Knowledge That Ignites
Original, Copyright-Free Notes aligned with Maharashtra Board syllabus
Complete coverage of thermal concepts, expansion, and heat transfer
Heat is a form of energy that flows from a body at higher temperature to a body at lower temperature. It is one of the most fundamental concepts in physics and plays a crucial role in our daily lives, influencing everything from weather patterns to cooking food.
Heat energy is responsible for various observable effects in matter. Understanding heat is essential not only for scientific knowledge but also for practical applications in engineering, technology, and everyday life.
Heating increases temperature, cooling decreases it. This affects our comfort, cooking, and industrial processes.
Most substances expand when heated, which is considered in construction, engineering, and manufacturing.
Heat can change solids to liquids (melting) and liquids to gases (vaporization), fundamental to many natural and industrial processes.
Heat always flows from hotter to colder objects
Although often used interchangeably in everyday language, heat and temperature are distinct concepts in physics. Understanding their difference is fundamental to studying thermal physics.
Definition: Energy in transit due to temperature difference
Nature: Form of energy
Flow Direction: Always from hotter to colder body
SI Unit: Joule (J)
Depends On: Mass, specific heat, temperature change
Analogy: Like total amount of water in a container
Definition: Degree of hotness or coldness of a body
Nature: Measure of thermal state
Flow Direction: Doesn't flow, it's a property
SI Unit: Kelvin (K)
Depends On: Average kinetic energy of particles
Analogy: Like level of water in a container
Scenario: A cup of boiling water and a bathtub of warm water
Temperature is measured using instruments called thermometers. Different temperature scales have been developed for various applications, each with its own reference points and unit divisions.
Inventor: Anders Celsius (1742)
Reference Points:
Usage: Most countries except USA, scientific work
Inventor: Daniel Fahrenheit (1724)
Reference Points:
Usage: USA, some Caribbean countries
Inventor: Lord Kelvin (1848)
Reference Points:
Usage: Scientific work (SI unit)
Celsius to Kelvin:
Kelvin to Celsius:
Celsius to Fahrenheit:
Fahrenheit to Celsius:
Problem: Convert 25°C to Kelvin and Fahrenheit.
Solution:
Result: 25°C = 298 K = 77°F
When heat is supplied to a substance, it can produce various observable effects. These effects form the basis for understanding thermal phenomena and have numerous practical applications.
Description: Increase in the degree of hotness of a body
Molecular Explanation: Heat energy increases the kinetic energy of molecules, making them move faster
Quantified by: Specific heat capacity (amount of heat needed to raise temperature)
Examples: Water heating on stove, metal getting hot in sun
Description: Increase in dimensions (length, area, volume) when heated
Molecular Explanation: Increased molecular motion causes molecules to occupy more space
Quantified by: Coefficients of linear, areal, and volume expansion
Examples: Railway tracks expand in summer, hot air balloons rise
Description: Transformation from one state of matter to another
Molecular Explanation: Heat overcomes intermolecular forces, allowing change in arrangement
Quantified by: Latent heat (heat needed for state change without temperature change)
Examples: Ice melting, water boiling, dry ice subliming
Most substances expand when heated and contract when cooled. This property, known as thermal expansion, occurs because increased thermal energy causes particles to vibrate more vigorously and occupy more space.
Definition: Increase in length when heated
Formula: ΔL = αL₀ΔT
Where:
Example: Railway tracks, bridges
Definition: Increase in area when heated
Formula: ΔA = βA₀ΔT
Where:
Example: Metal plates, glass windows
Definition: Increase in volume when heated
Formula: ΔV = γV₀ΔT
Where:
Example: Liquids, gases, solids
Problem: A steel rail is 10 m long at 20°C. If the coefficient of linear expansion of steel is1.2 × 10⁻⁵ /°C, how much will it expand when heated to 50°C?
Solution:
The rail will expand by 3.6 mm when heated from 20°C to 50°C.
Gaps left between rails to allow for expansion in summer, preventing buckling.
Liquid (mercury/alcohol) expands in a narrow tube, showing temperature change.
