Heat and temperature are two terms that we commonly use in our daily lives. Heat is a form of energy whereas temperature is referred to as an approach to measure the hotness or coldness of a body. It is an important chapter in Class 11 Physics and students will find a significant number of questions from this chapter. Students can follow this article to understand the concepts of heat and temperature better.

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Heat: Definition

We define heat as thermal energy that is transferred from one system to another system due to temperature differences.
(i) We call the body that is transferring this energy a hot body.
(ii) The body that is receiving this energy is a cold body.

Heat exists only when there is the transfer of thermal energy from one system to another system. For example:-A a cup of coffee is hot, but we do not say it contains heat. We feel the heat when it is transferred to our hand when we touch it. So we can say that heat is a form of energy in transient.

SI unit of heat is Joule \(\left( J \right).\) Another unit of heat is calories \(\left( {{\rm{cal}}} \right).\)
\(1\,{\rm{cal}} = 4.186\,J\)

CLEAR YOUR CONCEPTUAL DOUBTS ON HEAT & TEMPERATURE

What is Temperature?

Temperature is a physical quantity that tells us about the degree of hotness or coldness of a body.

While heat is the transfer of energy, the temperature tells about the potential of the body to transfer this heat.
(i) This means that a body or system with a higher temperature has a higher ability to transfer heat to the surrounding.
(ii) A body or system with a lower temperature has a higher ability to receive this energy.

SI unit of temperature is Kelvin \(\left( K \right).\) More common units are degree Celsius \(\left( {{}^{\rm{o}}C} \right)\) and degree Fahrenheit \(\left( {{}^{\rm{o}}F} \right).\)

What is the Relationship Between Heat and Temperature?

The temperature of a system will only change when there will be heat transfer taking place from the system or into the system. The more the temperature difference, the more will be the heat transfer.

The magnitude of heat transfer \(\left( Q \right)\) and temperature \(\left( T \right)\) are related by the equation,
\(Q = mc\Delta T\)
where,
\(m\) is the mass of the substance,
\(c\) is the specific heat capacity,
\(\Delta T\) is the temperature change.
\(\Delta T = {T_{{\rm{final}}}} – {T_{{\rm{initial}}}}\)

If \({T_{{\rm{final}}}} > {T_{{\rm{initial}}}},\) then there is an increase in temperature. In this case, the heat transfer will be into the system, and \(Q\) is taken as positive. The thermal energy of the system increases in this case.
If \({T_{{\rm{final}}}} < {T_{{\rm{initial}}}},\) then there is a decrease in temperature, and \(Q\) is taken negative. Thermal energy decreases in this case.

Let us learn about Specific heat capacity. Specific heat capacity is the amount of heat required to increase the temperature of \(1\,{\rm{kg}}\) mass by \(1\,{}^{\rm{o}}C.\) Its SI unit is \({\rm{JK}}{{\rm{g}}^{ – 1}}^{\rm{o}}{C^{ – 1}}\) or \({\rm{J}}/{\rm{K}}{{\rm{g}}^{\rm{o}}}C\). Values of Specific heat capacity of some materials are given in the below table.

Material	Specific Heat \(\left( C \right)\) in  \({\rm{JK}}{{\rm{g}}^{ – 1}}^{\rm{o}}{C^{ – 1}}\) or \({\rm{J}}/{\rm{K}}{{\rm{g}}^{\rm{o}}}C\)
Water	\(4186\)
Ice	\(2090\)
Wood	\(1700\)
Air	\(721\)
Silver	\(235\)
Gold	\(129\)

For example, to raise the temperature of \(1\,{\rm{kg}}\) of water by \(1\,{}^{\rm{o}}C,\,4186\) Joules of heat is needed. However, for gold, just \(129\,J\) of heat enough.

From the table, we see that water is one of the substances having very high specific heat. This is important for maintaining life balance in nature.

What is the Difference Between Heat and Temperature?

Heat \(\left( Q \right)\)	Temperature \(\left( T \right)\)
It is the measure of the transfer of energy between the particles of the system.	It is a measure of the average kinetic energy of the particles in the system.
It depends on the amount of substance in a material (mass).	It does not depend on the mass.
A hot body transfers heat out from itself to a colder body.	When heat is transferred out from the body, its temperature reduces.
A cold body receives heat from a hotter body.	When heat is received, its temperature increases.
It is measured using a calorimeter.	It is measured using a thermometer.

