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Pick a topic. New ones get added here as tests come up.
Chapter 5 — Electricity & Magnetism
Secondary 4 ST. Built from your review form and checklist. Work top to bottom — the timer tells you where you should be.
Self-check
10 minRate yourself honestly on each topic. Your weak spots show up at the bottom so you know where to slow down.
Charges & static electricity
20 minKey ideas
- Atoms have protons (+) and neutrons (no charge) in the nucleus, and electrons (−) around it.
- Only electrons move from one object to another. Protons never move.
- Gain electrons → negative. Lose electrons → positive. Neutral = equal numbers of + and −.
- Like charges repel. Opposite charges attract.
- Static electricity = charge built up on an object after electrons transfer. It stays put until it discharges (the shock from a doorknob, lightning).
Three ways to charge something
- Friction — rub two materials. The one that holds electrons more tightly (lower in the series below) takes them and turns negative; the other turns positive. Equal and opposite charges.
- Conduction — touch a charged object. Electrons flow between them, and both end up with the same sign.
- Induction — bring a charged object near without touching. Electrons inside the neutral object shift, so the near side gets the opposite charge — that's why a charged object attracts a neutral one. The object is still neutral overall: its charges have only separated, and they move back when the charged object is taken away.
Try it: rub two materials together
Top of the list gives up electrons most easily (becomes +). Bottom grabs electrons most (becomes −). If your teacher's table orders a few materials differently, use theirs on the test.
Practice
Circuits & Ohm's law
40 minQuantities
| Quantity | Symbol | Unit |
|---|---|---|
| Current intensity — charge flowing per second | I | ampere (A) |
| Potential difference — energy given to each charge | V | volt (V) |
| Resistance — how much something opposes current | R | ohm (Ω) |
Ohm's law: V = R × I → R = V ÷ I → I = V ÷ R
Same resistor, double the voltage → double the current. Bigger resistance, same voltage → less current.
Parts of a circuit
- Power supply (battery, outlet, generator) — supplies the energy that pushes charges around.
- Resistors / loads (bulb, heating element, motor) — resist the current and transform electrical energy into light, heat, motion…
- Conductors (wires) — carry the current between parts.
- Switch — opens or closes the circuit.
- Ammeter — measures current. Connected in series (current flows through it).
- Voltmeter — measures potential difference. Connected in parallel (across the part).
Series vs. parallel
| Series | Parallel | |
|---|---|---|
| Paths | One loop, end to end | Two or more branches |
| Current | Same everywhere: IT = I1 = I2 | Splits: IT = I1 + I2 + … |
| Potential difference | Shared: VT = V1 + V2 + … | Same on every branch: VT = V1 = V2 |
| Equivalent resistance | Req = R1 + R2 + … (gets bigger) | 1/Req = 1/R1 + 1/R2 + … (smaller than the smallest one) |
| One bulb burns out | All go out | Others stay lit |
| Add a bulb | All get dimmer | Others stay as bright |
Shortcut for any circuit: Req = Vsupply ÷ Isupply. Your house is wired in parallel, so every outlet gets the full 120 V.
What changes a wire's resistance
- Longer wire → more resistance
- Thicker wire (bigger diameter) → less resistance
- Hotter wire → more resistance
- Material — copper and silver are great conductors; iron and nichrome resist more. Rubber and plastic are insulators (that's why they cover cords).
Best wire for a fast fan: short, thick, copper, cool.
Current direction
- DC (direct current) flows one way only — batteries.
- AC (alternating current) keeps switching direction back and forth — wall outlets. In Canada it completes 60 back-and-forth cycles every second (60 Hz).
- Conventional current flows from + to − outside the supply. Electrons actually move the other way (− to +).
Circuit symbols
Practice
Power, energy & cost
40 minThe formulas
P = V × I power (W) = volts × amps
E = P × Δt energy = power × time
Cost = E (kWh) × price per kWh
Units must match
| Power in | Time in | Energy comes out in |
|---|---|---|
| W | seconds | joules (J) |
| kW | hours | kilowatt-hours (kWh) |
1 kWh = 3 600 000 J · 1 kJ = 1000 J · Energy units: J and kWh. Power unit: W.
