Master the universal language of electronics β standard circuit symbols used by engineers and scientists worldwide.
Learning Objectives
π Identify and draw the symbols for a cell and a battery
β‘ Recall symbols for resistor, LED, ammeter, and voltmeter
π Understand the symbols for switch, diode, LDR, and thermistor
π Read and interpret complete circuit diagrams
π Distinguish between series and parallel circuit arrangements
βοΈ Draw accurate circuit diagrams using standard AQA symbols
Why Do We Use Circuit Symbols?
Circuit diagrams are a universal language. Instead of drawing realistic pictures of components, physicists and engineers use standardised symbols that everyone worldwide can understand, regardless of their spoken language. These symbols are agreed internationally and used in all AQA examinations.
A circuit diagram shows how components are connected together using straight lines to represent wires. The lines always run horizontally or vertically β never diagonally β and corners are right angles. The diagram does not show the actual shape or size of the real circuit.
Circuit Diagram: A simplified drawing that uses standard symbols to represent components and straight lines to represent connecting wires.
π‘ Exam Tip: In the AQA exam you may be asked to identify symbols, complete a circuit diagram, or draw one from a description. All symbols must be drawn accurately β an ammeter must have an A inside a circle, a voltmeter a V inside a circle, etc.
Power Supply Symbols
Every circuit needs a source of electrical energy. The two main supply symbols are:
Cell
Single source of EMF. One long thin line (β) and one short thick line (+).
Battery
Two or more cells in series. Provides a higher combined voltage.
A cell has one pair of lines. A battery is two or more cells joined in series β shown by repeating the cell symbol. The longer line is always the positive terminal.
Symbol Feature
Meaning
Long thin line
Negative terminal (β)
Short thick line
Positive terminal (+)
Multiple pairs
Battery (multiple cells)
Resistors & Special Resistors
Resistors oppose the flow of current in a circuit. There are fixed resistors and two very important variable resistors whose resistance changes with physical conditions.
Rectangle with an arrow through it. Resistance can be adjusted manually.
LDR
Resistor with two arrows pointing inward. Resistance decreases as light intensity increases.
Thermistor
Resistor with a diagonal line or ΞΈ. Resistance decreases as temperature increases (NTC type).
LDR (Light-Dependent Resistor): A component whose resistance decreases when light intensity increases. Used in automatic lighting circuits.
Thermistor: A component whose resistance decreases when temperature increases (for NTC thermistors). Used in temperature sensors and thermostats.
π‘ Remember: Both LDR and thermistor have decreasing resistance as their respective input (light/temperature) increases. Think "more input β less resistance β more current."
Meters: Ammeter & Voltmeter
Meters measure electrical quantities in a circuit. It is vital to know not just their symbols, but how they are connected β an ammeter in series, a voltmeter in parallel.
Ammeter
Letter A in a circle. Connected IN SERIES. Measures current in amperes (A).
Voltmeter
Letter V in a circle. Connected IN PARALLEL. Measures voltage in volts (V).
An ammeter must have near-zero resistance so it does not reduce the current it is measuring. A voltmeter must have very high resistance so it does not divert current away from the component it measures across.
Switches, Diodes & LEDs
These components control the direction and flow of current through a circuit.
Switch (Open)
A gap in the line with dots at each end. Open = circuit broken = no current.
Switch (Closed)
A straight line connecting both dots. Closed = complete circuit = current flows.
Diode
A triangle pointing to a bar. Current flows only in the direction the triangle points.
LED
Diode with two arrows showing light emission. Emits light when current flows in forward direction.
Diode: A component that only allows current to flow in one direction β from anode (+) to cathode (β). It has very high resistance in the reverse direction.
LED (Light-Emitting Diode): A diode that emits light when current passes through it in the forward direction. Much more energy-efficient than filament bulbs.
π‘ Key Rule: The triangle in the diode symbol always points in the direction that conventional current is allowed to flow. The arrows on an LED point away from the component to show light being emitted.
Worked Examples
Example 1: A student connects a cell, a switch, an ammeter, and a resistor in series. A voltmeter is connected across the resistor. Draw and label a circuit diagram for this arrangement.
1Identify all components: Cell (power supply), switch (control), ammeter (measure current), resistor (component), voltmeter (measure voltage across resistor).
2Decide connections: Cell, switch, ammeter, and resistor are all in series (one loop). The voltmeter must be in parallel across the resistor only.
3Draw the circuit:
4Check the rules: Ammeter is in series β. Voltmeter is in parallel across the resistor β. Switch is in series to control the whole circuit β.
The circuit has one main loop (series) containing: cell β switch β ammeter β resistor β back to cell. The voltmeter branches off in parallel across the resistor only.
Example 2: Name the component represented by each symbol and state one use: (a) a rectangle with two inward-pointing arrows, (b) a triangle pointing to a bar with two outward arrows, (c) a circle with the letter A.
1Part (a) β Rectangle with two inward arrows: The rectangle is the resistor symbol. Two arrows pointing INTO the component indicate light entering. This is an LDR (Light-Dependent Resistor).
2Part (b) β Triangle + bar + outward arrows: A triangle pointing to a bar is a diode. Two arrows pointing AWAY from the component mean light is being emitted. This is an LED (Light-Emitting Diode).
3Part (c) β Circle with letter A: This is the symbol for an ammeter.
(a) LDR β used in automatic outdoor lighting (turns on when it gets dark). (b) LED β used in energy-efficient lighting, phone screens, traffic lights. (c) Ammeter β used to measure current in a circuit, connected in series.
