Year 6 Science: A fair test with cells, and safety
Part of the Year 6 Science cells, brightness and volume chapter. Chapter overview · whole chapter on one page
A fair test with cells, and safety
A fair test changes only one variable. In this test, the changed variable is the number of cells. The measured variable is the brightness or the loudness, rated out of 5. The fixed variables are kept the same: the same bulb, the same wires, the same bulb holder and the same type of cell.
Safety is part of planning: use low-voltage cells only, and always work with a teacher present.
Key ideas
- A fair test changes only the number of cells.
- The fixed variables are the same bulb, wires, holder and type of cell.
- Repeat each trial, and find the mean.
- Never connect a wire straight across the two ends of one cell.
Method. Find the thing that changes on purpose.
- Read the plan. Aisha changes the number of cells on purpose.number of cells
- Write the words. So the changed variable is the number of cells.number of cells
- Write the answer. the number of cells
Check. The bulb stays the same. ✓
Method. Count the changes.
- Count the changes. She changes the number of cells and also the bulb.two changes
- Decide. Changing two things is not fair, so the answer is no.no
- Write the answer. no
Check. Only one variable should change. ✓
Method. Count the fixed variables in the list.
- Count. The fixed variables are the bulb, the wires, the bulb holder and the type of cell.four
- Write the number. So there are four fixed variables.4
- Write the answer. 4
Check. Keeping four things the same makes the test fair. ✓
Watch out
Watch out: a short circuit can make a cell and its wires hot. Connect the wires to the cell ends only through the bulb and holder.
Exam tip
Exam tip. When you plan a test on cells, name the changed variable, the measured variable and two fixed variables.
Try it (with your teacher, or a grown-up at home). Plan a fair test, with a teacher, on the number of cells and the brightness of a bulb. Write the changed variable, the measured variable, and three fixed variables, then carry out the test with three repeats for each number of cells.
Exercise 9E A fair test with cells, and safety
Method. Read the plan.
- Read the plan. The number of cells is the thing we change on purpose.changed on purpose
- Write the words. So the changed variable is the number of cells.number of cells
- Write the answer. number of cells
Check. The brightness is the measured variable. ✓
Method. Read the plan.
- Read the plan. The brightness of the bulb is measured, rated out of 5.brightness
- Write the word. So the measured variable is brightness.brightness
- Write the answer. brightness
Check. The loudness is the measured variable for the buzzer test. ✓
Method. Read the fixed variables.
- Read the list. The fixed variables include the same bulb.same bulb
- Write the words. So the fixed variable is the same bulb.the same bulb
- Write the answer. the same bulb
Check. The bulb must not change in the test. ✓
Method. Read the warning.
- Read the warning. A wire straight across a cell is a short circuit, which makes the cell hot.hot
- Decide. So it is not safe, and the answer is no.no
- Write the answer. no
Check. Never connect a wire straight across a cell. ✓
Method. Read the safety rule.
- Read the rule. Only low-voltage cells and bulbs are used, with a teacher present.low-voltage
- Decide. So mains electricity is not used, and the answer is no.no
- Write the answer. no
Check. Mains electricity is dangerous. ✓
Method. Count the list.
- Count. The list has four items: the bulb, the wires, the bulb holder and the type of cell.four
- Write the number. So there are four fixed variables.4
- Write the answer. 4
Check. Four things are kept the same. ✓
Method. Count the changed variables.
- Count the changes. Ruby changes two things: the number of cells and the holder.two things
- Decide. Changing two things is not a fair test, so the answer is no.no
- Write the answer. no
Check. Only one variable should change. ✓
Method. Think about repeats.
- Think about repeats. Repeats let Tom find a mean, which is more reliable.mean
- Write one word. The word is mean.mean
- Write the answer. mean
Check. The mean evens out chance differences. ✓
Method. Read the warning.
- Read the warning. A wire across the two ends of a cell is a short circuit, which can make it hot.short circuit
- Decide. So it is not safe, and the answer is no.no
- Write the answer. no
Check. Never connect a wire straight across a cell. ✓
Method. Think about what must stay the same.
- Think about the buzzer. The buzzer, the wires and the type of cell must be the same.same
- Write one. For example, use the same buzzer each time.same buzzer
- Write the answer. Accept any one of: the same buzzer, the same wires, the same cells type, the same holder.
Check. Only the number of cells changes. ✓
Method. Think about the bulb.
- Think about a different bulb. A different bulb may be brighter for its own reasons.own reasons
- Think about the test. So the bulb must stay the same, so only the cells change the brightness.only the cells
- Write the answer. Accept any sentence that says a different bulb could be brighter for another reason, so the bulb must be kept the same.
Check. Fair tests change one thing at a time. ✓
Method. Think about safety.
- Think about the connections. A teacher can check each connection before the cells are used.check
- Think about the risk. This prevents short circuits and other harm.prevents
- Write the answer. Accept any sentence that says a teacher can check the connections and make sure no wire is joined across a cell, so the test is safe.
Check. Safety comes first in every practical. ✓