Methods of Transfer of Heat

We saw in the beginning that heat is due to the transfer of thermal energy between two systems. Heat transfer takes place only when there is a difference in temperature between the two systems.
There are three ways through which heat is transferred:
(a) Conduction
(b) Convection
(c) Radiation

Conduction	Convection	Radiation
Heat transfer in solids	Heat transfer in fluids	Heat transfer by electromagnetic radiation
No physical movement of particles	There is a physical movement of fluid particles	There is no need for any medium
Examples: Hot vessel on a stove, melting wax in a candle, ironing of clothes	Examples: Water boiling, ocean currents, the warmth of a fire	Examples: Heat from the sun, microwave oven

Thermal Equilibrium

When two objects are at the same temperature, they are said to be in thermal equilibrium. When the objects will be in thermal equilibrium, there will not be any heat transfer between them. This concept is applied in a thermometer. The thermometer reads the temperature when heat is transferred from the object to the mercury bulb/sensor of the thermometer. Only when the temperature of the thermometer equals that of the object, we get the reading.

When the two bodies are kept in contact with each other, they always try to come in thermal equilibrium. For example:-A a hot cup of tea and an ice block are kept on a table. After some time, the cup of tea becomes cold and the ice melts.
(a) The hot cup transfers its heat to the surrounding air of lower temperature, thereby reducing its temperature.
(b) The ice receives heat from the air, which has a higher temperature than itself. As the temperature of ice increases, its thermal energy increases and melts to become water.

Heat and Temperature – Sample Problems

Q.1. \(1\) litre of hot water is left to cool. It gives out \(167.44\,{\rm{kJ}}\) of heat while reducing from \({90^{\rm{o}}}\,C.\) What is its final temperature?
Ans:
Given,
The volume of water is \(1\) litre or \(1000\,{\rm{ml}}.\)
The density of water is \(1\,{\rm{kg}}\) per litre. Therefore, the mass of water is \(1\,{\rm{kg}}{\rm{.}}\)
Specific heat of water is \(4.186\,{\rm{kJ/k}}{{\rm{g}}^{\rm{o}}}C\)
Amount of heat going out, \(Q = 167.44\,{\rm{kJ}}\)
Since it is lost by the water, it will be taken with a negative sign.
Amount of heat transfer is given by:-
\( \Rightarrow Q = mc\Delta T\)
\( \Rightarrow Q = mc\left( {{T_{{\rm{final}}}} – {T_{{\rm{initial}}}}} \right)\)
\( \Rightarrow – 167.44 = 1 \times 4.186 \times \left( {{T_{{\rm{final}}}} – 90} \right)\)
\( \Rightarrow – 40 = \left( {{T_{{\rm{final}}}} – 90} \right)\)
\( \Rightarrow {T_{{\rm{final}}}} = {50^{\rm{o}}}C\)
\(1\) litre of water gives out \({\rm{167}}{\rm{.44}}\,{\rm{kJ}}\) of heat while reducing the temperature from \({\rm{9}}{{\rm{0}}^{\rm{o}}}C\) to \({\rm{5}}{{\rm{0}}^{\rm{o}}}C.\)

Summary

(a) Heat is the movement of energy. The temperature difference causes this movement.
(b) A material that allows heat to flow through it is a good conductor, while an insulator does not allow it.
(c) Heat is transferred by conduction, convection and radiation.
(d) Heat is measured in Joules. Temperature is measured in Celsius, Fahrenheit and Kelvin.

FAQs on Heat and Temperature

Frequently asked questions related to heat and temperature is listed as follows:

Q.1. Why do thermos flasks keep hot things hot or cold things cold for a long time?
Ans: Thermos flasks have two containers separated by a space with a vacuum. Heat transfer by conduction or convection needs a medium. They cannot happen in a vacuum, unlike radiation. There is no way for the heat to go out or in.

Q.2. What is the lowest temperature possible?
Ans: The lowest temperature possible in the universe is \( – {273.15^{\rm{o}}}C.\) It is \(0\) Kelvin. Temperature cannot go below this.

Q.3. Why do we wear warm clothes in cold weather? Do they give out heat?
Ans: Warm clothes do not create heat. They are made of materials that are bad conductors of heat. They prevent the heat of our body from escaping out into the air of lower temperature.

Q.4. Why do cooking vessels have handles made of plastic, rubber, or wood?
Ans: These are bad conductors of heat. They do not get hot quickly.

Q.5. When we touch the ice, we feel chilly. Does it transfer coldness to us?
Ans: Cold waves do not exist like heat waves. Coldness means low temperature. Our body is at a higher temperature than ice. Due to the large temperature gap, it quickly absorbs the heat of our hands.

PRACTICE QUESTIONS ON HEAT & TEMPERATURE

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