Example 3: A thermistor is used in a temperature sensor circuit. Explain what happens to the resistance of the thermistor and the current in the circuit when the temperature rises.
1Recall the thermistor's behaviour: An NTC (Negative Temperature Coefficient) thermistor β the standard type at GCSE β has resistance that decreases as temperature increases.
2Apply Ohm's Law: For a fixed supply voltage V, current I = V Γ· R. If R decreases, then I must increase (since V stays the same).
3State the relationship: Temperature β β Resistance β β Current β
When temperature rises, the thermistor's resistance decreases. By Ohm's Law (I = V/R), a lower resistance with the same voltage means a larger current flows in the circuit. This change in current can be used to trigger an alarm or switch a device on/off.
Example 4: A diode is connected in a circuit with a cell and a lamp. The student reverses the cell. Explain what happens to the lamp and why.
1Recall diode behaviour: A diode only allows current to flow in one direction β from its anode (+) to its cathode (β). In the reverse direction its resistance is extremely high.
2Original arrangement: Cell drives current in the forward direction through the diode β low resistance β current flows β lamp is ON.
3After reversing the cell: Current would need to flow in the reverse direction through the diode β the diode has very high resistance β virtually no current flows.
When the cell is reversed, the diode is in reverse bias. Its resistance becomes extremely large, so essentially no current flows in the circuit. The lamp goes out (turns off). This shows how a diode acts as a one-way valve for current.
Practice Questions
Q1. Which component has a rectangle as its symbol with two arrows pointing INTO it?
Q2. An ammeter is always connected ________ in a circuit.
Q3. A thermistor is placed in a cold room. What happens to its resistance compared to a warm room?
Q4. What letter appears inside the circle in the voltmeter symbol?
Q5. State the difference between a cell and a battery in terms of circuit symbols.
Q6. In which direction does conventional current flow through a diode?
Challenge Questions
Exam-style questions β attempt each one before revealing the answer.
Challenge 1. A student builds a circuit to investigate how the resistance of an LDR changes with light intensity. She connects a battery, a switch, an LDR, and an ammeter in series. She also connects a voltmeter in parallel across the LDR.
(a) Describe what happens to the ammeter reading as she increases the light intensity shining on the LDR. Explain your answer using ideas about resistance. [3 marks]
(b) The voltmeter reads 6 V and the ammeter reads 0.02 A. Calculate the resistance of the LDR at this light level. [3 marks]
(a) As light intensity increases, the resistance of the LDR decreases (1 mark). With lower resistance and the same voltage from the battery, by Ohm's Law (I = V/R), the current increases (1 mark). So the ammeter reading increases (1 mark).
Challenge 2. A circuit contains a battery (12 V), a switch, two lamps (Lamp A and Lamp B) in series, and an LED in parallel with Lamp B only. A diode is placed in series with the LED branch to protect it.
(a) Explain why the diode is needed in the LED branch. [2 marks]
(b) The switch is opened. State what happens to each lamp and the LED, giving a reason. [3 marks]
(c) Suggest one advantage of using an LED instead of a filament lamp in this circuit. [1 mark]
(a) The diode ensures current can only flow through the LED in the forward (correct) direction (1 mark). Without it, if the battery were reversed or connected incorrectly, reverse current could damage the LED (1 mark).
(b) When the switch is opened, the main series circuit is broken. No current can flow anywhere in the circuit (1 mark). Both Lamp A and Lamp B go off (1 mark). The LED also goes off because there is no complete circuit β even the parallel branch needs the main loop to be complete (1 mark).
(c) Any one of: LEDs use less energy (are more efficient) / LEDs last longer / LEDs produce less heat for the same light output / LEDs can be made very small. (1 mark)
(a) Calculate the current through the thermistor at 20Β°C. [2 marks]
(b) Calculate the current through the thermistor at 80Β°C. [2 marks]
(c) Explain how this change in current could be used to automatically switch off a heater when a room gets too warm. [3 marks]
(a) I = V Γ· R = 10 Γ· 5000 = 0.002 A (2 Γ 10β»Β³ A) β
(b) I = V Γ· R = 10 Γ· 200 = 0.05 A β
(c) As the room warms up, the thermistor's resistance decreases (1 mark), causing the current through it to increase (1 mark). This larger current can be used to trigger a relay or electronic switch that cuts power to the heater, turning it off automatically when the temperature reaches the set level (1 mark).
Challenge 4 β Extended Writing. A student is asked to identify whether an unknown component is an LDR or a thermistor, using only a battery, an ammeter, a voltmeter, and a light source with adjustable brightness.
Describe a method the student could use to identify the component. Include what measurements to take, what results would indicate each type of component, and how to make the test fair. [6 marks]
Mark scheme points (any 6):
β’ Connect the unknown component in series with the battery and ammeter; connect voltmeter in parallel across the component (1).
β’ Calculate resistance using R = V Γ· I at each test condition (1).
β’ Test 1: Change light intensity (bright, medium, dim) while keeping temperature constant (1).
β’ If resistance changes with light level β component is an LDR (1).
β’ Test 2: Change temperature (warm the component gently / cool it) while blocking all light changes (1).
β’ If resistance changes with temperature β component is a thermistor (1).
β’ Control variables: keep temperature constant when testing light; keep light constant when testing temperature; use same supply voltage throughout (1).
β’ Repeat readings and calculate a mean resistance for reliability (1). (Award up to 6 marks)