All 15 short questions of 9th Class Physics Chapter 7, Thermal Properties of Matter, with exam-ready answers in the book's words. Punjab Board new syllabus.
Exam-ready answers. Learn the meaning, then write them in your own words.
In a solid the intermolecular forces are very strong. They hold the molecules at fixed positions, where the molecules can only vibrate about their mean positions. Since the molecules cannot move from place to place, a solid keeps a definite shape and a definite volume.
Gas molecules are far apart and the forces between them are very weak. They move freely and randomly in all directions and spread out to fill whatever container they are in. So a gas has no fixed shape and no fixed volume.
| Solid | Liquid | Gas |
|---|---|---|
| Very close together, at fixed positions | Close together, but free to slide over each other | Far apart |
| Strongest forces | Weaker forces | Very weak forces |
Its molecules move faster and more vigorously, so their average kinetic energy increases. The liquid usually expands, and if heated enough the fastest molecules escape and it evaporates or boils.
The degree of hotness or coldness of a body. It decides the direction of heat flow and is a measure of the average kinetic energy of its molecules. SI unit: kelvin (K).
The energy that is transferred from one body to another because of a temperature difference between them.
It is called energy in transit because it exists as heat only while it is flowing; once it reaches the colder body it becomes part of that body's internal energy.
A property of a substance that changes appreciably and regularly with temperature, so it can be used to measure temperature.
| Scale | Ice point | Steam point |
|---|---|---|
| Celsius (centigrade) | 0 °C | 100 °C |
| Fahrenheit | 32 °F | 212 °F |
| Kelvin (absolute) | 273 K | 373 K |
The ability of a thermometer to detect small changes in temperature. A thermometer with marks 0.1 °C apart is more sensitive than one with marks 1 °C apart.
A direct proportional relationship between the change in temperature and the change in the scale reading over the whole range: equal rises in temperature move the thread by equal lengths.
The scale must be linear: equal changes in temperature must give equal increments on the scale, so the markings are evenly spaced over the whole range. This needs a capillary of uniform bore and a liquid that expands uniformly, like mercury.
Temperature. Heat always flows from the body at higher temperature to the body at lower temperature, whatever their sizes, and stops when their temperatures become equal.
| Heat | Internal energy |
|---|---|
| Energy transferred from a hot body to a cold body because of a temperature difference | Sum of the kinetic and potential energies of all the molecules of a body |
| Exists only while flowing (energy in transit) | Stored in the body all the time |
| Depends on a temperature difference | Depends on the temperature, mass and state of the body |
Energy travels from your hand to the cold surface, because your hand is at the higher temperature. There is no such thing as "cold" flowing; the hand feels cold because it is losing energy.
No, not reliably. The hand and the forehead are at nearly the same raised temperature, so the hand feels little difference. Touch only gives a rough idea of hot and cold and cannot measure temperature; a clinical thermometer is needed.
