Heat Capacity In Physics, Unit Of Heat, Calorimetry, Important Topics For JEE 2025

Heat Capacity In Physics : We will learn that Heat is defined as a process of transferring energy across the boundary of a system because of a temperature difference between the system and its surroundings. It is also the amount of energy Q transferred by this process. When you heat a substance, you transfer energy into it by placing it in contact with surroundings that have a higher temperature.

JEE Main Seat Matrix 2024 : Click to Check

This is the case, for example, when you place a pan of cold water on a stove burner. The burner is at a higher temperature than the water, and so the water gains energy by heating. The process of heat exchange between bodies and system is known as calorimetry.

IIT Allahabad Seat Matrix : Click To Check

What Is Heat

When two bodies at different temperatures are placed in contact, the hotter body cools down and the colder body warms up. Energy is thus transferred from a body at higher temperature to a body at lower temperature when they are brought in contact. The energy being transferred between two bodies or between adjacent parts of a body as a result of temperature difference is called heat. Thus, heat is a form of energy. It is energy in transit whenever temperature differences exist. Once it is transferred it becomes the internal energy of the receiving body. It should be clearly understood that the word “heat” is meaningful only as long as the energy is being transferred. Thus, expressions like “heat in a body” or “heat of a body” are meaningless.

Colleges Accepting JEE Main Score 2024 : Click to Check

Concept Of Specific Heat

Concept Of Specific Heat : As a student of Physics, you can always think and under stand that different masses of bodies made of same material require different amounts of heat to be supplied to them to raise their temperature by the same amount. Similarly,  equal masses of bodies of same material require different amounts of heat to be supplied to them to raise their temperature by different amounts and also equal masses of bodies of different material require different amount of heat to be supplied to them to raise their temperature by same amount. (Think of some practical examples!).

JEE Advanced Cutoff 2024 : Click To Check

From above, we conclude that amount of heat supplied Q to a body is directly proportional to mass m of body and rise in temperature ∆ T of the body i.e., Q ∝ m ∆ T

Q = mc ∆ T

Thermal Capacity Or Heat Capacity

The amount of heat required to raise the temperature (∆ T ) of a body through 1 unit. (= 1 unit change)

i.e., if ∆ T = 1 unit then Heat capacity = mc

So, heat capacity of a body of mass m , gram specific heat c is the product of m and c

JEE Advanced Result  2024 : Click To Check

Unit Of Heat

As heat is just energy in transit, its unit in SI is joule. However, another unit of heat “calorie” is in wide use. This unit was formulated much before it was recognised that heat is a form of energy. The old day definition of calorie is as follows:

• The amount of heat needed to increase the temperature of 1 g of water from 14.5°C to 15.5°C at a pressure of 1 atm is called 1 calorie.
• The amount of heat needed to raise the temperature of 1 g of water by 1°C depends slightly on the actual temperature of water and the pressure.
• That is why, the range 14.5°C to 15.5°C and the pressure 1 atm was specified in the definition. We shall ignore this small variation and use one calorie to mean the amount of heat needed to increase the temperature of 1 g of water by 1°C at any region of temperature and pressure. The calorie is now defined in terms of joule as 1 cal = 4. 186 joule. We also use the unit kilocalorie which is equal to 1000 calorie as the name indicates

JEE Question Banks : Click to Check

Principle Of Calorimetry

• A simple calorimeter is a vessel generally made of copper with a stirrer of the same material. The vessel is kept in a wooden box to isolate it thermally from the surrounding. A thermometer is used to measure the temperature of the contents of the calorimeter. Objects at different temperatures are made to come in contact with each other in the calorimeter. As a result, heat is exchanged between the objects as well as with the calorimeter. Neglecting any heat exchange with the surrounding, the principle of calorimetry states that the total heat given by the hot objects equals the total heat received by the cold objects.
• Calorimetry means 'measuring heat'. When two bodies (one being solid and other being liquid or both being liquid) at different temperatures are mixed, heat will be transferred from the body at higher temperature to the body at lower temperature till both acquire same temperature. The body at higher temperature releases heat while the body at lower temperature absorbs it, so that

It means that the temperature of mixture can never be lesser than lower temperature (as a body cannot be cooled below the temperature of cooling body) and greater than higher temperature (as a body cannot be heated above the temperature of heating body). Furthermore, usually rise in temperature of one body is not equal to the fall in temperature of the other body. Though, heat gained by one body is equal to the heat lost by the other. Mixing of two substances when temperature changes only: It means that there is no phase change.

JEE Main Previous Year Paper : Click to Check

Mechanical Equivalent Of Heat

In early days heat was not recognised as a form of energy. Heat was supposed to be something needed to raise the temperature of a body or to change its phase. Calorie was defined as the unit of heat. A number of experiments were performed to show that the temperature may also be increased by doing mechanical work on the system. These experiments established that heat is equivalent to mechanical energy and measured how much mechanical energy is equivalent to a calorie. If mechanical work W produces the same temperature change as heat H , we write

W = JH

where J is called mechanical equivalent of heat. It is clear that if W and H are both measured in the same unit then J = 1. If W is measured in joule (work done by a force of 1 N in displacing an object by 1 m in its direction) and H in calorie (heat required to raise the temperature of 1 g of water by 1°C) then J is expressed in joule per calorie. The value of J gives how many joules of mechanical work is needed to raise the temperature of 1 g of water by 1°C. We describe below a laboratory method to measure the mechanical equivalent of heat.

JEE Revision Books : Click to Check