Charge and current: what actually moves in a wire
By the end of this lesson you will be able to
- Explain static charge as the transfer of electrons, and describe attraction, repulsion, charging by induction and lightning in those terms.
- Distinguish conductors from insulators by what their electrons can do.
- Define current as the rate of flow of charge and calculate with I = Q ÷ t, using the coulomb and the ampere correctly.
- Describe voltage as the energy a cell gives to each coulomb, and outline what a cell, a battery and a switch do in a circuit.
- Draw circuit diagrams with the standard symbols, and state where an ammeter and a voltmeter must be placed.
The puzzle
Flick a light switch and the lamp is on at once — yet the electrons in the copper wire drift along at less than a millimetre per second. At that speed an electron leaving the switch would take hours to reach the bulb. So why is there no delay? And what is it, exactly, that the power station sends to your house, if it is not the electrons themselves?
Predict what is moving in each case, and whether it is created, transferred or already there: (1) a balloon rubbed on hair that then sticks to a wall; (2) the current in a torch wire when the switch is closed; (3) a lightning bolt from cloud to ground.
Compare your answer
In all three, the moving things are electrons — and none are created. Rubbing transfers a tiny fraction of the hair’s electrons on to the balloon, which is then negative; the wall’s electrons are pushed slightly away from its surface, leaving a positive surface that attracts the balloon. In the torch wire the electrons were already there — a copper wire is full of free electrons — and closing the switch lets the cell push them all along together, like water in a hose that is already full, which is why the lamp lights at once. Lightning is the same transfer as the balloon on a monstrous scale: charge separated inside a storm cloud jumps across kilometres of air.
The science — structured notes
1.1 Static charge: electrons that have moved
Every atom (Year 2, Unit 5) has a positive nucleus surrounded by negative electrons, and normally the two cancel exactly: matter is neutral. Rub two different insulators together and the outer electrons of one are dragged on to the other. The material that gains electrons becomes negatively charged; the one that loses them becomes positively charged by exactly the same amount. Charge is never created or destroyed — it is transferred.
There are only two kinds of charge, and one rule: like charges repel, unlike charges attract. Two rubbed balloons swing apart; a rubbed comb picks up scraps of paper; your hair follows the balloon.
| What you see | What the electrons are doing |
|---|---|
| A polythene rod rubbed with a cloth becomes negative | Electrons move from the cloth to the rod; the cloth is left positive |
| An acetate rod rubbed with the same cloth becomes positive | Electrons move from the rod to the cloth — which material wins the electrons depends on the pair |
| A charged balloon sticks to a neutral wall | Induction: the balloon’s negative charge pushes the wall’s electrons back from the surface, leaving the surface positive; unlike charges attract |
| A shock from a car door on a dry Kalahari day | Sliding across the seat charges you; the charge leaks across the gap to the metal as a spark |
| Lightning | Ice crystals and hail colliding in a storm cloud separate charge; the base of the cloud goes negative and induces a positive charge on the ground; when the air can no longer hold them apart, electrons flood across in a channel a few centimetres wide and kilometres long |
1.2 Conductors and insulators
Whether a material can carry charge depends on whether its electrons are free to move through it.
| Conductors | Insulators | |
|---|---|---|
| Examples | All metals (copper, aluminium, silver best of all), graphite, salt water, the human body | Plastics, rubber, glass, dry wood, dry air, pure water |
| Electrons | Each metal atom gives up one or two outer electrons to a shared “sea” that moves freely through the whole metal | Every electron is held to its own atom or molecule; none can travel |
| Static charge | Spreads over the whole surface at once, and leaks away to earth if there is a path — hard to charge by rubbing while you hold it | Stays where it was put — easy to charge by rubbing |
| In a circuit | The wires and the components that current must pass through | The coating on the wire, the plug casing, the switch handle |
1.3 Current: charge on the move
An electric current is a flow of charge. In a metal wire the charge that flows is the sea of free electrons; in salt water or in your nerves it is ions. The size of the current is the rate at which charge passes a point:
I = Qt current (amperes, A) = charge (coulombs, C)time (seconds, s)
The coulomb (C) is the unit of charge: one coulomb is the charge of about six million million million electrons (6.25 × 10¹⁸). The ampere (A) is one coulomb passing per second. Everyday currents: a phone charging, about 2 A; a kettle, about 9 A; a lightning strike, about 30 000 A for a few thousandths of a second.
