25.12 - Other Question Types You'll Meet
Short practical question types
Practical exam papers regularly include short questions worth one or two marks each.
| Type | Typical question |
|---|---|
| Why a step is in the method | "Explain why the student waits two minutes before taking the reading." |
| Measuring a small quantity by multiples | "Suggest why the student times 20 oscillations rather than one." |
| Measuring to a point you can't touch | "Describe how the student measures the length accurately using a set square." |
| Turning a claim into a procedure | "Suggest how the student could use the apparatus provided to test his idea." |
"Why is this step in the method?"
When asked why a step is included in an experimental method, explain what problem the step prevents - ask what would go wrong if this step were skipped?
Common steps and their reasons
| The step | Why it's there |
|---|---|
| Wait until the thermometer reading stops changing | The thermometer takes time to reach thermal equilibrium with the liquid |
| Stir the liquid before reading the temperature | Convection causes temperature variations; stirring ensures a uniform temperature throughout |
| Balance the bare metre rule before hanging anything on it | Places the pivot at the rule's centre of mass, so the rule's own weight exerts no turning effect |
| Switch the power supply off between readings | Prevents the wire from heating up, which would increase its resistance and affect results |
| Let the mass swing a few times before starting the stop-watch | The first swings after release are irregular; waiting lets the pendulum settle into steady oscillations |
| Take the reading with your eye level with the scale | Avoids parallax error |
| Use the same wire for every length | Ensures material and cross-sectional area remain constant, so only length affects resistance |
Worked example - Finding a balance point
The student finds the balance point of the metre rule by allowing it to tip slowly one way and then the other. Explain why the student does this. [1]
Answer: The rule appears level over a small range of positions. Tipping it each way finds the limits of that range, so the true balance point can be taken as the midpoint.
Measuring a small quantity by measuring multiples
When a single quantity is too small to measure accurately with standard instruments, measure a large number together and divide.
| What you want | What you measure | The calculation |
|---|---|---|
| Period of a pendulum | Time for 20 oscillations | Time ÷ 20 |
| Thickness of one sheet of paper | Thickness of a stack of 100 sheets | Thickness ÷ 100 |
| Diameter of a wire | Length of 20 turns wound tightly side by side | Length ÷ 20 |
| Volume of one ball bearing | Rise in water level when 10 identical ball bearings are added | Volume rise ÷ 10 |
Why it works
- The absolute error in the measurement (such as reaction time when using a stop-watch) stays roughly the same whether timing one oscillation or twenty.
- Timing multiple oscillations makes the measured time much larger, so the reaction-time error becomes a much smaller percentage of the total. Dividing by the count reduces the uncertainty in the single value by the same factor.
Worked example - Timing 20 oscillations
Suggest why the student measures the time for 20 oscillations rather than the time for one. [1]
Answer: A single oscillation is short, so human reaction time when starting and stopping the stop-watch causes a large percentage error. Timing 20 oscillations gives a much larger total time, reducing the percentage error from reaction time and giving a more accurate value for one period after dividing by 20.
Measuring to a point you cannot touch
Using a set square
- To read the position of a point not touching the rule (e.g. the bottom of a hanging mass or spring): hold a set square with one edge flat against the vertical rule and the other edge aligned with the point, then read the scale.
- This ensures the line from the object to the scale is strictly perpendicular (at right angles) to the rule, eliminating parallax error.
Finding a balance point
- A metre rule on a pivot balances across a narrow range rather than at one sharp point.
- Technique - Let the rule tip slowly one way, then slowly the other, and take the position midway between the two as the balance point.
- To find the centre of mass of a bare rule, balance it on the pivot with nothing attached; the balance point marks the centre of mass.
Worked example - Reading a spring length with a set square
The student measures the length of the stretched spring to the nearest 0.1 cm. Describe how she uses a set square to take this reading accurately. [1]
Answer: Place one edge of the set square flush against the vertical metre rule and align the perpendicular edge with the bottom of the spring. Read the scale on the rule where the set square meets it to ensure the line of sight is at right angles to the rule.
Testing a claim using given apparatus
Short planning questions (1-2 marks) give a claim and existing experimental apparatus, asking how to test the claim.
A student thinks three resistors , , and have the same resistance within the limits of experimental accuracy.
(i) Suggest how the student could use the apparatus provided to test his idea. [2]
(ii) Explain how the student can decide whether the values of resistance are the same within the limits of experimental accuracy. [1]
Answering the two parts
- Part (i) - The procedure - Repeat the same measurement on each item, keeping control variables constant.
- Answer: Connect each resistor in turn into the circuit, keeping the power supply voltage constant. Record the current and potential difference for each, and calculate resistance using .
- Part (ii) - The comparison - Compare the calculated values. If the values are within 10% of each other (or within experimental uncertainty), they can be considered equal.
One-mark test questions
Suggest how the student could check whether the period of a pendulum depends on the mass. [1]
Answer: Repeat the experiment with a different mass attached, keeping the length of the pendulum constant, and compare the measured periods.