Thermal Properties of Heat

Key Concepts

• Heat
• Units of Heat
• Internal Energy

Introduction:

When an individual rubs their palms against each other or air is pumped into a bicycle tube, heat is produced. Thus, heat is generated by doing work, i.e., from mechanical energy. Whereas in an electric iron, heat is obtained by passing electrical energy. And on heating charcoal, heat is obtained from the chemical energy within it. Heat is generated in all these processes. In this session we will be learning about heat. 

Explanation:

Heat: 

Heat is a form of energy. When we heat a body, it gains energy. At a microscopic level, its particles (atoms/molecules) gain energy. The particles of a solid vibrate at their respective places. They are tightly packed do not move from their places. The particles of a liquid move randomly at different speeds, staying close to each other at the same time. And, the particles of a gas move randomly at comparatively increased speeds and cover up the whole volume of the container. The particles have large spaces among themselves. 

When heat is supplied to the water molecules in solid state (ice), after some time the molecules start moving randomly, a pattern similar to the motion of water molecules in liquid form. 

On further heating, the molecules start moving more randomly at higher speeds and fill up the entire space of the container, a patter similar to the motion of water molecules in gaseous form. 

Conclusion: 

• All materials are made up of tiny particles (atoms, molecules). 
• The particles of liquids and gases are at random motion always. However, the particles of solids do not move as much. They vibrate at their own places. 
• When a substance is heated, its constituent particles move more randomly at increased speeds. 
• On supplying enough heat, the substance may also change its state and the motion and spacing between its particles also align with the state change. 

Thus, the particles of matter possess some kinetic energy due to their motion. The particles of a gas possess the highest kinetic energy, followed by liquids and then solids. Each molecule also exerts a force of attraction on another molecule. This interaction results in some potential energy. 

Thus, molecules possess energy and the heat energy of the body is the internal energy of its molecules. The total internal energy of a substance is the sum of the internal kinetic energy due to molecular motion and the internal potential energy due to molecular attractive forces. 

Internal energy = Kinetic energy + Potential energy 

Heat: 

A hot body of identical shape and size has a greater internal energy than that of a cold body. When such bodies are kept in contact with each other, the cold body warms up and the hot body cools down, i.e., the internal energy of the hot body decreases and that of the cold body increases. Thus, heat energy flows from a hot body to a cold body when placed in contact. 

On touching, a body appears to be cold or hot when the heat energy flows out of or into our hands respectively. When we hold ice cubes, the heat from our palms flow to the ice, making us feel cold. In contrast, when we hold a hot cup of coffee, the heat from the coffee enters our palms, making us feel hot. 

Units of Heat: 

The SI unit of heat is the same as that of energy, i.e., Joule (J). The CGS unit of heat is “erg”, which is given by, 

1 J = 10⁷ erg 

There are some other commonly used units of heat called calorie and kilocalorie. 

1 kcal = 1000 cal 

The units, calorie and joule are related as under. 

1 cal = 4.186 J or 4.2 J. 

Questions and answers: 

1. Which of the following refer to the total energy of a system? 

1. Total potential energy due to the interaction between its constituent molecules. 

2. Total kinetic energy is due to the motion of its constituent molecules. 

3. Sum of A and B. 

4. Difference of A and B. 

Answer: C. Sum of A and B. 

2. Convert 436 cal of energy into joules. 

Answer:  

Given that, 

Total energy = 436 cal 

1 cal = 4.2 J 

Therefore, 346 cal = 4.2 x 436 

= 1831.2 J 

1 kilocalorie = 1000 calories 

1 cal = 1/1000 kcal 

436 cal = 436/1000 kcal 

436 cal = 0.436 kcal 

Therefore, 436 cal = 0.436 kcal 

Summary

• Heat is generated by either doing work or by providing energy in a form other than heat It is a form of energy.
• The particles of liquids and gases are at random motion always. However, the particles of solids do not move as much. They vibrate at their own places.
• On supplying enough heat, the substance may also change its state and the motion and spacing between its particles also align with the state change.
• The total internal energy of a substance is the sum of the internal kinetic energy due to molecular motion and the internal potential energy due to molecular attractive forces. • Internal energy = kinetic energy + potential energy
• Heat energy flows from a hot body to a cold body when placed in contact.
• On touching, a body appears to be cold or hot when the heat energy flows out of or into our hands respectively.
• The unit of heat is the same as that of energy i.e., Joule (J). It is the S I unit of heat.