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Tools and Inquiry — IB Physics skills

I.1.3 Controlling Variables in Physics — IB Physics HL Revision Notes & Topical Questions

5 min read 10 parts Lesson 17 of 22 1 quiz

📚 IB Physics HL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Lesson summary

I.1.3 Controlling Variables in Physics — IB PHYSICS SL notes: Explain how to control variables in practice by calibration, constant conditions, insulation and reducing friction, resistance and background radiation.Controlling variables minimises systematic errors — errors that sh…

  • Force sensor
  • Motion sensor
  • Digital thermometer and probes
  • Digital balance
  • Ammeters and voltmeters
  • Conditions that matter

Part 1 of 8Purpose of controlling variables

Purpose of controlling variables
Controlling variables minimises systematic errors
  • Controlling variables minimises systematic errors errors that shift results consistently in one direction.
It gives more confidence that any effect is due to the independent variable.
It makes results valid and the test fair.

Part 2 of 8Calibrating apparatus

1Calibration
Calibration checking an instrument against a known reliable standard and adjusting if necessary.
2Force sensor
  • Force sensor zero with no load.
3Motion sensor
  • Motion sensor check its distance reading against a known object.
4Digital thermometer and probes
  • Digital thermometer and probes check in boiling distilled water (100.0 °C) and crushed ice (0.0 °C).
5Digital balance
  • Digital balance tare before use.
6Ammeters and voltmeters
  • Ammeters and voltmeters check that they read zero with no current or potential difference.

Part 3 of 8Constant environmental conditions

1Conditions that matter
  • Conditions that matter temperature, pressure, humidity, air currents and light intensity.
2Temperature affects density, gas pressure and electrical resistance
  • Temperature affects density, gas pressure and electrical resistance so it is often the most important to control.
3Draughts from windows or air conditioning cool substances
  • Draughts from windows or air conditioning cool substances close windows or use a draught shield.
4If a variable cannot be perfectly controlled, monitor and record it
  • If a variable cannot be perfectly controlled, monitor and record it then discuss its impact in the evaluation.

Part 4 of 8Insulating against heat loss or gain

1
Unwanted heat exchange is the biggest error in thermal experiments.
2Use a polystyrene cup instead of a glass beaker
  • Use a polystyrene cup instead of a glass beaker it is a much better insulator.
3Place the cup inside a larger beaker
  • Place the cup inside a larger beaker for stability and an insulating layer of air.
4Use a lid with holes for thermometer and stirrer
  • Use a lid with holes for thermometer and stirrer it reduces loss by evaporation and convection.

Part 5 of 8Reducing friction and electrical resistance

Reducing friction and electrical resistance
Friction reduces motion and wastes energy as heat and sound.
  • Lubricate pulleys and wheels.
  • Add bearings to stop components rubbing.
  • Use low-friction apparatus such as an air track, ideal for collisions.
Unwanted resistance in wires and connections causes heating.
  • Use shorter, thicker wires.
  • Keep the current as low as possible.
  • Make connections secure, because poor connections add resistance.

Part 6 of 8Background radiation

  • Background radiation must be accounted for in experiments with radioactive sources.
Background radiation
Record the background count rate with no source present, using a Geiger–Müller tube.
Subtract the background count rate from all subsequent measurements.
  • This ensures that only radiation from the source is measured.

Part 7 of 8Applying controls

Applying controls
Say how a variable is controlled and why
  • "an air track provides a near-frictionless surface" not just "reduce friction".
Pendulum controls.
  • Amplitude below 10° and constant, measured with a protractor, because T = 2π√(L/g) assumes the small-angle approximation.
  • A knife-edge pivot reduces friction damping; no draughts and a dense bob reduce air resistance.
Resistance of a wire controls.
  • A low potential difference with the supply off between readings avoids Joule heating.
  • The same wire, with diameter checked by micrometer, keeps the area constant.
  • Clean, firm crocodile clips avoid contact resistance.
Use the same equipment and the same person for readings
  • Use the same equipment and the same person for readings to reduce subjective error.
Prioritise the controls that affect the results most.

Part 8 of 8More from the specification

1Length
  • Resistance is directly proportional to the length of a wire.
2Period
  • Time for one revolution.
3Oscillation
  • Repeated back-and-forth motion.
4Hypothesis
  • Testable prediction with a scientific justification.
5Capacity
  • Charge a cell can supply, in A h or C.
6Methodology
  • Step-by-step procedure that another person can replicate.
7Simple pendulum
  • A small bob on a light string of length L
8Directly proportional
  • Two quantities increase together in a constant ratio, giving a straight line through the origin.
9Heating effect
  • Heating effect in a wire resistance experiment, switch the current on briefly for each reading so the wire does not warm and change resistance.
10Uniform wire
  • Uniform wire measure the diameter with a micrometer at several points along the wire to check it is uniform.

Key terms

TermMeaning
CalibrationChecking an instrument against a known standard.
Systematic errorError that shifts all results in the same direction.
InsulationMaterial or design that reduces heat transfer.
Air trackApparatus giving a near-frictionless surface.
Background count rateCount rate with no radioactive source present.
Fair testTest in which only the independent variable changes.

Avoid these mistakes

1
Saying "I will reduce friction" without stating how.
2
Forgetting to subtract background radiation in radioactivity experiments.
3
Not zeroing sensors, balances or meters before use.
4
Using a glass beaker with no lid for thermal experiments.
5
Ignoring uncontrolled variables instead of monitoring and recording them.

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