Section 1 of 5Power and Control Components
Cell
- Provides a source of potential difference, and a battery is two or more cells joined together.
Switch
- Opens or closes the circuit, which turns the current off or on.
Fixed resistor
- Limits the current in a circuit, and its resistance cannot be changed.
Variable resistor
- Has a slider that changes its resistance, and is used in dimmer switches and volume controls.
Section 2 of 5Output and Measuring Components
Diode
- Allows current to flow in one direction only, and is used to convert a.c. to d.c.
LED
- A diode that emits light when current flows through it in the forward direction, used in displays and lights.
Ammeter
- Measures the current in a circuit and is connected in series with the component.
Voltmeter
- Measures the potential difference across a component and is connected in parallel with that component.
Section 3 of 5Sensors and Circuit Checks
Sensors and Circuit Checks
Thermistor
- Its resistance falls as its temperature rises, so it is used in thermostats for radiators and kettles.
LDR
- Its resistance falls as the light intensity rises, so it is used in street lamps that switch on in the dark.
Symbol clues
- Two arrows pointing towards a symbol show that it is light-dependent, while arrows pointing away show light emission.
Circuit check
- A working circuit needs a source of potential difference, a closed loop, and a switch that can open the loop.
Section 4 of 5Reading Circuit Diagrams
Reading Circuit Diagrams
Series or parallel
- Components drawn on one loop are in series, while components drawn on separate branches of the same loop are in parallel.
Measuring devices
- Ammeters and voltmeters are drawn as a letter inside a circle and must be placed in the correct position to give valid readings.
Drawing carefully
- Use a ruler and straight lines when drawing circuits, and label each component clearly with its symbol.
Symbol arrows
- Arrows on the LED and LDR symbols show whether light is being emitted or detected, so check the arrow direction in every question.
Section 5 of 5Exam Checklist & Tips
5.1.3 – Key Points for the Exam
- Circuit symbol A standard drawing used to represent a component in a circuit diagram.
- Series A connection in which components share a single path for the current.
- Parallel A connection in which components are on separate branches with the same potential difference.
- Diode A component that allows current to flow in one direction only.
- Thermistor A resistor whose resistance depends on its temperature.
- LDR A light-dependent resistor whose resistance depends on the light intensity.
Common Mistakes
- Drawing an ammeter in parallel with a component; an ammeter must be connected in series.
- Connecting a voltmeter in series with a component, which changes the circuit and gives a wrong reading.
- Confusing the LED symbol with the LDR symbol, which has arrows pointing towards it rather than away.
- Stating that a thermistor's resistance rises with temperature;
- for a thermistor it falls as the temperature rises.