How are real electrical and electronic circuits built, and how does a potential divider work?
Practical electrical and electronic circuits: standard circuit symbols and components, input and output devices such as the LDR and thermistor, the potential divider, and the action of switches and simple control circuits.
An SQA National 5 Physics answer on practical electrical and electronic circuits, covering standard circuit symbols, input devices such as the LDR and thermistor and output devices such as the LED and motor, how a potential divider splits a voltage, and how these are combined into simple control circuits.
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What this key area is asking
The SQA wants you to recognise standard circuit symbols and components, describe input devices (such as the LDR and thermistor) and output devices (such as the LED and motor), explain how a potential divider splits a voltage, and combine these into simple control circuits.
Components and circuit symbols
You should recognise and draw the standard symbols for a cell and battery, a switch, a resistor, a variable resistor, a lamp, a light-emitting diode (LED), a motor, a voltmeter and an ammeter, an LDR and a thermistor. Components are classed as:
- Input (sensing) devices: the LDR (senses light), the thermistor (senses temperature) and the switch.
- Output devices: the lamp and LED (give out light), the motor (produces movement) and the buzzer (makes sound).
Input devices: the LDR and the thermistor
An LDR is used in automatic street lights and cameras; a thermistor is used in thermostats and temperature alarms. Because their resistance changes, they are usually placed in a potential divider so that a changing resistance produces a changing voltage that can switch something on or off.
The potential divider
Simple control circuits
By feeding the output of a potential divider into a control device, you build a circuit that switches automatically. For example, an LDR potential divider can switch on a light when it gets dark, and a thermistor potential divider can switch on a fan when it gets hot. At National 5 you should be able to describe the action of such a circuit in words: as the input condition changes, the sensor resistance changes, so the output voltage changes, which switches the output device.
Try this
Q1. State how the resistance of a thermistor changes as it gets hotter. [1 mark]
- Cue. Its resistance decreases.
Q2. A potential divider has two equal resistors across a supply. Find the voltage across one resistor. [2 marks]
- Cue. .
Q3. Name one use of an LDR. [1 mark]
- Cue. An automatic light switch, such as a street light or security light.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA N5 style4 marksA potential divider has a 200 ohm resistor in series with a 300 ohm resistor across a 10 V supply. Calculate the voltage across the 300 ohm resistor.Show worked answer →
Use the potential divider relationship, where the voltage across one resistor is its share of the total resistance times the supply voltage.
Relationship: .
Substitution: .
Markers reward the potential divider relationship, correct substitution, and a final answer in volts. (A check: the two voltages must add to , and .)
SQA N5 style3 marksDescribe how the resistance of a light-dependent resistor (LDR) changes with light level, and state one use of an LDR.Show worked answer →
As the light level increases, the resistance of an LDR decreases. In bright light its resistance is low; in darkness its resistance is high.
A use of an LDR is in an automatic light-sensing circuit, for example switching on a street light or a security light when it gets dark.
Markers reward the correct direction of the resistance change (more light, less resistance) and one valid use such as a light-operated switch.
Related dot points
- Electrical charge carriers: current as the flow of charge, the relationship between charge, current and time, the role of electrons as charge carriers, and how current divides in series and parallel circuits.
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Sources & how we know this
- SQA National 5 Physics Course Specification — SQA (2019)