Physics Lesson 13.2.6 - Bimetallic Strip. Thermostat

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Welcome to our Physics lesson on Bimetallic Strip. Thermostat, this is the sixth lesson of our suite of physics lessons covering the topic of Thermal Expansion, you can find links to the other lessons within this tutorial and access additional physics learning resources below this lesson.

Bimetallic Strip. Thermostat

A bimetallic strip is a system composed by two different metal strips, which are placed side by side and welded together. This process is carried out in normal temperature, so initially the system looks like this:

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Let's suppose that the coefficient of linear thermal expansion of metal 1 is greater than that of metal 2. This means the metal 1 can expand or contract more than the metal 2. As a result, when the bimetallic strip is heated up, it bends around the material with the smallest coefficient of linear thermal expansion (metal 2) as it remains shorter (metal 1 expands more than metal 2). On the other hand, when the bimetallic strip is cooled down, the metal 1 contracts more than the metal 2 because it has a greater coefficient of thermal expansion and contraction as well. As a result, the system bends around the metal 1 as shown in the figure.

This property of different amount of thermal expansion or contraction two metals experience when they are exposed to the same change in temperature, is used to construct thermostats, which are equipment that keep heaters at constant temperature by controlling the current flow in an electric circuit through bimetallic strips. Thus, if the heater is at the desired temperature, the bimetallic strip is at straight position, as shown in the figure.

When the heater is delivering more heat than needed, the bimetallic strip bends around the metal with the smallest coefficient of linear expansion. As a result, the contact is open, so the current flow in the circuit stops as shown in the figure below. This state lasts until the bimetallic strip cools down to the desired temperature and therefore it straightens again. Then the current starts flowing again through the circuit as the contact closes, and so on.

This is basically the operating principle of a thermostat.

Example 4

Two bimetallic strips made of different materials at room temperature (200

are shown in the figure below.

The coefficients for linear thermal expansion of the four materials are:

aluminum => 24 × 10^-6 K^-1
iron => 12 × 10^-6 K^-1
bronze => 18 K^-1
steel => 11 K^-1

What is the shape of the two bimetallic strips when they are heated up at a few hundred Celsius degrees?

Solution 4

When a bimetallic strip is heated, it bends around the material with the least coefficient of thermal expansion, as it expands less than the material with the smallest coefficient. Therefore, the first bimetallic strip will bend around the iron strip when heated, as the coefficient of thermal expansion of iron is smaller than that of aluminum, while the second bimetallic strip will bend around steel when heated, as steel has a smaller coefficient of thermal expansion than bronze. However, the bending in the second strip will not be of the same degree as in the first strip; the first bimetallic strip will bend more than the second one, as the difference in the coefficients of linear thermal expansion in the first bimetallic strip is much greater than in the second.

The two bimetallic strips will look as in the figure below when heated.

You have reached the end of Physics lesson 13.2.6 Bimetallic Strip. Thermostat. There are 7 lessons in this physics tutorial covering Thermal Expansion, you can access all the lessons from this tutorial below.

More Thermal Expansion Lessons and Learning Resources

Thermodynamics Learning Material
Tutorial ID	Physics Tutorial Title	Revision Notes	Revision Questions
13.2	Thermal Expansion
Lesson ID	Physics Lesson Title	Lesson	Video Lesson
13.2.1	What is Thermal Expansion and Thermal Contraction?
13.2.2	Linear Thermal Expansion and Contraction
13.2.3	Area Thermal Expansion and Contraction
13.2.4	Volume Thermal Expansion and Contraction
13.2.5	Thermal Expansion and Contraction in Daily Life
13.2.6	Bimetallic Strip. Thermostat
13.2.7	Types of Thermometers

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