Lesson A · 2.1–2.6
How I start this topic
Mains electricity can kill. Every safety sentence on the paper is a “because” sentence: the fuse melts because a large current heats the thin wire, which breaks the live connection. If you only write “the fuse blows,” you get zero.
2.1 · Units
| Quantity | Symbol | Unit | What I say in class |
|---|---|---|---|
| Current | $I$ | ampere (A) | How much charge passes each second |
| Charge | $Q$ | coulomb (C) | A packet of charge. $1\,\text{C}$ is a large amount |
| Energy | $E$ | joule (J) | What is transferred. Convert kJ $\times 1000$ |
| Resistance | $R$ | ohm (Ω) | How hard it is to push charge through |
| Time | $t$ | second (s) | Minutes $\times 60$. Hours $\times 3600$ |
| Voltage | $V$ | volt (V) | Energy per coulomb. $1\,\text{V} = 1\,\text{J/C}$ |
| Power | $P$ | watt (W) | Joules per second. kW $\times 1000$ |
Convert before you calculate
$250\,\text{mA} = 0.250\,\text{A}$. $4.8\,\text{kJ} = 4800\,\text{J}$. $2.3\,\text{kW} = 2300\,\text{W}$. Leaving milli- or kilo- in the formula is the fastest way to lose both marks.
2.2 · Safety
A UK plug has three wires: live (brown) carries the current in; neutral (blue) completes the circuit; earth (green/yellow) is a safety path to the ground. The fuse sits in the live wire.
| Feature | What it does | The “because” I want |
|---|---|---|
| Insulation | Plastic coating on live and neutral | Plastic is an insulator, so you cannot touch the live conductor |
| Double insulation | Plastic case; no earth wire. Symbol: a square inside a square | Even if a wire comes loose inside, the case cannot become live |
| Earthing | Earth wire joined to the metal case | If live touches the case, a large current flows to earth, the fuse melts, and the case is no longer live |
| Fuse | Thin wire that melts above its rating | Breaks the live connection so the current stops |
| Circuit breaker | Electromagnetic switch that trips | Breaks the circuit faster than a fuse and can be reset |
4-mark earthing answer
Live wire touches the metal case [1]. The case is connected to earth, so a large current flows in the earth wire [1]. The large current melts the fuse [1]. The live wire is disconnected, so the user cannot get a shock [1].
2.3–2.5 · Heating, power, energy
Electrons moving through a metal collide with vibrating ions. Those collisions transfer energy, so the temperature of the resistor rises. We use that on purpose in a kettle, toaster, hairdryer, electric heater and a filament lamp.
$$P = I \times V \qquad E = I \times V \times t$$Power is energy per second, so $E = P t$ as well. Always put $t$ in seconds.
A $800\,\text{W}$ toaster on $230\,\text{V}$: $I = 800/230 \approx 3.5\,\text{A}$. A $3\,\text{A}$ fuse would melt in normal use. Choose $5\,\text{A}$. A $13\,\text{A}$ fuse would not melt until a dangerously large current was already flowing — the appliance or the cable could overheat first.
Board example — energy
A $2.0\,\text{kW}$ heater runs for $15\,\text{minutes}$ on $230\,\text{V}$.
$P = 2000\,\text{W}$, $t = 900\,\text{s}$.
$$E = IVt = Pt = 2000 \times 900 = 1.8 \times 10^6\,\text{J}$$Or $I = 2000/230 \approx 8.7\,\text{A}$, then $E = 8.7 \times 230 \times 900$ — same idea, more rounding risk. I prefer $E=Pt$ when power is given.
2.6
Direct current (d.c.) flows in one direction. Cells and batteries supply d.c. On a voltage–time graph it is a horizontal line.
Alternating current (a.c.) keeps reversing direction. Mains is a.c. On a voltage–time graph it is a wave that crosses the time axis.
Watch
1.
An $800\,\text{W}$ toaster is used on $230\,\text{V}$. Calculate the current and state the correct fuse ($3\,\text{A}$, $5\,\text{A}$ or $13\,\text{A}$).
2.
Why must a fuse be in the live wire, not the neutral?