Chapter 3 — Electricity
Close a switch and a bulb lights at once. Electric charge, pushed round a closed loop by a cell, carries energy from the cell to everything in the circuit — and a handful of simple rules tell you exactly how much.
Press the switch by the door and the ceiling light comes on instantly, even though the switch is metres away from the bulb. Plug in a phone and, an hour later, the battery is full of energy it took from the wall socket. The kettle, the fan, the laptop and the electric train all run on the same thing: electric charge moving round a closed path. A cell or a power station does not make the charge — the wires are already full of it — it gives the charge a push, and the moving charge delivers energy wherever it meets a bulb, a heater or a motor.
In this chapter you learn to describe that flow in numbers. How much charge passes each second (the current)? How hard is the cell pushing (the potential difference)? How much does a wire resist the flow (the resistance), and why is a heater coil made of nichrome while the cable to it is made of copper? You will draw circuits with standard symbols, connect ammeters and voltmeters the right way round, combine resistors, and finally work out the energy your home uses and what it costs. Chapter 4 then shows that the same moving charges make magnetic fields.
| Everyday question | The idea that answers it | Section |
|---|---|---|
| What does "2 A" on a charger mean? | current = charge per second, $I=\frac Qt$ | 3.1 |
| Why does a torch need two cells, not one? | potential difference = energy per unit charge | 3.1 |
| Why are connecting wires copper but heater coils nichrome? | resistance, $R=\rho\frac lA$ | 3.2 |
| Why does one fused fairy light not switch off the whole house? | parallel connection | 3.3 |
| What is a "unit" on the electricity bill? | energy $=$ power $\times$ time, in kWh | 3.4 |
- explain electric current as a flow of charge, use $I=\frac Qt$ and count the electrons that make up a charge;
- draw circuit diagrams with standard symbols, and connect an ammeter in series and a voltmeter in parallel;
- define potential difference as work done per unit charge, $V=\frac WQ$, and use the volt;
- state Ohm's law, draw and read V–I graphs, and calculate with $V=IR$;
- explain how resistance depends on length, area and material, and calculate with $R=\rho\frac lA$;
- find the equivalent resistance of resistors in series and in parallel, and the currents and voltages in each;
- use Joule's law $H=I^2Rt$ and the power formulas $P=VI=I^2R=\frac{V^2}{R}$, and calculate energy and cost in kilowatt-hours.
Sections in this chapter
Electric Charge, Current and Potential Difference
Charge and the electron, current as charge per second, circuit symbols, potential difference, and how to connect an ammeter and a voltmeter.
Ohm's Law and the Factors Affecting Resistance
The V–I graph of a wire, resistance and the ohm, and why length, thickness and material matter.
Resistors in Series and in Parallel
Same current or same voltage? Equivalent resistance, and why houses are wired in parallel.
The Heating Effect of Current and Electric Power
Joule's law of heating, fuses, heaters and bulbs, electric power, and the kilowatt-hour on your bill.