Thermal Physics

Kinetic Particle Model

The Kinetic Particle Model describes matter as containing particles which are in constant motion (except at absolute zero.)

Entropy

Entropy represents the 'degree of randomness' in a system, or the 'amount of disorder'. The laws of thermodynamics make some assertions about properties of entropy.

Laws of Thermodynamics

0. Transitivity of Thermal Equilibrium: If $A$ and $B$ are in thermal equilibrium and $B$ and $C$ are in thermal equilibrium then $A$ and $C$ are in thermal equilibrium.
1. Law of Conservation of Energy: Energy cannot be created or destroyed
2. The entropy of a system always increases or Heat can never flow from a colder to a warmer body
3. The entropy of a system approaches zero as temperature approaches zero

Temperature

Temperature is a measure of the average kinetic energy of a system, i.e.:

$$T\propto \overline{E_k}$$

Specific Heat Capacity

The Specific Heat Capacity represents the heat required to raise the temperature of some mass of a substance by a given amount, normally $1K$.

The SHC of liquid water is $4180Jk{g}^{-1}{K}^{-1}$.

We use the following formula:

$$Q=mc\Delta T$$

where $Q$ is energy, $m$ is mass, $c$ is SHC and $\Delta T$ is the difference in temperature.

Latent Heat

The latent heat of a substance is the heat required to turn a solid into a liquid or a liquid into a vapour without changing temperature, i.e. the amount of energy required to undergo a state change.

There are two types. Latent Heat of Fusion, $L_f$, is the energy required to change a solid into a liquid. Latent Heat of Vapourisation, $L_v$, is the energy required to change a liquid into a vapour.

Water has the following values:

$$L_f=3.34\ast {10}^{5}Jk{g}^{-1}{L}_v=2.26\ast {10}^{6}Jk{g}^{-1}$$

We use the following formula:

$$Q=mL$$

where $Q$ is energy, $m$ is mass, and $L$ is latent heat. (In calculations, be sure to specify $L_f$ or $L_v$.)

Calculations using specific and latent heat

To solve for, for example, the final temperature of a mixture of substances, we use the second law of thermodynamics. Essentially what the second law of thermodynamics means in this context is:

$$Q_1+Q_2+Q_3+\cdots =0$$

We can then expand out each of the $Q$s and solve for whatever is needed.

Heat Transfer Methods

Conduction

Conductive heat transfer occurs between bodies in contact.

Convection

Convection is the transfer of heat through the movement of fluids. During convective heat transfer, particles heat up, causing the volume to increase, reducing the density and causing the fluid to fall back down again, creating a 'convection current' with warmer particles higher up and cooler particles further down.

Radiation

Radiative heat transfer involves two bodies that do not have to be in contact, and occurs when energy is transmitted through electromagnetic radiation.

Efficiency

Efficiency can be determined by the equation:

$$\eta =\frac{\text{energy out}}{\text{energy in}}\ast 100\%$$