Introduction
Generally, heat is anything that provides warmth but scientifically, heat is the flow of energy from a warmer object to a cooler object in comparison till both the objects attain equilibrium. Every matter on earth has some amount of heat energy stored in it. Heat energy flows due to the difference in temperature of the two bodies. In this article, students will learn about the units and conversions of heat energy but first let's look at a few terms, definitions and concepts.
Heat: Scientifically, heat is defined as the energy that is spontaneously transferred from one object to another due to differences in temperatures. Heat transfer occurs until the bodies attain equilibrium.
Temperature: Temperature is defined as the kinetic energy of molecules of a body.
Internal Energy: The total energy of all the molecules of a body is the internal energy within the object.
Specific Heat: Specific heat, also known as heat capacity, is the amount of energy required to produce a unit change in its temperature.
Difference between Temperature, Heat, and Internal Energy
Temperature is the kinetic energy of the molecules of a body. The average kinetic energy of individual molecules is termed temperature.
The total energy of all the molecules is the internal energy within the object. Internal energy is an extensive property.
Heat is defined as the energy that is spontaneously transferred from one body to another due to its temperature difference.
For example, if a 5 kg of steel, at 100°C, is placed in contact with a 500 kg of steel at 20°C, heat flows from the cube at 300°C to the cube at 20°C, even though, the internal energy of the 20°C cubes is much greater because there is so much more of it. Mathematically heat can be expressed as:
\[C=\frac{Q}{m\times \Delta T}\]
Where m = mass of the body,
C = specific heat,
Δ T = temperature difference.
Q = heat
SI Unit of Heat:
As all the energy is represented in Joules (J), therefore, heat is also represented in Joules. Hence, the SI unit of heat is Joules. Joules can be defined as the amount of energy required to raise the temperature of a given mass by one degree. To increase the temperature of one unit weight of water by one degree, we require 4.184 joules of heat.
Other Heat Units:
Other heat units are:
BTU
Calorie
Joules
BTU:
BTU is a British thermal unit. It is the amount of energy required to raise the temperature of one pound of water by 10 F at sea level.
Conversion:
1 BTU = 1055.06 J = 2.931 x 10-4 kWh = 0.252 kcal = 778.16 ft lbf = 1.055 x 1010 ergs = 252 cal = 0.293 watt-hours
Calorie:
The amount of energy required to raise the temperature of one gram of water by 10 C.
Conversion:
1 kcal = 4186.8 J = 426.9 kp m = 1.163 x 10-3 kWh = 3.088 ft lbf = 3.9683 BTU = 1000 cal
Joule:
Joule is the SI unit of heat.
Conversion:
1 J = 0.1020 kpm = 2.778 x 10-7 kcal = 0.7376 ft lb = 1 kg m2 / s2 = 1 watt second = 1 Nm = 9.478 x 10-4 BTU
Conversion Table:
Temperature Conversion :
Celsius to Kelvin
K = C+273
For Example:
1000C = 100+273 = 373 K
Kelvin to Celsius
C = K – 273
For Example:
273 K = 273 – 273 = 00C
Celsius To Fahrenheit
0F = 9/5 (0C ) + 32
Kelvin to Fahrenheit
0F = 9/5 (K-273) +32
Fahrenheit to Celsius
0C = 5/9 (0F-32)
Fahrenheit to Kelvin
K = 5/9 (0F-32) + 273
Example 1: An electric kettle contains 1.5 kg of water. The specific heat capacity of water is 4180 J kg-1 K-1. Calculate the amount of energy required to raise the temperature of the water from 15 0C to 100 0C.
Solution:
Given:
Specific heat (C) = 4180 J kg-1 K-1
T1 = 15 0C = 15+273 = 288 K
T2 = 100 0C = 100+273 = 373 K
m = 1.5 kg
Q = m x Δ T x C
Q = 1.5 x 4180 x (373-288)
= 533 kJ
Example 2: Calculate the energy needed to raise the temperature of the water from 20 0C to 90 0C.
Solution: Q = mcΔθ
= (0.7) (4200) (90-20) = 205.8 kJ
Methods of Transfer of Heat Energy
Convection: Transfer of heat energy via fluids. When fluids get heated, they form vapours and rise higher up in the environment.
Conduction: Transfer of heat energy through direct contact between two bodies. Such a method of transfer of heat is generally observed in solids.
Radiation: Radiation from hot objects (such as the sun) warms up the air in all directions which are absorbed by molecules all around.
