Controlling Variables: notes and practice questions
- This topic covers the practical skills for identifying and managing variables in scientific investigations.
- Key considerations include calibrating measuring apparatus, including sensors.
- Maintaining constant environmental conditions of systems is often essential.
- Insulating against heat loss or gain helps control temperature variables.
- Reducing friction and electrical resistance minimizes unwanted energy dissipation.
- Taking background radiation into account is important in relevant experiments.
How it is examined
Paper 1B, usually 1 or 2 marks. Subtracting background count rate before plotting a decay curve is the single most examinable item on the list. A question asking for a control that is not one of these six (a chemistry-style "keep the concentration constant") is off-syllabus for Physics.
- calibrate measuring apparatus, including sensors
- maintain constant environmental conditions of systems
- insulate against heat loss or gain
- reduce friction
Practice questions
2 questions · 2 mediumQuestion 1
MediumPaper 2 · calculator5 marksIn a blacksmith's workshop, a 2.00 kg iron horseshoe is heated in a furnace. It is then plunged into a large, insulated barrel containing 5.00 kg of water at 15.0 °C. The system reaches a final equilibrium temperature of 100 °C, by which time 0.120 kg of the water has turned to steam.
(a) Calculate the initial temperature of the iron horseshoe.
Data:
Specific heat capacity of water = 4200 J kg⁻¹ °C⁻¹
Specific latent heat of vaporization of water = 2.26 × 10⁶ J kg⁻¹
Specific heat capacity of iron = 450 J kg⁻¹ °C⁻¹
(b) The barrel is made of wood, but in reality, it is not a perfect thermal insulator and will absorb some energy. Suggest and explain how this would affect the calculated value for the initial temperature of the horseshoe, assuming the calculation does not account for the barrel.
Apply the principle of conservation of energy. The total heat energy lost by the hot horseshoe must equal the total heat energy gained by the water. Remember that the water gains energy in two distinct ways: its temperature increases, and some of it changes state.
Consider all the places the energy from the hot horseshoe can go. In the ideal calculation, it only goes to the water. In the real experiment, where else does it go? How does this change the amount of energy that you thought the horseshoe lost, versus what it actually lost?
Question 2
MediumPaper 2 · calculator10 marksA student conducts an experiment to investigate the relationship between the period of a simple pendulum and its length . The student measures the time taken for 20 complete oscillations for various lengths of the pendulum string.
(a) State:
(i) the independent variable.
(ii) the dependent variable.
(iii) one variable that should be controlled.
(b) The student uses a digital stopwatch to measure the time. They have a consistent reaction time, starting the stopwatch 0.15 s after the pendulum bob is released from its highest point. State and explain whether this is a random or a systematic error.
(c) The student measures the time for 20 oscillations rather than for a single oscillation. Suggest why this experimental technique improves the measurement of the period .
(d) For a pendulum of length 1.00 m, the accepted value for the period is 2.01 s. The student takes three measurements of the time for 20 oscillations and calculates the period for each. The results are 2.15 s, 2.16 s, and 2.14 s. Distinguish between accuracy and precision, using the student's results as an example.
Think about what the student is deliberately changing, what they are measuring as a result, and what other factors could affect the outcome that need to be kept constant.
Does this error affect each measurement randomly, or does it introduce a consistent offset in one direction?
Consider the effect of the uncertainty in starting and stopping the timer. How does measuring a longer total time interval affect the fractional uncertainty of the final calculated period?
Accuracy relates to the 'true' value, while precision relates to the consistency of repeated measurements. How do the student's results compare to the accepted value and to each other?
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Where marks are lost
- Stopping one step short of the conclusion. Two numbers and no sentence is two marks out of three.
- Answering a procedure question with a platitude.
- Losing precision in Paper 1B. Uniquely to this paper, quoting the right number badly loses marks.
