Power and the Grid

Different from what

Ask somebody why the live wire is dangerous and you will usually be told that it carries the electricity. Ask why a bird can sit on a power line unharmed and you will get a different answer altogether. They are the same answer, and it is worth more than either fact on its own. A voltage is never a property of one place. It is always a difference between two places. Saying a wire is at 230 volts means nothing until you say what it is 230 volts different FROM. For mains wiring, the answer is almost always: from earth. And you are standing on earth. So the one question to carry through this module is different from what? It explains which wire is dangerous, why it stays dangerous when the switch is off, why the bird is fine, what the earth wire is actually for, and, at the far end, why electricity crosses the country at a voltage that sounds insane.

Three wires and two words

Three wires run inside a mains cable, and each has a different job. Read them as jobs for now. What each one sits at comes next, and that is where the marks are.

Which box does which job

Tap the place where the voltage is raised, so that the same power can travel along the line as a smaller current.

The lamp is switched off

A table lamp is plugged in and switched off at the lamp. An electrician will still treat the live wire inside it as dangerous. Why?

• The switch breaks the circuit but does not change what the live wire sits at. It is still at a large potential difference from earth, and a person touching it is connected to earth
• Charge is stored in the wire after the switch is opened, and it takes some time to drain away
• The wire is still hot from carrying current, so touching it would burn
• It is not dangerous once switched off, and the electrician is simply being over-cautious

The same question, asked of three wires

Ask each wire what it sits at, what it is for, and what happens if you touch it. Once the first column is answered, the other two follow.

Five things people find strange

Work out what the cable wastes

A power station sends 2000000 watts along a transmission line of total resistance 4 ohms, at a transmission voltage of 100000 volts. Work out the current in the line, then use it to find the power wasted heating the cable. Give the power wasted, in watts.

Two sentences to strike out

Five statements from a set of revision notes. Tap the TWO that are wrong.

• The live wire is at a large potential difference from earth
• The earth wire carries the current back to the supply during normal use
• UK mains is an alternating supply, while a battery gives a direct current
• The grid uses a high voltage because a higher voltage is wasted less easily than a low one
• The neutral wire sits at close to earth potential

Why the line runs at a voltage nobody wants near a house

A power station has to move a fixed amount of power a long way. It can choose to send it as a big current at a low voltage, or a small current at a high voltage. The power arriving is the same either way. What is not the same is how much is lost on the journey. Energy is wasted heating the transmission cables, and the power wasted is the current squared times the resistance of the cable. Because the current is squared, cutting the current to a tenth cuts the waste to a hundredth. That is why a step-up transformer raises the voltage before the line, and a step-down transformer lowers it again before it reaches anybody. The high voltage is not the goal. It is the price of a small current, and it is worth paying. Notice the shape of that answer. A weak answer says the grid uses high voltage to reduce energy loss. A strong one says high voltage means low current, and the loss depends on the current squared. Where these power equations come from, and how circuits behave, are other questions for other modules on this topic.

Name the wire, five in a row

Each description belongs to exactly one of the three wires. Three lives.

The rope that does not move

Two teams take hold of a rope, one at each end, and both pull as hard as they possibly can. The rope does not move. Ask how hard the left team is pulling and you have asked a question that cannot explain anything, because the answer would be the same whether the rope moved or not. What moves the rope is the difference between the two pulls, and nothing else. Two teams pulling enormously and equally produce exactly as much movement as two teams barely pulling at all: none. Somebody watching only one end could never work out which situation they were looking at. Now tie one end of the rope to a wall. The wall does not pull. But it does not give either, and that is enough: any pull at all from the other end now counts for something, because there is finally a difference. The wall changed nothing about how hard anybody was pulling. It changed what the pulling was measured against, which turns out to be the only thing that ever mattered.

Put the proper words back

Mains electricity in the United Kingdom is an _____ current, which means its direction keeps reversing many times every second. Three wires run into an appliance. The _____ wire sits at a large potential difference from earth and is the dangerous one. The _____ wire sits close to earth potential and completes the circuit. The _____ wire is a safety wire, connected to the metal case, which gives a fault a path that is not through a person. On the National Grid a _____ raises the voltage before transmission, so the same power travels as a smaller _____, and far less energy is wasted heating the cables.

True of a mains supply

Select every statement that is true of the mains supply in a United Kingdom home.

• The supply is alternating, so the direction of the current keeps reversing
• The fuse is fitted in the live wire, so that it disconnects the dangerous one when it blows
• The earth wire is connected to the metal case so that a fault current has a path that avoids a person
• A battery would supply the same kind of current as the mains
• The neutral wire is at the same large potential difference from earth as the live wire

Four things that need explaining

Four situations, each of which somebody has got wrong. Each stage sets out where things stand and hands you the decision.

• A student says that a bird on a bare overhead line must be insulated from it somehow, because otherwise it would be killed. Decide what the better explanation is.
• The same student says that if the bird is safe, a person standing on the ground and touching that line would be safe too, since it is only one wire either way. Decide what has changed.
• A metal-cased appliance is found to have a broken earth wire, though it still works perfectly. Its owner sees no reason to repair it. Decide what to tell them.
• A different student writes that the grid uses a high voltage because high voltages lose less energy than low ones. Decide how to correct that sentence.

Build the answer an examiner wants

Assemble the sentence that gets full marks when a question asks why the National Grid transmits at a high voltage.

Explain it all from one idea

Explain why the live wire is dangerous even when an appliance is switched off, why a bird on an overhead line is not harmed, and why the National Grid transmits at a very high voltage. Begin with the idea that all three have in common.

• Start by explaining what a potential difference actually is, and why the word difference matters
• Explain what the live wire sits at, and why a person touching it completes something the bird does not
• Explain why switching an appliance off does not remove that danger
• Explain what the earth wire is for, and why it carries nothing in normal use
• Finish with the grid: say what raising the voltage does to the current, and what the wasted power depends on