Chapter 6: Electricity
The chapter problem: the school garden cabin
A school has built a cabin at the edge of its field, with a steel frame and a timber outer wall. The cabin is used for gardening and for science club. It is wired to the mains supply, and it also has a battery circuit for a water pump.
These are the facts to use all the way through the chapter.
- The cabin is connected to the mains supply: 230 V alternating, at 50 Hz.
- The heater in the cabin takes a current of 5.0 A.
- The lamp takes a current of 0.20 A.
- The heater and the lamp are connected in parallel across the supply.
- The heater is switched on for 30 minutes.
- A 12 V battery runs the water pump. The pump circuit contains a 6.0 Ω resistor and a 2.0 Ω resistor in series.
- The cabin's clear plastic window is rubbed with a dry cloth. After rubbing, the window attracts dust.
By the end of the chapter you will work out the power and the resistance of the heater and of the lamp. You will find the total current drawn from the supply, and the energy the heater transfers in 30 minutes. You will work out the current and the potential differences in the pump circuit. You will explain why the two appliances are connected in parallel rather than in series, and why the earth wire makes the cabin safer. You will also explain the charge on the plastic window and the force it exerts on the dust.
First contact: no working yet. Estimate. The heater takes a current of 5.0 A from the 230 V supply. Is its power closer to 12 W, 120 W or 1200 W? Write your estimate down.
If you are not sure where to start
Power is the energy transferred each second. Each coulomb of charge that flows carries energy, and 5.0 coulombs flow each second. Multiply the potential difference by the current to find the power.
By the end of this chapter
You will be able to:
- describe an electric current as a flow of charge, and use the equation ;
- use the equation to link potential difference, current and resistance;
- describe how the resistance of a lamp, a diode, a thermistor and an LDR changes;
- describe the difference between series and parallel circuits;
- calculate currents, potential differences and resistances in series circuits;
- explain why adding resistors in series raises the total resistance, and why adding resistors in parallel lowers it;
- recall that the UK domestic supply is alternating, at 50 Hz and about 230 V;
- describe the live, neutral and earth wires, and explain the dangers of the live wire;
- use the equations , , and ;
- describe how the National Grid transfers power at high potential difference;
- describe how static electricity is produced, and explain it using electric fields.
Where it is used
Electricity carries energy to almost every home and workplace. It lights rooms, cooks food, charges phones and drives motors. Engineers choose how to connect each device, and how to keep people safe around a supply that can kill. The same ideas explain a spark from a car door, a lightning strike and the high-voltage cables that cross the countryside.
Your turn to connect it
Where in your own day do you use electricity? Write two examples. One should be a device that changes electrical energy into movement or heat. One should be a time when you had to be careful about electricity, or when you noticed a spark or a shock.