Thermal Expansion & Heat Transfer

Thermal Expansion & Heat Transfer

Thermal Expansion

When a body is heated, its length, surface area, and volume increase. The increase in length, area, and volume with the increase in temperature is measured in terms of:

Coefficient of Linear Expansion or Linear Expansivity (α) Coefficient of Superficial Expansion or Superficial Expansivity (β)

• Coefficient of Cubical Expansion or Cubical Expansivity (γ)

Relation between α, β, and γ

• β = 2α

• γ = 3α

α : β : γ = 1 : 2 : 3

Anomalous Expansion of Water

• Almost every liquid expands with the increase in temperature. But when the temperature of water is increased from 0°C to 4°C, its volume decreases.

• If the temperature is increased above 4°C, its volume starts increasing.

• Clearly, the density of water is maximum at 4°C.

Transmission of Heat

The transfer of heat from one place to another is called the transmission of heat.

There are three modes of heat transfer:

1. Conduction

2. Convection

3. Radiation

Conduction

In this process, heat is transferred from one place to another by the successive vibrations of the particles of the medium without the bodily movement of the particles of the medium.

• In solids, heat transfer takes place by conduction.

• Metals are, in general, good conductors of heat, whereas brick, concrete, felt, ice, wood, sawdust, paper, air, glass, and cloth are bad conductors of heat.

• This is why ice is packed in sawdust to prevent it from melting.

• Diamond is a good conductor of heat.

Convection

In this process, heat is transferred by the actual movement of particles from one place to another. Due to the movement of particles, a current of particles is set up, which is called a convection current.

• In liquids and gases, heat transfer takes place by convection.

• Example: Earth’s atmosphere is heated by convection.

Radiation

• In this method, transfer of heat takes place at the speed of light without affecting the intervening medium.

Newton's Law of Cooling

• The rate of loss of heat by a body is directly proportional to the difference in temperature between the body and the surroundings.

Kirchhoff's Law

According to Kirchhoff's law, the ratio of emissive power to absorptive power is the same for all surfaces at the same temperature and is equal to the emissive power of a black body at that temperature.

• Kirchhoff's law signifies that good absorbers are good emitters.

• Example: If a shining metal ball with some black spots on its surface is heated to a high temperature and seen in the dark, the shining ball becomes dull, but the black spots shine brilliantly because the black spots absorb radiation during heating and emit it in the dark.

Stefan's Law

The radiant energy emitted by a black body per unit area per unit time (i.e., emissive power) is directly proportional to the fourth power of its absolute temperature.

• Formula: E = σT⁴, where σ is Stefan's constant.

Change of State

Any material can remain in any of its three states: solid, liquid, or gas. To change the substance from one state to another is called a change of state. For this, either the substance is heated or heat is extracted.

• A change of state takes place at a fixed temperature.

Fusion or Melting

• The process by which a substance is changed from a solid state to a liquid state is called fusion.

• Fusion takes place at a fixed temperature called the melting point (M.P.)

Freezing or Solidification

The process by which a substance is changed from a liquid state to a solid state is called freezing. Freezing takes place at a fixed temperature called the freezing point (F.P.)

• The M.P. of a substance changes with a change in pressure.

  • The melting point of substances that contract in fusion (e.g., ice) decreases with the increase in pressure.

  • The melting point of substances that expand in fusion (e.g., wax) increases with the increase in pressure.

• With the addition of impurity (e.g., salt in ice), the melting point decreases.

Vaporization

The process by which a substance is changed from a liquid state to a vapor state is called vaporization.

• Vaporization takes place by two methods:

  1. Evaporation

  2. Boiling or Ebullition

Evaporation

The process of vaporization, which takes place only from the exposed surface of liquid and at all temperatures, is called evaporation.

• The rate of evaporation increases with:

  • Increase in the temperature of the liquid.

  • Increase in the open surface area of the liquid.

• At a given temperature, the smaller the B.P. of the liquid, the larger the rate of evaporation.

Boiling

The process of vaporization, which takes place at a fixed temperature and from the whole part of the liquid, is called boiling.

• The temperature at which boiling takes place is called the boiling point.

• A liquid boils at a temperature at which its vapor pressure becomes equal to the air pressure.

• The boiling point of a liquid increases with an increase in pressure.

• The boiling point of a liquid increases with the addition of impurity.

• In a pressure cooker, the temperature at which food is cooked depends mainly on the area of the hole in the lid and the weight of the lid.

Condensation

Condensation: The process by which a substance is changed from a vapor state to a liquid state is called condensation.

Final Thoughts

When a body is heated, its length, area, and volume increase, measured by coefficients of expansion: linear (α), superficial (β), and cubical (γ). Water is unique, shrinking from 0°C to 4°C before expanding, making it densest at 4°C.

Heat transfers through conduction in solids, convection in fluids, and radiation without any medium. Newton’s law of cooling and Kirchhoff’s law explain how heat is lost and emitted.

Substances also change state—melting, freezing, boiling, evaporation, and condensation—usually at fixed temperatures, showing heat’s role in daily life.