These cells can be connected into an electric circuit in various ways. Look carefully at the two pictures below and you will see the difference straight away: in one the cells are lined up nose to tail in a single row, and in the other they sit side by side with loops of wire joining their ends.
The method in which two or more cells are used together is the combination of cells. Combination of cells can generally be done in two ways: series combination and parallel combination.
Series combination
A series combination can be made by joining the negative pole of one cell to the positive pole of the second cell, and the negative pole of the second cell to the positive pole of the third cell, in order.
Parallel combination
The parallel combination of cells is made by connecting the negative poles of two or more cells to one side and the positive poles to the other side.
Do not learn these as two definitions to be recited, because there is a single question that separates them instantly. Look at where two neighbouring cells are joined and ask: are the poles that meet the same, or opposite? In a series combination the poles alternate, so the negative of one always meets the positive of the next, and the whole row runs nose to tail like people standing in a queue. In a parallel combination all the like poles are joined together, so every positive goes to one wire and every negative goes to the other, and the cells sit side by side like people standing in a line facing you. That is the whole difference, and it is exactly what Activity 8.2 asks you to look for inside the cell box of a torch, a wall clock and a television remote. Once you can do that test in a second, everything else in this topic follows from it.
What each arrangement actually buys you
In a series combination, as cells go on being added the voltage goes on increasing. That is, the total voltage is equal to the sum of the voltages of all the cells. Three cells of 1.5 volts joined in series therefore give 4.5 volts, and a bulb connected to them glows more brightly than it would with one cell.
From a parallel connection the bulb lights for a very long time. When cells are connected in parallel, however many cells you increase them to, the brightness of the bulb does not increase. In this arrangement the electric pressure does not increase, but the time for which the electricity flows does increase. In a parallel combination the total voltage of the circuit is equal to the voltage of each single cell.
| Series combination | Parallel combination | |
|---|---|---|
| How the cells are joined | Negative of one to positive of the next, in a single line | All the positives to one wire, all the negatives to the other |
| Total voltage | The sum of all the cells: 1.5 + 1.5 + 1.5 = 4.5 V | The same as one cell: 1.5 V, however many you add |
| Brightness of the bulb | Increases as more cells are added | Does not change at all |
| How long it lasts | About the same as a single cell would | Much longer, since the cells share the work |
| Used when you want | More push: a brighter bulb, a device needing higher voltage | More endurance: a small steady current for a long time |
The book tells you what happens but not why, and the why is easy once you picture the cells as people. In a series combination the cells stand one behind the other, all pushing the same load in the same direction, like three people pushing one cart in a line. Every push adds to the push in front of it, so the total push, which is what voltage measures, is three times as big. But notice what else is true: the whole load passes through every single person, so nobody is spared any work and all three tire at about the same rate as one person would. In a parallel combination the cells stand side by side and each takes its own share of the same load, like three people carrying one heavy sack between them. The push is no greater than one person could manage, so the bulb is no brighter, but each person is only doing a third of the work, so all three last about three times as long. Series adds the pushes. Parallel shares the work. There is no arrangement that does both, and that is not a limitation of the wiring, it is simple arithmetic.
Uses of the series combination of cells
| a | This kind of combination is useful for increasing the brightness of a bulb. |
| b | This combination is used in many appliances, including torches, radios and remotes. |
Uses of the parallel combination of cells
| a | This connection is used in order to make a cell or a battery last for a long time. |
| b | Such a combination can be used where a small current has to be drawn steadily for a long time. |
Worked example: choosing the right combination
You have a torch that needs 3 volts of electricity, and two cells of 1.5 volts each. What kind of combination should you make, and what is the benefit of doing it that way?
| Step | Working |
|---|---|
| What is needed | 3 V, and each cell gives only 1.5 V |
| Which combination adds voltages? | Only series. In parallel the total would stay at 1.5 V |
| Check the arithmetic | 1.5 + 1.5 = 3 V, which is exactly what the torch needs |
| How to join them | Negative pole of the first cell to the positive pole of the second |
| The benefit | The torch gets the full 3 V it was designed for, so the bulb is properly bright |
A radio that needs 3 volts and has two 1.5 volt cells is exactly the same problem with a different appliance, and the answer is the same: join them in series so that the voltages add. Now try the reverse question. Suppose you had a clock that needs only 1.5 volts, but you wanted it to run for as long as possible on two cells. Series would give it 3 volts, which is twice what it wants and may damage it. Parallel keeps the voltage at 1.5 volts, which is correct, and makes the pair last roughly twice as long as one cell would. So the arrangement is not chosen by habit; it is chosen by asking what the appliance needs.
Here is a useful consequence of the rule that series voltages add. If two 1.5 volt cells are joined in series the correct way, negative to positive, the total is 3 volts. But if one of them is put in the other way round, the two cells are now pushing against each other, and instead of adding you have to subtract: 1.5 minus 1.5 leaves 0 volts, and the torch does absolutely nothing. This is the reason a torch with one cell reversed behaves exactly like a torch with flat cells, and it is worth checking before you throw away perfectly good cells. It also explains why every cell box has a small plus and minus printed inside it. Those markings are not decoration. They are telling you which way round each cell has to face so that the pushes add instead of fighting.
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Using two or more cells together is called the combination of cells.
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Question 1 of 10
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One Question Tells Them Apart
- Look at where two neighbouring cells are joined
- Are the poles that meet OPPOSITE, or the SAME?
- Opposite, nose to tail in one line: that is SERIES
- Same, all positives on one wire: that is PARALLEL