Heat and Thermodynamics Notes for SSC Exams

Heat and Thermodynamics is one of the most important topics in the Physics part of SSC General Science. Questions are conceptual and use everyday examples, and many are repeated across years: the Celsius-Fahrenheit relation, thermometers, specific heat, modes of heat transfer and the laws of thermodynamics. These notes explain every concept in simple language with formulas, tables and practice questions, in the style of a Physics teacher revising the chapter before the exam.

Heat and Temperature

• 1 calorie = 4.186 joule (about 4.2 J). 1 kilocalorie = 1000 calories (the "Calorie" of food).
• Temperature is a measure of the average kinetic energy of molecules.

Temperature Scales

• Conversion: C/5 = (F - 32)/9 = (K - 273)/5.
• Celsius and Fahrenheit give the same reading at -40.
• Celsius and Kelvin differ by 273 only, so a change of 1 degree C is equal to a change of 1 K.
• The normal human body temperature is 37 degree C = 98.6 degree F.

Thermometers

• Mercury thermometer: mercury is used because it has a uniform expansion and is a good conductor. It cannot measure below -39 degree C (freezing point of mercury) so alcohol is used for very low temperatures.
• Clinical thermometer: reads 35 to 42 degree C (94 to 108 degree F), has a kink that prevents mercury from falling back so that the reading can be noted.
• Pyrometer: measures very high temperatures (like furnaces and the Sun) without touching the body, using radiation.
• Platinum resistance thermometer: used for a wide range of temperatures. Thermocouple: based on the Seebeck effect.
• Six's maximum-minimum thermometer records the highest and lowest temperature of a day.

Specific Heat, Heat Capacity and Calorimetry

• Heat capacity = heat needed to raise the temperature of the whole body by 1 degree C. Specific heat capacity (c) = heat needed to raise 1 kg of the substance by 1 degree C.
• Formula: Q = m c (T2 - T1).
• Specific heat of water is the highest among common liquids (4200 J/kg K or 1 cal/g degree C). Therefore water is used in hot-water bottles, radiators and as a coolant.
• Sea breeze and land breeze, and the moderate climate of coastal areas, are due to the high specific heat of water.
• Principle of calorimetry: heat lost by the hot body = heat gained by the cold body (no heat loss to the surroundings).

Change of State and Latent Heat

• Steam at 100 degree C causes more severe burns than boiling water at 100 degree C because of its higher latent heat.
• Ice at 0 degree C cools a drink better than water at 0 degree C because it absorbs latent heat of fusion.
• The boiling point increases with pressure (pressure cooker) and decreases with fall in pressure (cooking at high altitude is difficult).
• Impurities (like salt) lower the freezing point of water and raise its boiling point. Salt is spread on icy roads for this reason.
• Evaporation takes place at all temperatures from the surface and causes cooling (sweating, earthen pot). Boiling takes place at a fixed temperature throughout the liquid.

Thermal Expansion

• Solids expand in length (linear), area (superficial) and volume (cubical) on heating. Coefficients are related as beta = 2 alpha and gamma = 3 alpha.
• Gaps are left in railway tracks and in bridges to allow expansion. Telephone wires sag in summer.
• Liquids expand more than solids, and gases expand the most.
• Anomalous expansion of water: water contracts from 0 to 4 degree C and expands above 4 degree C. Hence water has maximum density at 4 degree C.

Modes of Heat Transfer

• Good conductors: silver (best), copper, aluminium. Poor conductors (insulators): wood, plastic, air, glass, wool.
• Stefan-Boltzmann law: the energy radiated by a black body is proportional to the fourth power of its absolute temperature (E is proportional to T^4).
• Wien's displacement law: the wavelength at maximum emission shifts to shorter values as temperature rises (a hotter star looks blue-white).
• Newton's law of cooling: the rate of cooling is proportional to the temperature difference between the body and the surroundings.
• Thermos flask (Dewar flask) stops all three modes: vacuum between double walls (conduction and convection) and silvered walls (radiation).
• Woollen clothes keep us warm because they trap air, which is a poor conductor.

Gas Laws

Laws of Thermodynamics

Thermodynamic Processes

• Isothermal: temperature constant. Isobaric: pressure constant. Isochoric (isometric): volume constant (no work done). Adiabatic: no heat exchange (Q = 0).
• A heat engine converts heat into work. Efficiency = work done / heat absorbed. The Carnot engine has efficiency = 1 - T2/T1.
• A refrigerator takes heat from a cold body and gives it to the surroundings with the help of work. Its working substance is a refrigerant such as freon.

