9th Class Physics Unit No.8 Thermal Properties of Matter Notes
9th Class Physics Unit 8 Notes on Thermal Properties of Matter, Short questions, MCQ’s and numerical solution.
Temperature and Heat
Temperature of a body indicates its degree of hotness or coldness.
Heat is the energy transferred from one body to another due to the temperature difference between them.
The sum of kinetic and potential energies associated with the atoms, molecules, and particles of a body is its internal energy.
Thermometer
A device used to measure the temperature of a body.
Thermometric materials change with temperature, and liquids are commonly used.
Mercury-in-glass thermometers are widely used in laboratories and clinics.
Temperature Scales
Celsius scale: Interval between 0°C (lower fixed point) and 100°C (upper fixed point) is divided into 100 equal parts.
Fahrenheit scale: Interval between 32°F (lower fixed point) and 212°F (upper fixed point) is divided into 180 equal parts.
Kelvin scale: Interval between 273K (lower fixed point) and 373K (upper fixed point) is divided into 100 equal parts. Absolute zero is 0K.
- Chapter No.1 Introduction to Biology
- Chapter No. 2 Solving a Biological Problem
- Chapter No.3 Biodiversity
- Chapter No.4 Cells and Tissues
- Chapter No.5 Cell Cycle
Conversion of Temperature
From Celsius to Kelvin: Add 273 to the Celsius temperature.
From Kelvin to Celsius: Subtract 273 from the Kelvin temperature.
From Celsius to Fahrenheit: Use the equation F = (C × 1.8) + 32.
From Fahrenheit to Celsius: Use the equation C = (F – 32) ÷ 1.8.
Specific Heat Capacity
Specific heat capacity (c) is the amount of heat required to raise the temperature of 1 kg mass of a substance by 1K.
The equation is Q = mcΔT, where Q is the heat absorbed, m is the mass, c is the specific heat capacity, and ΔT is the temperature change in Kelvin.
Specific heat capacities of some common substances are listed in Table 8.1.
Specific heat capacity is an essential property used in various applications, such as cooling systems in engines, heating systems, and cooking processes. The knowledge of temperature and heat, along with specific heat capacity, helps us understand and manage thermal processes efficiently.
The large specific heat capacity of water plays a crucial role in various natural processes and practical applications. Specific heat is the amount of heat energy required to raise the temperature of a unit mass of a substance by one degree Celsius (or one Kelvin).
The specific heat capacity of water is 4200 Jkg-1K-1, while that of dry soil is only about 810 Jkg-1K-1. This significant difference in specific heat capacity has several important implications:
Climate Regulation: Bodies of water, such as lakes and seas, have a tremendous capacity to store thermal energy due to their high specific heat. During the day, water absorbs and stores heat from the sun, preventing the surrounding areas from getting too hot. At night, water releases the stored heat, moderating the temperature and keeping the surroundings warmer. As a result, places near large water bodies experience milder temperature variations between summer and winter compared to areas far from the sea.
Thermal Buffering: The high specific heat of water makes it an excellent coolant in various systems. In automobiles, water is used in the cooling system to carry away excess heat generated by the engine. Water absorbs the thermal energy and dissipates heat through the radiator, preventing the engine from overheating.
Central Heating Systems: Hot water is used in central heating systems to carry thermal energy through pipes from the boiler to radiators inside buildings. The water releases heat to warm up the living spaces, providing a comfortable environment during colder months.
Climate Stability: The presence of large bodies of water helps stabilize the climate in coastal regions. The water’s ability to absorb and release heat creates more stable and pleasant weather conditions, contributing to a more habitable environment.
Thermal Regulation in Organisms: Water’s high specific heat capacity also plays a vital role in the thermal regulation of living organisms. Bodies of water in the environment help moderate temperature fluctuations, providing a more stable habitat for aquatic life and supporting diverse ecosystems.
Overall, the large specific heat capacity of water is crucial for maintaining stable environments, supporting life, and enabling various essential processes in both the natural world and human-made systems.
Evaporation is the process of changing a liquid into vapors (gaseous state) from the surface of the liquid without the need for heating. Unlike boiling, which occurs at a specific fixed temperature (the boiling point), evaporation can take place at any temperature as long as the liquid’s molecules have enough kinetic energy to escape from the surface.
During evaporation, the molecules of the liquid are in constant motion and possess kinetic energy. Some of these fast-moving molecules manage to escape from the surface of the liquid and enter the atmosphere as vapor. This process continues until equilibrium is reached, and the rate of evaporation equals the rate of condensation.
Several factors influence the rate of evaporation
Temperature: Higher temperatures provide more kinetic energy to the liquid molecules, increasing their escape rate and speeding up evaporation.
