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Unit of Heat - SI Unit, CGS Unit & Other Heat Units, FAQs

Unit of Heat - SI Unit, CGS Unit & Other Heat Units, FAQs

Edited By Vishal kumar | Updated on Jul 02, 2025 04:50 PM IST

Introduction
Heat is a form of energy possessed by a body, also called thermal energy of the body. The S.I unit of heat is Joule (J) just like all other forms of energy. It flows from one body to another or from one system to another having different temperatures. Hence both heat and temperature are very closely related although their concepts are not the same. Temperature is measured in Kelvin which is the S.I unit. In fact Kelvin is also the C.G.S unit of temperature. Heat is measured in C.G.S unit also in addition to the S.I unit of heat energy. C.G.S unit of heat is also very commonly used. C.G.S unit of heat is Calorie (Cal). Another unit of heat is B.T.U and the B.T.U full form is British Thermal Unit. It is more of a traditional unit of heat which is still used in Britain as well as in U.S.A in some fields. Generally the symbol for heat is represented by Q. Another important term related to heat is heat capacity. The heat capacity unit is Joule per Kelvin. We will discuss heat capacity later in the article.

Unit of Heat - SI Unit, CGS Unit & Other Heat Units, FAQs
Unit of Heat - SI Unit, CGS Unit & Other Heat Units, FAQs

How do we define heat and where does it come from?

There are three states of matter- solids, liquids and gases, each of which are made up of molecules. These molecules possess kinetic energy due to their translational motion, vibrational motion and rotational motion (see fig 2).

All the 3 forms are made up of tiny molecules having random motion.The motion gets restricted as we go from gases to solids(Fig-1)


The figure shows translational,rotational and vibrational motion of the molecules(Fig-2)

The kinetic energy associated with these random motions of molecules and the configuration of these molecules with respect to each other (potential energy) is called the internal energy of the matter and the heat energy is part of this internal energy.

So we can define heat energy as a part of internal energy which flows between two bodies or systems when they have difference in temperature between them and it flows from higher temperature to the body at lower temperature.

Examples- 1) When we put a cup of hot coffee in the table, it gets cold in a matter of time. This is because there is difference in temperature between the coffee and the air around it. Thus the heat flows from coffee to the atmosphere.

heat transferred from hot coffee to surrounding air(Fig-3)

2) When there is friction between two bodies heat energy is produced. For example rubbing our hands together. In this case the mechanical energy is transformed into heat energy.

here mechanical movement of hands transforms to heat due to friction. (Fig-4)

3) If we compress a gas, the molecules collide with each other more often which increases their internal energy (pressure is increased). Hence the gas gets hotter due to the heat energy produced. See the figure below.

In the figure, the gas is shown to be contained in a cylindrical containerwhere a piston is used to compress it.(Fig-5)

Thus heat energy either comes from internal energy of the body or is produced when any another form of energy like mechanical energy, chemical energy or electrical energy gets transformed into heat energy.

Also read -

Units of heat

S.I. unit of heat is Joules (J).James Prescott Joule found that when mechanical work (W) is converted to heat energy (Q), W by Q ratio comes out to be constant which is represented by unit joule.

Background wave

i.e. W/Q=J

Dimensional formula for joules is (kgm2s-2) –> [ML2T-2].

C.G.S. unit of heat is Calorie (Cal).

1 Cal= 4.186 J

We can define 1 calorie of heat energy as the quantity of heat energy required to raise temperature of 1 gram of water by 1 ̊C.

However, in B.T.U. - 1 btu =1055.05585 J

How temperature and heat are closely related?

Temperature and heat is not the same thing although they are very closely related. In fact, the measurement of heat is called temperature. It measures the degree i.e. how cold or how hot the body is.

Example- When we have a high fever, we use thermometer that actually records how much our body is heated up, that is the temperature of the body.

thermometer(Fig-6)

When any two bodies of different temperatures are in contact with each other, heat flows from the body at higher temperature to the body at lower temperature and this heat flow continues till their temperatures become equal. This is called temperature equilibrium. Thus we can define temperature equilibrium between two bodies as follows- two bodies are said to be in thermal equilibrium if no heat is transferred between them when they are in contact. Based on this concept of thermal equilibrium, an important law of thermodynamic (study of dynamic/flow of heat) called Zeroth law of thermodynamics is there which states that if 2 bodies A and B are in thermal equilibrium and B and C are in thermal equilibrium, then A and C are also in thermal equilibrium.

