4.2.2 Electric current
- Syllabus
- 0625–2026–2027
- Topic
- 4.2.2
- Level
- —
Learning objectives
4.2.2.1Electric current is related to the• Know: electric current is related to the flow of charge4.2.2.2Use of ammeters (analogue and digital)• Describe the use of ammeters (analogue and digital) with different ranges4.2.2.3Electrical conduction in metals in• Describe electrical conduction in metals in terms of the movement of free electrons4.2.2.4Difference between direct current• Know the difference between direct current (d.c.) and alternating current (a.c.)4.2.2.5Electric current as charge passing a• Define electric current as charge passing a point per unit time; recall/use: I = Q/t4.2.2.6Conventional current is from positive• State: conventional current is from positive to negative and that the flow of free electrons is from negative to positive
Relate electric current to the flow of charge
An electric current exists when electric charge flows. A larger current means that more charge passes a point each second.
| Situation | Current? |
|---|---|
| charge flows continuously around a complete circuit | yes |
| charge is present but does not flow | no |
| circuit path is broken | no continuous current |
In a complete series path, charge is not used up by a component: the same continuing flow passes successive points, while the component transfers energy.
Current is related to moving charge, not simply to the amount of charge stored on an object. The quantitative rate definition and equation are introduced in objective 5.
Use analogue and digital ammeters correctly
An ammeter measures electric current. Connect it in series so that the charge flowing through the component also flows through the meter.
| Step | Safe, accurate action |
|---|---|
| choose range | begin above the expected current; move to a lower suitable range for greater resolution |
| connect | put the meter in series with correct terminals/polarity for d.c. |
| analogue meter | check zero, read the correct scale at eye level and avoid parallax |
| digital meter | select A or mA and the appropriate d.c./a.c. setting; read sign and units |
A reading off-scale needs a higher range; a very small reading on a high range is less precise and may need a lower range. Record the value with A or mA and convert units when required.
Never connect an ammeter directly in parallel across a cell or component: its low resistance can cause a very large current and damage the meter.
Explain electrical conduction in metals
A metal contains fixed positive ions in a lattice and free electrons that can move through the metal. These mobile electrons carry charge through a wire.
| Condition | Electron behaviour |
|---|---|
| no electric driving effect | electrons move randomly with no net drift |
| complete circuit with a source | electrons acquire a net drift through the metal |
| circuit opened | continuous drift and current stop |
The drift of many free electrons produces an electric current even though each electron's drift is slow. The metal ions remain in their lattice positions.
Protons and metal nuclei do not travel around the wire. Electrons already present throughout the conductor move; the source does not have to send one electron through the whole circuit before a lamp responds.
Distinguish direct and alternating current
Direct current (d.c.) flows in one direction only. Alternating current (a.c.) repeatedly reverses direction.
| Current–time graph | Classification |
|---|---|
| stays on one side of zero | d.c.; direction does not reverse |
| crosses zero and alternates between positive and negative | a.c.; direction reverses |
| horizontal line above or below zero | steady d.c. |
A d.c. magnitude may be steady or may vary while remaining in the same direction. An a.c. waveform can have different shapes, but it must reverse direction.
The defining difference is direction, not whether the graph is curved or whether the magnitude changes. A varying current that never reverses is still d.c.
Define and calculate electric current
Electric current is the charge passing a point per unit time.
| Find | Relationship | Units |
|---|---|---|
| current | I = Q / t | A = C/s |
| charge | Q = I t | C |
| time | t = Q / I | s |
Example: 7.0 C passes in 5.0 min = 300 s, so I = 7.0 / 300 = 0.023 A. Convert time to seconds and current prefixes to amperes before substitution.
Current is a rate, so do not multiply Q by t when finding I. One ampere means one coulomb per second; it does not mean one coulomb in total.
Compare conventional current and electron flow
In the external circuit, conventional current is defined from the positive terminal to the negative terminal. Free electrons in a metal flow from the negative terminal to the positive terminal.
| Description | Direction through the external metal circuit |
|---|---|
| conventional current | positive terminal → components → negative terminal |
| electron flow | negative terminal → components → positive terminal |
Circuit arrows labelled I normally show conventional current. To add an electron-flow arrow, reverse the conventional-current direction along the same wire.
Opposite directions do not mean two different currents exist. The same metal conduction is described using the historical positive-charge convention and the actual motion of negative electrons.