Energy Interaction

  1. - We know that for closed systems, energy transfer between the system and the environment can be in the form of heat or work.

    -We know that for open systems or control volumes, energy transfer between the system and the environment can in the form of heat, work, or mass flow.

    Let's say we have a well-insulated oven with a heating element inside. We consider the heating element and the air inside the oven to be the system. When the oven is turned on, energy is transferred in the form of work into the system. This is because the electrons flowing in the heating element crosses into the boundary.

    I'm struggling to understand how the energy transferred is in the form of work instead of heat.

    1) Would the flow of electrons be considered mass flow, making it an open system?
    2) If the answer to question #2 is "yes", would mass flow be considered work?

    Questions not directly relevant to the problem:
    1) In an open system, can energy transferred be both in the form of heat AND work?

    Last edited: Jan 10, 2011
  2. jcsd
  3. Mapes

    Mapes 2,532
    Science Advisor
    Homework Helper
    Gold Member

    A flow of electrons is strictly mass flow, but the electron mass flux in a problem like this is absolutely negligible compared to the mass of the system. Most physicists would assume that the system is essentially closed in terms of mass. The much more important consideration is that the power source is doing work on the system in the form of moving electrical charge.

    No, mass flow is not generally equated with work. But in this case, the electrons represent a negligible amount of mass transfer and a considerable amount of electrical charge transfer.

    Sure; energy can enter/leave in the form of heat, work, and/or mass simultaneously.
  4. K^2

    K^2 2,470
    Science Advisor

    For simplest example of open system loosing energy to work, consider a gas-filled cylinder with a piston as your system. When the gas inside moves the piston, it is the work done by the closed system against whatever is pushing from the other side.
  5. Thanks guys! I think I understand now.
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