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# Heat

Heat is important for animals and planets. Without heat, living things die, the right amount of heat is necessary for all living things.

This desert is very hot in the daytime. Most plants and animals can't live there because it is too hot and too dry.

When heat is transferred to the system from the surroundings, it means that the system temperature is lower than the temperature of its surroundings. When the heat is transferred from the system to the surroundings, it shows that the system is at the higher temperature.

As stated heat is the transfer of energy and the transfer of energy always takes place from the region of the higher temperature to the region of the lower temperature. Heat is the energy in flow due to temperature difference.

## What is Heat?

Heat is stated as the transfer of energy across the boundary of a system because of the difference in temperature between the system and its surroundings.
Example: We have a hot cup of tea. Let’s say that the initial temperature of the tea is 80°C. If we do not drink the tea and keep it as such, after sometime its temperature would start decreasing and the tea will become cold. Here, the heat gets transferred from the tea to the surroundings which are at lower temperature than the tea until equilibrium is reached between tea and its surroundings.

## Heat Measurement Units

There are three main units of heat :
• Calorie – It is defined as the amount of heat needs to be transferred to increase the temperature of 1 gram of water by 1°C. It is represented by "Cal".
• BTU – It is stated as the British Thermal Unit. It is defined as the amount of heat needs to be transferred to increase the temperature of one pound of water through 1°F. It is represented by BTU.
• Joules – Joules is the SI unit of heat. It is the widely used unit of heat. It is equal to the work done when a force of one Newton is applied for a distance of one meter. It is represented by J.

## Heat Capacity

For a particular substance it is defined as the amount of energy required to increase the temperature of that substance by 10°C.

According to the definition, if heat Q given to the substance, increases the temperature of the substance by $\Delta$T, then
$Q = C \Delta T$

## Heat Conduction

Heat Conduction is defined as the process of the transfer of energy which takes place because of the temperature difference between the two substances. Heat conduction is also termed as the Thermal conduction.At the atomic level, the energy transfer between two substances or between the system and the surroundings can be viewed as the transfer of kinetic energy between microscopic elements (molecules, atoms, and electrons). Energy transfer takes place from the microscopic particles at higher kinetic energy to the particles at lower kinetic energy due to collisions. This is also termed as Heat Diffusion.

Example:
Take one long piece of metal. Put first end of this metal in the flame. Gradually, you will find that the temperature of the other end of the metal in your hand starts increasing. Energy gets transferred from the first end under flame to the second end of the metal in your hand by conduction.

## Heat Conduction Equation

Thermal Conduction takes place only if there is a temperature difference between two parts of a conducting medium.

Consider a piece of metal having thickness $\Delta$x and cross-sectional area A. If first face of metal slab is at a temperature T1 and the other face of the metal is at temperature T2, T2 > T1. If heat Q is transferred during the time interval of $\Delta$t from the part of the metal at temperature T2 to the other end, energy Q transferred in a time $\Delta$t from the hotter end to the end at lower temperature. Let, the temperature difference between the two ends be $\Delta$T.
Then,
$\frac{Q}{\Delta t}$ $\propto$ A $\frac{\Delta T}{\Delta x}$

$\rho$ = $kA\left | \frac{dT}{dx} \right |$

Where $\rho$ = $\frac{Q}{\Delta t}$. It is also termed as the Law of the Thermal conduction. “k” is defined as the thermal conductivity of the material.

## Specific Heat

Specific heat c of a substance is defined as the Heat Capacity of the substance per unit mass of the substance. Therefore, if energy Q is given to the substance having mass m, and it results in the change in the temperature of the substance by $\Delta T$, Specific Heat of the substance is
Specific Heat Formula : $c$ = $\frac{Q}{m\Delta T}$

Specific heat is essentially a measure of how thermally insensitive a substance is to the addition of energy. If the specific heat capacity of a substance is more, then for a given mass m of a substance, for a particular $\Delta T$ temperature change, more energy Q needs to be transferred to the substance as compared to the second substance having less specific heat capacity. Therefore, more the specific heat capacity, more energy needs to be transferred to the substance if the other conditions (Q, m, $\Delta T$ ) are same.

Specific Heat Units – SI unit of Specific Heat is Joules per Kilogram Kelvin. $(\frac{J}{K})$.

When energy is given to a substance and the substance is not performing any work, it results into increase in temperature of the substance. (If the temperature of the substance is not increased, when the heat is given to it, and also the substance is not performing any work, it could lead to change in the phase of the substance Phenomenon termed as Phase Transition). Amount of heat which is required to raise the temperature of a substance by a certain amount, depends on the properties of the substance and this amount of heat varies from substance to substance.

For example, the amount of energy or heat required to increase the temperature of 1 kg of water by 1°C is 4.186 J, but the amount of heat required to increase the temperature of 1 kg of copper by 1°C is only 387 J. There are two methods to increase the temperature of the substance:
1. By transferring heat or energy to it.
2. By doing Work on it.

## Latent Heat

Latent Heat is the heat absorbed or released by a system that occurs without changing the temperature of the system.

## Temperature

Temperature of a substance is defined as the measure of the internal energy contained in the substance. Temperature is directly related to the Kinetic energy of the particles, Atoms and molecules of the substance. It is the average Kinetic energy of the particles contained in the system. It determines the direction of transfer of heat when two substances at different temperatures come in contact.

For example, heat always flows from the substance at the higher temperature to the substance at the lower temperature. In simple terms it can also be stated as the degree of hotness or coldness of the system.

## Heat Transfer

In simple terms it is defined as the heat moving or flowing from the warmer substance to the cooler substance. When heat is transferred to the system from the surroundings, it means that the system temperature is lower than the temperature of its surroundings. When the heat is transferred from the system to the surroundings, it shows that the system is at the higher temperature. As stated heat is the transfer of energy and the transfer of energy always takes place from the region of the higher temperature to the region of the lower temperature. Heat is the energy in flow due to temperature difference.

According to second law of Thermodynamics heat can flow from a substance at higher temperature to the lower one if work is done on the substance at higher temperature.
Rate of Heat Transfer (Q) :

The rate at which heat is transferred or conducted through a substance is directly proportional to the Area of the surface (A) perpendicular to the flow of heat and the temperature gradient $(\frac{\Delta T}{d})$ along the path of heat transfer. For a one dimensional steady state heat transfer rate of Heat Transfer is expressed by Fourier’s equation:

$Q$ = $kA$ $\frac{\Delta T}{d}$

Types of Heat Transfer / Methods of Heat Transfer

There are three main types of heat transfer.
1. Conduction
2. Convection

## Internal Energy

It is the energy of the system which is related to the energy of the microscopic components of the system like atoms and molecules present in it.