A torch lamp passes 24 C of charge in 2 minutes. Find the current. Then find how much charge a phone charger delivering 2.0 A passes in 1 hour.
Convert first: 2 minutes = 120 s. I = Qt = 24 C120 s = 0.20 A.
Rearrange: Q = I × t = 2.0 A × 3600 s = 7200 C. That is about 4.5 × 10²² electrons — and not one of them was made by the charger; they were already in the cable.
1.4 Cells, batteries and voltage
A current needs a push. A cell uses a chemical reaction to move electrons from one terminal to the other, so that one terminal has too many (negative) and one too few (positive). Connect a wire between them and the electrons flow round to even things out, giving energy to whatever they pass through. A battery is two or more cells joined; cells in series add their voltages (four 1.5 V cells in a torch make a 6 V battery).
Voltage (potential difference) is the energy transferred per coulomb. A 1.5 V cell gives each coulomb 1.5 joules; a 12 V car battery gives each coulomb 12 joules; the 230 V mains gives each coulomb 230 joules — which is why it can kill and a torch cell cannot.
V = EQ voltage (volts, V) = energy (joules, J)charge (coulombs, C)
A 9 V battery pushes 20 C through a buzzer. How much energy does the buzzer receive? How does the number change for a 1.5 V cell pushing the same charge?
E = V × Q = 9 V × 20 C = 180 J.
For the 1.5 V cell: E = 1.5 × 20 = 30 J. Same charge, six times less energy per coulomb, six times less energy — the buzzer would barely sound. Voltage is about energy per coulomb; current is about coulombs per second. Keep them apart.
1.5 Circuit symbols
A circuit diagram is a map, not a picture: straight lines for wires, right-angle corners, and one agreed symbol for each component. Learn these — the unit test asks for them, and every wiring diagram in a car, a house or a solar system uses them.
| Component | Symbol | Function in the circuit |
|---|---|---|
| Cell | Gives energy to each coulomb; the long thin line is the positive terminal | |
| Battery | Two or more cells; voltages add | |
| Switch (open) | Breaks or completes the path; open = no current anywhere in that loop | |
| Lamp | Transfers energy to light (and heat) as current passes through its filament | |
| Resistor | Limits the current; transfers energy to heat | |
| Variable resistor | Resistance that can be changed — a dimmer, a volume control, a fan speed | |
| Ammeter | Measures current, in A; placed in series (in the path) | |
| Voltmeter | Measures voltage, in V; placed in parallel (across a component) | |
| Fuse | A thin wire that melts and breaks the circuit if the current is too large | |
| LED | Light-emitting diode: lights only when current flows in the direction of the arrow; very efficient | |
| Motor | Transfers electrical energy to movement (kinetic energy) |
1.6 Where the meters go
“A current of 3 A means 3 electrons pass each second.”
Reveal answer
False. 3 A means 3 coulombs per second, and one coulomb is about 6 × 10¹⁸ electrons — so roughly 2 × 10¹⁹ electrons pass every second. The coulomb is a large unit precisely because a single electron’s charge is so tiny; a charged balloon carries perhaps a hundred-millionth of a coulomb.
Calculate, converting units first.
(a) A car starter motor draws 150 A for 3.0 s. How much charge passes?
(b) A lightning strike transfers 15 C in 30 ms. What is the current?
(c) The 12 V car battery in (a) gives how much energy to the starter motor?
