Class 9 Physics | Punjab Curriculum and Textbook Board Syllabus 2025
Matter is composed of very small particles called molecules, which are always in motion. Their motion may be vibrational, rotational, or linear. A mutual force of attraction called intermolecular force exists between the molecules. This force depends on the distance between molecules and decreases as the distance increases.
In solids, the intermolecular forces are very strong, so molecules are held at fixed positions, showing only vibrational motion. That is why solids have a definite shape and volume.
In liquids, the intermolecular forces are weaker, allowing molecules to slide over each other in random directions. Therefore, liquids have a definite volume but no definite shape, and take the shape of the container.
In gases, the molecules are far apart and the intermolecular forces are very weak. Gas molecules move freely in all directions, so a gas has no definite shape or volume.
Plasma is a gas in which most of the atoms are ionized, containing positive ions and electrons. These particles move freely within the volume of the gas. Because of the presence of these charged particles, plasma is a conducting state of matter, allowing electric current to pass through it.
Temperature of a body is defined as the degree of its hotness or coldness.
(Alternate definition in terms of thermal energy): Temperature can be defined as a physical quantity which determines the direction of flow of thermal energy.
Heat is the energy which is transferred from one object to another due to the difference of temperature between the two bodies.
The sum of kinetic and potential energies of the molecules of an object is called its internal energy.
A thermometer is an instrument used for the exact measurement of the hotness or temperature of a substance.
Liquids expand on heating. So, expansion in the volume of a liquid can be used for the measurement of temperature. This is known as a liquid-in-glass thermometer.
For the measurement of temperature, two reference temperatures called fixed points are required.
This is the steam point, slightly above the boiling point of water at standard atmospheric pressure.
This is the melting point of pure ice, also known as the ice point.
Lower fixed point: 0℃; Upper fixed point: 100℃.
Lower fixed point: 32℉; Upper fixed point: 212℉.
Lower fixed point: 273 K; Upper fixed point: 373 K.
Absolute zero is the temperature at which molecular motion ceases, and the average kinetic energy of particles becomes zero. On the Kelvin scale, it is defined as 0 K, which is equivalent to -273.15°C (rounded to -273℃ for calculations). It is the lowest possible temperature in the universe.
Thermocouple thermometer is based on the flow of electric current between two junctions of two wires of different materials due to difference of temperatures at the junctions.
Sensitivity of a thermometer refers to its ability to detect small changes in the temperature of an object.
This refers to the span of temperature, from low to high, over which the thermometer can measure accurately.
This refers to a direct proportional relationship between the temperature and scale reading across the entire range of measurement.
Temperature difference determines the direction of heat flow. Heat always flows from a body at higher temperature to one at lower temperature.
Mercury is preferred because it has a uniform expansion, is opaque and shiny (making it easy to read), does not stick to glass, and remains in liquid state over a wide temperature range.