Humidity and Other Important Points

• Humidity is the water vapour content of air, measured by a hygrometer. Relative humidity is the ratio of vapour present to the maximum vapour the air can hold at that temperature. Dew point is the temperature at which air becomes saturated and dew forms.
• The triple point of water is 273.16 K (0.01 degree C).
• Greenhouse effect: gases like CO2 and methane trap the heat radiated by the Earth and warm the atmosphere.
• Heat is transferred from the Sun to the Earth by radiation as there is no medium in space.
• The temperature of the Sun's surface is about 5500 degree C and of the core about 15 million degree C.

Units and Quick Revision

Practice Questions With Answers

1. At what temperature is the reading same on the Celsius and Fahrenheit scales? Answer: -40
2. Which liquid has the highest specific heat among common liquids? Answer: Water
3. At what temperature is water the densest? Answer: 4 degree C
4. Heat from the Sun reaches the Earth by: Answer: Radiation
5. Which metal is the best conductor of heat? Answer: Silver
6. Which law of thermodynamics defines temperature? Answer: Zeroth law
7. What is the SI unit of temperature? Answer: Kelvin
8. Which thermometer is used to measure the temperature of a furnace? Answer: Pyrometer
9. What is the efficiency of a Carnot engine working between 400 K and 300 K? Answer: 25 percent (1 - 300/400)
10. Camphor disappears slowly in air because of: Answer: Sublimation

Revise the other physics topic in Physics Mechanics notes, and practise with our previous year papers. For science questions of the General Science subject see the General Science notes.

Frequently Asked Questions

What is the difference between heat and temperature?

Heat is a form of energy that flows from a hot body to a cold body, measured in joule or calorie. Temperature is the degree of hotness or coldness of a body and tells the direction of heat flow, measured in degree Celsius, Fahrenheit or kelvin.

What is the SI unit of heat and temperature?

The SI unit of heat is joule (J) and the SI unit of temperature is kelvin (K). One calorie is equal to about 4.186 joule.

How do you convert Celsius to Fahrenheit and Kelvin?

Use F = (9/5) x C + 32 and K = C + 273 (more precisely 273.15). The common formula is C/5 = (F - 32)/9 = (K - 273)/5. For example, 100 degree C = 212 degree F = 373 K.

At what temperature are the Celsius and Fahrenheit readings equal?

At -40 degrees both the Celsius and the Fahrenheit scales show the same reading.

What is absolute zero?

Absolute zero is the lowest possible temperature, 0 K or -273.15 degree C, at which the molecular motion of an ideal gas theoretically stops. It cannot be reached in practice (third law of thermodynamics).

What is specific heat capacity?

It is the amount of heat needed to raise the temperature of 1 kg of a substance by 1 K (or 1 degree C). Water has a very high specific heat of about 4200 J/kg K, which is why it is used as a coolant and why coastal areas have a moderate climate.

What is latent heat?

Latent heat is the heat absorbed or given out by a substance during a change of state at constant temperature. The latent heat of fusion of ice is about 334 kJ/kg (80 cal/g), and the latent heat of vaporisation of water is about 2260 kJ/kg (540 cal/g).

What are the three modes of heat transfer?

Conduction (in solids, through direct contact of particles), convection (in liquids and gases, by movement of the fluid) and radiation (through electromagnetic waves, no medium needed). The Sun's heat reaches the Earth by radiation.

Why do dark surfaces get hotter in sunlight?

Dark and rough surfaces are good absorbers (and good emitters) of radiation, while light and shiny surfaces are good reflectors. This is why we wear light-coloured clothes in summer.

What is the anomalous expansion of water?

Between 0 degree C and 4 degree C, water contracts on heating instead of expanding, so water has its maximum density at 4 degree C. This keeps lakes from freezing at the bottom in winters and protects aquatic life.

What does the zeroth law of thermodynamics state?

If two bodies are each in thermal equilibrium with a third body, they are in thermal equilibrium with each other. This law forms the basis of temperature measurement.

What does the first law of thermodynamics say?

It is the law of conservation of energy for heat: the heat supplied to a system is equal to the increase in its internal energy plus the work done by it, that is Q = delta U + W.

What does the second law of thermodynamics say?

Heat cannot flow by itself from a colder body to a hotter body, and no heat engine can convert all the heat into work. In any natural process the entropy of the universe increases.

What is a Carnot engine and its efficiency?

A Carnot engine is an ideal reversible heat engine working between two temperatures T1 (source) and T2 (sink), with efficiency 1 - T2/T1 (temperatures in kelvin). No real engine can be more efficient than a Carnot engine.

Why does a pressure cooker cook food faster?

The raised pressure inside the cooker increases the boiling point of water above 100 degree C, so food cooks faster at a higher temperature.

Which heat and thermodynamics topics are asked in SSC exams?

Temperature scales, thermometers, specific and latent heat, modes of heat transfer, expansion of solids and water, laws of thermodynamics, evaporation and humidity, and everyday applications. These notes cover all of them with tables and MCQs.