Expansion joints allow for thermal expansion and contraction.
Hot rivets contract when cooled, creating tight joints in metal structures.
Bimetallic strips bend when heated, completing circuit to trigger alarm.
Heating metal lid expands it more than glass, making it easier to open.
Matter exists in three primary states: solid, liquid, and gas. Change of state refers to the transformation from one state to another when heat is added or removed. These changes occur at specific temperatures and involve energy transfer.
Properties: Fixed shape and volume
Particle Arrangement: Regular, closely packed
Motion: Vibrational only
Properties: Fixed volume, takes container shape
Particle Arrangement: Random, less closely packed
Motion: Vibrational + rotational + translational
Properties: No fixed shape or volume
Particle Arrangement: Very far apart, completely random
Motion: High speed in all directions
Definition: Change from solid to liquid
Temperature: Melting point (fixed for pure substances)
Energy: Absorbs heat (latent heat of fusion)
Examples: Ice to water, butter melting, wax melting
Special Cases:
Definition: Change from liquid to gas throughout liquid
Temperature: Boiling point (depends on pressure)
Energy: Absorbs heat (latent heat of vaporization)
Examples: Water boiling, alcohol boiling
Special Cases:
Definition: Slow change from liquid to gas at surface
Temperature: Occurs at any temperature
Energy: Absorbs heat from surroundings (cooling effect)
Examples: Drying clothes, sweat cooling, puddles drying
Factors Affecting:
| Process | Change | Heat | Example |
|---|---|---|---|
| Freezing | Liquid → Solid | Releases heat | Water to ice |
| Condensation | Gas → Liquid | Releases heat | Dew formation, clouds |
| Sublimation | Solid → Gas | Absorbs heat | Dry ice, naphthalene balls |
| Deposition | Gas → Solid | Releases heat | Frost formation |
| Aspect | Evaporation | Boiling |
|---|---|---|
| Temperature | Occurs at any temperature | Occurs at fixed boiling point |
| Location | Only at surface | Throughout the liquid |
| Rate | Slow process | Fast process |
| Bubbles | No bubbles formed | Bubbles formed throughout |
| Cooling Effect | Causes cooling | No cooling effect |
| Examples | Drying clothes, sweat cooling | Water boiling, cooking |
Latent heat is the heat energy required to change the state of a substance without changing its temperature. The term "latent" means "hidden" because this heat doesn't cause temperature rise but is absorbed or released during state changes.
Observation: During state changes (B and D), temperature remains constant while heat is absorbed.
Definition: Heat required to change 1 kg of solid into liquid at its melting point without temperature change
Formula: Q = m × Lf
Where:
Examples:
Definition: Heat required to change 1 kg of liquid into vapor at its boiling point without temperature change
Formula: Q = m × Lv
Where:
Examples:
Problem: How much heat is required to melt 2 kg of ice at 0°C? (Latent heat of fusion of ice = 3.34 × 10⁵ J/kg)
Solution:
Using formula: Q = m × Lf
Q = 2 kg × 3.34 × 10⁵ J/kg
Q = 6.68 × 10⁵ J = 668,000 J
Thus, 668,000 J of heat is needed to melt 2 kg of ice at 0°C.
Specific heat capacity is a fundamental property of substances that determines how much heat energy is required to raise the temperature of a unit mass by one degree. Different substances have different abilities to store thermal energy.
Where:
Definition of Specific Heat Capacity:
The amount of heat required to raise the temperature of 1 kg of a substance by 1°C (or 1 K).
| Substance | Specific Heat Capacity (J/kg°C) | Significance |
|---|---|---|
| Water | 4186 (approximately 4200) | Very high; excellent for temperature regulation |
| Ice | 2100 | About half of liquid water |
| Aluminium | 900 | High for a metal; used in cookware |
| Iron/Steel | 450 | Moderate; heats up relatively quickly |
| Copper | 385 | Low; excellent conductor of heat |
| Lead | 130 | Very low; heats up very quickly |
| Air (at constant pressure) | 1005 | Moderate; affects weather patterns |
Problem: How much heat is required to raise the temperature of 5 kg of water from 20°C to 80°C? (Specific heat of water = 4200 J/kg°C)
Solution:
Thus, 1.26 million joules of heat is needed.