Note-Temperature is one of the seven fundamental quantities along with length, time, mass, ampere, mole and candela (used to measure luminous intensity).

Units of temperature

Generally, a thermometer is used to measure temperature. However different types of thermometers are there to be used in different situations. Other than thermometers, we also use thermocouples, infrared sensors, silicon diodes, change of state sensors etc. There are 2 fixed reference points while measuring temperature- freezing point of water and boiling point of water.

The different units of measurement are-

  1. Celsius scale ( ̊ C) - (Designed by Celsius) In this scale freezing point is 0 ̊ C and boiling point is 100 ̊C. Between these 2 points, there are 100 equal divisions.
  2. Fahrenheit scale ( ̊ F) - In this scale freezing point is 32 ̊ F and boiling point is 212 ̊ F.
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Between these two points there are 180 divisions.

Relation between Celsius and Fahrenheit scale is- ̊ C = ( 5/9) ( ̊ F - 32) eq-(1)

  1. Kelvin scale (K) - This is the S.I. unit of thermodynamic temperature. At 0K it is absolute zero temperature. Therefore it is also called the absolute scale. On this scale the freezing point is 273.15K ~ 273K and the boiling point is 373K.

Relation between Celsius and Kelvin scale is – K = ̊ C + 273 eq-(2)

In both C.G.S. and M.K.S unit for temperature, we use Kelvin scale. See the figure below and compare the freezing and boiling points of water on different scales.

Temperature scales(Fig-7)

See the graph of pressure vs. temperature of a gas below. Upon extrapolating this graph it meets absolute zero at -273.15 ̊ C. At this point, the internal energy of the gas becomes zero. All molecular motions cease to exist and below this temperature, volume of the gas becomes negative which is impossible. Thus the lowest temperature one can reach is 0K. In fact, it is also practically not possible to reach absolute zero. The closest that scientists have reached to absolute zero is 150 Nano Kelvin but not 0K.

Pressure vs. Temperature graph of a gas

(Fig-7)

Remember - the S.I. unit of heat and temperature are J and kelvin respectively.

Also Read:

Heat capacity

Heat capacity is the amount of heat required to raise temperature of a gas by 1 ̊ C. However we usually use the quantity ‘specific heat capacity’ that is the amount of heat required to raise temperature of a unit mass of the substance by 1 ̊ C. Hence heat capacity is an intensive thermodynamic quantity, that is, it depends on mass of the substance while specific heat capacity is an extensive thermodynamic quantity as it doesn’t depend on mass. Mathematically we can find the formula for specific heat as-

Experiments show that if ΔQ is a small amount of heat required to raise temperature of ‘m’ mass of a substance through temperature ΔT, then we can write

ΔQ ∝ m and ΔQ ∝ ΔT

  • ΔQ ∝ mΔT
  • ΔQ=C(mΔT)
  • C=ΔQ/ ( mΔT), this is the formula for specific heat capacity.

Also check-

NCERT Physics Notes:

Frequently Asked Questions (FAQs)

1. At which temperature do we get the same reading in both Celsius and Fahrenheit scale?

Using equation (1), we put x in place of the temperature in degree Fahrenheit and degree Celsius and solve for x. We will get -40 as our answer.

2. What is a calorimeter?

A calorimeter is a device used for measuring the amount of heat absorbed or evolved during any process. It can also measure heat capacity for us.

3. What is molar heat capacity?

Molar heat capacity is the amount of heat required to be added to 1 mole of a substance to increase its temperature by 1 degree unit.

4. In which measuring scale of temperature, the rise in temperature is the smallest?

As there are 180 divisions in Fahrenheit scale and 100 divisions in Celsius scale and Kelvin scale. Thus 1 ̊F is smaller than the other two.

