Use expansion in thermometers and bimetal devices
Describe a liquid-column indicator and two bonded strips that bend differently on heating.
Core and Extended; Physics 0625, 2026–2028, section 2.2.1. Describe thermal expansion at constant pressure and everyday applications and consequences. The relative magnitude explanation using particles belongs to the labelled Supplement lesson. Written practice is self-checked.
Before you begin
- Describe thermal expansion without changing state.
- Distinguish a measured device response from an assumed universal calibration.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What makes a liquid-column height a temperature reading?
What you will learn
- Describe thermal expansion as a basis for a calibrated liquid thermometer.
- Predict bimetal bending toward the less-expanding layer in a specified free-bending model.
A liquid column provides an observable response
In a suitable liquid-in-glass thermometer, the liquid's net expansion relative to the container makes its column rise in a narrow tube as temperature increases. Marks calibrated for that instrument associate column position with temperature. It is the stated device response and calibration that justify a reading, not an assumption that any liquid in any vessel has the same temperature scale.
The bulb and tube also expand, and different liquids and designs have different responses. A narrow tube makes a small volume change easier to observe as a column-height change. Do not treat a change in uncalibrated height as a known number of degrees or propose opening or constructing a thermometer from hazardous materials.
Bonded strips cannot expand independently
A bimetal strip joins two layers with different free expansions over the chosen temperature range. On heating, the layer that would expand more needs the longer outer side of the bent strip. The strip bends toward the less-expanding layer, which lies on the shorter inner side. The diagram specifies the top layer as the more-expanding one and shows bending toward the lower layer.
A suitably designed thermostat can use that bending to move a contact as temperature changes. Whether a contact opens or closes depends on where the contact is placed; bending direction alone does not establish the device's wiring or switching outcome. Cooling in the specified reversible model tends to undo the heating bend. This teaching schematic is not a certified component design.
Pause and explain
A top bonded layer expands more than the lower layer on heating. Which way does the free strip bend?
Worked example
Two bonded layers initially lie straight, with A above B. A expands more on heating. Predict the bend and say whether every contact must open.
Show the worked solution
- A needs the longer outside of the bent strip.
- The strip bends toward the less-expanding lower layer B.
- Contact opening or closing also depends on contact placement, which is not supplied.
Answer It bends toward B; a contact's switching outcome is not determined without its placement.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Predict the heating bend for top layer A expanding more than lower layer B.
Give me a hint
Assign the longer outer side to the layer needing greater free length.
Compare my reasoning
- A would expand more if free.
- In the bonded strip A takes the longer outer arc.
- The strip bends toward less-expanding B.
Toward B, with A on the outer arc.
Look for these in your work
- Identify the unequal expansions.
- Keep inside and outside consistent.
Explain why a narrow calibrated liquid thermometer tube helps observation, then predict the bend if the lower bimetal layer expands more than the upper one.
Give me a hint
Separate the liquid-column response from the reversed layer order.
Compare my reasoning
- A narrow tube turns a small net volume change into a more noticeable height change.
- Calibration is still needed to associate height with temperature.
- With the lower layer expanding more, the strip bends toward the less-expanding upper layer.
A narrow tube makes the height response easier to see; the reversed strip bends toward the upper layer.
Look for these in your work
- Retain calibration.
- Reverse the bimetal direction correctly.
A diagram shows a strip bending on heating but gives no contact positions. A student claims every such thermostat must close its electrical circuit. Evaluate it.
Give me a hint
Bending is only one part of a switching device.
Compare my reasoning
- Unequal expansion can cause bending.
- Opening or closing depends on how contacts are placed and connected.
- The supplied bend alone does not establish a circuit outcome or certify a device.
The switching outcome is unspecified; contact placement and design are needed.
Look for these in your work
- Separate mechanical response and electrical outcome.
- Avoid universal device claims.
Common mistakes
- Thermometer readings need calibration.
- Bend toward the less-expanding layer.
- Do not infer contact wiring from bending alone.
What can you explain now?
Which bimetal layer lies on the longer outer arc after heating in this model?
Compare with the explanation
The more-expanding layer.
Bonding makes the layers bend together, with different arc lengths matching their unequal expansions.
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Make it stick
Explain calibrated thermometer response and bimetal bending while keeping device-specific design limits explicit.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.