Surface Area: Evaporation occurs only from the surface of the liquid. A larger surface area allows more molecules to escape, increasing the evaporation rate.
Wind: Moving air (wind) sweeps away the vapor molecules near the surface, allowing more liquid molecules to evaporate. This is why wet clothes dry faster on a windy day.
Nature of the Liquid: Different liquids have different rates of evaporation based on their molecular interactions and forces. For example, liquids with weaker intermolecular forces evaporate faster.
Importance of Evaporation
Cooling Effect: Evaporation causes cooling. As faster molecules escape from the liquid’s surface, the average kinetic energy of the remaining molecules decreases, resulting in a decrease in temperature. This cooling effect is commonly experienced when sweating cools down our bodies in hot weather.
Drying of Wet Objects: Wet objects, such as clothes, dry up rapidly due to evaporation. When wet surfaces are exposed to air, the water molecules on the surface evaporate, causing the object to dry over time.
Natural Water Cycle: Evaporation plays a vital role in the water cycle. Water bodies, such as oceans, lakes, and rivers, continuously undergo evaporation, leading to the formation of clouds and subsequent precipitation.
In summary, evaporation is a natural process that helps regulate temperature, dry wet objects, and is a fundamental part of the water cycle, contributing to the Earth’s climate and weather patterns.
Why does heat flow from hot body to cold body?
Heat flows from a hot body to a cold body because of the second law of thermodynamics, which states that heat naturally flows from regions of higher temperature to regions of lower temperature. This is due to the fact that heat is a form of energy, and energy tends to disperse or spread out. When two bodies at different temperatures are in contact or connected, the hotter body has higher thermal energy, and the energy spontaneously moves to the colder body until both reach thermal equilibrium.
Define the terms heat and temperature.
Heat: Heat is the energy transferred between two substances or objects due to a temperature difference. It flows from a hotter object to a cooler one until thermal equilibrium is reached.
Temperature: Temperature is a measure of the average kinetic energy of the molecules or particles in a substance. It indicates how hot or cold an object is and determines the direction of heat flow.
What is meant by internal energy of a body?
The internal energy of a body refers to the total kinetic and potential energy possessed by its particles or molecules. It includes the energy associated with the motion and interactions of the microscopic constituents of the body. The internal energy is responsible for the temperature of the body and is related to its thermal energy.
How does heating affect the motion of molecules of a gas?
When a gas is heated, its molecules gain kinetic energy, causing them to move faster. The increased kinetic energy leads to more vigorous and rapid movement of gas molecules. As a result, the pressure of the gas increases due to the increased collisions of molecules with the walls of the container.
What is a thermometer? Why mercury is preferred as a thermometric substance?
A thermometer is a device used to measure temperature. It consists of a narrow, uniform tube containing a liquid (thermometric substance) that expands or contracts with changes in temperature. The temperature is determined by reading the level of the liquid in the tube.
Mercury is preferred as a thermometric substance because it has several advantageous properties, such as high thermal expansion, visible meniscus, and wide temperature range before freezing. It also adheres well to the glass tube, ensuring accuracy and consistency in readings.
Explain the volumetric thermal expansion.
Volumetric thermal expansion refers to the increase in volume of a substance due to an increase in its temperature. When a substance is heated, its molecules gain kinetic energy, leading to increased movement and more space between them. This results in an expansion of the substance, causing an increase in its volume. The volumetric expansion is characterized by the coefficient of volume expansion (β), which measures the fractional change in volume per unit change in temperature.
Define specific heat. How would you find the specific heat of a solid?
Specific heat is the amount of heat energy required to raise the temperature of a unit mass (usually one gram) of a substance by one degree Celsius (or one Kelvin).
To find the specific heat of a solid, you can perform an experiment called the method of mixtures. It involves taking a known mass of the solid at a known initial temperature and immersing it in a known mass of water at a known initial temperature. The heat lost by the solid equals the heat gained by the water, and by measuring the temperature change, you can calculate the specific heat of the solid using the formula: Specific heat = (mass of water × specific heat of water × temperature change of water) / (mass of solid × temperature change of solid).
Define and explain latent heat of fusion.
Latent heat of fusion is the amount of heat energy required to change a unit mass of a solid substance into a liquid state without any change in temperature. During fusion, the solid absorbs heat energy to overcome the intermolecular forces holding its molecules together in a regular pattern. The heat energy breaks the forces, and the molecules start sliding past each other, transitioning into the liquid state. The latent heat of fusion is released when the liquid solidifies back into a solid.
Define latent heat of vaporization.
Latent heat of vaporization is the amount of heat energy required to change a unit mass of a liquid substance into vapor or gas without any change in temperature. During vaporization, the liquid absorbs heat energy to overcome the intermolecular forces and change its state to gas. The latent heat of vaporization is released when the gas condenses back into a liquid state.