Reveal answer
(a) Q = I × t = 150 A × 3.0 s = 450 C.
(b) 30 ms = 0.030 s. I = Q ÷ t = 15 ÷ 0.030 = 500 A — a small strike; big ones reach 30 000 A or more.
(c) E = V × Q = 12 V × 450 C = 5400 J, delivered in 3 seconds — a power of 1800 W, which is why starter cables are as thick as your finger.
A student draws a circuit to measure the current through a lamp and the voltage across it. She puts the ammeter across the lamp and the voltmeter in the loop next to the cell. What happens when she closes the switch, and how should the diagram be redrawn?
Reveal answer
The voltmeter’s very high resistance is now in the only path, so almost no current flows and the lamp stays dark — the voltmeter reads nearly the whole cell voltage and the ammeter reads about zero. If she then “fixes” it by removing the voltmeter, the ammeter across the lamp becomes a short cut of almost zero resistance: the current bypasses the lamp, rises sharply, and the ammeter or the cell can be damaged. Redraw with the ammeter in series in the loop and the voltmeter connected by two leads to the two sides of the lamp only.
A charged polythene rod is held near a thin stream of water from a tap and the stream bends towards it. What if the rod were positively charged instead? What if the water were replaced by a stream of dry sand?
Reveal answer
A positive rod bends the stream just as much and in the same direction — towards the rod — because the effect is induction: the rod pulls the unlike charge in each water molecule to the near side and pushes the like charge away, so the near side is always the opposite of the rod and is attracted. (Water molecules are also naturally lopsided, positive at one end, and rotate to face the rod.) Dry sand hardly bends: its grains are insulators whose charges cannot separate much, and each grain is heavy for the tiny force, so the deflection is too small to see.
Try it — meet the ammeter and the voltmeter
This lab is a circuit built to measure a wire: a supply, an ammeter in the loop, and a voltmeter across the length of wire being tested. It reports the ratio of the voltmeter reading to the ammeter reading — the wire’s resistance, which is the subject of the next lesson. For now use it to see the two meters in their proper places, and to practise the arithmetic of charge. Set the supply to 2 V and the length to 50 cm, run it, and read the current from the result.
Suppose your 2 V, 50 cm run gave a current of 1.02 A.
(a) Calculate the charge passing through the wire in 60 s and the energy the supply transfers to it.
(b) The wire warms up during a long run. Explain, using electrons and ions, where the energy goes.
(c) Explain why the voltmeter must be connected across the wire and not across the supply if you want the voltage across the wire alone.
Reveal answer
(a) Q = I × t = 1.02 × 60 = 61.2 C (61 C); E = V × Q = 2 × 61.2 = 122 J.
(b) The free electrons are pushed through a lattice of vibrating metal ions and collide with them repeatedly; each collision hands some of the electrons’ energy to the ions, which vibrate more — the wire’s thermal energy store rises and its temperature climbs. The 122 J from the supply ends up almost entirely as heat in the wire and, from there, in the air.
(c) A voltmeter reads the energy per coulomb between its two leads. Across the supply it reads what the supply gives; across the wire it reads what the wire takes — and the two differ by whatever the ammeter, the connecting leads and the supply’s own internal resistance take. If you want the wire’s share, the leads go to the two ends of the wire.
Common mistakes to avoid
- “Charge is created by rubbing” — electrons are transferred; one object gains what the other loses.
- “Current is used up in the lamp” — the same current leaves as enters; it is energy that is transferred.
- “Volts flow” — current (A) flows; voltage (V) is energy per coulomb between two points.
- Forgetting to convert — minutes to seconds, ms to s, before I = Q ÷ t.
- Ammeter across, voltmeter in series — ammeter in the path, voltmeter across the component.
- “Electricity travels at the speed of light” — the push does; the electrons drift at under a millimetre per second.
