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Thermal Physics

Heat as energy in transit

  • 3 things to know
  • Component 1

Overview

Heat in the thermodynamic sense, condition under which heat enters a system and condition under which heat leaves a system.

What you need to know

Answer each one before you reveal it.

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Key term

Explain why a vacuum flask keeps its contents hot.

Key term

Define heat in the thermodynamic sense.

Key term

Why is it wrong to say that a hot object contains a lot of heat?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    Define heat in the thermodynamic sense.

    Model answer

    Heat is energy that flows across the boundary of a system because of a temperature difference between the system and its surroundings.

  2. 2

    Why is it wrong to say that a hot object contains a lot of heat?

    Model answer

    Heat is energy in transit across a boundary and exists only while that transfer is happening, so what an object contains is internal energy, not heat.

  3. 3

    State the condition under which heat enters a system.

    Model answer

    Heat enters a system when the temperature of the system is lower than that of its surroundings.

  4. 4

    State the condition under which heat leaves a system.

    Model answer

    Heat leaves a system when the temperature of the system is higher than that of its surroundings.

  5. 5

    Through what part of a system does heat pass?

    Model answer

    Heat passes through the boundary of the system, which in practice is the wall of its container or the surface of the body.

  6. 6

    What sign convention is used for the heat term Q in thermodynamics?

    Model answer

    Q is taken as positive when heat flows into the system and negative when heat flows out of it.

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