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UNIT 8: MEASUREMENT OF TEMPERATURE

  • Temperature: It is a measure of the average kinetic energy of the atoms or molecules in a body. (It tells us how hot or cold something is.)

  • Heat: Heat is the energy that is exchanged between two objects due to their difference in temperature.

  • Calibrating a thermometer:

    • Step 1: To measure temperature we need a physical quantity that is affected by temperature. For example volume, e.m.f., resistance or pressure.

    • Step 2: Mark 0oC (using pure melting ice) and 100oC (by placing just above boiling water).

    • Step 3: Divide the temperature range between the two marks into 100 equal parts.

  • Thermometric liquids:

    Mercury

    Alcohol

    Advantages

    • Good conductor

    • Does not wet glass

    • High boiling point

    • Not poisonous

    • Cheap

    • Low freezing point

    Disadvantages

    • Poisonous

    • Expensive

    • High freezing point (-39oC)

    • Bad conductor

    • Wets glass

    • Low boiling point (78oC)

  • Some definitions:

    • Responsiveness: It is about how quickly you get the result.

    • Sensitivity: It is about how much does the thermometric property (e.g., volume) change per degree change in temperature.

    • Range: The minimum and maximum temperatures that can be measured.

    • Linearity: It is about whether each degree is an equal distance or not.

  • Liquid-in-glass thermometers: In comparison to a laboratory thermometer, a clinical thermometer has:

    • a constriction

    • less range

    • greater sensitivity (divisions further apart)

    • triangular cross-section (for magnification)

  • Thermocouple thermometers:

    • Construction: You should be able to draw figure 8.13.

    • Defining equation: e.m.f. = Δ θ

    • Advantages:

      • Large range

      • Measures temperature at a point

      • High responsiveness

      • Can be connected to computers because of electrical output


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