Heat energy and temperature
Over the past few centuries scientists
have put forward some very strange theories concerning the nature of heat. One of these
was that heat was some sort of fluid that you added to a body to make it hot and removed
from a body to cool it down! Whatever heat was, the result of its addition or removal was
clear - the temperature of the body rose or fell.
We must therefore consider the
change in temperature of a body to be related to the change in the heat content of that
body.
During the last century two men, Rumford and Joule, proposed that heat was
related to energy, indeed that heat was itself a form of energy. Davy showed that even cold
objects like blocks of ice could be melted if they were rubbed together. In 1843 Joule
performed his classic paddle wheel experiment, in which water was heated by friction from a
rotating paddle wheel driven by the loss of potential energy from a falling mass. We can
summarise their results as:
To heat up a body requires energy. This energy
increases the internal energy of the body by increasing the kinetic energy of its molecules,
and so the temperature of the body rises
Fixed
points
Standard reference temperatures (fixed points) are used when
calibrating thermometers.
Primary reference
temperatures
| Equilibium helium triple point |
-259.97 oC |
| Oxygen boiling point |
-182.962 oC |
| Water, triple point |
+0.01 oC |
| Water, boiling point (standard pressure) |
+100.00 oC |
| Freezing point of silver |
+960.5 oC |
| Freezing point of gold |
+1063 oC |
Secondary
reference temperatures
The following temperatures are used as practical fixed points in
different temperature ranges:
| Equilibium between gaseous and liquid oxygen |
-182.97 oC |
| Carbon dioxide sublimation point point |
-78.476 oC |
| Water, ice point |
0 oC (273.15 K) |
| Equilibrium between liquid sulphur and its vapour (standard pressure) |
+444.60 oC |
| Freezing point of aluminium |
+660.37 oC |
| Freezing point of copper |
+1084.5 oC |
| Freezing point of tungsten |
+3387 oC |
Student investigation
Devise experiments to measure the following temperatures:
(a) the temperature at which paper burns
(b) the temperature of the base of a domestic electric iron for various settings.
Questions
Suggest appropriate ways of measuring the following temperatures. Give examples of the difficulties that might be experienced in each measurement.
(a) The temperature of the human body.
(b) The temperature of the surface of the Sun.
(c) The temperature of liquid helium.
(d) The temperature of the exhaust gases in a jet engine.
(a) The temperature of the plasma in a fusion reactor.
(f) The melting point of gold.
(g) The change in temperature as water is slowly added to anhydrous copper sulphate.
