This part of the web document deals with energy and temperature.





Energy is found in Nature in many different forms, A basic definition of energy is 'the capacity to do work', and the different forms of energy can often be converted from one to another. The SI unit of energy is the Joule (symbol J). Like the Newton and Hertz, this unit is named after a famous scientist and its symbol is capitalized. In everyday life, you are more likely to meet another, non-SI unit of energy, the calorie (cal). Food labelling shows the energy content of different foodstuffs, usually in units of kcal (1000 cal - also sometimes written confusingly as Calorie). One calorie is just over 4 Joules, and is equivalent to the heat energy needed to raise the temperature of 1 gramme of water by 1°C (see below). So both the Joule and calorie are quite small units.


In biosciences you will meet not only measurements of the quantity of energy, but also the rate at which energy is used or transferred. The Watt (symbol W) is an energy change or transfer of 1 Joule per second (1 J s-1 - see the section on Compound units). Again, we often meet larger or smaller units in everyday life. For instance, the rating of domestic electrical heaters and ovens is given in kilowatts (1 kW = 1000 W).


Various forms of energy may have their own special units, often relating to the effects of the energy source. These include light energy in relation to photosynthetic growth or other biological response, and the effect of exposure to radioactive decay.




Temperature scales define how hot or cold objects are. They are established by giving values to the temperatures of specific physical events, such as the freezing or melting points of pure materials. In everyday life, we now use the Celsius temperature scale (sometimes incorrectly called Centigrade). The 'fixed points' for this scale are the freezing point of pure water (0°C) and the boiling point of pure water (100°C).


You may also meet so-called 'absolute' temperatures. These have importance for measurement of physical processes. The unit on the absolute temperature scale is called a Kelvin (symbol K, not °K). One Kelvin represents the same temperature change as one Celsius degree, but the fixed points are different. Thus the freezing point of water (0°C) has a temperature of 273.15 K, whilst the boiling point of water (100°C) is 373.15 K. A temperature of 0 K is commonly called 'absolute zero', at which a physical system neither emits or absorbs energy.


Temperatures on the Fahrenheit scale are still used sometimes, for instance alongside the Celsius scale in TV weather forecasts, and remain commonplace in everyday life in the US. The corresponding fixed points are: freezing point of water = 32°F and boiling point of water = 212°F. Unlike absolute scale, the Fahrenheit degrees are not the same interval as Celsius degrees (1 Celsius degree = 1.8 Fahrenheit degrees).