S3 Physics – Electricity 2: Electronics and Control

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Overview

In Electricity 2 we move from “just circuits” to electronics and control. You’ll learn how a potential divider can turn changes in light or temperature into a useful output, how to power an LED safely, and how a circuit can switch at the right moment.

The same ideas keep coming back: resistance changes → voltages change → output changes. By the end of this unit, you’ll be reading the hidden language of control circuits — using diagrams, measurements, and real builds you can test.

Key links

Start with the Knowledge Organiser (symbols + rules), then use your booklet.

Knowledge Organiser Start hereSymbols, divider rules and testing. The booklet Do a section, get it checked, then move on. Physics Past Paper Finder Mixed questions. Use it after the KO and the methods. Electronics PPQ Section C: symbols, switching, simulation, safety and tools.
What to attempttap to open
  • Physics: focus on Voltage, Ohm’s Law, Circuit rules.
  • Electronics: symbols & functions, resistor values, series/parallel, switching circuits, voltage dividers, simulation/testing, safety, errors, layout/diagrams.
  • Note: you may see a few unfamiliar symbols (mix of S3 + N5 Electronics).

Video revision (watch only what you need)

Open a video to load it. Watch → then go straight to booklet questions.

Voltage Divider Theorytap to load

Voltage Divider Theory

How changing resistor values changes the output voltage.

Video © The Other Mr Stewart (YouTube @mrstewartphysics), via YouTube

LEDs – Basictap to load

LEDs – Basic

Why LEDs need a protective resistor.

Video © The Other Mr Stewart (YouTube @mrstewartphysics), via YouTube

Thermistors and LDRstap to load

Thermistors and LDRs

How sensors change resistance.

Video © The Other Mr Stewart (YouTube @mrstewartphysics), via YouTube

Transistor Switching Circuitstap to load

Transistor Switching Circuits

Using a transistor as a switch.

Video © The Other Mr Stewart (YouTube @mrstewartphysics), via YouTube

Stripboard Schematicstap to load

Stripboard Schematics

Plan track cuts and links before soldering.

Video © Mr Bell – Practical Electronics, via YouTube

Test details (no spoilers)

What to expect

  • 3 sections in total.
  • Section 1: 5 multiple choice questions.
  • Section 2: Physics written questions — 15 marks.
  • Section 3: Electronics written questions — 10 marks.
  • Total time: 45 minutes.

Time strategy

  • Do the easy marks first (build confidence).
  • If stuck: write what you do know (often earns marks).
  • Leave ~5 minutes at the end to check units, polarity and sensible answers.
General advice: Use the KO for symbols and rules, and practise explaining control/switching in full sentences.

Success criteria (G / A / R) — saved on this device

Tap each statement and choose G, A or R. Use the summary to decide what to do next.

Tip: If you mark something Red, pick one action: KO → worked example → practise → ask for help.

LED tool (protective resistor ↔ current)

Use this to either: (1) calculate the protective resistor for an LED, or (2) calculate the current if you already know the resistor.

Step 1: VR = Vs − VLED = —
Step 2: I (A) = I (mA) ÷ 1000 = —
Step 3: R = VR ÷ I = —
Suggested resistor: next highest available → —
If Vs ≤ VLED, the LED won’t run properly from that supply.
LED practice (random + check your answer)tap to open

Try working it out yourself first, then check. (We accept ±5%.)

Switching circuits (transistor + MOSFET)

SQA wording tip: in explanations, talk about Vout at the junction (the base voltage for a transistor, or gate voltage for a MOSFET). Aim for the chain: condition → resistance → Vout → switching/threshold voltage → output.

Use the sentence builder to practise the exact cause → effect style the marking instructions reward.

Switching sentence builder (SQA-friendly)tap to open
Switching circuit diagram (temperature or light) for the selected circuit
Circuit diagram — choose a circuit type to view Circuit 1–4.
1. When the temperature , the resistance of the thermistor .
2. This means Vout (the output voltage — the voltage across the bottom resistor/sensor in the left-hand potential divider; i.e. the voltage at the junction / base (or gate) voltage) .
3. When Vout reaches the , the transistor switches , so the LED switches on.
Quick science remindertap to open
  • LDR: more light → lower resistance.
  • NTC thermistor: higher temperature → lower resistance.
  • Exam wording: say Vout at the junction (base/gate voltage) changes — that’s what the transistor/MOSFET “sees”.
  • Voltage word: BJT uses switching voltage; MOSFET uses threshold voltage.
Copy-paste model answers (SQA style)tap to open

3-mark template (switch ON)

  1. When [condition] [increases/decreases], the resistance of the [LDR/thermistor] [decreases/increases].
  2. This means Vout (output voltage at the junction / base (or gate) voltage) [increases/decreases].
  3. When Vout reaches the switching voltage (BJT) / threshold voltage (MOSFET), the transistor/MOSFET switches on, so the [LED/motor/relay] switches on.

If relay present: “the relay coil is energised and the contacts close, switching on the device.”