5. What is the molar heat capacity of water?

75.32J per mol per degree celsius

6. What is the SI unit of heat?
The SI unit of heat is the joule (J). It represents the amount of energy transferred as heat and is consistent with the fundamental principle that heat is a form of energy.
7. Why isn't calorie the SI unit of heat?
While the calorie was historically used to measure heat, it's not the SI unit because the joule is the standard unit for all forms of energy in the SI system. This ensures consistency across different branches of physics and simplifies calculations involving energy conversions.
8. How does the joule relate to other energy units?
One joule equals one newton-meter (N·m) of work. It's also equivalent to one watt-second (W·s) of power. These relationships highlight the interconnectedness of different energy concepts in physics.
9. What is the CGS unit of heat?
The CGS (centimeter-gram-second) unit of heat is the erg. One erg is equal to 10^-7 joules, making it a much smaller unit than the joule.
10. How does the calorie relate to the joule?
One calorie is approximately equal to 4.184 joules. This relationship allows for easy conversion between the two units and is important when dealing with nutritional information or older scientific literature.
11. What is the relationship between heat and temperature?
Heat is the transfer of thermal energy between systems, while temperature is a measure of the average kinetic energy of particles in a substance. Adding heat to a system generally increases its temperature, but they are distinct concepts.
12. Can heat be measured directly?
Heat, being energy transfer, cannot be measured directly. Instead, we measure its effects, such as temperature change or phase transitions. Calorimetry is a method used to indirectly measure heat transfer by observing these effects.
13. Why do different materials have different specific heat capacities?
Different materials have different atomic and molecular structures, affecting how they absorb and store thermal energy. This leads to varying specific heat capacities, which is why some substances heat up or cool down faster than others.
14. What's the difference between heat and internal energy?
Heat is energy transfer due to temperature difference, while internal energy is the total energy contained within a system. Heat can change a system's internal energy, but they are not the same thing. Both are measured in joules in the SI system.
15. Why is it important to distinguish between heat and temperature in physics?
Heat and temperature are often confused but are distinct concepts. Heat is energy transfer, measured in joules, while temperature is a measure of average particle kinetic energy, typically in Kelvin or Celsius. Understanding this distinction is crucial for accurate thermal analysis.
16. How does the concept of heat relate to energy conservation?
Heat, measured in joules, is a form of energy transfer. The principle of energy conservation states that energy cannot be created or destroyed, only converted. This fundamental law governs all heat transfer processes and is crucial for understanding thermal systems.
17. How does the unit of heat relate to the concept of heat death in cosmology?
The heat death of the universe is a theoretical end state where all energy is evenly distributed. Expressing this concept in terms of joules and entropy (J/K) allows cosmologists to model long-term universal evolution and explore concepts of time and thermodynamics on a cosmic scale.
18. What's the difference between a calorie and a kilocalorie?
A kilocalorie (kcal) is 1000 calories. Confusingly, the "Calorie" (with a capital C) used in nutrition is actually a kilocalorie. This distinction is crucial for accurate energy calculations in dietary and metabolic studies.
19. Why do we still use calories in some contexts if joules are the SI unit?
Calories are still used in nutrition and some fields of chemistry due to historical precedent and public familiarity. However, scientific literature increasingly uses joules to maintain consistency with the SI system.
20. What is the British Thermal Unit (BTU)?
The BTU is a non-SI unit of heat, defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. It's commonly used in the United States for rating air conditioning and heating systems.
21. How does the BTU compare to the joule?
One BTU is approximately equal to 1055 joules. This conversion is important when working with heating and cooling systems, especially when comparing international standards.
22. What is specific heat capacity and how is it related to the unit of heat?
Specific heat capacity is the amount of heat required to raise the temperature of one unit mass of a substance by one degree. It's typically expressed in J/(kg·K) in SI units, directly relating to the joule as the unit of heat.
23. How does the unit of heat relate to the concept of latent heat?
Latent heat is the energy absorbed or released during a phase change without temperature change. It's measured in the same units as heat (joules in SI), representing the energy needed to break or form intermolecular bonds.
24. How does the unit of heat relate to the laws of thermodynamics?
The unit of heat (joule) is fundamental to the laws of thermodynamics. It's used to quantify energy transfer in the First Law, and entropy changes (measured in J/K) in the Second Law, linking heat to the broader concepts of energy and disorder.