Scenario: Equal masses of water and copper heated with same heat source
| Aspect | Water | Copper |
|---|---|---|
| Specific Heat | 4200 J/kg°C | 385 J/kg°C |
| Heating Rate | Slow (high specific heat) | Fast (low specific heat) |
| Cooling Rate | Slow to cool down | Fast to cool down |
| Practical Implication | Good for storing heat | Good for transferring heat |
Heat can be transferred from one place to another through three distinct mechanisms: conduction, convection, and radiation. Each method operates differently and is dominant in different situations and materials.
Mechanism: Direct transfer through collisions between adjacent particles
Medium Required: Solid materials (best in metals)
Particle Movement: Particles vibrate but don't change position
Example: Metal spoon in hot soup gets hot at handle
Mechanism: Transfer by actual movement of heated substance
Medium Required: Fluids (liquids and gases)
Particle Movement: Particles move from one place to another
Example: Hot air rises, cool air sinks (room heating)
Mechanism: Transfer by electromagnetic waves
Medium Required: No medium needed (works in vacuum)
Particle Movement: No particle movement involved
Example: Heat from Sun reaching Earth
Conduction is the transfer of heat through a material without any net movement of the material itself. It occurs when faster-moving particles collide with slower-moving particles, transferring kinetic energy.
Metal cooking utensils have metal handles that conduct heat:
Convection is the transfer of heat by the actual movement of the heated substance (fluid - liquid or gas). It involves bulk movement of molecules from one place to another.
Radiation is the transfer of heat by electromagnetic waves, primarily infrared radiation. Unlike conduction and convection, radiation doesn't require any medium and can travel through vacuum.
The Sun's energy reaches Earth through radiation:
Materials vary greatly in their ability to conduct heat. This property determines whether a material is suitable for applications requiring rapid heat transfer (conductors) or heat retention (insulators).
Definition: Materials that allow heat to pass through easily
Properties:
Examples:
Uses: Cooking utensils, heat exchangers, radiators
Definition: Materials that don't allow heat to pass through easily
Properties:
Uses: Cooking utensils, heat exchangers, radiators
Definition: Materials that don't allow heat to pass through easily
Properties:
Examples:
Uses: Handles of utensils, thermal insulation, clothing
Setup: Rods of different materials (copper, iron, aluminium, wood) with wax attached at ends are heated at one end.
Observation: Wax melts at different distances from heat source:
Conclusion: Different materials have different thermal conductivities.
| Material | Thermal Conductivity (W/mK) | Rank as Conductor |
|---|---|---|
| Silver | 429 | Best |
| Copper | 401 | Very Good |
| Aluminium | 237 | Good |
| Iron | 80 | Moderate |
| Glass | 1.0 | Poor |
| Wood | 0.1 | Very Poor |
| Air | 0.024 | Excellent Insulator |
Understanding heat transfer mechanisms allows us to design systems for heating, cooling, and insulation. These principles are applied in everyday objects, buildings, and industrial processes.
Principle: Conduction
Metal bottoms for good heat conduction; wooden/plastic handles as insulators.
Principle: Conduction + Convection
Wool traps air (poor conductor), reducing heat loss from body in winter.
Principle: All three methods minimized
Double walls with vacuum (stops conduction/convection); silvered surfaces (reduce radiation).
Principle: Conduction + Convection
Insulating materials in walls/roofs reduce heat transfer, saving energy.
Principle: Conduction + Convection
Metal fins conduct heat from engine; airflow carries heat away by convection.
Principle: Conduction + Convection
Insulated walls minimize heat entry; coolant circulates absorbing heat.
Principle: Radiation + Conduction
Black pipes absorb solar radiation; water circulates transferring heat.
Principle: Convection
Natural cooling near coasts due to differential heating of land and sea.
Heat Transfer:
Latent Heat:
Linear Expansion:
Temperature Conversion:
Heat Capacity:
Volume Expansion: