Mechanical Engineering Principles

212 MECHANICAL ENGINEERING PRINCIPLES

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 exercise

Exercise 96 Further problems on temper-

ature 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]

19.2 The measurement of temperature

Athermometeris an instrument that measures tem-

perature. 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 resis-

tance or in colour. Examples of temperature mea-

suring devices include:

(i) liquid-in-glass thermometer, which uses the

expansion of a liquid with increase in temper-

ature 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 24,

page 267.

19.3 Specific heat capacity

Thespecific heat capacityof a substance is the

quantity of heat energy required to raise the temper-

ature of 1 kg of the substance by 1°C. The symbol

used for specific heat capacity iscand the units are

J/(kg°C) or J/(kg K). (Note that these units may also

be written as J kg−^1 °C−^1 or J kg−^1 K−^1 ).

Some typical values of specific heat capacity for

the range of temperature 0°C to 100°C include:

Water 4190 J/(kg°C), Ice 2100 J/(kg°C)

Aluminium 950 J/(kg°C), Copper 390 J/(kg°C)

Iron 500 J/(kg°C), Lead 130 J/(kg°C)

Hence to raise the temperature of 1 kg of iron by

1 °C requires 500 J of energy, to raise the tempera-

tureof5kgofironby1°C requires (500×5) J of

energy, and to raise the temperature of 5 kg of iron

by 40°C requires( 500 × 5 × 40 )J of energy, i.e.

100 kJ.

In general, the quantity of heat energy,Q,re-

quired to raise a massmkg of a substance with a

specific heat capacitycJ/(kg°C) from temperature

t 1 °Ctot 2 °C is given by:

Q=mc(t 2 −t 1 )joules