Everything in this module is built on one idea: some particles carry electric charge, and a circuit is an arrangement for pushing them round a loop. By the end of this lesson you should be able to say what charge is and what its unit means, work out how much charge has flowed from a current and a time and run that calculation backwards, explain why a metal carries a current and a plastic does not, and say why the current is the same at every point of a single loop.
Charge, and the two kinds of it
Electric charge is a property that some particles have, in the way that all particles have mass. There are two kinds, called positive and negative, and the names are labels rather than descriptions — nothing is being added or taken away. Ordinary matter contains both in equal measure: an atom has a nucleus containing positively charged protons, surrounded by an equal number of negatively charged electrons, and because the two charges are equal and opposite the atom as a whole is neutral, meaning it has no net charge. Almost everything you handle is neutral, which is why charge is invisible in daily life until you disturb the balance.
Disturb it and you get a force. Two charges of the same kind push each other apart, and two of opposite kinds pull each other together: like charges repel, unlike charges attract. This is a genuine force, measured in newtons as any force is, and it acts at a distance without the two charges touching — which is what makes it possible to run a current down a wire without anything being physically dragged along. Rub a balloon on your sleeve and you scrape electrons off one surface onto the other; the balloon now has more electrons than protons and is negatively charged, the sleeve has fewer and is positively charged, and the two attract each other while two rubbed balloons repel. Notice that only the electrons moved. Protons are locked into nuclei, so in solid matter charge is transferred by electrons going somewhere, never by protons.
Charge is measured in coulombs, written C, and one coulomb is a large amount of it. A single electron carries a charge of about 0.000 000 000 000 000 000 16 coulombs, so it takes something like six million million million electrons to add up to one coulomb. That is worth knowing only for the sense of scale it gives: the charges that build up when you rub a balloon are millionths of a coulomb, while a coulomb passing along a wire every second is a perfectly ordinary current in a household appliance.
Why some materials carry a current and others do not
For charge to flow through a material, something charged inside it has to be free to move. In a metal that condition is met in an unusual way: the atoms sit in a fixed lattice, but each of them gives up one or more of its outermost electrons, and those electrons are not attached to any particular atom any more. They drift about through the whole lump of metal, which is why they are called free electrons. The metal is still neutral overall — the electrons are all still inside it — but if you push on that sea of electrons from one end, it can move. That is what makes copper, aluminium, silver, steel and every other metal a conductor, meaning a material that lets charge pass through it.
An insulator is a material with no such free charges. In glass, rubber, dry wood, most plastics, oil and dry air, every electron is bound tightly to a particular atom or molecule, so there is nothing available to carry a current and charge stays where you put it. Two honest qualifications. First, an insulator is not a barrier for ever: push hard enough on the charge inside it and electrons can be torn loose, which is exactly what a spark is. Second, a couple of materials sit awkwardly in the scheme. Graphite, a form of carbon, conducts well despite not being a metal, and pure water barely conducts at all while salty water conducts quite well, because dissolving salt fills the water with charged particles free to drift. That last point is why wet hands make an electric shock far more dangerous than dry ones.
Current is a rate of flow
Now put a conductor between the two ends of a supply so that the free charge inside it is pushed steadily in one direction. Electric current is the name for that flow, and the quantity you measure is not how much charge is in the wire but how fast it is going past — how much charge crosses a chosen point each second. That makes current a rate, in exactly the way that speed is a rate: speed is metres per second, current is coulombs per second. One coulomb per second is given its own name, the ampere, written A and usually shortened to amp in speech.
Because current is charge per second, multiplying a current by a time gives you the charge that went past during it. Two amps means two coulombs every second, so in ten seconds twenty coulombs have gone by.
Charge, current and time
- the charge that flows past a point, in coulombs (C)
- the current, in amperes (A) — one ampere is one coulomb per second
- the time the current flows for, in seconds (s)
The two rearrangements are worth deriving once rather than memorising separately. Divide both sides of that relationship by the time: on the right the times cancel and leave the current, so — which is just the definition of current written out again, and a useful check that the formula says what it should. Now divide both sides of the original by the current instead: the currents on the right cancel and leave the time, so . Three arrangements, one fact.
Two points of housekeeping, because items exploit both. The time must be in seconds, so a duration given in minutes has to be multiplied by sixty before it goes anywhere near the formula. And currents smaller than an amp are usually quoted in milliamps, written mA, where the prefix milli- means one thousandth: 1 mA is 0.001 A, so to turn milliamps into amps you divide by a thousand. A torch bulb might draw 300 mA, which is 0.3 A.
The loop has to be complete
A current will not flow along a wire that goes nowhere. Charge has to have a complete path from one terminal of the supply, through whatever the circuit contains, and back to the other terminal — because the supply is not making charge, only pushing round the charge that is already in the wires. Break that path anywhere and everything stops, everywhere, instantly: a switch left open at one corner of the loop stops the current in the far corner just as effectively, which is exactly why a switch works at all. This is also why every electrical device has at least two connections rather than one.
The figure carries the single most important consequence, and it is the one candidates get wrong. The current is the same at every point of a single loop. It is not larger leaving the cell than returning to it, and it is not reduced by having gone through the lamp. Think about what the alternative would mean: if two coulombs a second left the cell and only one came back, charge would be piling up somewhere in the circuit at a coulomb a second, and since charge repels itself that build-up would very quickly stop the flow altogether. Nothing of the sort happens, because nothing accumulates. What the lamp takes from the charges passing through it is energy, not charge — the charges leave the lamp in exactly the same number they entered it, just carrying less energy. Keeping charge and energy apart is what the rest of this module rests on.
Which way it actually goes
One piece of honesty about a convention. Long before anyone knew what was moving inside a wire, current was defined as flowing from the positive terminal of a supply, round the external circuit, to the negative terminal, and every circuit diagram, every arrow in this module and every rule you will meet is written in those terms. It later turned out that in a metal the things actually moving are electrons, which are negative, and which therefore drift the other way — from the negative terminal towards the positive one. Nobody reversed the convention, because nothing depends on it: the arrangement of a circuit and the size of its current are unaffected by which sign you attach to the carriers. So the arrows in these figures show conventional current, from positive to negative, and you should read them that way without worrying that the electrons disagree.
Why it matters in aviation
Everything electrical on an aircraft is a complete circuit fed from a busbar, and the current each service draws is what the generators must supply between them. That is why an electrical load is quoted and watched in amperes: the figure is a count of coulombs per second leaving the bus, and the sum of the individual demands is what the generation has to meet.
Battery capacity is quoted in ampere-hours for the same reason, and the unit is worth reading literally. A current multiplied by a time is a charge, so an ampere-hour rating describes a store of charge, and dividing that store by the current the essential services draw is what indicates how long they could be run if generation were lost. The requirement for a complete path also shows up in the airframe itself. Metal structure, and the equipment mounted on it, are deliberately bonded together by conducting straps so that current — including charge that collects on the skin in flight — has a defined route back rather than finding one by jumping across a gap.
What the test asks
The distractor to watch for is the idea that current is used up as it goes round — an item will offer you two ammeter readings, one before a lamp and one after it, and invite you to pick the smaller for the second. There is no such reduction: the charge that goes into the lamp comes out of it, and what the lamp consumes is the energy those charges are carrying. Expect also a straightforward Q = It calculation in which the entire difficulty is a unit, with the time given in minutes or the current in milliamps and a distractor sitting exactly where the unconverted answer would land. And expect one item asking which of a list of materials conducts, where the reliable trap is either water on its own, which barely conducts until something is dissolved in it, or graphite, which conducts although it is not a metal.