Week – 4 L2 Presentation Principles and Definitions 1

Principles and Definitions A liquid -in -glass thermometer is a temperature-measuring instrument consisting of a thin-walled glass bulb (the reservoir for the thermometer liquid) attached to a glass stem (the capillary tube through which the liquid moves with a change in temperature), with the bulb and stem system sealed against its environment. 2

This is a common type of thermometers in everyday use. The bulb is usually filled with mercury or alcohol that expands into a capillary tube when heated. In this case the thermometric property is the change in volume of the thermometric liquid. Any temperature change can be defined as being proportional to the change in length of the liquid column. 3

Expansion  Many substances expand when heated  When heated, the particles (atoms or molecules) in a substance vibrate more  This means they need more space  So the substance expands Heat 4

A scale is provided to indicate the height to which the liquid column rises in the stem, and this reading is made to indicate closely the temperature of the bulb. 5

The operation of a liquid-in-glass thermometer thus depends on the expansion of the liquid being greater than that of the containing glass bulb. Before any thermometer is marked with a scale, it is calibrated to ensure the correct measurement of the temperature. 6

Worksheet 1  Draw this diagram ! Heat 7

Plenary  How does a thermometer work?  Why substances expand with heat?  Examples of matter expansion in daily life ? 8