How we found out — amber, a kite and a very small particle
The Greek word for amber is elektron. Around 600 BCE Thales of Miletus noted that rubbed amber picks up chaff, and for two thousand years that was the whole of electrical science. In 1600 William Gilbert showed that many materials do the same and coined electricus. In 1733 the French chemist Charles du Fay found there were two kinds of charge — rubbed glass repelled rubbed glass but attracted rubbed resin — and that like repelled like. Benjamin Franklin in Philadelphia proposed in 1747 that there was really one electrical “fluid” and that objects had too much or too little of it, which he named positive and negative; his famous kite experiment of 1752 showed that lightning was the same thing as the spark from a rubbed rod, and led directly to the lightning conductor — a pointed metal rod on the roof, wired to the ground, that gives the charge a safe path. Franklin’s guess about which way the fluid moved was, by bad luck, backwards; we still use his labels, which is why conventional current runs opposite to the electrons.
The push was measured next. In 1785 Charles-Augustin de Coulomb hung a charged sphere on a fine wire and measured how the twist changed with distance and charge, giving the force law that bears his name and the unit that bears it too. In 1800 Alessandro Volta stacked discs of zinc, copper and brine-soaked card and produced the first steady current — the first cell — and in 1820 André-Marie Ampère showed that two wires carrying current pushed on each other magnetically, giving a way to measure current and, later, his name to its unit. The particle itself came last. In 1897 J. J. Thomson in Cambridge bent a beam of cathode rays with electric and magnetic fields and showed that they were tiny negative particles, two thousand times lighter than a hydrogen atom, the same whatever gas or metal they came from: the electron, the thing that had been moving all along. It took 2500 years to go from amber to the particle, and 250 more to fill the world with wires.
Franklin’s positive-and-negative labels were kept even after Thomson showed the moving particle is negative. Explain why physicists did not simply relabel the terminals, and why it does not matter for calculations.
Reveal answer
By 1897 a century of measurements, instruments, textbooks and engineering (telegraphs, dynamos, Edison’s power stations) had been built on Franklin’s convention; relabelling every meter and diagram would have caused far more confusion than keeping an arrow that points the “wrong” way. It does not matter for calculations because a flow of negative charge to the left is, in every equation, identical to a flow of positive charge to the right: the current, the energy transferred and the heating are the same. The direction only matters when you ask about the particles — and then you say so.
Where it is used now
- Lightning in Botswana. Summer thunderstorms bring frequent lightning across the country, and lightning kills people and cattle every rainy season. Lightning conductors on schools and clinics, and the advice to stay away from lone trees and out of open water, all follow from induction: the charge concentrates on the nearest high point.
- Fuel and static. Petrol flowing through a hose charges by friction, and a spark near petrol vapour is fatal. Tankers are earthed with a cable before they unload, and aircraft are bonded to the fuel truck before the hose is connected — the charge leaks away instead of jumping.
- Cleaning the smoke. A coal station such as Morupule must take the ash out of its flue gas before it leaves the stack, and one standard way to do it is an electrostatic precipitator: wires at tens of thousands of volts charge the ash particles, which are then attracted to earthed plates and collected.
- Printing and painting. Photocopiers and laser printers place charge on a drum so that toner sticks only where the image is; spray-painting charges the droplets so they wrap round the car body and none is wasted.
- Your screen. A touchscreen is a grid of conductors that senses the tiny change in charge when a conducting finger comes near — it ignores a plastic pen, which is why gloves do not work.
Science now — and the scientist you need to be
What is happening in 2026
Charge is now watched from orbit. The lightning imager on Europe’s newest weather satellite, stationed over Africa, records every flash across the continent many times a second, and forecasters use the maps to warn where storms are turning severe — data that Botswana’s meteorological service can see minutes after the flash. On the ground, the newest triboelectric generators do Thales’s experiment for a living: layers of polymer rubbing together in a shoe sole or a rolling tyre harvest enough charge to power a sensor without a battery. And in the factories that make the chips behind every AI system, static is the enemy — a spark you cannot feel carries enough energy to destroy transistors a few atoms wide, so every worker, floor and tool is earthed, and the air itself is ionised to bleed charge away. The oldest electrical effect is the one that modern electronics guards against most carefully.
