Comprehensive notes, solved MCQs, and Short Questions for Class 9 Physics Chapter 8 Thermal Properties of Matter. Covers Temperature, Heat, Specific Heat, Latent Heat, and Thermal Expansion.
Matter: Matter is composed of very small particles called molecules that are always in motion. Their motion may be vibrational, rotational, or linear.
Intermolecular Force: There exists a mutual force of attraction between the molecules known as intermolecular force. This force depends upon the distance between the molecules; it decreases with increasing distance.
Kinetic Energy & Temperature: Molecules possess kinetic energy due to motion and potential energy due to forces of attraction. When a substance is heated, its temperature rises, and molecular motion becomes more vigorous, increasing the kinetic energy. Thus, temperature depends on the average kinetic energy of molecules.
Definition: Plasma is a state of matter where atoms are ionized, consisting of positive ions and free electrons moving freely.
Formation: At very high temperatures, collisions between atoms become so violent that they tear electrons off atoms. It is the conducting state of matter found in stars (like the Sun), lightning streamers, and neon discharge tubes.
Temperature: The degree of hotness or coldness of a body. It determines the direction of heat flow.
Heat (Thermal Energy): The energy transferred from one body to another to thermal contact due to a temperature difference. It flows from a hotter body to a colder one.
Internal Energy: The sum of kinetic energy (due to motion) and potential energy (due to attractive forces) of all molecules in a substance. Heating increases internal energy, usually raising temperature.
Definition: A device used to measure temperature.
Principle: Uses a thermometric property of a substance that changes uniformly with temperature (e.g., thermal expansion of liquid, change in electrical resistance).
Ideal Thermometric Liquid Properties:
Construction: Consists of a glass stem with a narrow capillary tube and a bulb filled with mercury or alcohol.
Working: Liquid expands on heating and rises in the capillary. The level indicates temperature.
Mercury vs Alcohol: Mercury is commonly used due to its high boiling point (357°C) and low freezing point (-39°C). Alcohol is used for very low temperatures (freezing point -112°C).
Scales are defined by two fixed points: Lower Fixed Point (Ice Point) and Upper Fixed Point (Steam Point).
Sensitivity: Ability to detect small temperature changes. Improved by a narrower bore or larger bulb.
Range: The span between the minimum and maximum measurable temperatures.
Linearity: A direct proportional relationship between temperature and the thermometric property (e.g., equal expansion for equal temp rise), ensuring an evenly spaced scale.
Structure: Consists of two wires of different metals (e.g., copper and iron) joined at two junctions.
Working: When junctions are at different temperatures, a potential difference (voltage) is generated, causing current to flow. The voltage magnitude depends on the temperature difference.
Application: Suitable for measuring very high temperatures and rapidly changing temperatures due to low thermal mass.