Grade 10 Chapter 5: Heat

Introduction

Heat is a form of energy that flows from a body at a higher temperature to a body at a lower temperature. It plays a crucial role in our daily lives, from cooking food to powering engines. This chapter explores the nature of heat, its effects, methods of heat transfer, and various applications of heat energy.

Nature of Heat

Heat as Energy

Heat is a form of energy associated with the random motion of particles in a substance. When a substance is heated: - The particles (atoms or molecules) gain energy - They move faster or vibrate more vigorously - The average kinetic energy of the particles increases

Difference Between Heat and Temperature

Although related, heat and temperature are different concepts:

• Heat: The total energy of molecular motion in a substance; measured in joules (J) or calories (cal)
• Temperature: A measure of the average kinetic energy of the particles in a substance; measured in degrees Celsius (°C), Kelvin (K), or Fahrenheit (°F)

An analogy: If we compare heat to the amount of water in a container, temperature would be like the water level. A large container with a low water level (large body at low temperature) can contain more water (heat) than a small container with a high water level (small body at high temperature).

Temperature Scales

Three common temperature scales are:

1. Celsius Scale (°C):
2. Water freezes at 0°C and boils at 100°C at standard atmospheric pressure

3. Commonly used in everyday life and scientific work

4. Kelvin Scale (K):

5. The SI unit of temperature
6. Absolute zero (the lowest possible temperature) is 0 K, which equals -273.15°C
7. No negative values on this scale

8. Conversion: K = °C + 273.15

9. Fahrenheit Scale (°F):

10. Water freezes at 32°F and boils at 212°F at standard atmospheric pressure
11. Commonly used in the United States
12. Conversion: °F = (°C × 9/5) + 32

Effects of Heat

Thermal Expansion

When a substance is heated, it generally expands. This phenomenon is called thermal expansion.

Linear Expansion

When a solid rod is heated, its length increases. The increase in length depends on: - Original length (L) - Change in temperature (ΔT) - Coefficient of linear expansion (α)

The formula for linear expansion is: ΔL = L × α × ΔT

Where: - ΔL is the change in length - L is the original length - α is the coefficient of linear expansion (different for different materials) - ΔT is the change in temperature

Volume Expansion

When a substance is heated, its volume increases. For solids and liquids, the formula for volume expansion is: ΔV = V × β × ΔT

Where: - ΔV is the change in volume - V is the original volume - β is the coefficient of volume expansion (β ≈ 3α for solids) - ΔT is the change in temperature

Anomalous Expansion of Water

Water exhibits an unusual property: it contracts when heated from 0°C to 4°C and then expands when heated above 4°C. This is called the anomalous expansion of water.

Consequences of this anomaly: - Water has maximum density at 4°C - Ice floats on water because water expands when it freezes - Lakes freeze from the top, allowing aquatic life to survive underneath

Applications and Consequences of Thermal Expansion

1. Thermometers: Mercury and alcohol expand when heated, allowing temperature measurement
2. Bimetallic strips: Used in thermostats and circuit breakers
3. Expansion joints: Gaps left in bridges, railway tracks, and buildings to allow for expansion
4. Fitting metal rims on wooden wheels: The metal rim is heated, expanded, placed on the wheel, and then cooled to contract and fit tightly
5. Cracking of glass: Uneven heating causes different parts to expand at different rates, creating stress that can crack the glass

Heat Capacity and Specific Heat Capacity

Heat Capacity

Heat capacity is the amount of heat energy required to raise the temperature of an entire object by one degree. It is measured in joules per kelvin (J/K) or joules per degree Celsius (J/°C).

Specific Heat Capacity

Specific heat capacity is the amount of heat energy required to raise the temperature of one kilogram of a substance by one degree. It is measured in joules per kilogram per kelvin (J/kg·K) or joules per kilogram per degree Celsius (J/kg·°C).

The formula for calculating heat energy is: Q = m × c × ΔT

Where: - Q is the heat energy in joules (J) - m is the mass in kilograms (kg) - c is the specific heat capacity in J/kg·K - ΔT is the change in temperature in kelvin (K) or degrees Celsius (°C)

Specific heat capacities of some common substances: - Water: 4,186 J/kg·°C - Ice: 2,090 J/kg·°C - Steam: 2,010 J/kg·°C - Aluminum: 900 J/kg·°C - Iron: 450 J/kg·°C

Water has a relatively high specific heat capacity, which means it can absorb or release a large amount of heat with relatively small changes in temperature. This property makes water an excellent coolant and moderator of climate.

