Laboratory Thermometer
Key idea: Learn how a liquid-in-glass laboratory thermometer works, how to take accurate readings, and how to reduce errors like parallax and poor contact.
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The core idea
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Learning objectives
- Compare physical properties of solids, liquids and gases
- Explain state properties using particle arrangement, motion, forces and separation
- Infer random molecular motion from a Brownian-motion experiment
- Relate temperature rise to increased average kinetic energy of particles
- Explain gas pressure using particle collisions with container walls
- Explain heating from higher to lower temperature until thermal equilibrium
- Describe conduction in solids through particle vibration and mobile electrons
- Describe convection in fluids through density changes and bulk motion
- Explain that energy transfer by electromagnetic radiation needs no material medium
- Explain how surface colour, texture, temperature and area affect radiation transfer rate
- Apply conduction, convection and radiation in everyday systems
- describe internal energy as an energy store that is made up of the total kinetic energy associated with the random motion of the particles and the total potential energy between the particles in the system
- Define heat capacity and specific heat capacity
- Apply energy transferred = mass × specific heat capacity × temperature change
- describe melting/solidification and boiling/condensation as processes of energy transfer without a change in temperature
- Explain the difference between boiling and evaporation
- Define latent heat and specific latent heat
- Apply energy transferred for a change of state = mass × specific latent heat
- Explain latent heat using particle behaviour
- Sketch and interpret a cooling curve
This lesson is assessed practical technique: the G3 Physics (K323) practical paper asks you to measure temperature with a laboratory thermometer, and Topic 1 asks you to choose an instrument for its range and precision. How the thermometer is built is not a separate learning outcome in Thermal Physics Topics 7–9.
1. Definition
A. Laboratory thermometer
A laboratory thermometer is a liquid-in-glass thermometer used to measure temperature in experiments.
It uses a thermometric property: the length of the liquid column changes continuously with temperature.
2. Key Ideas
- For a correct measurement, the thermometer must reach thermal equilibrium with the object.
- The bulb should be in good thermal contact with what you are measuring.
- Avoid parallax error: read the scale at eye level.
- Choose a thermometer with a suitable range and resolution. Resolution is the smallest scale interval that can be distinguished.
3. Detailed Explanations
A. Correct method (how to take a reading)
- Immerse the bulb fully in the liquid.
- Ensure the bulb does not touch the container walls (otherwise heat can flow through the wall and affect the reading).
- Stir gently when instructed so that the liquid has a more uniform temperature.
- Wait until the liquid column becomes steady.
- Read the top of the column at eye level, while the bulb is still immersed, and record the value with its unit.
A thermometer gives the correct temperature only after it reaches thermal equilibrium with the object.
B. Range and sensitivity (what matters at O Level)
- Range: the temperatures the thermometer can measure (limited by the liquid used).
- Sensitivity: how much the liquid column changes for a 1°C change in temperature.
- A narrower capillary makes the column rise more for the same temperature increase → higher sensitivity.
- Resolution: the smallest change that can be read from the scale. Closely spaced, clearly distinguishable divisions improve resolution.
- Response time: how quickly the thermometer approaches the object’s temperature. A smaller bulb usually responds faster because less energy is needed to change its temperature.
Sensitivity and resolution are related but not identical: sensitivity describes the movement per degree, whereas resolution describes the smallest readable change.
C. Common measurement errors (and how to reduce them)
| Error / issue | What happens | How to reduce it |
|---|---|---|
| Parallax | reading too high/low | read at eye level |
| Not in equilibrium | reading changes while you record | wait until steady |
| Bulb touches container | reading affected by container temperature | keep bulb away from walls |
| Temperature varies through liquid | reading depends on bulb position | immerse consistently and stir gently if allowed |
Use the instrument’s precision. For a thermometer marked every 1°C, a mark-aligned reading may be recorded as, for example, 22.0°C when the practical instructions expect interpolation between divisions.
4. Common Mistakes
- Recording before the reading is steady (no thermal equilibrium).
- Reading the scale from an angle (parallax error).
- Lifting the thermometer out before reading it, so the bulb begins exchanging energy with the air.
- Calling the smallest scale division the thermometer’s accuracy. It indicates resolution, not necessarily closeness to the true value.
- Forgetting the unit (°C) in the final answer.
5. Exam Tips
- In practical questions, always write: “wait until the thermometer reading is steady”.
- If asked about “reducing error”, mention: eye level and bulb not touching the container.
- If asked “why use a narrow tube?”, say it increases sensitivity.