Try Yourself:
Calculate heat required to evaporate 1kg of water at the atmospheric pressure (p = 1.0133 bar) also at the temperature of 100°C.
Calculate heat required to evaporate 1 kg of feed water at the pressure of 6 MPa (p = 60 bar) and the temperature of 275.6°C.
Calculate the specific heat of a 100 kg mass of water if the temperature changes from 150 C to 1000 C. Heat required is 130 BTU.
Calculate the heat required to raise the temperature of 60-milligram mass from 22 K to 273 K. Specific heat given 223 J/K.
Calculate the specific heat of a 20 dkg mass of water if the temperature changes from 150 C to 260 C. Heat required is 137 BTU.
Calculate the heat required to raise the temperature of 200 kg mass from 2320 C to 300 K. Specific heat given 203 J/K.
Calculate the specific heat of a 1000 kg mass of water if the temperature changes from 15 K to 100 K. Assume the rest data.
Calculate the heat required to raise the temperature of 29 kg mass from 220 C to 273 K. Assume the rest data.
Calculate the specific heat of a 20 kg mass of water if the temperature changes from 1500 C to 1000 C. Heat required is 130 cal.
Calculate the heat required to raise the temperature of 505 kg mass from 320 C to 273 K. Specific heat given 320 J/K.
Explain how heat is transferred in the body?
Name the other methods for transferring heat.
What is the SI unit of heat?
FAQs on Unit of Heat
1. What is the SI unit of heat?
The SI unit of heat is the Joule (J). This is because heat is a form of energy transfer, and the Joule is the standard unit of energy in the International System of Units. Using Joules allows for consistency when relating heat to other forms of energy like mechanical work or electrical energy.
2. What are the other common units for measuring heat besides the Joule?
Besides the Joule, other common units used to measure heat include:
- Calorie (cal): The amount of energy needed to raise the temperature of one gram of water by one degree Celsius.
- Kilocalorie (kcal): Equal to 1000 calories, this unit is commonly used in the context of food energy.
- British Thermal Unit (BTU): The amount of energy required to raise the temperature of one pound of water by one degree Fahrenheit, often used in heating and cooling industries.
3. How is heat different from temperature?
Heat and temperature are related but distinct concepts. Temperature is a measure of the average kinetic energy of the molecules within a substance, indicating its degree of hotness or coldness. In contrast, heat is the transfer of thermal energy from a hotter object to a colder one. Temperature is an intrinsic property of an object, while heat is energy in transit between objects.
4. What is the CGS unit of heat and how is it defined?
The CGS (Centimetre-Gram-Second) unit of heat is the calorie (cal). One calorie is precisely defined as the amount of heat energy required to raise the temperature of one gram of water by one degree Celsius (°C) at a pressure of one standard atmosphere.
5. How do you convert between the Celsius, Kelvin, and Fahrenheit temperature scales?
As per the 2025-26 NCERT syllabus, understanding temperature conversions is essential for heat calculations. The primary formulas are:
- Celsius to Kelvin: K = °C + 273.15
- Kelvin to Celsius: °C = K - 273.15
- Celsius to Fahrenheit: °F = (°C × 9/5) + 32
- Fahrenheit to Celsius: °C = (°F - 32) × 5/9
6. Why is the Joule, a unit for energy, also used as the SI unit for heat?
The Joule is the SI unit for heat because scientific experiments, notably by James Prescott Joule, established that heat is a form of energy in transit. This discovery, known as the mechanical equivalent of heat, proved that heat and work are interconvertible. Using the same unit (Joule) for both heat and other forms of energy (like kinetic or potential) unifies the concept of energy within physics, making calculations and principles consistent across different branches of science.
7. In the specific heat formula Q = mcΔT, what are the standard units for each component?
In the specific heat capacity formula, Q = mcΔT, using consistent SI units is crucial for accurate calculations. The standard units for each variable are:
- Q (Heat transferred): Joules (J)
- m (mass of the substance): kilograms (kg)
- c (specific heat capacity): Joules per kilogram per Kelvin (J/kg·K)
- ΔT (change in temperature): Kelvin (K) or degrees Celsius (°C), as a change of 1K is equal to a change of 1°C.
8. Can an object at a lower temperature have more internal energy than an object at a higher temperature?
Yes, an object at a lower temperature can possess more total internal energy. Internal energy is the sum of all kinetic and potential energies of the molecules in an object, so it depends on both temperature and mass. For instance, a large iceberg at 0°C has far more molecules and thus a much greater total internal energy than a small cup of boiling water at 100°C, despite its lower temperature.