Change of State

Matter can exist in three states: solid, liquid, and gas. The change from one state to another is called a change of state or phase change.

Types of Phase Changes

1. Melting (Fusion): Solid → Liquid
2. Occurs at the melting point
3. Requires heat energy (endothermic process)

4. Temperature remains constant during melting

5. Freezing (Solidification): Liquid → Solid

6. Occurs at the freezing point (same as melting point)
7. Releases heat energy (exothermic process)

8. Temperature remains constant during freezing

9. Vaporization: Liquid → Gas

10. Can occur as evaporation (surface phenomenon) or boiling (throughout the liquid)
11. Requires heat energy (endothermic process)

12. Temperature remains constant during boiling

13. Condensation: Gas → Liquid

14. Occurs when a gas is cooled below its boiling point
15. Releases heat energy (exothermic process)

16. Temperature remains constant during condensation

17. Sublimation: Solid → Gas (directly)

18. Occurs in substances like dry ice (solid CO₂) and iodine

19. Requires heat energy (endothermic process)

20. Deposition: Gas → Solid (directly)

21. Occurs when water vapor forms frost
22. Releases heat energy (exothermic process)

Latent Heat

Latent heat is the heat energy absorbed or released during a change of state without a change in temperature.

1. Latent Heat of Fusion: The amount of heat energy required to convert 1 kg of a solid into liquid at its melting point without a change in temperature.

2. For water: 334,000 J/kg or 334 kJ/kg

3. Latent Heat of Vaporization: The amount of heat energy required to convert 1 kg of a liquid into gas at its boiling point without a change in temperature.

4. For water: 2,260,000 J/kg or 2,260 kJ/kg

The formula for calculating heat energy during a phase change is: Q = m × L

Where: - Q is the heat energy in joules (J) - m is the mass in kilograms (kg) - L is the latent heat in joules per kilogram (J/kg)

Heat Transfer

Heat can be transferred from one place to another through three mechanisms: conduction, convection, and radiation.

Conduction

Conduction is the transfer of heat through a substance without the movement of the substance itself. It occurs primarily in solids.

Process: 1. Particles at the hotter end vibrate more vigorously 2. They transfer energy to neighboring particles through collisions 3. This process continues, transferring heat from the hotter end to the colder end

Factors affecting the rate of conduction: - Temperature difference (higher difference, faster conduction) - Cross-sectional area (larger area, faster conduction) - Length of the conductor (shorter length, faster conduction) - Material of the conductor (metals are good conductors, non-metals are poor conductors or insulators)

The formula for the rate of heat conduction is: Q/t = (k × A × ΔT) / L

Where: - Q/t is the rate of heat transfer in watts (W) or joules per second (J/s) - k is the thermal conductivity of the material in W/m·K - A is the cross-sectional area in square meters (m²) - ΔT is the temperature difference in kelvin (K) or degrees Celsius (°C) - L is the length of the conductor in meters (m)

Convection

Convection is the transfer of heat through the movement of a fluid (liquid or gas). It occurs in liquids and gases.

Process: 1. The fluid near the heat source gets heated and expands 2. The heated fluid becomes less dense and rises 3. Cooler, denser fluid moves in to take its place 4. This creates a continuous flow called a convection current

Natural convection is driven by buoyancy forces due to density differences. Forced convection is driven by external means like fans or pumps.

Examples of convection: - Sea and land breezes - Room heaters and radiators - Boiling water - Atmospheric circulation

Radiation

Radiation is the transfer of heat through electromagnetic waves. It does not require a medium and can occur in a vacuum.

