Thermal Physics: Heat, Temperature, and Energy Transfer
Internal Energy, Heat, and Temperature
Internal energy is the total kinetic and potential energy of the particles within a substance.
Heat is the transfer of thermal energy.
Temperature is the average kinetic energy of the particles in a substance.
Heating is the transfer of thermal energy from a hotter substance to a colder substance.
0°C = 273K
Laws of Thermodynamics
Zeroth Law
If thermal equilibrium exists between two objects, then they have the same temperature. No thermal energy will flow between them.
First Law
ΔU = Q – W
Where:
- ΔU = Change in internal energy of a system
- Q = Heat energy added (+Q for heating) or removed (-Q for cooling)
- W = Work done on the system (-W) or by the system (+W)
Specific Heat Capacity
Specific heat capacity (c) is the amount of energy that must be transferred to change the temperature of 1 kg of a material by 1°C.
Q = mcΔT
Where:
- Q = Heat energy transferred (J)
- m = Mass (kg)
- ΔT = Change in temperature = Tfinal – Tinitial (°C)
- c = Specific heat capacity of the material (Jkg-1°C-1)
Latent Heat
Fusion/Melting
Latent heat of melting is the energy added to increase the potential energy of the particles and reduce the intermolecular forces.
Latent heat of fusion is the energy released by the particles to reduce the potential energy of the bonds.
Latent heat of fusion = Latent heat of melting.
Q = mL
Where:
- Q = Heat energy transferred (J)
- m = Mass (kg)
- L = Latent heat of melting/fusion (Jkg-1)
Vaporization/Condensation
Latent heat of vaporization is the energy added to increase the potential energy of the particles and break the intermolecular bonds.
Latent heat of condensation is the energy released by the particles to allow the intermolecular forces to create bonds between the particles.
Latent heat of vaporization = Latent heat of condensation.
Q = mL
Where:
- Q = Heat energy transferred (J)
- m = Mass (kg)
- L = Latent heat of vaporization/condensation (Jkg-1)
Evaporation Rate Factors
The rate of evaporation depends upon:
- Type of liquid
- Surface area of the liquid
- Temperature of the liquid
- Humidity of the air
- Air movement
Heat Transfer
Heat transfer occurs through conduction, convection, and radiation.
Conduction
Particles with greater kinetic energy transfer their energy by collisions with other particles and free electrons.
Where:
- Q = Heat energy transferred through a material (J)
- t = Time (s)
- k = Thermal conductivity of the material (Wm-1K-1)
- ΔT = Temperature difference (K or °C)
- L = Thickness of the material (m)
- A = Surface area (m2)
Convection
Heat transfer by convection involves the mass movement of particles within a system over a distance.
When a liquid or gas is heated, it becomes hotter and less dense, so it rises. The colder, denser fluid will fall. As this fluid heats up, it in turn will rise, creating a convection current.
Radiation
Radiation is a means of transferring heat without the movement of matter.
Electromagnetic radiation is emitted by all objects that are at a temperature above absolute zero (0 K or -273°C).
The wavelength and frequency of the emitted radiation depend on the internal energy of the object. The higher the temperature of the object, the higher the frequency and the shorter the wavelength of the radiation emitted.