3-mark template (switch OFF)

  1. When [condition] [increases/decreases], the resistance of the [LDR/thermistor] [decreases/increases].
  2. This means Vout (output voltage at the junction / base (or gate) voltage) [increases/decreases].
  3. When Vout falls below the switching voltage (BJT) / threshold voltage (MOSFET), the transistor/MOSFET switches off, so the [LED/motor/relay] switches off.

If relay present: “the relay coil is de-energised and the contacts open, switching off the device.”

Do / Don’t (mark-friendly wording)

  • Do say: “Vout at the junction (base/gate voltage) increases/decreases.”
  • Do say: “When Vout reaches the switching/threshold voltage → switches on.”
  • Do say (relay): energised/de-energised; contacts close/open.
  • Don’t say: “voltage through the resistor/sensor”
  • Don’t rely on: “current increases so it turns on” unless asked about current.
Variable resistor purpose (1 marker): The variable resistor is used to set the light level/temperature at which the circuit switches (the switching point / sensitivity).

Soldering guide + checklist (saved on this device)

Use this before you build on PCB or stripboard. Tick items as you go — ticks are saved on this device.

Big rule: never solder a circuit while it is powered. Power off / unplug before changing anything. If you’re using a 230 V plug-in supply: dry hands, switch off at the socket before adjustments, and never open/modify the supply.
Open the soldering checklisttap to open
1) Safety + setup (before you start)must-do
2) Soldering technique (perfect joints)how-to

What you do (every joint)

  • Heat the joint: tip touches pad/track + component lead together.
  • Feed solder to the joint (not to the iron).
  • Remove solder first, then remove the iron.
  • Keep it still for a few seconds while it cools.

What it should look like

  • Shiny joint.
  • Small “volcano” shape (not a blob).
  • No spikes, no cracks, no bridges.
Croc clip heat sink (for transistors/LEDs if you’re slow): clip the crocodile clip onto the component lead between the component body and the solder joint. Solder the joint, then leave it still for a moment so heat flows into the clip instead of the component.
3) PCB vs stripboard (key differences)build
Polarity = direction/orientation matters. LED: current flows from anode (+) to cathode (−). Backwards LED won’t light. Transistor: the pins have specific roles (e.g. base/collector/emitter) — wrong orientation/pinout means it won’t switch correctly.
4) Component order (S3 Electronics & Control)sequence

General rule: smallest/lowest parts first, then taller parts, then wires. For stripboard, tidy planning reduces mistakes.

  1. Resistors + thermistor (and LDR/variable resistor if your design includes them).
  2. LED(s) (check polarity).
  3. Transistor (check pinout/orientation; solder quickly / use heat sink if needed).
  4. Wire links (single core or tinned copper where safe).
  5. External wiring (use stranded wire for anything that moves).
5) How to desolder (fix mistakes safely)repair
Before desoldering: power OFF, let parts cool, and wear eye protection.

Desoldering pump (solder sucker)

  1. Prepare the pump: push the plunger down until it locks.
  2. Heat the joint until solder melts.
  3. Bring the pump nozzle to the molten solder and press the release button.
  4. Repeat if needed to clear the hole.

Solder braid (wick)

  1. Place braid on the joint.
  2. Press iron on top until solder flows into braid.
  3. Remove braid first, then iron.
  4. Cut off the used braid end.
6) If you burn yourself (class procedure)first aid
If burned: put the iron in the holder, go straight to the sink, run the burn under cold water, then tell the teacher.
  • Cool under gently running cold water for at least 5 minutes (longer is even better).
  • Do not apply creams/ointments.
  • Teacher will decide next steps / medical help if needed.
7) Quick pre-power checks (before switching on)check

Electronics skills & testing (must-know)

These are common written questions (short bullet points = full marks).

Testing, meters, safety, toolstap to open

Pre-power checks (examples)

  • Correct component values (especially resistors).
  • Correct polarity (LED/diode/capacitor/transistor/MOSFET).
  • No short circuits / no solder bridges.
  • Rails correct (V+ and 0V continuous).

Functionality tests (examples)

  • Supply voltage correct at rails.
  • Vout changes as expected with light/temp.
  • Output switches at the intended point.
  • Nothing heats unexpectedly; current not excessive.

Meter rules

  • Voltmeter: in parallel (measures V across a component).
  • Ammeter: in series (measures current through a circuit).
  • Resistance: power OFF to measure.

Tool functions

  • Track cutter: breaks copper track on stripboard.
  • Heat sink: protects heat-sensitive parts while soldering.
  • Solder sucker: removes molten solder to fix mistakes/clear holes.
Mark-friendly writing: answer practical questions in short bullet points (2 clear points = full marks).

Practical Electronics Assignment — Overheating Warning System

This is your checklist hub. Tick items as you complete them. Ticks are saved on this device.
Assignment checklist (interactive)tap to open
Planning & simulation1–4
Build & layout5–7
Testing evidence8–10
Logbook & evaluation11–13

Sources & credits: The RS Electricity B2 booklet © R Stewart, 2025. The PPQ is reproduced with permission of J Keegan, 2025. The Past Paper Finder is compiled by Mr Davie (September 2026 update by Mr White); past-paper questions © Qualifications Scotland (SQA).