Thermal Properties of MatterIntroduction
Riya's village lohar is fitting a new iron tyre on a bullock-cart wheel. The ring is a few millimetres too small for the wooden rim, and that is on purpose. He lays it in a bed of burning charcoal, waits, then lifts it with long tongs and drops it over the rim. A splash of water, a hiss of steam, and the ring is locked on so tightly that no nail is needed. Riya wants to know how a fire can make solid iron bigger.
That question is where this chapter starts. We will pin down what temperature and heat really are, how a thermometer measures one of them, and what heat does to matter: it makes things expand, warms them by different amounts, melts and boils them, and travels from place to place in three different ways.
Drag the ring's temperature and watch its diameter on the graph.
Ring temperature:
The blue line shows the ring's diameter growing steadily with temperature. It meets the dashed rim line at the purple dot, near 197 °C. Heating made the iron only about 0.2% bigger, but that was just enough. Cooling on the wheel, the ring tried to shrink back and squeezed the rim with an enormous force. This is thermal expansion, and in Example 10.2 you will work out that temperature yourself.
The Plan for This Chapter
First we separate two words that everyday speech mixes up, temperature and heat, and see how a thermometer turns hotness into a number on the Celsius, Fahrenheit and Kelvin scales. Gases lead us to the ideal-gas equation and to absolute zero, the coldest anything can be. Then we follow what heat does to matter: thermal expansion, warming measured by specific heat capacity, mixing in calorimetry, and melting and boiling, where heat goes in with no change in temperature at all. Finally we see how heat travels, by conduction, convection and radiation, and how fast a hot cup cools, by Newton's law of cooling.
A pan of water on the stove has reached 100 °C and is boiling hard. You turn the flame up. What happens to the temperature of the water while it keeps boiling?
It stays at 100 °C. The extra heat turns more water into steam instead of making the water hotter. The section on change of state shows where that heat goes.