The mindshift
What you need to be able to do
Criterion A questions in this unit start here: define current and voltage in words that separate them, calculate with I = Q ÷ t and E = V × Q after converting units, draw symbols from memory, and explain any static effect with electrons transferred or induced. The examiners’ reports say the same thing every year: an explanation that never mentions electrons, energy or charge is a description, and scores as one.
See an example
Cling film: pulling it off the roll transfers electrons between layers, so the sheet is charged and induces the opposite charge on the bowl — it clings. Fuel pump: the black strap earths the nozzle so charge from the flowing petrol leaks away rather than sparking. Dusty screen: the charged screen induces charge in neutral dust and attracts it. Charger label: 5 V, 2 A — in one hour Q = 2 × 3600 = 7200 C and E = 5 × 7200 = 36 000 J. The AI first said “electricity travels at nearly the speed of light”; asked how fast the electrons move, it corrected itself to a drift of about 0.1 mm/s and explained that the wire is already full of electrons.
Key terms
| Charge | A property of particles; electrons are negative, protons positive; measured in coulombs (C). |
| Static electricity | Charge that has been transferred by rubbing and stays on an insulator. |
| Induction | The separation of charge in a neutral object caused by a charged object nearby, so that the near side is oppositely charged. |
| Conductor / insulator | A material whose electrons are free to move through it / one whose electrons are held to their atoms. |
| Current (I) | The rate of flow of charge: I = Q ÷ t, in amperes (A); 1 A = 1 C per second. |
| Coulomb (C) | The unit of charge; about 6.25 × 10¹⁸ electrons. |
| Conventional current | The direction from positive to negative shown on diagrams; electrons flow the other way. |
| Voltage (potential difference) | The energy transferred per coulomb between two points: V = E ÷ Q, in volts (V); 1 V = 1 J per C. |
| Cell / battery | A source that uses a chemical reaction to give energy to charge / two or more cells joined together. |
| Ammeter / voltmeter | Measures current, in series, low resistance / measures voltage, in parallel, high resistance. |
Think like a scientist
Criterion A Read the scenario, then answer every part. Write your answers in full before revealing the model.
At Sir Seretse Khama airport a fuel tanker draws up beside an aircraft. Before the hose is attached, a technician clips a bonding cable from the tanker to a metal point on the aircraft. Fuel then flows through the hose at about 20 litres per second for eight minutes. A notice on the tanker reads: “Static discharge hazard — bond before fuelling.” During a storm the previous week, a strike on the airfield mast carried a current of 20 000 A for 0.50 ms.
(a) Explain, in terms of electrons, how fuel flowing through the hose can leave the aircraft charged. [2]
(b) Explain why a spark between the hose nozzle and the aircraft would be dangerous, and how the bonding cable prevents it. [3]
(c) Calculate the charge transferred by the lightning strike on the mast. [2]
(d) The aircraft battery is 28 V. State what “28 V” means in terms of energy and charge, and calculate the energy given to 150 C of charge that the battery pushes through a starter motor. [3]
(e) The bonding cable is copper with a rubber sleeve. Explain the function of each material in terms of its electrons. [2]
Model answer
(a) As the fuel rubs against the inside of the hose, electrons are transferred between the fuel and the hose material (friction charging); the fuel carries its charge into the aircraft’s tank, so the aircraft gains a net charge while the hose and tanker gain the opposite charge. No charge is created — it is separated.
(b) The aircraft and tanker end up with opposite charges, so there is a voltage between them; if it becomes large enough, electrons jump across the air gap as a spark. A spark transfers energy to the air and heats it to thousands of degrees, and fuel vapour around the nozzle is flammable, so it can ignite an explosion. The bonding cable is a conductor joining the two before fuelling starts, so any charge that separates flows back along the cable continuously and the voltage between them never builds up — there is nothing left to jump.
