Support
By Grant Laidlaw
GRANT LAIDLAW
Grant Laidlaw is currently the owner of the Air Conditioning and Refrigeration Academy ( ACRA ) in Edenvale . He holds a Bachelor of Business Administration and an associate degree in educational administration . He has a National Technical Diploma and completed an apprenticeship with Transnet . He has dual-trades status : refrigeration and electrical . He has been involved with SAIRAC for over two decades and served on the Johannesburg committee as chairman and was also president between 2015 and 2018 . Currently he is the SAIRAC national treasurer .
UNDERSTANDING THE BASICS
Getting to grips on the difference between energy , heat and temperature .
Stefan asks : Grant , Thank you for your contributions , we find that many people working in the field do not understand the basics , for example energy , heat and temperature . When I even mention latent heat the wheels come off . Perhaps this would be of value in getting some basic understanding in place . Thanks .
Hi Stefan , yes what you say is correct . Let us then go back to basics - starting with temperature .
The temperature of a body is how hot or how cold the body is ; it is measured with a thermometer . In South Africa it is typically indicated in degrees Celsius (° C ).
The two fixed points on the thermometer are 0 ° C for the temperature at which pure water will freeze and 100 ° C for the temperature at which water will boil at average sea level atmospheric pressure .
Temperature is a measure of the heat intensity of a body . It is a function of the internal kinetic energy and as such is an index of the average velocity of the molecules . Thus the colder a body is , the less the movement of the molecules and the lower the internal energy .
Scientists have calculated the point at which there will be no movement of molecules and thus no internal energy . This point is called the absolute zero temperature ( -273 o C ). The unit of absolute temperature is the Kelvin ( K ). Its symbol is T .
The customary temperature ( t ) is given in degrees Celsius ( o C ).
When expressing a temperature interval or a temperature difference , both units can be used ( K or o C ). Note that when K is used , the o sign is not used .
For instance if the outside air temperature is 35 ° C and the temperature inside a room is 25 ° C the temperature difference = 35 ° C - 25 ° C = 10K .
Note : Temperature and internal energy must not be confused . A body can have a very high temperature but , due to its low mass , have a very low internal energy .
Temperature provides an indication of the direction in which internal energy flows . When two bodies are in contact , internal energy flows from the body at higher temperature to the body at lower temperature regardless of the amount of internal energy in each one . ( The energy flowing is called heat .) Conversion of temperatures . The conversion of customary temperature to absolute temperature and vice versa are as follows : Degrees C = degrees Kelvin - 273 , or ° C = K – 273 . Degrees Kelvin = degrees Celsius + 273 , or K = ° C + 273 .
HEAT Heat is a form of energy . It can be made to do work . An example is the steam engine where the heat energy produces steam , which can be used to produce work ; or in a petrol engine , heat is used to expand the fuel / air mixture , which pushes down the piston , which moves the car .
Any substance or body has internal energy due to the movement of molecules . The rate of movement depends on its temperature . When energy is transferred due to temperature difference , the energy is called heat .
Thus heat is energy that flows . It simply describes an energy transfer resulting from a temperature difference . If there is no temperature difference , there is no sensible heat transfer . Heat energy is given the symbol Q . To indicate a rate of heat transfer , a dot is placed over the symbol .
. Thus : Q = heat transfer rate .
Heat being a form of energy is also expressed in Joules ( J ) or kilojoules ( kJ ). Heat transfer is Joules per second , which equals Watts .
1 J / s = 1Watt – and - 1 kJ / s = 1kW .
SENSIBLE AND LATENT HEAT Let ’ s look at water being changed from ice to water and water to water vapour .
Continued on page 43
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