Where might the electricity that is needed to run the electrical goods in your house and school have come from? What might such electricity be called? Discuss whether the electricity used in the house and the electricity used in the school are the same kind or different kinds.
The four things a circuit needs
For electricity to flow, a source of electricity, a conducting wire, a switch and a load are needed. The regular path made by joining these together is called an electric circuit.
| Part | The question it answers | Examples |
|---|---|---|
| Source of electricity | What keeps the electrons moving? | A cell, a battery, a generator, the mains supply |
| Conducting wire | Which road do they travel along? | Copper or aluminium wire, usually covered in plastic |
| Switch | How do we open and close the road? | A wall switch, the button on a torch, a knob on a fan |
| Load | What does the useful work? | A bulb, a fan, a heater, a bell, a motor |
The electricity in which electrons flow continuously from one place to another in a conducting wire is current electricity.
A circuit is a loop, and that changes everything
Look at the picture of the closed circuit and notice its shape. It is a loop, and it has to be. This is the single most useful thing to understand about current electricity, and almost everybody gets it wrong to begin with. Most people picture electricity as something that flows out of a cell, travels up a wire, gets used up inside the bulb and stops there, rather like water poured into a glass. If that were true, one wire would be enough. Look at any bulb, any fan or any charger in your house and count the wires going into it. There are always two, and now you know why. The electrons leave the source through one wire, go round through the load, and come back to the source through the other, and the loop never breaks. Nothing is consumed on the journey. What the load takes is not the electrons but their energy, and it hands that energy on as light, heat, sound or movement. The same electrons come home, tired but all present.
Once you see the circuit as a loop, a switch stops being mysterious. A switch does not hold the electricity back or store it up while it is off. It simply lifts a small piece of metal out of the way and leaves a gap, and since air does not conduct, the loop is no longer complete and nothing can go round at all. That is why a single break anywhere in a circuit stops the whole circuit, and not just the part after the break. It is also why a torch with a loose battery, a broken wire or a blown bulb all show exactly the same symptom, which is nothing happening.
Which way does the current go?
Inside the wire it is the electrons that move, and they carry a negative charge. They are pushed out of the negative pole of the cell, travel round the circuit through the load, and arrive back at the positive pole. So the flow of electrons from the negative pole of a cell towards the positive pole is what we mean by the flow of electric current.
What the load does with the energy
Bulbs, radios, televisions, computers, water pumping motors, fans, heaters, electric bells and so on, which are used in houses, schools and factories, are appliances that run on electricity, that is electrical appliances. With the help of those very appliances, electrical energy can be converted into heat, light, sound, magnetic and other kinds of energy.
| Load | Electrical energy is converted into |
|---|---|
| Bulb | Light energy, and a good deal of heat as well |
| Heater, iron, rice cooker | Heat energy |
| Radio, television, electric bell | Sound energy |
| Fan, water pumping motor | Movement, that is mechanical energy |
| Electromagnet in a crane or a doorbell | Magnetic energy |
Read that table across and you should recognise what is happening, because you spent a whole unit on it. Energy is never made and never destroyed; it is only converted from one form into another, and the load is simply the place where the conversion happens. A generator turns movement into electrical energy at the power house, wires carry that energy across the country, and a fan in your room turns it back into movement. Notice how honest the first row is. A bulb is described as producing light and a good deal of heat, and that heat is not a bonus, it is waste. Put a hand near a working bulb and you can feel the electrical energy that never became light at all. This is why an LED bulb costs less to run than an old filament bulb: it is not making more light out of nothing, it is simply wasting less of the same energy as heat.
Cells and batteries
A cell is one of the sources of electricity. In a cell there are a positive pole and a negative pole. A group of cells is called a battery. Cells are of various shapes and kinds, such as an ordinary cell, a dry cell and a lead acid cell.
In everyday speech people call the single cell in a torch a battery, and there is no harm in that at the shop. In an examination there is. Read the definition again: a group of cells is called a battery, so a battery means two or more cells working together. The little cylinder you buy for a wall clock is one cell. The flat rectangular block with two terminals on top, the kind used in a smoke alarm, is a battery, because inside its casing there are several small cells joined together. A car battery is a battery for the same reason, with six cells inside one box. This distinction matters more than it looks, because the whole of the next topic is about what changes when you put cells together and in which arrangement you put them.
Drawing a circuit instead of painting one
When you are asked to show a circuit in a symbol diagram, do not try to draw a picture of the real thing. Nobody wants a sketch of a torch. Draw a plain rectangle of straight lines to stand for the wire, and set the symbols into the sides of that rectangle wherever those parts belong. It takes less time, it is far easier to read, and every scientist in the world will understand it. Two habits are worth building from the start. Always mark which end of a cell is positive, since the long thin line is the positive pole and the short thick line is the negative one. And always draw a switch clearly open or clearly closed, because a reader has to be able to tell at a glance whether your circuit is meant to be working or not.
Static and current: the same charge, doing two different things
| Static electricity | Current electricity | |
|---|---|---|
| What the charge is doing | Sitting still where the rubbing left it | Flowing continuously along a wire |
| Where it is produced | On non-conducting substances, by friction | In conducting wires, from a source such as a cell |
| How long it lasts | Builds up, then discharges all at once in a burst | Keeps going for as long as the loop is closed |
| Examples | A rubbed comb, a crackling sweater, lightning | A torch, a fan, a bulb, everything on the mains |
It is worth saying plainly that these are not two different kinds of electricity. The charge is the same charge and the electrons are the same electrons. The only difference is whether they are stuck in one place or moving round a loop, and what a cell really does is keep them moving so that the flow never stops. Lightning is the exception that proves the point: for a fraction of a second, static electricity becomes an enormous current, and then it is over.
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For electricity to flow, four things are needed: a source, a conducting wire, a switch and a load.
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Four Parts, Four Questions
- SOURCE: what keeps the electrons moving? A cell, a generator
- WIRE: which road do they travel along? Copper, usually covered
- SWITCH: how do we open and close that road?
- LOAD: what does the useful work? A bulb, a fan, a heater