Components in Series & Parallel CircuitsEdexcel GCSE Physics: Revision notes
Section 1
How are series test circuits designed?
A series circuit for testing a component connects the cell, an ammeter and the component in a single loop, one after another.
- The ammeter is always connected in series with the component being tested, so the same current flows through both.
- A voltmeter is connected in parallel with the component, to measure the potential difference across it without changing the current in the main circuit.
- Circuit diagrams should use standard symbols for cells, batteries, switches, voltmeters, ammeters, resistors, variable resistors, lamps, motors, diodes, thermistors, LDRs and LEDs.
Core Practical: circuits can be built to (a) investigate how potential difference, current and resistance are related for a resistor and a filament lamp, and (b) test series and parallel circuits using resistors and filament lamps — by varying the variable resistor/power supply and recording ammeter and voltmeter readings.
Section 2
How does current vary with potential difference for different components?
| Component | Current–potential difference behaviour |
|---|---|
| Fixed resistor (at constant temperature) | Current is directly proportional to potential difference — resistance stays constant |
| Filament lamp | As current increases, the filament heats up and its resistance increases, so current increases less than proportionally with potential difference |
| Diode | Allows current to flow easily in one direction only (forward bias, very low resistance); in the reverse direction, resistance is very high and almost no current flows |
These behaviours can be investigated by varying the potential difference across a component and recording the resulting current, then examining how resistance (V ÷ I) changes.
Don't say a filament lamp 'doesn't obey' resistance rules — its resistance simply isn't constant, because temperature changes with current.
Section 3
How do LDRs and thermistors behave?
- Light-dependent resistor (LDR): its resistance decreases as light intensity increases (more light → lower resistance → more current for a given voltage).
- Thermistor (negative temperature coefficient type only, as used at GCSE): its resistance decreases as temperature increases (higher temperature → lower resistance → more current for a given voltage).
Circuits can be designed to explore how resistance varies for LDRs and thermistors, for example by using a fixed potential difference and measuring current at different light intensities or temperatures, then calculating resistance from V = I × R.
LDR and thermistor both follow the same pattern to remember: 'more stimulus (light/heat) = less resistance'.
Must Know
- In a series test circuit, the ammeter is in series with the component; the voltmeter is in parallel across it.
- Fixed resistor at constant temperature: current directly proportional to potential difference (constant resistance).
- Filament lamp: resistance increases as it heats up, so current increases less than proportionally with p.d.
- Diode: very low resistance in forward direction, very high resistance in reverse direction.
- LDR: resistance decreases as light intensity increases.
- Thermistor: resistance decreases as temperature increases.
- Circuit designs can be used to explore how resistance varies for filament lamps, diodes, thermistors and LDRs.
That's the notes covered.
Carry on to the next subtopic.