What are the Thermal Properties?
Thermal properties of engineering materials are a study of those substances which have a direct relation to the temperature fluctuations. Whenever materials come in contact with heat or thermal change, they tend to change in dimensions such as length or volume. Moreover, there is a fluctuation in the temperature of solids when it absorbs heat energy.
The properties of materials help to ascertain how it will respond when it comes in contact with heat. For instance, when a metal comes in contact with heat, its temperature rises, and it tends to melt at high temperature as it reaches its melting point.
What are the Components of Thermal Properties?
The thermophysical properties of materials consist of the following:
Point 1: The capacity of heat: It is the measure of heat that is necessary to raise the temperature of the material. It is the ratio of heat energy transferred into a substance that results in the temperature change.
Mathematically, it can be stated as the following:
C= ∆Q/∆T=dQ/dT
Where C = Specific heat which gets expressed as per mole,
T = Temperature
Q = Energy
dT = Fluctuations in temperature
dQ =Energy transferred (added or deducted) to cause a change in temperature.
One can measure heat capacity under two conditions which are constant pressure and volume. The formula is:
CV = (δq/dT)V - heat at constant volume
CP = (δq/dT)P - heat at constant pressure
For solids and liquids, there is a small difference between CP and CV since the expansion is less in these substances after they are subject to heating. However, for gases, CV is always less than CP as the material expands when it absorbs heat.
Fun fact: The heat capacity of aluminium is 900 kJ per kg atoms (degree C) at room temperature!
The Heat Capacity of Some Materials at kJ/kg Atoms (Degree C) are as Follows:
Point 2: Thermal expansion: When any substance absorbs heat or faces temperature changes, there is a change in its volume and length. Thermal expansion occurs in materials when heat passes through the atoms, and they move from their equilibrium.
The formulas of linear and volume expansion are:
Linear: (lf - l0)/l0 = ∆l/l0 = αl (Tf -T0) = α1-∆T
Where l0 = initial length, lf = final length.
Volume: (Vf -V0)/V0 = ∆V/V0 = αV (TV -T0) = α1- ∆T
Where V0 = initial volume, Vf = final volume.
Point 3: Thermal conductivity: One of the vital components of the thermal properties of materials is its conductivity. Thermal conductivity is the measure of heat transferring capability of a material. Through it, one can ascertain how substances can conduct energy and the direction of its flow.
According to Fourier’s law, energy transfer through solids can be expressed as:
q = -k (dT/dx)
Where q = quantity of heat flowing from a unit area in a specific time
K denotes thermal conductivity
T = temperature
X = direction of energy flow
Fun fact: In insulators, heat conductivity is relatively lower than metals!
Point 4: Thermal stresses: It is the tension created in materials resulting from temperature fluctuations or energy absorption. This category of thermal properties of materials can result in deformation or fracture occurring from dimensional changes.
The study of thermal properties of nanomaterials is vast, and it gets confined to a limited area of knowledge. Moreover, it has broad applications in the field of thermodynamics and study of heat conductivity, etc.
You can learn about this topic and its related concepts in further detail from the study material provided by us at Vedantu. Furthermore, now you can also download our Vedantu app for convenient access to online interactive classes.
FAQs on Thermal Properties of Materials
1. What are the primary thermal properties of materials studied in Class 11 Physics?
In Class 11 Physics, the primary thermal properties of materials describe how they respond to heat. The key properties include:
- Thermal Expansion: The tendency of matter to change its shape, area, and volume in response to a change in temperature.
- Heat Capacity: The amount of heat required to raise the temperature of a substance by a certain amount.
- Latent Heat: The energy absorbed or released by a substance during a phase change (like melting or boiling) without changing its temperature.
- Thermal Conductivity: A material's ability to conduct or transfer heat.
2. What is the difference between heat capacity and specific heat capacity?
Heat capacity and specific heat capacity both relate to how a substance absorbs heat, but they differ in scale. Heat capacity (J/K) is the heat needed to raise the temperature of an entire object by 1°C or 1K. It depends on the object's mass. In contrast, specific heat capacity (J/kg·K) is an intrinsic property of a substance itself, defined as the heat needed to raise the temperature of 1 kg of the substance by 1°C or 1K.
3. Why does water exhibit anomalous expansion, and what is its real-world significance?
Most substances expand when heated and contract when cooled. Water behaves differently between 4°C and 0°C; it expands as it cools in this range. This is known as the anomalous expansion of water. Its significance is crucial for aquatic life in cold climates. As a lake's surface water cools, it becomes denser and sinks until the entire lake reaches 4°C. Further cooling makes the surface water less dense, so it stays on top and freezes, forming an insulating layer of ice that protects the aquatic life in the liquid water below.
4. How do the three modes of heat transfer—conduction, convection, and radiation—differ?
The three modes of heat transfer are distinct in their mechanisms and requirements:
- Conduction is the transfer of heat through direct molecular collision within a medium, without the bulk movement of the medium itself. It is the primary mode of heat transfer in solids.
- Convection is the transfer of heat through the actual movement of a fluid (liquid or gas). Warmer, less dense fluid rises, and cooler, denser fluid sinks, creating convection currents.
- Radiation is the transfer of heat through electromagnetic waves, which requires no medium. This is how the Sun's heat reaches Earth.
5. What is meant by the latent heat of a substance?
Latent heat is the heat energy absorbed or released by a substance during a change of state (e.g., from solid to liquid or liquid to gas) that occurs at a constant temperature. There are two main types:
- Latent Heat of Fusion: The heat absorbed to change a substance from a solid to a liquid at its melting point.
- Latent Heat of Vaporisation: The heat absorbed to change a substance from a liquid to a gas at its boiling point.
6. Why does a metal object feel colder than a wooden object at the same room temperature?
A metal object feels colder because of a difference in thermal conductivity, not temperature. Both objects are at the same temperature, but metal is an excellent thermal conductor. When you touch it, it rapidly transfers heat away from your hand, creating a sensation of cold. Wood is a poor thermal conductor (an insulator), so it transfers heat much more slowly, and therefore doesn't feel as cold.
7. How are the coefficients of linear, area, and volume expansion related for an isotropic solid?
For an isotropic solid (which expands uniformly in all directions), the coefficients of thermal expansion are related. The coefficient of area expansion (β) is approximately twice the coefficient of linear expansion (α), so β ≈ 2α. Similarly, the coefficient of volume expansion (γ) is approximately three times the coefficient of linear expansion, so γ ≈ 3α. This relationship helps in calculating changes in area or volume if only the linear expansion coefficient is known.
8. What scientific principles allow a thermos flask to keep liquids hot or cold for a long time?
A thermos flask is designed to minimise all three forms of heat transfer. It consists of a double-walled glass container with a vacuum in between.
- The vacuum between the walls prevents heat transfer by conduction and convection.
- The walls are silvered (shiny), which reflects thermal energy, thus minimising heat loss or gain through radiation.
- The stopper, typically made of an insulating material, reduces heat transfer through the opening.
