Thermodynamics - Physics

Table of Contents
1. Definition
2. Laws of Thermodynamics
3. Heat Transfer (Introduction and Applications)
4. Perfectly Black Body
5. Memorable Points
6. Latent Heat
7. Temperature
8. Thermal Expansion
9. Anomalous Expansion of Water
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Definition

Thermodynamics is the branch of physics that studies the relations between heat, work, temperature and energy. It deals with the macroscopic behaviour of systems containing large numbers of particles, and formulates general laws that govern energy transfer and transformations.

Laws of Thermodynamics

First Law of Thermodynamics

The first law of thermodynamics is the law of conservation of energy applied to thermodynamic systems: energy can be transformed from one form to another, but it cannot be created or destroyed. For a closed system undergoing a process, the heat supplied to the system is used to change its internal energy and to do work on the surroundings. This is commonly expressed by the relation

Q = ΔU + W

where Q is the heat added to the system, ΔU is the change in internal energy and W is the work done by the system (sign convention may vary; the above is a commonly used convention in physics).

First Law of Thermodynamics 

The Second Law of Thermodynamics

The second law of thermodynamics places a direction on thermodynamic processes. It can be stated in several equivalent ways:

• Clausius statement: Heat cannot spontaneously flow from a colder body to a hotter body without external work being done.
• Kelvin-Planck statement: It is impossible to construct a heat engine that, operating in a cycle, produces no other effect than the absorption of heat from a reservoir and the performance of an equal amount of work.
• Entropy formulation: For an isolated system, the entropy either increases or remains constant; it never decreases. For a reversible process ΔS = ∫(δQ_rev / T); for an isolated irreversible process ΔS > 0.

Entropy is therefore a measure of irreversibility or the disorder of a system, and the second law explains why certain processes are irreversible in practice.

Heat Transfer (Introduction and Applications)

One important application of thermodynamics is the study of heat transfer - the transfer of thermal energy between bodies or within a body. Heat transfer appears in many engineering devices and natural phenomena, such as heat exchangers, evaporators, condensers, radiators, coolers, heaters, refrigeration systems, and environmental processes.

Second Law of Thermodynamics 

Modes of Heat Transfer

Heat is transferred by three principal modes: conduction, convection and radiation. Each mode has governing laws and typical engineering formulas.

1. Conduction

Conduction is heat transfer through a material without bulk motion of the material. The transfer occurs by microscopic collisions and energy exchange between particles or by free-electron transport in metals. The one-dimensional steady Fourier law for conduction is

q = -kA (dT/dx)

where q is the heat flow rate, k is the thermal conductivity of the material, A is the cross-sectional area and dT/dx is the temperature gradient. Examples: heat flow along a metal rod, heat loss through walls, cooking on a metal pan.

2. Convection

Convection is heat transfer by the bulk motion of a fluid (liquid or gas). Convection may be natural (driven by density differences caused by temperature gradients) or forced (driven by pumps, fans or winds). A commonly used approximate relation for convective heat transfer is Newton's law of cooling:

Q̇ = h A (T_surface - T_fluid)

where h is the convective heat transfer coefficient, A is the area and T's are temperatures. Examples: heating of a room by warm air, cooling of electronic components by forced air, chimney-driven flows, land and sea breezes.

3. Radiation

Radiation is the transfer of energy by electromagnetic waves; it does not require a material medium. All bodies emit thermal radiation according to their temperature and emissivity. The total power radiated per unit area of a perfect black body is given by the Stefan-Boltzmann law:

E = σ T⁴

where σ is the Stefan-Boltzmann constant and T is the absolute temperature (in kelvin). For real surfaces an emissivity factor ε (0 ≤ ε ≤ 1) modifies the expression: E = ε σ T⁴. Example: heat from the Sun reaches Earth by radiation; radiative heat exchange between hot and cold surfaces; thermal insulation by reflective surfaces.

Perfectly Black Body

A perfectly black body is an idealised object that absorbs all incoming electromagnetic radiation at all wavelengths and does not reflect or transmit any radiation. Such a body is a perfect emitter when heated and is often used as a standard for radiative heat transfer. 
Key facts:

• Black bodies absorb all incident radiation and therefore appear black when cold.
• A perfect black body, when at temperature T, emits radiation according to Planck's law and has a total emissive power given by the Stefan-Boltzmann law E = σ T⁴.
• Real objects are characterised by an emissivity ε (0 < ε ≤ 1); highly polished surfaces have low emissivity (they are poor absorbers and poor emitters but good.

Memorable Points

• All metals are generally good conductors of heat. Among metals, silver has the highest thermal conductivity.
• Good conductors of heat are frequently good conductors of electricity, because both heat and electrical conduction in metals are carried largely by free electrons.
• Mica is an electrical insulator and is used where electrical insulation is required; compared with many non-metallic insulators it can withstand heat, but it is not a metal-like conductor of electricity.
• Cooking utensils are provided with wooden or ebonite handles because wood and ebonite are poor conductors of heat and reduce the risk of burns.
• A metal object feels colder than a wooden object at the same temperature because the metal conducts heat away from the hand more rapidly.
• In a room, ventilators are often placed near the ceiling so that hot air, which rises by convection, can escape efficiently.
• The principle of a chimney (draught) is based on convection: hot gases rise and are replaced by cooler air.
• Land and sea breezes arise from differential heating and cooling of land and water and are examples of natural convection and pressure-driven flows.
• The upper part of a flame is hotter than its sides because rising hot gases carry heat upwards.
• Highly polished surfaces are weak absorbers and emitters of thermal radiation but good reflectors.
• If a vacuum flask (thermos) containing tea is shaken vigorously, the temperature of the tea may rise slightly because of internal friction and mixing that converts macroscopic mechanical energy to internal energy.
• If the door of a refrigerator is left open in a closed room, the room will not be cooled; the refrigerator draws heat from the room and rejects additional heat to the room via its condenser so the net effect is to increase room temperature.
• If the heat rejection portion of the refrigerator is placed outside a closed room (for example, the condenser outside), opening the refrigerator door can gradually cool the room.

