1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Thermal Physics: Questions on change of state

  1. Mar 14, 2015 #1
    Hello I am trying to learn about the change of state between solids, liquids and gases but I don't understand exactly how it works. I've been reading material given by my school, but I got these two questions to ask because I can't answer them. These are not questions on the sheet they just questions that I made which is something I should know but don't...

    How is it possible for two particles to be at the same temperature but have different internal energy?
    The material says ''it is possible for two objects/substances to be at the same temperature but have different internal energies'' but then goes we will go into this further next lesson in 'the specifics' but i found no explenation of this on that sheet...

    How can potential energy increase when bonds are broken during change of state?
    I thought that the more bonds there are/ the more rigid they are the more potential energy there is so if you break those bonds wouldn't PE decrease instead of increasing, which my material says?

    If it increases, does that mean that there will be more PE when boiling than melting? And if i were asked 'in which stage would a substance have the most internal energy?' in which of the four stages (solid --> melting --> boiling ---> gas) would it have the most? or would the internal energy stay constant throughout since lost pe = gained ke if energy was to be conserved?

    SORRY for not being able to do this thanks for help :(

  2. jcsd
  3. Mar 14, 2015 #2


    User Avatar

    Staff: Mentor

    Temperature is a collective phenomenon. You can't talk about the temperature of a single particle, only about its energy. Imagine an ideal gas in a closed, isolated container. Initially, give all the particles the same amount of kinetic energy. As the particles collide, the energy will be redistributed among the many particles, and no two will have the same energy. The system will however have a temperature, and the distribution of kinetic energy (or speed) will follow a well-known cure, the Maxwell-Boltzmann distribution.

    Normally, there will be more potential energy say in a liquid than in a gas, as the density is much greater then. Can you site with the material says about that?

    Internal energy usually refers to energy that is not in the form of kinetic energy.
  4. Mar 14, 2015 #3
    I think you are confusing binding energy with potential energy.

    Potential energy would refer to the distance between the molecules.
    As energy is added to the substance, the intermolecular distance increases during the phase change and thus so does the potential energy.
    Normally known as latent heat, as there is no temperature increase.
  5. Mar 14, 2015 #4
    It says the fallowing: ''At melting point the particles do not vibrate any faster, meaning that the kinetic energy and temperature are constant. The bonds that keep the particles in a rigid shape are broken and the potential energy increases''

    Also, I thought there will be the most potential energy in a solid since the sheet says that ''the potential energy (of the solid) is from the bonds between them (particles)'' and for a liquid it states that ''since they (particles) are free to slide past each other the potential energy is less than that of it in solid form.''

    Says on this sheet that the internal energy ''of a substance is due to the vibrations/movement energy of the particles (kinetic) and the energy due to the bonds holding them together (potential).''
    Last edited: Mar 14, 2015
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook