Thermodynamics is the branch of Physics that studies heat, work, temperature, energy and entropy. It’s a lot of stuff, but we’ll analyse every concept step-by-step!
All gases that we study in thermodynamics will be called “ideal”. Ideal means that the interactions between molecules of said gas are negligible. The ideal gas law states that $pV=KNT$, where p is the pressure exerted by the gas, V is the volume of the container in which the gas resides, K is the Boltzmann constant and has the value of $1.38*10^-23 J/K$, N is the number of atoms and T is the absolute temperature of the gas (measured in Kelvin).
It can also be written as $pv=nRT$, where n is the number of moles and R is a constant with value of $8.31 J/mol*K$. We can rewrite it as such because $n=N/N_a$, where $N_a=6.022 mol^-1$ is the number of Avogadro.
To heat up something, you will need some heat that is proportional to the mass of what you want to heat up, to its specific heat and to the difference between the final and the initial temperature. How do we write this? This will be our formula: $Q=mcΔT$, where m is the mass, ΔT is the delta of the temperature and c is the specific heat, measured in J/kg*K (this depends on the material).
To make something change its state, we need some heat to melt it or to vaporize it. This heat is defined like this: $Q=m*L$, where L is the Latent heat of fusion in the case of fusion or Latent heat of vaporization in the case of vaporization, and m is the mass of the object.
Conduction is the process in which heat is transferred in an object from the hotter side to the colder side. This process is defined by the following formula: $Q/t=k*(ΔTA/d)$, where Q is the heat transferred in the unit of time t, k is the thermal conductivity of the material and is measured in W/mK, ΔT is the difference between the two temperatures, A is the surface area of the object and d is its thickness.
Convection is the process in which both heat and matter move; it happens mainly in fluids because particles aren’t free to move in a solid. The animation below accurately shows the process.
Radiation is the process in which an object transfers heat to another object through electro-magnetic waves and is represented by this formula: $E=σT^4$, where σ is the constant of Stefan-Boltzmann and has value of $5.6710^-8 W/m^2*K^4$ and T is the temperature of the object. E is the released energy.
$ΔU=Q+W$ is the first law of thermodynamics. It states that the change in internal energy of the system is equal to the difference between the heat Q transferred into the system and the work W done by the system. It’s an extension of the principle of the conservation of energy.
| ΔU | Q | W |
|---|---|---|
| is + if temperature T increases | is + if heat enters gas | is + if gas is compressed |
| is - if temperature T decreases | is - if heat exist gas | is - if gas expands |
| is 0 if temperature T is constant | is 0 if no heat is exchanged | is 0 if volume is constant |