(c) t = 0.50 ms = 0.00050 s. Q = I × t = 20 000 A × 0.00050 s = 10 C.
(d) 28 V means the battery transfers 28 joules of energy to every coulomb of charge that passes through it. E = V × Q = 28 V × 150 C = 4200 J.
(e) Copper is a metal whose outer electrons form a sea of free electrons that can move through the whole cable, so charge flows along it easily — it is the path. Rubber’s electrons are locked in bonds to their atoms and cannot move, so the sleeve stops charge leaking from the cable to whatever touches it and protects the technician — it is the barrier.
Probe deeper — exam-style questions
Attempt every question before opening the answer. Marks in brackets show what a full answer is worth.
A plastic ruler is rubbed with a woollen cloth and becomes negatively charged. Explain what has happened to the ruler and the cloth, and predict what the cloth will do if brought near the ruler.
Reveal answer
Electrons have been transferred from the cloth to the ruler [1]; the ruler has gained electrons (negative) and the cloth has lost the same number, leaving it positive [1]; unlike charges attract, so the cloth and ruler attract each other [1].
A current of 0.25 A flows through an LED for 4.0 minutes. Calculate the charge that passes, and the energy transferred if the LED has 2.0 V across it.
Reveal answer
4.0 min = 240 s [1]; Q = I × t = 0.25 × 240 = 60 C [1]; E = V × Q = 2.0 × 60 = 120 J [1].
Define current and voltage in a way that makes clear how they differ.
Reveal answer
Current is the rate of flow of charge — the number of coulombs passing a point each second, in amperes [1]; voltage is the energy transferred per coulomb between two points, in joules per coulomb (volts) — a rate of flow versus an energy per unit of charge [1].
A student writes: “The cell makes electrons and sends them round the circuit at the speed of light; the lamp uses them up, so fewer come back.” Identify three errors.
Reveal answer
The cell does not make electrons — the wire is already full of free electrons, and the cell gives them energy [1]; the electrons drift at well under a millimetre per second; it is the push (the electric field) that is set up at nearly the speed of light [1]; charge is not used up — the same current leaves the lamp as enters it; the lamp transfers the energy the electrons carry [1].
Draw (or describe precisely) a circuit containing a battery of two cells, a switch, a lamp, an ammeter measuring the lamp’s current and a voltmeter measuring the voltage across the lamp.
Reveal answer
One loop: battery (two cell symbols), switch, lamp and ammeter in series, in any order, with correct symbols [1]; voltmeter connected by two leads to the two sides of the lamp only, in parallel with it [1].
A lightning conductor is a thick copper strip from a pointed rod on the roof down to a plate buried in the ground. Explain how it protects the building, using induction and conduction.
Reveal answer
The negative base of the storm cloud induces a positive charge on the ground and on everything on it, most strongly on the highest point — the rod [1]; the copper strip is a conductor, so charge can flow between the ground and the rod freely, and the pointed tip lets charge leak into the air, reducing the build-up that leads to a strike [1]; if a strike does come, it takes the low-resistance copper path to the earth plate instead of through the walls, so the building’s materials are not heated or blown apart by the current [1].
Summary
- Static charge is electrons transferred by rubbing; like charges repel, unlike attract; a charged object attracts a neutral one by induction.
- Conductors have free electrons; insulators do not. Metals are conductors, plastics insulators.
- Current is the rate of flow of charge: I = Q ÷ t, in amperes; 1 A is 1 coulomb per second. Convert time to seconds first.
- Voltage is energy per coulomb: V = E ÷ Q. A cell gives energy to charge that is already in the wire; charge is not used up.
- Ammeters go in series (low resistance); voltmeters go across a component (high resistance). Learn the symbols.
Next time → Series circuits: one loop, one current, voltages that add — and the law Georg Ohm was mocked for.