Circuit Symbols, Charge & Current

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Standard circuit symbols

You must be able to draw and interpret circuit diagrams using the standard symbols

The standard circuit symbols in a grid, each labelled: cell, battery, switch (open), lamp, fuse, voltmeter, ammeter, diode, resistor, variable resistor, LED, LDR, thermistor

Component	What it does
Cell / battery	provides the potential difference; a battery is two or more cells
Switch	opens or closes the circuit
Resistor / variable resistor	fixed / adjustable resistance, used to control current
Lamp	filament glows; resistance rises as it heats
Diode / LED	current in one direction only; an LED lights when current flows
Thermistor	resistance falls as temperature rises
LDR	resistance falls as light intensity rises
Ammeter	measures current; connected in series
Voltmeter	measures potential difference; connected in parallel across the component
Fuse	thin wire that melts if the current is too large

Charge and current

For charge to flow through a closed circuit there must be a source of potential difference
Electric current is a flow of electrical charge. The size of the current is the rate of flow of charge (recall this equation):

Q = I t

$Q$ charge flow in coulombs, C; $I$ current in amperes (amps), A; $t$ time in seconds, s
Worked example: a current of 0.5 A for 2 minutes. $Q = 0.5 \times 120 = 60$ C
The current has the same value at any point in a single closed loop: charge is not used up
In metal wires the charge is carried by electrons, which are negative and flow from negative to positive; conventional current is drawn from positive to negative