Unit 11:  Heat and temperature                                    1   2   3   4   5   6

2. Temperature

Temperature is the measurement of a substance’s thermal energy.

So, when we say that a body has a higher temperature than another body,

what we really mean is that its atoms or molecules are moving more quickly.

a) Temperature measurement

We use thermometers to measure temperature. Most thermometers contain liquids which expand and contract when there are changes in temperature.

There are different types of thermometer: mercury, coloured alcohol, or digital.

 - Mercury thermometers

- Alcohol thermometers

- Digital thermometers

b) Temperature scales

To measure temperature we can use three different scales: the Celsius scale,

the Fahrenheit scale and the Kelvin scale.

- Celsius or centigrade scale (°C)

            This is a scale we use to measure normal temperatures.

       In this scale the value:

- 0 is the temperature at which water freezes in conditions

  of normal atmospheric pressure.

- 100 is the temperature at which water boils in conditions

   of normal atmospheric pressure.

          The interval between these two values is divided into one hundred equal parts;

     each of the parts corresponds to one degree Celsius (1°C).

- Kelvin or absolute scale (K)

        This is the real or physical scale that is directly linked to the movement

    of particles. The kelvin (K) is the unit of temperature in the International

    System of Units (S. I.)

          The value 0 in this scale corresponds to the temperature at which

     there is no movement in particles: when a substance’s thermal energy is zero.

     Absolute zero or zero kelvin (0 K) indicates the natural limit of temperatures:

     lower temperatures are not possible, because temperature is the measurement

     of the movement of particles.

           On this scale, water freezes at around 273 K, so 0°C = 273 K.

           The divisions of this scale are the same size as those of the Celsius scale,

      so the temperature at which water boils is 100°C = 373 K.

           To convert degrees Celsius into kelvin we have to add on 273.

T (K) = t (°C) + 273

- Fahrenheit scale (°F)

          This scale is usually used in Anglo-Saxon countries instead of Celsius scale.

      The freezing point of water is 32°F and its boiling point 212°F.

      The degrees of this scale are different of those of the Celsius scale.

      To convert degrees Celsius into degrees Fahrenheit we have to use

      this expression:

t (°C) = t (°F) - 32 / 1.8

Animation: Scales of temperature (Tutorvista)

2.1. Answer the questions:

a. What properties of matter do mercury and digital thermometers use?

b. Why must the tube of liquid inside a traditional thermometer be very thin?

c. How many degrees Celsius does absolute zero (0 K) correspond to?

d. What type of thermometer will we use in polar regions where temperatures

    can reach -40°C? Why?

2.2. Change these temperatures into kelvin (K). Are any of them impossible?

        Why?

a) -270°C                        c) 25°C                           e) -534°C

b) 14,000°C                     d) -190°C                        f) 425°C