Lesson 1 of 13 · 6 min
Temperature, heat and thermometer scales
NCERT §10.1, §10.2, §10.3
Tara is packing in Chennai for a winter week at her Nani's house in a village near Manali. Her mother reads a fever thermometer before they leave: 37 °C, all clear. Her cousin in the hills texts back that his old thermometer says 98.6. Both are right.
The story this chapter follows: Tara's winter week near Manali
The lesson in notes
In short
Temperature says how hot or cold a body is, in numbers. Touch alone is a poor guide: skin can judge only roughly, so a measuring device is needed.
A glass of iced water warms up on a hot day and a cup of hot tea cools down on the same table. In both cases energy moves from the hotter body to the colder one.
Heat is the energy that passes between two systems, or between a system and its surroundings, because their temperatures differ. Its SI unit is the joule (J).
The SI unit of temperature is the kelvin (K); the degree Celsius (°C) is the everyday unit. Heating a body can raise its temperature, make it expand or change its state.
A thermometer uses some property of a material that changes with temperature. Liquid-in-glass thermometers use the volume of mercury or alcohol, which changes nearly linearly with temperature over a wide range.
A standard scale needs two fixed reference points. Every material changes size with temperature, so the fixed points are tied to events that always recur at one temperature: pure water freezing (ice point) and boiling (steam point) under standard pressure.
On the Fahrenheit scale the ice and steam points are 32 °F and 212 °F, with 180 equal divisions between them. On the Celsius scale they are 0 °C and 100 °C, with 100 divisions.
A graph of t_F against t_C is a straight line, which gives (t_F − 32)/180 = t_C/100, or t_F = (9/5)t_C + 32.
A change of 1 °C equals a change of 1.8 °F. Body temperature 37 °C is (9/5)(37) + 32 = 98.6 °F; −40 °C and −40 °F are the same temperature.