point per second.
Coulomb (C)
The unit of charge: the quantity of charge carried past a point by one ampere in one second.
Volt (V)
The unit of voltage: one joule of energy transferred per coulomb of charge.
Ohm (Ω)
The unit of resistance: one volt per ampere.
Joule (J)
The unit of energy, and of work done.
Watt (W)
The unit of power: one joule of energy transferred per second.
Second (s)
The unit of time, and the unit every electrical equation on this course assumes.

What is actually happening in a metal wire

  • A metal has DELOCALISED ELECTRONS that are free to move through the whole lattice
  • The positive metal IONS stay in place and only vibrate — they do not flow
  • A current is the DRIFT of those free electrons in one overall direction
  • Electrons are NEGATIVE, so in a metal they move from the negative terminal towards the positive one
  • CONVENTIONAL CURRENT is drawn from positive to negative, a convention fixed before the electron was discovered
  • The drift is very slow — under a millimetre per second — but the lamp lights at once because the wire is already full of electrons

Conventional current against electron flow

A. CONVENTIONAL CURRENT: from the POSITIVE terminal to the negative. This is what circuit diagrams show and what every equation assumes.

B. ELECTRON FLOW: from the NEGATIVE terminal to the positive, because electrons are negatively charged. This is what physically moves in a metal.

Model answer [3 marks]

A current of 3.0 A flows for 5.0 minutes. Calculate the charge that passes. [3]

The time must be in seconds, so 5.0 minutes = 5.0 × 60 = 300 s. Using Q = I × t, the charge is 3.0 × 300 = 900 C.

Model answer [3 marks]

Explain why the current is the same at every point in a series circuit. [3]

Charge is conserved, so electrons are neither created nor destroyed anywhere in the circuit. The circuit is a complete loop and there is nowhere for charge to leave it. The number of electrons passing any one point each second must therefore equal the number passing every other point each second, so the current has the same value throughout.

Not this: Current is used up as it goes round a circuit, which is why the lamp furthest from the battery is dimmer.

This: Current is the same everywhere in a series circuit, because charge is conserved. What a lamp uses is ENERGY, not charge — the same electrons carry on round the loop having given up energy on the way.

Mark-losing trap. Convert minutes to SECONDS before using Q = I × t. The ampere is defined per second.

Mark-losing trap. Current is not used up. A component transfers energy from the charge, not the charge itself.

Mark-losing trap. Electrons flow − to +. Conventional current is drawn + to −. Both are true; say which one you mean.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

  1. Grade 6 · State [1 mark] — What is an electric current?
  2. Grade 7 · Calculate [3 marks] — A current of 2.5 A flows through a wire for 3.0 minutes. Calculate the charge that passes.
  3. Grade 8 · Explain [5 marks] — A lamp lights the instant a switch is closed, even though individual electrons drift through the wire at less than a millimetre per second. Select every statement that belongs in a full-mark explanation.
  4. Grade 9 · Calculate [4 marks] — A charge of 1.2 kC passes through a heater in 8.0 minutes. Calculate the current.
  5. 9+ · Analyse [6 marks] — A student says: 'Electrons flow from negative to positive, so all the arrows on circuit diagrams are drawn the wrong way round and physics teachers should fix them.' Select every statement that belongs in a full-mark analysis.

The people behind this science

Two ways into the same idea — the one who gave the quantities their names before anyone knew what carried them, and the one who established that the carriers are real, countable objects. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Michael Faraday — the one who gave the quantities their names before anyone knew what carried them

Faraday spent years on the question this page opens with: what is actually being transferred when a current flows? He could not see electrons — nobody would for another sixty years — so he measured what current DID, and found that the mass of a substance deposited in electrolysis is proportional to the quantity of electricity passed. That is charge, measured before anyone knew charge came in units. He also coined much of the vocabulary this topic uses, including ion, electrode, anode and cathode.

  • “How can you measure a quantity of electricity without knowing what it is made of?”
  • “What did electrolysis tell you about how much charge had passed?”
  • “Why did you invent so many new words for this subject?”
  • “Did you suspect electricity came in fixed, indivisible amounts?”
  • “What is actually travelling along a wire, in your view?”

Albert Einstein — the one who established that the carriers are real, countable objects

For most of the nineteenth century it was entirely respectable to treat charge as a continuous fluid and atoms as a useful fiction. Einstein's 1905 papers on Brownian motion and the photoelectric effect made the discrete, countable picture almost impossible to resist: matter is grainy, energy arrives in packets, and charge is carried by particles you could in principle count. This page depends on that — it says a coulomb is a definite number of electrons, which is only meaningful if electrons are objects rather than a way of speaking.

  • “How do we know electric charge is carried by countable particles?”
  • “Why did so many scientists doubt that atoms were real?”
  • “What convinced you that matter comes in discrete grains?”
  • “Does it matter whether we picture a current as particles or as a fluid?”
  • “How slowly can something move and still deliver energy instantly?”

Then defend it

On Incandio a lesson is not finished when the questions come out right. You teach the idea back to Ember, an AI apprentice who asks the awkward question, and then you argue it against J. J. Thomson in a structured debate marked against descriptors you can read before you enter. Learn it, teach it, then defend it — all three happen on this page once the app loads.

Carry on through the course