Thermal Energy: Understanding Temperature, Heat, and Expansion

Thermal Energy

Thermal energy is the average kinetic energy of the atoms or molecules in a substance.

Thermal Energy and Changes of State

When a change of state occurs, the energy transferred in heating up the substance is used to break the bonds between the molecules. The energy transferred does not increase the thermal energy but rather increases the internal energy of the substance.

Key Concepts

• The particles of a substance are in constant motion, and this increases as the substance heats up.
• Thermal energy is the average kinetic energy of the particles of a substance.
• During a change of state, the energy transferred in the heating process is used to break the bonds between molecules without increasing its thermal energy.

Temperature: Measurement and Scales

Temperature is the measurement of a substance's thermal energy.

The Absolute Scale or Kelvin

To convert degrees Celsius into Kelvin, we have to add 273.

T (kelvin) = t (^oC) + 273

To convert Kelvin temperatures into degrees Celsius, we subtract 273.

t (^oC) = T (kelvin) - 273

Key Takeaways

• Temperature is the measurement of a substance's thermal energy.
• Temperature is usually measured in the Celsius and Kelvin scales.

Thermal Expansion of Solids, Liquids, and Gases

Thermal expansion is the increase in volume that occurs in physical bodies and systems when their temperature rises.

Expansion in Solids and Liquids

The linear expansion of solids and liquids is expressed in the following equation:

L = L_O (1 + α ⋅ ΔT)

Charles's Law

The volume of a gas is directly proportional to its absolute temperature.

The ratio between the volume of a gas and its absolute temperature is constant:

Heat and Thermal Equilibrium

When two bodies or systems at different temperatures come into contact, the one with the higher temperature transfers energy to the one with the lower temperature until they reach a state of thermal equilibrium.

Heat is the transfer of energy from one body or system with a higher temperature to another with a lower temperature.

Units of Measurement of Heat

Heat is measured in units of energy. So, in the International System of Units, the unit for heat is the joule (J).

A calorie is defined as the amount of heat needed to heat a gram of water by one degree Celsius.

Thermal Conductors and Insulators

Thermal conductors are substances that transfer thermal energy quickly from one point to another.

Thermal insulators are substances that transfer thermal energy slowly from one point to another.

Convection

Convection is the process by which thermal energy is transferred from one point in a fluid (liquid or gas) to another because of the movement of the fluid.

In convection, thermal energy is transmitted through the transport of matter.

Radiation is the process of heat transfer by which bodies emit energy that can move through a vacuum.

Thermal Efficiency and Saving Energy

The speed at which heat is transferred by thermal conduction through the walls of a material depends on:

• The difference between the temperatures on either side of the wall.
• The area of the walls.
• The thermal conductivity of the material used in the walls.
• The thickness of the walls.