Current as flow of charge

1. Electric current is the rate of flow of charge, which means the quantity of charge that passes a point in the circuit each second.
2. Current is given the symbol $I$, and its unit is the ampere, written $A$ and often shortened to the amp.
3. A current of $1A$ means that $1C$ of charge passes a point every second, so $1A=1{C\; s}^{-1}$.
4. A larger current therefore means that more charge passes each second, and a smaller current means that less charge passes in the same time.
5. Typical values give a sense of scale: an indicator LED draws about $0.020A$, a torch lamp about $0.30A$ and a kettle element about $10A$.
6. Because a current is a flow, it is measured through a point in the circuit rather than across a component.

What actually moves

1. In a metal wire the charge is carried by delocalised electrons, the outer electrons that have broken away from individual atoms and can move through the whole metal.
2. Before a supply is connected these electrons are already moving quickly, but at random and in all directions, so no net charge passes any point and there is no current.
3. Once a potential difference is applied, the electrons gain a slow steady drift in one direction on top of their random motion, and it is that drift which is the current.
4. The positive ions of the metal are locked in position, so the metal itself stays where it is even though charge is flowing through it.
5. In an electrolyte such as copper sulfate solution the charge is carried by ions instead, with positive ions drifting one way through the liquid and negative ions drifting the other way.
6. Both cases count as a current, because in each case charge passes a point every second, whatever particle happens to be carrying it.
7. The charge itself is not used up on its way round the circuit. What the components take from it is the energy it carries, not the charge.

Conventional current

1. Conventional current is taken to pass from the positive terminal of the supply, round the external circuit, to the negative terminal.
2. The convention was agreed before anyone knew what carried the charge in a metal, and the direction chosen was the one in which positive charge would be pushed.
3. The electrons that actually move in a metal are negatively charged, so they are pushed the other way, out of the negative terminal and round to the positive terminal.
4. Conventional current and electron flow are therefore in opposite directions in a metal wire.
5. The two descriptions are equivalent, because negative charge moving one way has the same effect as positive charge moving the other way.
6. Every arrow on a circuit diagram at this level shows conventional current, so arrows are drawn leaving the positive terminal of the cell.

Measuring current

1. An ammeter measures the current at the point where it is placed, so it has to become part of the path that the charge follows.
2. It is therefore connected in series: the wire is broken at that point and its two ends are joined to the meter's terminals, so every unit of charge passing along the wire passes through the meter.
3. An ammeter is built with a very low resistance, so that inserting it disturbs the circuit it is measuring as little as possible.
4. Connecting an ammeter across a component instead would place a nearly resistance free branch beside that component, so a very large current would pass through the meter and it could be damaged.
5. Choose a range whose maximum lies just above the current expected: too high a range wastes resolution, while too low a range overloads the meter.
6. The resolution is the smallest change the meter can show, so a display reading to $0.01A$ cannot reveal a change of $0.001A$ and a milliammeter is used for very small currents.
7. A meter that does not read $0A$ with the switch open has a zero error, and every reading it gives is shifted by that amount until the meter is adjusted or the error is subtracted.
8. An analogue ammeter is read with the eye square on to the pointer, and its terminals are marked so that the meter is connected the correct way round.

A complete circuit

1. Charge only flows when there is an unbroken conducting path all the way from one terminal of the supply back to the other.
2. Opening a switch leaves a gap that the charge cannot cross, so the current in that loop falls to $0A$ immediately.
3. Air is an insulator, so at the potential differences used in a school circuit even a gap of a millimetre is enough to stop the flow completely.
4. A loose crocodile clip, a snapped lead or a broken lamp filament has exactly the same effect as an open switch.
5. A complete loop is not enough on its own, because a potential difference is also needed to push the charge, so a closed loop with no cell in it carries no current.
6. A lamp lights the instant the switch closes because the charge is already spread through every part of the wire and begins to drift throughout the loop at once, rather than travelling from the cell to the lamp first.

Using the charge equation

1. Because current is the charge passing each second, the charge that has passed in a given time is $Q=It$.
2. In this equation $Q$ is the charge in coulombs, $I$ is the current in amperes and $t$ is the time in seconds.
3. The rearrangements are $I=\frac{Q}{t}$ for the current and $t=\frac{Q}{I}$ for the time.
4. Times given in minutes are multiplied by $60$ and times given in hours are multiplied by $3600$, so $4.0$ minutes becomes $240s$ and $2.0$ hours becomes $7200s$.
5. Currents given in milliamperes are divided by $1000$, so $150mA=0.150A$.
6. At a steady current the charge that has passed is directly proportional to the time, so doubling the time doubles the charge and a graph of $Q$ against $t$ is a straight line through the origin.