Pick one before you read on. It is fine to be wrong.
By the end you can
Explain voltage as the push that makes current flow
Predict how adding cells changes the brightness of a bulb
In the last chapter the cell pushed the charges round the loop. How hard does it push? We measure the push as voltage.
We measure voltage in volts. We write V. One torch cell gives about 1.5 V. A bigger push makes a bigger current.
A model. The same beads are pushed harder in the bottom tube, so more beads pass each second.
Voltage is the push. Current is the flow that the push makes. They are not the same thing, and they have different units.
A battery is two or more cells joined in a line. The + end of one cell touches the − end of the next. Their pushes add up.
Draw it first
Draw one, two and three cells
1 Draw a loop with 1 cell and a bulb. The push is 1.5 V. The current is 0.3 A.
2 Add a second cell. The push is now 3 V. The current is about double, 0.6 A, and the bulb is brighter.
3 Add a third cell. The push is 4.5 V. The current is about three times as much, 0.9 A, and the bulb is brightest.
The push adds as you add cells, but only when they face the same way. Each cell has a + end and a − end. Join + to − all along the line.
A cell turned the wrong way pushes against the others. It cancels one of them.
A voltmeter measures the push across a part. You join it across the part, side by side with it. It does not go in the loop.
The voltmeter is across the bulb, not in the loop. It reads the push across the bulb.
See it three ways
As a picture
In words
Each new cell adds 1.5 V. The same bulb takes more current when the push is bigger, so it is brighter. In our sums a bulb keeps the same resistance. A real filament gets hotter as the current grows. So its current grows a little less than the sums.
In symbols
volts = cells × 1.5. For this bulb, 1 cell gives 0.3 A, so 3 cells give about 3 × 0.3 = 0.9 A.
A bulb is made for a certain push. Too many cells send too much current through its thin wire, and it burns out. Use only the cells the bulb is made for.
The loop for the next example. A resistor has the same resistance whatever the current.
Worked example
Kofi's resistor takes 0.2 A on one cell. He joins 4 cells in a line, each 1.5 V. How many volts push, and how many amps flow?
Cells in a line add their volts. 4 × 1.5 = 6 V.
The push is 4 times bigger than for one cell.
For the same resistor the current grows in step. 4 × 0.2 = 0.8 A.
Answer: 6 V and 0.8 A
Try it
Make a bulb brighter with a second cell
You need: two small torch cells, 1.5 V each, a small bulb made for 2.5 V or 3 V, two strips of kitchen foil, folded long and thin, tape.
Be safe: Join the strips for only a few seconds. The foil and the cells can get warm, so let them cool before you touch them. Never let the two strips touch each other.
Tape the + end of one cell to the − end of the other. Now they are in a line.
Touch one foil strip to the bulb side and the other to the bulb tip.
Touch the free ends of the strips to the cell you are using, for 3 seconds only. Look at the bulb.
Try one cell alone, then the two cells in a line. Compare the glow.
What you will see: One cell gives a dim glow. Two cells in a line make the bulb brighter. A weak cell gives less push, so an old cell gives a dimmer light too. Your looks may differ from ours.
If you cannot try it, this is what happens.
A wrong idea many people have
Voltage and current are the same thing.
Why it feels right: Both seem to be "how much electricity" a cell gives, and both grow when you add cells.
The test that breaks it: A 1.5 V cell with a 5 Ω bulb gives 0.3 A. A 1.5 V cell with a 10 Ω bulb gives only 0.15 A. The volts are the same, but the amps are not.
What is true: Voltage is the push, in volts. Current is the flow the push makes, in amps. The same push can make a bigger or smaller current.
Key words
voltage
the push that makes a current flow round a circuit.
volts
the unit of voltage. We write V.
battery
cells joined in a line, + to −. Their volts add up.
voltmeter
a meter that reads the push across a part. It goes across the part.
Back to your guess
The answer is A: Cells in a line add their push, so more current flows and the bulb is brighter.
Cells joined in a line add their push. A bigger push makes more current flow round the bulb, so it is brighter. The extra cells are not spares.
Check yourself
Try each one before you look. A wrong answer tells you where to go next.
What does the voltage of a cell tell you?
See the answer
How hard it pushes the charges round the loop.
Voltage is the push, measured in volts. The current is the flow that the push makes.
If you chose A: A big cell and a small cell can both give 1.5 V. Voltage is the push, not the size. Go to S3.10.2.
If you chose C: That is the current, in amps. Voltage is the push that makes the current. Go to S3.10.1.
4 cells of 1.5 V are joined in a line, + to −. What is the push in all?
See the answer
6 V
Each cell adds 1.5 V. 4 × 1.5 = 6 V.
If you chose A: You may think the volts stay the same. Cells joined in a line add their volts. Go to S3.10.2.
If you chose B: You added the number of cells to the volts. Multiply the cells by the volts of each cell. Go to S3.10.2.
If you chose C: That is the number of cells, not the volts. Each cell adds 1.5 V. Go to S3.10.2.
Circuits A and B use the same bulb and the same kind of cells. Which bulb is brighter?
See the answer
Circuit B
B has two cells, so a bigger push and a bigger current. A bigger current makes the bulb brighter.
If you chose A: You may think one cell is stronger. Two cells in a line add their push, so B has the bigger push. Go to S3.10.2.
If you chose B: The bulbs are the same, but the push is not. More push makes more current, and a brighter bulb. Go to S3.10.2.
Ana joins one 1.5 V cell to a 3 Ω resistor. The current is 0.5 A. She swaps in a 6 Ω resistor and the current falls. What is the push of the cell now?
See the answer
1.5 V
The cell is the same, so its push is still 1.5 V. The resistor changed, so the current changed, from 0.5 A to 0.25 A.
If you chose B: You halved the volts because the current fell. Voltage and current are not the same thing. The cell pushes as hard as before. Go to S3.10.2.
If you chose C: You doubled the volts because the resistor doubled. A bigger resistor slows the current, but it does not change the push of the cell. Go to S3.10.2.
From an earlier chapter
Review: Mei puts an ammeter on each side of a bulb. The first reads 0.4 A. What does the second read?
See the answer
0.4 A
The current is the same all round the loop, so the second ammeter also reads 0.4 A.
If you chose A: You may think the bulb uses up half the current. It does not. The same current goes all round the loop. Go to S3.10.1.
If you chose C: If the bulb used it all up, none would come back. But the same current goes all round the loop. Go to S3.10.1.
Two 1.5 V cells are in a line with a bulb. One cell is turned the wrong way round. What push do they give?
See the answer
0 V, so the bulb stays dark
The turned cell pushes against the other one with the same strength, so the pushes cancel.
If you chose A: You added both cells. The turned cell pushes the other way, so it cancels the first. Go to S3.10.2.
If you chose B: Only one cell does not count. The turned cell works against the first, so they cancel. Go to S3.10.2.
Joseph's resistor takes 0.5 A on one cell. How many amps does it take with 3 cells in a line?
See the answer
1.5 A
3 cells give 3 times the push, so 3 times the current. 3 × 0.5 = 1.5 A.
Keep it
Copy this into your exercise book.
Voltage is the push, in volts. Cells in a line add their volts. A bigger push means a bigger current and a brighter bulb. A voltmeter goes across a part.
Draw: Three cells in a line, labelled 1.5 V each, with 4.5 V in all. Mark + and − on each cell.