25. How does the concept of heat capacity differ from specific heat capacity?
Heat capacity is the amount of heat required to raise an object's temperature by one degree, measured in J/K. Specific heat capacity is heat capacity per unit mass (J/(kg·K)). The distinction is important when comparing objects of different sizes or masses.
26. What is the significance of the unit of heat in studying phase changes?
The unit of heat (joule) is crucial in studying phase changes as it quantifies the energy required for transitions like melting or vaporization. This energy, called latent heat, doesn't change the temperature but alters the material's phase.
27. How does the choice of heat unit affect calculations in thermodynamics?
Using SI units (joules) for heat simplifies calculations in thermodynamics, especially when dealing with work and other forms of energy. It eliminates the need for constant unit conversions and maintains consistency across different areas of physics.
28. What is thermal conductivity and how is it related to the unit of heat?
Thermal conductivity measures a material's ability to conduct heat. It's typically expressed in W/(m·K), which can be converted to J/(s·m·K), directly relating to the joule as the unit of heat transfer over time and distance.
29. How does the unit of heat apply in radiation heat transfer?
In radiation heat transfer, energy is transmitted as electromagnetic waves. The amount of radiant energy transferred is measured in joules, consistent with other forms of heat transfer. This is crucial in understanding phenomena like solar radiation and thermal imaging.
30. What role does the unit of heat play in understanding the greenhouse effect?
The greenhouse effect involves trapping heat in the Earth's atmosphere. Quantifying this heat in joules helps scientists model and understand climate change, as it allows for precise calculations of energy balance in the Earth's system.
31. How is the unit of heat relevant in studying chemical reactions?
Chemical reactions often involve heat release (exothermic) or absorption (endothermic). Measuring this heat in joules allows chemists to quantify reaction energetics, crucial for understanding and predicting chemical processes.
32. Why is it important to use consistent units when dealing with heat in different contexts?
Using consistent units, preferably SI units like joules, ensures accuracy in calculations and facilitates communication across scientific disciplines. It prevents errors that can arise from unit conversion and allows for easier comparison of data from different sources.
33. What is the relationship between the unit of heat and power?
Power is the rate of energy transfer, measured in watts (W), which is equivalent to joules per second (J/s). This direct relationship between heat (energy) and power is crucial in many applications, from engineering to physiology.
34. How does the unit of heat apply in biological systems?
In biology, heat is often measured in calories due to historical reasons, but joules are increasingly used. Understanding energy transfer in joules is crucial for studying metabolism, thermoregulation, and ecosystem energetics.
35. What is the significance of the unit of heat in geothermal energy applications?
Geothermal energy harnesses heat from the Earth. Quantifying this heat in joules is essential for assessing geothermal resources, designing power plants, and calculating energy output, making the unit of heat crucial in this renewable energy field.
36. How does the choice of heat unit affect our understanding of the universe's thermal history?
Using joules to measure cosmic heat allows cosmologists to track the universe's thermal evolution consistently. It's crucial for understanding phenomena like the cosmic microwave background and the heat death of the universe, linking microscopic and macroscopic physics.
37. What is the relationship between the unit of heat and entropy?
Entropy, a measure of disorder, is closely related to heat. It's typically measured in J/K (joules per Kelvin), directly incorporating the unit of heat. This relationship is fundamental to the Second Law of Thermodynamics and our understanding of energy dissipation.
38. How does the unit of heat apply in studying climate systems?
Climate systems involve complex heat transfers between the atmosphere, oceans, and land. Using joules to quantify these heat flows allows for precise modeling of climate patterns and changes, crucial for understanding and predicting global climate phenomena.
39. What role does the unit of heat play in engineering thermal systems?
In engineering, the joule is essential for designing and analyzing thermal systems like engines, refrigerators, and heat exchangers. It allows engineers to calculate energy efficiency, heat loss, and performance metrics consistently across different applications.
40. How does the concept of heat capacity impact our choice of materials in everyday life?
Heat capacity, measured in J/K, affects how quickly materials heat up or cool down. This property influences material choices in everything from cookware (high heat capacity for even cooking) to building materials (for thermal regulation), demonstrating the practical importance of understanding heat units.
