Year 6 Science: Brightness of a bulb and the number of cells
Part of the Year 6 Science cells, brightness and volume chapter. Chapter overview · whole chapter on one page
Brightness of a bulb and the number of cells
The brightness of a bulb depends on the voltage across it. More cells joined end to end give a higher total voltage. A higher voltage makes more current flow through the bulb, so the bulb is brighter.
The class rated each bulb brightness out of 5. A rating of 5 is the brightest.
The table gives Priya's three trials for each number of cells, and the mean of the trials.
| Cells | Trial 1 | Trial 2 | Trial 3 | Mean brightness (out of 5) |
|---|---|---|---|---|
| 1 | 2 | 2 | 2 | 2 |
| 2 | 4 | 4 | 4 | 4 |
| 3 | 5 | 5 | 5 | 5 |
Key ideas
- More cells give a higher total voltage.
- A higher voltage makes a bulb brighter.
- Brightness is recorded as a rating out of 5, and repeats give a mean.
- Keep the bulb, the wires and the holder the same in a fair test.
Method. Read the mean column.
- Read the row. The row for two cells gives three trials of 4, 4 and 4.4, 4, 4
- Write the mean. The mean brightness is 4 out of 5.4
- Write the answer. 4 out of 5
Check. 4 + 4 + 4 = 12, and 12 divided by 3 is 4. ✓
Method. Subtract the one-cell rating from the three-cell rating.
- Read the ratings. The three-cell bulb is rated 5, and the one-cell bulb is rated 2.5 and 2
- Subtract. The difference is 5 minus 2.5 - 2 = 3
- Write the answer. 3
Check. 2 + 3 = 5. ✓
Method. Find the highest rating.
- Read the ratings. The ratings are 2, 4 and 5 for one, two and three cells.5
- Write the answer. The highest rating is 5, for three cells.3
- Write the answer. 3 cells
Check. The bulb with three cells is the brightest. ✓
Watch out
Watch out: brightness is a rating out of 5 in this table. Do not compare it with a voltage; say the bulb is brighter because its total voltage is higher.
Exam tip
Exam tip. When you link brightness to the cells, say that more cells give a higher voltage, and that makes the bulb brighter.
Try it (with your teacher, or a grown-up at home). With a teacher, build a circuit with one cell and one bulb in a holder. Compare the brightness with a second cell added, joined end to end. Use the same bulb each time. Give each bulb a rating out of 5, and repeat the test three times. Switch off the circuit between tests.
Exercise 9B Brightness of a bulb and the number of cells
Method. Read the bar for two cells.
- Find the bar. The middle bar is for two cells.two cells
- Read the rating. That bar is rated 4 out of 5.4
- Write the answer. 4
Check. Two cells are brighter than one cell. ✓
Method. Read the table.
- Read the table. The one-cell bulb is rated 2 out of 5.2
- Write the answer. 2
Check. One cell is the least bright of the three. ✓
Method. Find the highest rating.
- Read the ratings. The highest rating, 5, is for three cells.5
- Write the number. So three cells give the brightest bulb.3
- Write the answer. 3
Check. Three cells is the brightest in the table. ✓
Method. Add and divide by three.
- Add. The total is 5 + 5 + 5 = 15.15
- Divide. Divide 15 by 3.15 divided by 3 is 5
- Write the answer. 5
Check. The bulb is always rated 5 with three cells. ✓
Method. Add and divide by three.
- Add. The total is 2 + 2 + 2 = 6.6
- Divide. Divide 6 by 3.6 divided by 3 is 2
- Write the answer. 2
Check. Each trial is 2 out of 5. ✓
Method. Subtract.
- Read the ratings. Two cells give 4 and one cell gives 2.4 and 2
- Subtract. The difference is 4 minus 2.4 - 2 = 2
- Write the answer. 2
Check. 2 + 2 = 4. ✓
Method. Use the cells table.
- Find the cells. The brightest bulb had three cells.3 cells
- Find the voltage. Three cells give 4.5 volts.4.5
- Write the answer. 4.5 volts
Check. 3 x 1.5 = 4.5. ✓
Method. Compare the ratings.
- Read the ratings. Two cells are rated 4, and three cells are rated 5.4 and 5
- Decide. So three cells give the brighter bulb, and Ali is not right.no
- Write the answer. no
Check. Three cells give more voltage, so a brighter bulb. ✓
Method. Subtract the first rating from the second.
- Read the ratings. The ratings are 2 and 4.2 and 4
- Subtract. The rise is 4 minus 2.4 - 2 = 2
- Write the answer. 2
Check. 2 + 2 = 4. ✓
Method. Think about a fair test.
- Think about the test. Only the number of cells changes.number of cells
- Write one fixed variable. For example, use the same bulb each time.same bulb
- Write the answer. Accept any one of: the same bulb, the same wires, the same bulb holder, the same type of cell.
Check. Keeping the others the same makes the test fair. ✓
Method. Think about the voltage.
- Think about the voltage. Three cells give a higher total voltage than one cell.higher voltage
- Think about the current. A higher voltage makes more current flow, so the bulb is brighter.more current
- Write the answer. Accept any sentence that says three cells give a higher total voltage, so more current flows through the bulb and it is brighter.
Check. The bulb is brighter because of the higher voltage. ✓
Method. Think about repeats.
- Think about one trial. One trial may be judged differently by chance.chance
- Think about the mean. The mean of three trials evens out these differences.evens out
- Write the answer. Accept any sentence that says repeats show whether the rating is consistent, and the mean evens out chance differences in judging.
Check. Repeats make the result more trustworthy. ✓