Current as a rate of flow of charge
Definition
Electric current
The rate of flow of electric charge.
Definition
Free electron
An electron in a metal that is not bound to any one atom and can move through the whole metal.
- An electric current is the rate of flow of charge, which means it measures how much charge passes a point each second.
- The word rate matters, because current tells you how quickly charge is flowing, not how much charge there is in total.
- A large current means a lot of charge passes a point every second, and a small current means only a little charge passes each second.
- Current is measured in amperes, usually shortened to amps, with the unit symbol A\text{A}A.
- In a metal wire, the charges that flow are electrons.
- A metal is made of positive metal ions held in a fixed arrangement, surrounded by electrons that are not attached to any particular atom.
- These free electrons drift through the metal when a current flows, while the positive ions stay where they are.
- The free electrons are already in every part of the wire before the circuit is switched on, so a cell does not supply the charge but makes the charge that is already there move.
- Electrons are negative, so they drift from the negative terminal of a cell, round the circuit, to the positive terminal.
- Circuit diagrams show current flowing the opposite way, from positive to negative, and this agreed direction is called conventional current.
Conditions for charge to flow
Definition
Closed circuit
A complete, unbroken loop of conductors through which charge can flow.
Definition
Electrical conductor
A material that allows electric charge to flow through it easily.
- Charge only flows continuously when two conditions are both met.
- The first condition is a source of potential difference, such as a cell, a battery or a power supply.
- The source provides the push that makes the free electrons drift in one direction, in a similar way to a pump driving water round a central heating system.
- The second condition is a closed circuit, a complete loop of conducting material with no gaps that runs from one terminal of the source, through the components and back to the other terminal.
- The loop must be made of conductors, such as copper wire, because insulators such as plastic have no free electrons to carry the charge.
- If the loop has a gap, charge cannot cross it, so the flow of charge stops.
- An open switch is a deliberate gap, so opening a switch stops the current and closing it completes the circuit again.
- A broken wire, a loose connection or a broken lamp filament also leaves a gap and stops the current.
- A complete loop without a source of potential difference has no current, because nothing pushes the free electrons in one direction.
- A source connected to an incomplete loop has no current either, because the charge has no complete path to flow around.
- A flat cell can no longer provide a potential difference, so a torch with flat cells stays dark even when its switch is closed.
Practical
Testing which materials let charge flow
- Aim: to find out which materials complete a circuit and allow a current, and which do not.
- Apparatus: a low-voltage power supply set to about 3 V3\ \text{V}3 V or two 1.5 V1.5\ \text{V}1.5 V cells in a holder, a lamp in a holder, an ammeter, three connecting leads, two crocodile clips, and test samples such as copper wire, an iron nail, aluminium foil, a pencil lead, a plastic ruler, a wooden splint, a rubber band, a glass rod and a strip of dry paper.
- Method, setting up:
- Connect the power supply, the lamp and the ammeter in one loop using the leads, leaving two free ends with a crocodile clip on each.
- Touch the two crocodile clips together to close the circuit, and check that the lamp lights and the ammeter shows a reading.
- Separate the clips again and check that the lamp goes out and the ammeter reads zero, which confirms that the gap has stopped the current.
- Method, testing the samples:
- Clip the first sample between the two crocodile clips so that it bridges the gap, with the clips gripping clean, bare parts of the sample.
- Look at the lamp and record whether it lights, then read the ammeter and record the current to the nearest 0.01 A0.01\ \text{A}0.01 A.
- Unclip the sample straight away, so that the lamp and the wires do not heat up, and move on to the next sample.
- Test each sample three times, reconnecting the clips each time, to check that the result is repeatable.
- Record the results in a table with columns for the material, whether the lamp lit and the ammeter reading.
- Results: the copper, iron, aluminium and pencil lead all light the lamp and give a current, while the plastic, wood, rubber, glass and dry paper give no current.
- Conclusion: metals and graphite are conductors, because they contain free charges that can flow, and the other materials are insulators, which leave a gap in the circuit.
- Watch out: a dirty or oxidised surface can stop a metal sample from conducting, and a poor grip by the clips can give a false zero, so clean the samples with sandpaper and check each connection.
- Safety: use only a low-voltage supply, switch it off between samples, and do not touch the wire samples while they are connected, because thin wires can become hot.
Example
Why a torch lights
- A torch contains two cells, a switch and a lamp joined by metal strips in one loop.
- With the switch open there is a gap in the loop, so even though the cells provide a potential difference, no charge flows and the lamp stays off.
- Closing the switch presses two metal contacts together and makes a closed circuit.
- The cells now push the free electrons in every part of the loop, so they drift from the negative terminal, through the lamp, to the positive terminal.
- Charge flows through the lamp, so there is a current and the lamp lights.
- If the lamp stays dark with the switch closed, either the loop has a gap, such as a broken filament, or the source has failed, such as flat cells.
Common Mistake
- Do not define current as “the flow of electrons” on its own, because current is the rate of flow of charge.
- Do not say that the cell stores the electrons that make up the current, because the free electrons are already in every part of the circuit.
- Do not say current passes through an open switch or a plastic part of a circuit, because charge needs a complete path made of conductors.
Exam technique
Answering questions on current
- For a definition, write “current is the rate of flow of charge”, and keep the word rate, because without it you have described something else.
- When asked which particles move through a metal wire, answer electrons, not protons or ions.
- When asked why there is no current, name the missing condition: either there is no source of potential difference or the circuit is not closed.
- Tie the missing condition to the fault in the diagram, such as “the switch is open, so there is a gap and the circuit is not complete”.
Self review
- Define electric current.
- Name the charged particles that flow through a metal wire.
- State the two conditions needed for charge to flow continuously.
- Explain why opening a switch stops the current in a circuit.
- Explain why a plastic ruler placed across a gap in a circuit does not light a lamp.
