| Concept of temperature |
| The Term Temperature |
| Define the term temperature |
| Temperature is property of a body, which decides which way heat will flow when it is placed in |
| contact with another body. |
| The SI Unit of Temperature |
| State the SI unit of temperature |
| Temperature refers to the degree of coldness or hotness of a body. The SI unit of temperature is |
| Kelvin (K) known as absolute zero or thermodynamic scale. |
| Commonly in thermometers we encounter the unit Celsius degree ( |
| 0 |
| C) another scale is called |
| Fahrenheit scale. |
| Measurement of Temperature |
| Measurable Physical Properties that Change with Temperature |
| Identify measurable physical properties that change with temperature |
| A reliable measurement of temperature is done by a thermometer. Thermometers use measurable |
| physical properties that change linearly with temperature to give temperature reading. |
| Physical properties that change with temperature are called thermometric properties of a |
| thermometer which include the following: |
| 1. |
| Expansion of liquid when heated. E.g. Alcohol and mercury |
| 2. |
| Expansion of strip of two metals. |
| 3. |
| Generation of an electric current when heated |
| 4. |
| Change in resistance of a wire |
| 112 |
| The Fundamental Interval of a Thermometer |
| Define the fundamental interval of a thermometer |
| When you want to construct a thermometer you must establish two constant temperatures called |
| fixed points. The fundamental interval of thermometer is the difference between the upper fixed |
| point and lower fixed points. |
| The lower fixed point is the temperature of pure melting ice. Note that impurities lower the |
| melting point of ice. |
| The Mode of Action of Liquid-in-glass Thermometer |
| Describe the 'mode' of action of liquid-in-glass thermometer |
| The working of this type of thermometer is based on the fact that liquid expands when heated |
| and contracts when cooled. Examples of Liquid in glass thermometers are mercury and alcohol |
| thermometers. |
| These two thermometers are called |
| 1. |
| Mercury in glass thermometer |
| 2. |
| Alcohol in glass thermometer |
| These thermometers have bulbs, which are reservoirs of liquid, and stems with fine bodies |
| through which liquid rises and falls during the variation of temperatures. |
| The liquid used in these thermometer types is called thermometric liquids. |
| Comparison between the thermometric liquids: |
| Mercury |
| Alcohol |
| i. It is good conductor of heat i.It is fairly a good conductor of heat. |
| ii. It expands linearly ii. It expands rapidly |
| iii. It is clearly seen iii. It is colourless |
| 113 |
| iv. It boils at 360 |
| c iv. It boils at 78 |
| c |
| 0 |
| 0 |
| v. It freezes at 39 |
| c v. It freezes at – 112 |
| c |
| 0 |
| 0 |
| vi. It does not wet a glass vi. It wets the glass |
| Why water is not used as a thermometric liquid |
| Even though water is very readily available and it is cheap compared to mercury or alcohol, it is |
| not used as thermometric liquid because: |
| 1. |
| Its volume expansion is not linear |
| 2. |
| It wets the glass |
| 3. |
| It has high heat capacity |
| The Temperature of a Body |
| Measure accurately the temperature of a body |
| A clinical thermometer is a typically a mercury in glass thermometer used to measure human |
| body temperature. As the temperature rises the mercury expands and flows up the capillary tube. |
| However, clinical thermometers have some limitations, they are delicate and can break easily, |
| they may spread infection if not properly sterilized and they do not necessarily reflect the core |
| body temperature. |
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