0

I have a few questions about heat and work:

  1. Can we have a change in temperature without heat entering and exiting the system? For example doing work to change the pressure, changing the internal energy of the system, and increasing the temperature?

  2. Why do we not consider heat transfer as one object 'doing work' to another to increase it's temperature?

  3. Can heat, by changing the internal energy of the system change properties other than it's temperature?

user1007028
  • 263

2 Answers2

1

The answer to question 1 is Yes.

The answer to question 2 is Terminology. We call transfer of heat from one entity to another as a result of a temperature difference Heat Flow.

The answer to question 3 is Sure. Pressure can change, volume can change, entropy can change, enthalpy can change, Helmholtz free energy can change, Gibbs free energy can change, etc.

Chet Miller
  • 33,586
  • 3
  • 21
  • 48
  • 1
    It would probably be useful to point out that question 1 is a very common ideal situation (perfect thermal insulation) and we should give OP the keyword "adiabatic process". – electronpusher May 05 '22 at 16:18
0

Chet Miller has already directly answered your specific questions. This is only intended to supplement it by providing further background on the concepts involved, in particular with regard to your first question.

Heat and work are two different and distinct means of transferring energy between a system and its surroundings. Heat $Q$ is energy transfer due solely to temperature difference. In order for it to occur there must be a temperature difference and the nature of the boundary between the system and its surroundings has to permit the flow of energy in the form of heat. Work $W$ is energy transfer due to force times displacement.

Although the two means of energy transfer are different, the result of the transfer on the change in internal energy $\Delta U$ can be identical as they are related by the first law which, for a closed system (no mass transfer) is given by

$$\Delta U=Q-W$$

Where $Q$ is positive if heat is added to the system and $W$ is positive if work is done by the system. Applying the above to your first question, using the example of an ideal gas contained in a cylinder fitted with a piston:

Let the piston be movable and the piston/cylinder be perfectly thermally insulated so that no heat can transfer between the gas and the environment outside the piston/cylinder. So in the first law equation $Q=0$. Work is then done on the gas by compressing it. We call this an adiabatic compression. Thus from the first law $\Delta U=-W$.

For an ideal gas, any process, the change in internal energy depends only on the change in temperature per the equation $\Delta U=nC_{v}(T_{f}-T_{i})$ where $T_i$ and $T_f$ are the initial and final temperatures of the gas. Therefore the first law becomes

$$nC_{v}(T_{f}-T_{i})=-W\tag{1}$$

(Since work is done on the gas, $W$ is negative making $\Delta U$ positive).

Now if instead we don't have a thermally insulated piston/cylinder and we lock the piston in place so that it cannot move, no work can be done i.e., $W=0$. We immerse the piston/cylinder in a constant temperature heat bath where $T=T_f$. Heat transfer to the gas increases the temperature from $T_i$ to T_f$. So now, from the first law

$$nC_{v}(T_{f}-T_{i})=Q\tag{2}$$

Equations (1) and (2) demonstrate that we can have the same change in temperature of the system by doing either adiabatic work or by heat transfer.

Hope this helps.

Bob D
  • 71,527
  • Would the best definition of work vs heat is that for work a 'force' must be required. – user1007028 May 06 '22 at 13:12
  • @user1007028 Not sure it makes sense to "define" work vs heat. Definitions generally apply to a single word. You could just as well say for heat a temperature difference is required. – Bob D May 06 '22 at 13:22
  • @user1007028 If you want to compare heat to work you could say for heat is a temperature difference is necessary and for work a net force is necessary. Note I say "necessary" because neither is a sufficient condition. For heat you also need a boundary that permits heat flow and for work you also need a displacement. – Bob D May 06 '22 at 13:29
  • I understand, actually kind of enlightening that, I realize that some things I would have called 'work' like heating something with electricity is more 'heat', my only problem is that power is generally defined for things where no work is done by this definition, like an oven for example. – user1007028 May 06 '22 at 13:39
  • @user1007028 Your original thinking about heat and electricity is actually thermodynamically correct, but heat is the consequence of electrical work. The sequence is roughly (1) work is done by the electric field to push electrons through a resistor (2) this results in an increase in molecular motion of the resistor increasing its internal energy and the temperature of the resistor, (3) the increase in resistor temperature relative to its environment results in heat transfer from the resistor to its environment. – Bob D May 06 '22 at 14:09
  • Thus what is called "Joule heating" or "resistance heating" is actually the consequence of electrical work. – Bob D May 06 '22 at 14:21
  • Okay, so all forms of work require some form of motion over displacement, and work is done. – user1007028 May 06 '22 at 17:30
  • @user1007028 Careful! Motion + displacement doesn't always equal work because motion doesn't always require a force – Bob D May 06 '22 at 18:45
  • of course, it's force over a displacement. – user1007028 May 06 '22 at 18:50