Circuit Symbols, Charge and Current

Electric current is the rate of flow of electric charge round a circuit. Charge is carried by tiny particles; in a metal wire the charge carriers are free electrons, which drift through the metal when a source pushes them.

For any charge to flow at all, two things must be true:

• The circuit must be closed (complete), forming an unbroken loop.
• The circuit must contain a source of potential difference, such as a cell or a battery, to push the charge round.

If the loop is broken, for example by an open switch, no charge can flow and the current is zero.

Current is measured in amperes (amps, symbol A) using an ammeter. The ammeter is connected in series, so the charge being measured flows straight through it.

Potential difference, which does the pushing, is measured in volts (V) using a voltmeter connected in parallel across the component you are testing.

Circuit diagrams use standard symbols so that any physicist can read them. You must be able to draw and interpret each of the following components. The diagram below shows a simple series circuit built from four of them.

The full set of symbols you must know for AQA 8463:

Component	What it does
Cell	Single source of potential difference (longer line = positive)
Battery	Two or more cells joined together
Switch (open / closed)	Breaks or completes the circuit
Fuse	Melts and breaks the circuit if the current gets too high
Resistor	Fixed opposition to current
Variable resistor	Resistance can be changed to control current
Diode	Lets current flow one way only
LED (light-emitting diode)	A diode that emits light when current flows
Lamp	Transfers energy to light (and heat)
Voltmeter	Measures potential difference (connected in parallel)
Ammeter	Measures current (connected in series)
Thermistor	Resistance changes with temperature
LDR (light-dependent resistor)	Resistance changes with light intensity

The amount of charge that has flowed is linked to the current and the time for which it flows:

$Q=It$

where $Q$ is charge in coulombs (C), $I$ is current in amperes (A) and $t$ is time in seconds (s).

You must recall and apply this equation. It is not given on the Physics equation sheet.

Reading the equation as words: the larger the current, or the longer it flows, the more charge passes. One coulomb is the charge that flows when a current of one ampere flows for one second.

To find charge, multiply current by time. To find current or time instead, rearrange:

$I=\frac{Q}{t}t=\frac{Q}{I}$

The single most common error is leaving time in minutes. Time must be in seconds before you multiply.

A current of 0.5 A flows through a torch bulb for 2 minutes. Calculate the charge that flows.

Step 1 — Convert the time to seconds.

$t=2\times 60=120\text{ s}$

Step 2 — Write the equation.

$Q=It$

Step 3 — Substitute and calculate.

$Q=0.5\times 120=60\text{ C}$

The charge that flows is 60 C.

Notice how skipping Step 1 would give $0.5\times 2=1$ C, which is 60 times too small. Converting minutes to seconds first is the step that saves the marks.

A charge of 180 C flows through a heater in 90 s. Calculate the current.

Step 1 — Rearrange Q = It to make I the subject.

$I=\frac{Q}{t}$

Step 2 — Substitute the values (time is already in seconds).

$I=\frac{180}{90}$

Step 3 — Calculate.

$I=2\text{ A}$

The current is 2 A.

A quick sanity check: $2\text{ A}\times 90\text{ s}=180\text{ C}$, which matches the charge given, so the answer is consistent.

In a single closed loop (a series circuit) the current has the same value at every point. Charge is never used up or lost as it flows; the same charge simply keeps moving round.

Picture the electrons as beads on a loop of string: if you pull the string, every bead moves at once and the same number pass each point each second. This is why an ammeter placed anywhere in a single loop reads the same value.

The current is the same at every point in a single closed loop. Components in that loop share out the potential difference and transfer energy, but they do not "use up" the current.

A frequent misconception is that a lamp "uses up" current so that less flows out than in. It does not. The lamp transfers energy carried by the charge, but the rate of flow of charge (the current) leaving it equals the rate entering it.

1. Leaving time in minutes or hours

$Q=It$ needs time in seconds. Convert first: multiply minutes by 60, hours by 3600. This is the error examiners see most often.

2. Connecting the meters the wrong way

An ammeter goes in series (current flows through it); a voltmeter goes in parallel across the component. Swapping them is a guaranteed lost mark on a circuit-drawing question.

3. Thinking current is "used up" by components

The current is the same at every point in a single loop. What a component changes is the energy the charge carries (the potential difference), not the rate of flow of charge.

4. Confusing the units and quantities

Quantity	Symbol	Unit	Unit symbol
Charge	Q	coulomb	C
Current	I	ampere	A
Time	t	second	s
Potential difference	V	volt	V

Writing charge in amps or current in coulombs loses marks even when the arithmetic is right.

5. Forgetting the circuit must be complete

No closed loop and no source of potential difference means no current. Always check the loop is unbroken and that a switch is closed before doing any calculation.