This chapter defines sensible and latent heat and provides the appropriate formulae to calculate the amount of energy required to convert a solid to a gas and vice-versa, and also for other combinations of solids, liquids and gases. This information is often required by engineers if they are required to design an artefact (say) to convert ice to steam via the state of liquid water. An example of a household requirement of when this type of calculation is required is that of the simple domestic kettle. When the designer is required to design a domestic electric kettle, it is important that the design is such that the powering arrangement is (just) enough to boil the required amount of water in a reasonable time. If the powering were too low, you may have great difficulty in boiling the water when the kettle is full. Similar calculations are required for large water containers, which are required to boil large quantities of water for other uses, including for kitchens in schools, to make tea/ coffee, etc, and for large hotels, which have many uses for hot water. The chapter also describes the three main methods of heat transfer, namely conduction, convection and radiation, together with their uses.
At the end of this chapter you should be able to:
• distinguish between heat and temperature
• appreciate that temperature is measured on the Celsius or the thermodynamic scale
• convert temperatures from Celsius into Kelvin and vice versa
• recognise several temperature measuring de- vices
• define specific heat capacity, c and recognise typical values
• calculate the quantity of heat energy Q using
Q = mc(t2 – t1)
• understand change of state from solid to liquid to gas, and vice versa
• distinguish between sensible and latent heat
• define specific latent heat of fusion
• define specific latent heat of vaporisation
• recognise typical values of latent heats of fusion and vaporisation
• calculate quantity of heat Q using Q = mL
• describe the principle of operation of a simple refrigerator
Introduction
Heat is a form of energy and is measured in joules. Temperature is the degree of hotness or coldness of a substance. Heat and temperature are thus not the same thing. For example, twice the heat energy is needed to boil a full container of water than half a container – that is, different amounts of heat energy are needed to cause an equal rise in the temperature of different amounts of the same substance.
Temperature is measured either (i) on the Celsius (°C) scale (formerly Centigrade), where the temperature at which ice melts, i.e. the freezing point of water, is taken as 0°C and the point at which water boils under normal atmospheric pressure is taken as 100°C, or (ii) on the thermodynamic scale, in which the unit of temperature is the kelvin (K). The kelvin scale uses the same temperature interval as the Celsius scale but as its zero takes the ‘absolute zero of temperature’ which is at about – 273°C. Hence,
kelvin temperature = degree Celsius + 273
K = (°C) + 273
Thus, for example, 0°C = 273 K, 25°C = 298 K and 100°C = 373 K
Problem 1. Convert the following temperatures into the Kelvin scale:
(a) 37°C
(b) – 28°C
From above, Kelvin temperature = degree Celsius + 273
(a) 37°C corresponds to a Kelvin temperature of 37 + 273, i.e. 310 K
(b) –28°C corresponds to a Kelvin temperature of– 28 + 273, i.e. 245 K
Problem 2. Convert the following temperatures into the Celsius scale:
(a) 365 K
(b) 213 K
From above, K = (°C) + 273
Hence, degree Celsius = Kelvin temperature –273
(a) 365 K corresponds to 365 – 273, i.e. 92°C
(b) 213 K corresponds to 213 – 273, i.e. – 60°C
Now try the following Practise Exercise
Practise Exercise 106 Further problems on temperature scales
1. Convert the following temperatures into the Kelvin scale:
(a) 51°C
(b) –78°C
(c) 183°C
[(a) 324 K (b) 195 K (c) 456 K]
2. Convert the following temperatures into the
Celsius scale:
(a) 307 K (b) 237 K
(c) 415 K
[(a) 34°C (b) –36°C (c) 142°C]
The measurement of temperature
A thermometer is an instrument that measures temperature. Any substance that possesses one or more properties that vary with temperature can be used to measure temperature. These properties include changes in length, area or volume, electrical resistance or in colour. Examples of temperature measuring devices include:
(i) liquid-in-glass thermometer, which uses the expansion of a liquid with increase in temperature as its principle of operation,
(ii) thermocouples, which use the e.m.f. set up, when the junction of two dissimilar metals is heated,
(iii) resistance thermometer, which uses the change in electrical resistance caused by temperature change, and
(iv) pyrometers, which are devices for measuring very high temperatures, using the principle that all substances emit radiant energy when hot, the rate of emission depending on their temperature.
Each of these temperature measuring devices, together with others, are described in Chapter 25, page 281 .