- Match improvements to problems: a narrower capillary improves sensitivity; a thinner-walled, smaller bulb shortens response time; calibration addresses a systematic scale error.
6. Worked Examples
Modelled example 1
Identifying an error
Problem
Study the worked solution
Identify the unwanted contact
Method
The bulb is in thermal contact with both the water and the glass wall.Reason
Touching the wall creates an additional energy-transfer path.Working
bulb ↔ water and bulb ↔ glassTrace the error mechanism
Method
The bulb can gain or lose energy by conduction through the glass.Reason
The container may not be at exactly the same temperature as the nearby water.Working
conduction through wall ⇒ biased bulb temperatureState the improvement
Method
Keep the bulb fully immersed but away from the container walls and base.Reason
This makes the reading represent the water more reliably.Working
bulb contacts measured liquid only
Guided practice 2
Choosing a thermometer
Problem
Try this before viewing the solution
Hints
Hint 1: compare maximum values
View solution step by step
Reject the insufficient range
Method
The 50°C thermometer is unsuitable.Reason
The expected water temperature can exceed its maximum reading.Working
80°C > 50°CSelect the suitable range
Method
Choose the 110°C thermometer.Reason
Its range includes the maximum expected temperature.Working
80°C < 110°C
Common misconception 3
Parallax error
Learner claim
Try this before viewing the solution
View solution step by step
Separate level from appearance
Method
The liquid level can stay fixed while its apparent position relative to the scale changes.Reason
The column and scale are viewed along different lines of sight from above or below.Working
fixed column + angled sight line ⇒ apparent offsetName the error
Method
This is parallax error.Reason
The apparent alignment depends on viewing position.Working
view above/below ⇒ reading too high or lowCorrect the method
Method
Place the eye level with the top of the liquid column and look perpendicular to the scale.Reason
This removes the angle-dependent apparent shift.Working
eye level ⇒ correct alignment
Examiner practice 4
Waiting for a steady reading
Examination question
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View solution step by step
Identify the initial temperature difference
1 markMethod
The water is initially hotter than the thermometer.Reason
A temperature difference is needed for net energy transfer.Working
T_water > TₜₕₑᵣₘₒₘₑₜₑᵣExplain the rising reading
1 markMethod
Energy transfers from the water to the thermometer, raising its temperature and reading.Reason
Net thermal transfer is from the hotter body to the cooler one.Working
E: water → thermometerExplain the steady reading
1 markMethod
The reading becomes steady when thermal equilibrium is reached.Reason
At equal temperature there is no net energy transfer between water and thermometer.Working
T_water = Tₜₕₑᵣₘₒₘₑₜₑᵣ ⇒ no net transfer
Self-mark with the mark scheme
Compare your response with each mark point. Select a point only when your response contains that evidence.
Self-mark the temperature difference, transfer direction and equilibrium condition.
Challenge 5
Sensitivity
Independent transfer
Try this before viewing the solution
Hints
Hint 1: same expanded volume
View solution step by step
Hold expansion fixed
Method
The same liquid and temperature change give the same volume expansion.Reason
The compared thermometers differ only in capillary width under the prompt’s conditions.Working
Δ V = sameCompare column movement
Method
The narrower capillary produces a larger length change.Reason
For the same volume, reducing cross-sectional area increases column length change.Working
Δ V = AΔ L ⇒ A↓, Δ L↑Identify sensitivity
Method
The narrower-capillary thermometer is more sensitive.Reason
Its liquid column moves farther for each 1°C temperature change.Working
sensitivity = Δ L/Δ T
7. Mind Stretchers
Mind stretcher 1: Thermometer affects the objectExtension
Why should a thermometer be “small compared with the object” you are measuring?
Show Answer
When the thermometer is placed in contact with an object, energy is transferred between them until thermal equilibrium is reached. If the thermometer is large, it can change the object’s temperature significantly, giving an inaccurate result.
Mind stretcher 2: ResolutionExtension
Two thermometers have the same range, but one has smaller scale divisions. Which gives a more precise reading, and why?
Show Answer
The one with smaller scale divisions, because you can read temperature in smaller increments (better resolution), so the measurement is more precise.
8. Practice and next step
Apply this method when collecting data in the cooling-curve lesson. For the Thermal Physics theory outcomes, return to the Topic 7–9 syllabus checklist.
Continue with the next resource in this course.
Course and syllabus information
- Course
- SEC G3 Physics
- Edition
- SEC G3 Physics 2027