Latent Heat

Latent heat is the amount of heat absorbed or released by a substance during a change of state without change in temperature. It is usually given per unit mass and there are two principal types:

(1) Latent Heat of Fusion

The latent heat of fusion of a substance is the amount of heat required to change unit mass of the substance from solid to liquid at the melting point, without change in temperature.

(2) Latent Heat of Vaporization

• The latent heat of vaporization is the quantity of heat required to change unit mass of a liquid into vapour at its boiling point, without change in temperature.
• The latent heat of vaporization of water is commonly quoted as approximately 536 cal/g (often approximated as 540 cal/g in many tables); the exact value depends on temperature and pressure.
• The latent heat of vaporization generally decreases as the temperature approaches the critical point; it also depends on pressure (latent heat decreases with increasing pressure if the boiling point rises).
• Steam at 100°C can cause more severe burns than water at 100°C because steam carries latent heat in addition to its sensible heat; when steam condenses on skin, it releases latent heat, increasing injury severity.
• The melting point of solids that contract on melting (for example, ice) decreases with increase in pressure, which is why ice melts under pressure; these solids often float on their liquid (ice on water).
• The melting point of substances that expand on melting (for example, wax, glass in certain conditions) tends to increase with pressure.
• Boiling point increases with increasing pressure (for a given substance).
• Supercooling is cooling a liquid below its normal freezing point without solidification (e.g., pure water can be supercooled below 0°C under suitable conditions).
• Superheating is heating a liquid above its boiling point without it turning to vapour; with disturbances (nucleation) it may then boil violently.

Temperature

Temperature is a measure of the average kinetic energy of the particles of a body and determines whether bodies are in thermal equilibrium. Two bodies in contact are in thermal equilibrium if no net heat flows between them; temperature is the property that is equal for bodies in thermal equilibrium (zeroth law of thermodynamics).

Mercury Thermometer

• Mercury thermometers commonly use two temperature scales: the Celsius scale (from 0°C, the freezing point of water, to 100°C, the boiling point of water at standard pressure) and the Fahrenheit scale (from 32°F to 212°F for the same reference points).
• The Celsius and Fahrenheit scales are related by the linear relation:

C = (5/9)(F - 32)

F = (9/5) C + 32

• In a mercury thermometer, mercury is contained in a bulb attached to a narrow capillary tube; mercury is used because it has a uniform thermal expansion over a wide temperature range, is visible, and has a suitable boiling point for many laboratory measurements.

Kelvin (Absolute) Scale

• On the kelvin or absolute scale the freezing point of water is 273.15 K and the boiling point of water (at standard atmospheric pressure) is 373.15 K.
• Conversion relations:

K = 273.15 + °C

°C = K - 273.15

• If °C = -273.15 then K = 0. This temperature is known as absolute zero, the theoretical point at which a classical description of thermal motion reaches its minimum.

Thermal Expansion

Thermal expansion is the tendency of matter to change its dimensions in response to a change in temperature. Expansion may occur in length, area or volume and is quantified by coefficients of expansion.

• Linear expansion: For a rod of length L, when temperature changes by ΔT the change in length ΔL is

ΔL = α L ΔT

• where α is the coefficient of linear expansion.
• Superficial (area) expansion: The fractional change in area is approximately 2α ΔT.
• Volume expansion: The fractional change in volume for isotropic solids and for most liquids is approximately 3α ΔT; the coefficient of volumetric expansion is often denoted by β or γ and ≈ 3α for solids.
• Applications and precautions: gaps in railway tracks, expansion joints in bridges and structures, bimetallic strips in thermostats, choice of materials in precision instruments.

Anomalous Expansion of Water

Most liquids expand on heating, but water shows anomalous behaviour between 0°C and 4°C: it contracts when heated from 0°C to 4°C and reaches a minimum volume (maximum density) at about 4°C. Above 4°C water expands on further heating.

This anomaly arises from the hydrogen-bonded structure of water; as ice melts and water warms from 0°C to 4°C, structural rearrangements allow molecules to occupy a smaller average volume. Consequences and examples:

• Ice is less dense than liquid water and therefore floats; this insulates lakes and ponds, allowing aquatic life to survive beneath the ice in winter.
• Because of this anomaly, the density of water is maximum at about 4°C (approximately 1.000 × 10³ kg m⁻³).
• The anomaly affects natural convection patterns, stratification of lakes and behaviour of freezing and thawing in the environment.
• Practical concerns include frost heave and the behaviour of pipelines and hydraulic systems in cold climates.

Summary 

This chapter summarises core thermodynamics concepts: conservation of energy (first law), directionality and entropy (second law), modes of heat transfer (conduction, convection, radiation), latent heat (fusion and vaporisation), temperature scales (Celsius, Fahrenheit, Kelvin), thermal expansion and the anomalous behaviour of water. These principles underpin engineering applications across civil, electrical and computer engineering fields - from heat exchangers and climate control to material selection and structural design.