41. What is the significance of the unit of heat in nuclear physics?
In nuclear physics, energy released or absorbed in reactions is often enormous and best expressed in joules or electron volts (easily convertible to joules). This is crucial for understanding nuclear power generation, fusion processes, and the energy output of stars.
42. How does the unit of heat relate to the concept of thermal equilibrium?
Thermal equilibrium is reached when net heat transfer between systems stops. Understanding this in terms of joules helps explain why objects at different temperatures eventually reach the same temperature when in contact, a fundamental concept in thermodynamics.
43. What is the importance of the unit of heat in studying phase diagrams?
Phase diagrams show how temperature, pressure, and composition affect a substance's physical state. The heat involved in phase transitions, measured in joules, is crucial for constructing and interpreting these diagrams, which are essential in materials science and chemistry.
44. How does the choice of heat unit impact our understanding of star formation and evolution?
In astrophysics, the enormous amounts of heat involved in star formation and evolution are typically expressed in joules or related units. This allows for consistent calculations of stellar energy output, fusion rates, and the life cycles of stars across a wide range of scales.
45. What is the relationship between the unit of heat and the concept of thermal radiation?
Thermal radiation, the emission of electromagnetic waves due to an object's temperature, is quantified using joules. This relationship is crucial in understanding blackbody radiation, the Stefan-Boltzmann law, and phenomena like the greenhouse effect.
46. How does the unit of heat apply in the study of thermochemistry?
In thermochemistry, heat changes in chemical reactions are typically measured in joules or kilojoules. This allows for precise calculations of reaction enthalpies, bond energies, and the energetics of chemical processes, crucial for understanding and predicting chemical behavior.
47. What is the significance of the unit of heat in understanding the human body's energy balance?
The human body's energy balance involves heat production and dissipation, typically measured in joules or calories (easily convertible to joules). This is crucial for understanding metabolism, thermoregulation, and nutritional requirements in physiology and medicine.
48. How does the choice of heat unit affect our understanding of heat engines and their efficiency?
Heat engines convert thermal energy to mechanical work. Using joules for both heat input and work output allows for direct calculation of efficiency, crucial for designing and optimizing engines in various applications, from automobiles to power plants.
49. What role does the unit of heat play in studying the Earth's internal heat and plate tectonics?
The Earth's internal heat, measured in joules, drives plate tectonics. Quantifying this heat is essential for understanding mantle convection, volcanic activity, and the formation of geological features, linking the fields of geophysics and thermodynamics.
50. What is the importance of the unit of heat in studying superconductivity?
Superconductivity occurs at extremely low temperatures where electrical resistance disappears. Understanding the heat involved in cooling materials to these temperatures, measured in joules, is crucial for developing and applying superconducting technologies.
51. How does the choice of heat unit impact our understanding of quantum thermodynamics?
In quantum thermodynamics, heat and energy are often dealt with at microscopic scales. Using joules allows for consistent treatment of quantum and classical systems, crucial for developing technologies like quantum computers and understanding fundamental physical processes.
52. What is the significance of the unit of heat in studying atmospheric phenomena?
Atmospheric processes involve complex heat transfers, crucial for weather patterns and climate. Quantifying these in joules allows meteorologists and climatologists to model and predict atmospheric behavior, from local weather to global climate trends.
53. How does the unit of heat apply in the field of cryogenics?
Cryogenics deals with very low temperatures. Understanding heat flow in joules is crucial for designing cooling systems, maintaining low temperatures, and studying material properties in extreme cold, with applications ranging from MRI machines to space exploration.
54. What role does the unit of heat play in understanding the physics of cooking?
Cooking involves heat transfer processes like conduction, convection, and radiation. Expressing these in joules helps explain why different cooking methods and materials yield different results, bridging the gap between everyday experience and scientific principles.
55. How does the concept of heat and its unit relate to the study of black holes?
Black holes are extreme objects where classical thermodynamics meets quantum mechanics and general relativity. Quantifying their properties, including Hawking radiation, in terms of energy (joules) and entropy is crucial for understanding these mysterious cosmic phenomena and pushing the boundaries of theoretical physics.

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