Electrical Components and Their CharacteristicsAQA GCSE Physics: Revision notes

You need to be able to draw and interpret circuit diagrams using standard symbols, including: switch, lamp, fuse, cell, battery, voltmeter, ammeter, diode, resistor, variable resistor, thermistor, LDR, and LED. Ammeters are always connected in series (to measure current through a component) and voltmeters in parallel (to measure potential difference across a component).

For an ohmic conductor at constant temperature, the current is directly proportional to the potential difference, so resistance stays constant. On an I-V graph this gives a straight line through the origin.

For a filament lamp, resistance increases as the filament gets hotter. On an I-V graph this gives a curve that flattens out as current increases, because temperature rises with current.

A diode is designed to let current flow in one direction only. Its I-V graph shows almost no current in the reverse direction, then a rapid rise in current once a small forward voltage is exceeded.

A thermistor is a resistor whose resistance decreases as temperature increases - used in applications like a thermostat, where the resistance change signals a temperature change.

An LDR (light-dependent resistor) has resistance that decreases as light intensity increases - used in applications like automatic (dusk-to-dawn) lighting.

Use I-V graphs to decide whether a component is linear (ohmic, straight line through origin) or non-linear (curved, e.g. filament lamp or diode). The gradient of an I-V graph relates to resistance - a steeper gradient (more current for a given pd) means lower resistance.