Circuit Diagrams — AQA GCSE Combined Science Synergy 8465 - Physical Sciences Revision Notes & Topical Questions

Section 1 of 11Circuit diagrams and symbols

Circuit diagram

Circuit diagram a drawing that uses standard symbols to show how components are connected

Drawing rules

• Drawing rules wires are drawn as straight lines with right-angle corners
• Every component is drawn with its standard symbol.
• The circuit must form a complete loop, with values such as e.m.f. and resistance labelled.

Section 2 of 11Power supplies and functional components

Power supplies and functional components

Power supplies

• Power supplies cells, batteries, power supplies and generators all supply current to the circuit

Battery

Battery two or more cells connected in series

Switch

• Switch opens or closes a circuit and so decides whether current can flow

Functional components

• Functional components each does a specific job when current passes through it
• A motor rotates.
• A lamp emits light.
• A heater transfers thermal energy.
• A bell emits sound.

Section 3 of 11Meters

Ammeter

• Ammeter measures current and is always connected in series with the component

Voltmeter

• Voltmeter measures potential difference and is always connected in parallel across the component

Section 4 of 11Resistors

Fixed resistor

• Fixed resistor has a constant resistance and limits the current

Variable resistor

• Variable resistor its resistance can be changed to adjust the current
• Increasing its resistance decreases the current.
• It is used in dimmer switches and volume controls.

Section 5 of 11Thermistor and LDR

1Thermistor

• Thermistor a non-ohmic, temperature-dependent resistor
• As temperature increases its resistance decreases.
• It is used as a temperature sensor.

2Light-dependent resistor (LDR)

• Light-dependent resistor (LDR) a non-ohmic resistor whose resistance depends on the light falling on it
• As light intensity increases its resistance decreases.
• Its resistance is high in the dark and low in bright light.

Section 6 of 11Diodes and LEDs

1Diode

• Diode allows current in one direction only
• Current flows in the direction the triangle of the symbol points.
• If the diode is reversed it blocks the current.

2Light-emitting diode (LED)

• Light-emitting diode (LED) behaves like a diode and also emits light when current flows through it

Section 7 of 11Fuses and electromagnetic components

Fuses and electromagnetic components

Fuse

• Fuse protects components from current surges and acts as a safety measure against fire

Relay

• Relay uses a small current in one circuit to switch on a much larger current in another circuit

Transformer

• Transformer steps current and potential difference up or down

Magnetising coil

• Magnetising coil uses the magnetic effect of a current

Section 8 of 11A potential divider sensor circuit

1

Connect a fixed resistor in series with an LDR across the supply.

2

The supply p.d. is shared in proportion to the resistances, so the larger resistance takes the larger p.d.

3

When light falls, the LDR resistance increases and the p.d. across it rises.

4

This rising p.d. can switch on a lamp circuit, giving an automatic street light.

Section 9 of 11More from the specification

More from the specification

Domestic electrical mains wiring

• Domestic electrical mains wiring protective earth conductors and circuit breakers provide a safety means when connecting consumer appliances to AC mains.

Current at junctions

• Current at junctions the total electrical current entering any junction equals the total current flowing out through each connected branch.

Earth wire and fuses

• Earth wire and fuses an earth wire directs fault currents safely into the ground, causing fuses to melt and protect the appliance.

Electrical insulation

• Electrical insulation non-conductive plastic coating wraps around live wires to prevent direct contact with metal casings.

Current at Circuit Junctions

• According to Kirchhoff's first law, the total electrical current entering any junction must equal the total current flowing out along connected branches.

Fuses and Electrical Safety

• A fuse contains a thin wire rated to melt and break the circuit if excessive current flows, protecting the appliance and preventing electrical fires.

Earth Wire and Double Insulation

• The earth wire provides a low-resistance path to ground in metal-cased appliances,
• while double insulation encases internal live parts in plastic to prevent electric shock.

Parallel Circuit Characteristics

• In parallel circuits, each branch receives the full voltage of the electrical source,
• allowing independent control of appliances without affecting other branches.

Source

• A part of the plant where substances are made or released.

Branch

• One of the paths in a parallel circuit.

Junction

• A point where branches meet; the current entering equals the current leaving.

Insulation

• A material that reduces thermal energy loss.

Potential divider

• Two resistors in series across a supply that share the supply p.d. in the ratio of their resistances.

Section 10 of 11Key terms

Term	Meaning
Circuit diagram	A drawing using standard symbols to show how components are connected.
Variable resistor	A resistor whose resistance can be adjusted to control current or p.d.
Thermistor	A resistor whose resistance decreases as its temperature increases.
Light-dependent resistor (LDR)	A resistor whose resistance decreases as light intensity increases.
Diode	A component that allows current in one direction only.
LED	A diode that emits light when current flows through it.
Fuse	A thin wire that melts and breaks the circuit when the current exceeds its rating.
Relay	A switch operated by a small current that switches a much larger current in another circuit.

Section 11 of 11Exam technique

1

Connecting a voltmeter in series — it must be connected in parallel across the component

2

Thinking a thermistor or LDR resistance rises with temperature or light — in both, resistance decreases as the quantity increases

3

Thinking a diode conducts in both directions — it conducts only in the direction the symbol arrow points

4

Confusing a fuse symbol with a resistor — the fuse symbol has a wire running through it