The 4-step cost recipe
- Power in kW. Divide watts by 1000. If you're given volts and amps (a rating plate), first do P = V × I.
- Total time in hours. Time per day (or week) × number of days (or weeks). Minutes ÷ 60. A "5-day work week" means 5 days, not 7.
- Energy: E = P × Δt, in kWh.
- Cost: kWh × the price.
Example: a 1200 W microwave runs 20 min a day for 30 days at $0.07/kWh.
- P = 1200 ÷ 1000 = 1.2 kW
- Δt = 20 ÷ 60 × 30 = 10 h
- E = 1.2 × 10 = 12 kWh
- Cost = 12 × $0.07 = $0.84
Energy in joules
A kettle on 120 V draws 12.5 A for 3 minutes.
- P = 120 × 12.5 = 1500 W
- Δt = 3 × 60 = 180 s
- E = 1500 × 180 = 270 000 J
Given resistance? Find I first
A 60 Ω heater on 120 V runs 10 minutes.
- I = V ÷ R = 120 ÷ 60 = 2 A
- P = 120 × 2 = 240 W
- Δt = 600 s → E = 240 × 600 = 144 000 J
Practice
Magnetism & the live wire
25 minMagnets
- Every magnet has a north (N) and a south (S) pole. Cut one in half and you get two smaller magnets, each with N and S.
- Opposite poles attract, like poles repel.
- Field lines leave the N pole and enter the S pole (outside the magnet). They never cross.
- The closer together the lines, the stronger the field — strongest at the poles.
- A compass is a tiny magnet. Its N end points along the field lines — toward a magnet's S pole.
- On Earth the compass's N end points north because there's a magnetic S pole near the geographic North Pole.
- Horseshoe magnet: lines go straight across the gap from N to S — a strong, even field.
- Disc / ring magnet: poles are on the two flat faces. Lines leave the N face and curve around to the S face.
Magnetic field of a live wire
- Current in a wire makes a magnetic field shaped like circles around the wire.
- Right-hand rule: grab the wire with your right hand. Thumb = conventional current (+ → −). Curled fingers = direction of the field lines.
- Stronger field: more current (a lower-resistance wire lets more through), get closer to the wire, or coil the wire into loops.
- Current off → field disappears. Reverse the current → the field (and every compass) flips.
| Compass near… | Bar magnet | Live wire |
|---|---|---|
| Points | From N toward S along the lines | Around the wire in a circle |
| Can switch off? | No — always there | Yes — only while current flows |
Compasses around a bar magnet
Red tip = the compass's N end. It points away from N and toward S.
Looking down on a live wire
Practice
Conservation & efficiency
15 minLaw of conservation of energy
Energy is never created or destroyed — it only changes form or moves from one place to another.
Energy in = useful energy out + wasted energy (wasted is usually heat, sometimes sound).
| Device | Transformation |
|---|---|
| Toaster, kettle | electrical → thermal |
| Fan, drill | electrical → mechanical (motion) |
| Speaker | electrical → sound |
| Light bulb | electrical → light (+ heat) |
| Flashlight battery | chemical → electrical → light |
| Generator, dynamo | mechanical → electrical |
Energy efficiency
The share of the energy consumed that becomes useful work.
Efficiency (%) = useful energy ÷ energy consumed × 100
A motor uses 500 J and does 350 J of useful work → 350 ÷ 500 × 100 = 70 %. The other 150 J is wasted as heat.
Always below 100 %. To improve it:
- Swap incandescent bulbs for LEDs
- Choose ENERGY STAR appliances
- Reduce friction — clean and lubricate moving parts
- Insulate things that heat (kettle, oven, water heater)
Practice
Mock test
30 min25 questions in the same style as the review form, with a 30-minute timer. Use loose-leaf paper for the calculations like on the real test. You'll get a score and the full solution for every question at the end.