In thermodynamics, heat and temperature are distinct but fundamentally interrelated concepts. While temperature describes the thermal state of a single body, heat describes the process of energy transfer between two or more bodies.
Kinetic Theory of Matter
According to the Kinetic Theory, all matter is composed of continuously moving atoms or molecules.
- Microscopic View: These particles possess kinetic energy due to their random motion (linear translation, rotation, and vibration).
- Thermal Energy: The total kinetic energy of all the particles inside a sample of matter is defined as its thermal energy.
Concept of Temperature
Temperature is a macroscopic, statistical measure of the average kinetic energy of the constituent particles in a substance. It determines the degree of hotness or coldness of an object.
Key Characteristics of Temperature
- Direction of Flow: Temperature is a intensive property (independent of the mass of the substance). It determines the direction of spontaneous heat flow, which always occurs from a body at a higher temperature to a body at a lower temperature.
- Thermal Equilibrium: When two bodies in contact reach the same temperature, net heat transfer stops, achieving thermal equilibrium. This forms the basis of the Zeroth Law of Thermodynamics.
Temperature Measurement Scales
Temperature is measured using thermometers calibrated to specific scales based on fixed reproducible points (like the freezing and boiling points of water).
| Scale | Symbol | Freezing Point of Water | Boiling Point of Water | Number of Divisions |
| Celsius | °C | 0°C | 100°C | 100 |
| Fahrenheit | °F | 32°F | 212°F | 180 |
| Kelvin (SI) | K | 273.15 K | 373.15 K | 100 |
Temperature Scale Conversion Formula
The mathematical relationship used to convert temperatures between different scales is derived from their fixed points:
Absolute Zero
Absolute Zero (0 K or -273.15°C) is the lowest theoretically possible temperature. At this point, the translational kinetic energy of molecules becomes zero, and molecular motion ceases entirely. The Kelvin scale is an absolute scale because it begins at absolute zero and contains no negative values.
Concept of Heat
Heat is the form of energy that transfers across a boundary between thermodynamic systems due to a temperature difference. It exists only as energy in transit and cannot be stored by a system as “heat”. Once absorbed, it converts into internal energy (kinetic or potential energy of the molecules).
Key Characteristics of Heat
- Extensive Property: Heat energy depends on the mass, material, and temperature change of the substance.
- Sign Convention: In thermodynamics, heat added to a system is taken as positive (+Q), whereas heat lost or extracted from a system is taken as negative (-Q).
Units of Heat Energy
- Joule (J): The standard SI unit of heat and all forms of energy.
- Calorie (cal): The amount of heat required to raise the temperature of 1 gram of water by 1°C (specifically from 14.5°C to 15.5°C).
- Mechanical Equivalent of Heat (J): Established by James Prescott Joule, it connects mechanical work (W) and heat energy (Q) via the relation W = JQ, where J ≈ 4.186 J/cal.
Key Differences Between Heat and Temperature
| Parameters | Heat | Temperature |
| Definition | Energy transferred due to a temperature gradient. | Measure of the average kinetic energy of molecules. |
| Nature | It is energy in transit (a path function). | It is a thermal state of matter (a state function). |
| Property Type | Extensive property (depends on mass). | Intensive property (independent of mass). |
| SI Unit | Joule (J) | Kelvin (K) |
| Measuring Device | Calorimeter | Thermometer |
| Capability | Can perform mechanical work directly. | Cannot perform work directly; only indicates thermal state. |
Specific Heat Capacity and Latent Heat
The absorption or release of heat by a system results in either a change in temperature or a change in the physical state (phase) of the matter.
Specific Heat Capacity (c)
Specific heat capacity is the amount of heat required to raise the temperature of a unit mass of a substance by one degree Celsius or Kelvin.
- Mathematical Formula: Q = m · c · Δ T (where m is mass, c is specific heat, and Δ T is change in temperature).
- SI Unit: J/kg·K or J/kg°C.
- Water’s High Specific Heat: Water has an exceptionally high specific heat capacity (≈ 4186 J/kg·K or 1 cal/g°C). This enables oceans to regulate global climates and makes water an excellent industrial coolant.
Molar Heat Capacity (C)
Molar heat capacity is the amount of heat required to raise the temperature of one mole of a substance by one degree. It is expressed as C = Q/n·Δ T, where n is the number of moles. Gases have two distinct molar heat capacities: at constant volume (Cv) and at constant pressure (Cp).
Latent Heat (L)
Latent heat (“hidden heat”) is the heat absorbed or released by a substance during a change in its physical state without any change in its temperature. The energy is utilized entirely to break or form intermolecular bonds.
- Mathematical Formula: Q = m · L
- Latent Heat of Fusion (Lf): Heat required to change a substance from solid to liquid at its melting point. For ice, it is 3.33 × 105 J/kg (80 cal/g).
- Latent Heat of Vaporization (Lv): Heat required to change a substance from liquid to gas at its boiling point. For water, it is 2.26 × 106 J/kg (540 cal/g).
Modes of Heat Transfer
Heat transfers spontaneously from high-temperature zones to low-temperature zones via three distinct mechanisms.
Conduction
Conduction is the transfer of heat through direct molecular collisions and the migration of free electrons within a medium, without any bulk movement of the matter itself.
- Medium Requirement: Requires a material medium; occurs most efficiently in solids (especially metals due to free electrons).
- Fourier’s Law: The rate of heat flow (Q/t) is directly proportional to the cross-sectional area (A) and the temperature gradient (Δ T/Δ x).
Convection
Convection is the transfer of heat through the actual bulk movement of the fluid (liquid or gas) molecules themselves.
- Natural Convection: Driven by buoyancy forces resulting from density differences caused by temperature variations (e.g., land and sea breezes).
- Forced Convection: Fluid is forced to move by external agents like pumps or fans (e.g., human blood circulation system, car radiators).
Radiation
Radiation is the transfer of heat energy via electromagnetic waves (specifically infrared waves) without requiring any material medium.
- Speed: Travels at the speed of light (3 × 108 m/s) through a vacuum.
- Governing Laws: * Prevost’s Theory of Exchanges: All bodies at all temperatures above absolute zero continuously emit and absorb thermal radiation.
- Stefan-Boltzmann Law: The total energy radiated per unit surface area of a black body per unit time is directly proportional to the fourth power of its absolute temperature (E ∝ T4).
Fact-Rich Trivia for UPSC Prelims
- Triple Point of Water: The unique temperature and pressure at which the solid, liquid, and gaseous phases of water coexist in thermodynamic equilibrium. It occurs at exactly 273.16 K (0.01°C) and a pressure of 611.657 Pa.
- Anomalous Expansion of Water: Unlike most substances that expand upon heating, water contracts when heated from 0°C to 4°C. Consequently, water attains its maximum density at 4°C. This anomaly allows aquatic life to survive at the bottom of frozen lakes during winter.
- Cryogenics: The study of production and behavior of materials at ultra-low temperatures (below -150°C or 123 K). It is crucial for satellite launch vehicles using liquid hydrogen and liquid oxygen as propellants.
- Pyrometer: A non-contact thermometer used to measure extremely high temperatures (such as the surface of the Sun or molten metals) by detecting the intensity of emitted radiation. Standard thermometers cannot be used here as they would melt.
- Why Steam Burns are More Severe: Steam at 100°C causes much more severe burns than boiling water at 100°C because steam contains an additional 540 calories of latent heat of vaporization per gram, which is released upon condensing on the skin.