Characteristics of thermal radiation: - All objects emit thermal radiation - Hotter objects emit more radiation than cooler ones - Dark, rough surfaces are better absorbers and emitters of radiation than light, smooth surfaces - Thermal radiation includes infrared radiation, which we feel as heat

The rate of heat radiation is given by the Stefan-Boltzmann law: P = e × σ × A × T⁴

Where: - P is the power radiated in watts (W) - e is the emissivity of the surface (0 ≤ e ≤ 1) - σ is the Stefan-Boltzmann constant (5.67 × 10⁻⁸ W/m²·K⁴) - A is the surface area in square meters (m²) - T is the absolute temperature in kelvin (K)

Examples of radiation: - Heat from the sun reaching Earth - Heat from a fire warming your hands - Infrared lamps used in physiotherapy

Applications of Heat

Thermos Flask (Vacuum Flask)

A thermos flask is designed to minimize heat transfer through all three mechanisms: - Conduction: Minimized by using poor conductors like glass and plastic - Convection: Prevented by creating a vacuum between the double walls - Radiation: Reduced by using silvered surfaces that reflect radiation

Solar Heating

Solar heating systems use the sun's radiation to heat water or air: - Solar collectors absorb radiation - The heat is transferred to water or air through conduction and convection - The heated fluid is then used for space heating or hot water supply

Heat Engines

Heat engines convert heat energy into mechanical energy. Examples include: - Steam engines - Internal combustion engines - Gas turbines

The efficiency of a heat engine is given by: Efficiency = (Work output / Heat input) × 100%

The maximum theoretical efficiency is given by: Efficiency = (1 - T₂/T₁) × 100%

Where: - T₁ is the absolute temperature of the hot reservoir in kelvin (K) - T₂ is the absolute temperature of the cold reservoir in kelvin (K)

Refrigerators and Air Conditioners

Refrigerators and air conditioners are heat pumps that move heat from a cooler place to a warmer place (against the natural direction) using external work: - They use a refrigerant that evaporates at low temperatures - The evaporation absorbs heat from inside the refrigerator - The refrigerant is then compressed, raising its temperature - The hot refrigerant releases heat to the surroundings through condensation - The cycle repeats

Global Warming and Greenhouse Effect

Greenhouse Effect

The greenhouse effect is a natural process that warms Earth's surface: 1. The sun's radiation passes through the atmosphere and warms the Earth's surface 2. The Earth emits infrared radiation 3. Greenhouse gases in the atmosphere (CO₂, CH₄, N₂O, water vapor) absorb some of this radiation 4. This trapped heat warms the atmosphere and the Earth's surface

Global Warming

Global warming refers to the long-term increase in Earth's average temperature due to enhanced greenhouse effect: - Burning fossil fuels releases CO₂, increasing its concentration in the atmosphere - Deforestation reduces the absorption of CO₂ by plants - Industrial processes release other greenhouse gases

Consequences of global warming: - Rising sea levels due to melting ice caps and thermal expansion of water - Changes in precipitation patterns - More frequent and intense extreme weather events - Shifts in plant and animal ranges - Ocean acidification

Mitigation strategies: - Reducing fossil fuel consumption - Increasing use of renewable energy sources - Improving energy efficiency - Reforestation and afforestation - Carbon capture and storage

Summary

Heat is a form of energy that flows from higher to lower temperatures. It causes effects like thermal expansion and changes of state. Heat can be transferred through conduction, convection, and radiation. Understanding heat and its properties is essential for many applications, from everyday devices like thermometers and refrigerators to addressing global challenges like climate change.

Practice Questions

1. Explain the difference between heat and temperature.

2. A metal rod of length 1 m is heated from 20°C to 70°C. If the coefficient of linear expansion of the metal is 1.1 × 10⁻⁵ /°C, calculate the increase in length of the rod.

3. Why does water at 4°C sink to the bottom of a lake?

4. Calculate the heat energy required to raise the temperature of 2 kg of water from 20°C to 80°C. (Specific heat capacity of water = 4,186 J/kg·°C)

5. Explain why the temperature remains constant during the melting of ice, even though heat is being supplied continuously.

6. Compare and contrast the three methods of heat transfer: conduction, convection, and radiation.

7. How does a thermos flask minimize heat transfer through conduction, convection, and radiation?

8. Explain the greenhouse effect and its role in global warming.

9. Why are metals good conductors of heat while materials like wood and plastic are poor conductors?

10. A 500 g block of ice at 0°C is converted to water at 0°C. Calculate the heat energy required for this change. (Latent heat of fusion of ice = 334 kJ/kg)

References: 1. Maharashtra State Board 10th Standard Science Syllabus 2025-26 2. NCERT Science Textbook for Class 10 3. Thermal Physics: Concepts and Practice 4. Heat Transfer: Principles and Applications