Quick question 1st law thermodynamics

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SUMMARY

The discussion focuses on the first law of thermodynamics, specifically regarding isothermal processes. The equation for heat flow in an isothermal system is given as ΔQ = ΔU + PΔV, where ΔU represents the change in internal energy and PΔV accounts for work done. It is established that if a system returns to its initial state on a PV graph, the total change in internal energy is zero, as both pressure and temperature remain constant, confirming that internal energy is dependent on temperature alone.

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  • Understanding of the first law of thermodynamics
  • Familiarity with the ideal gas law (PV=nRT)
  • Knowledge of internal energy concepts (U = nC_vT)
  • Basic grasp of PV diagrams and isothermal processes
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  • Study the implications of the first law of thermodynamics in various thermodynamic cycles
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ice87
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What is the equation to find the change in heat of a system that is isothermal?

And also, for a PV graph, if the state of the system loops back onto itself, as in it starts at a and ends at a, is the total change in internal energy always 0?
 
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ice87 said:
What is the equation to find the change in heat of a system that is isothermal?
Since PV=nRT, if T does not change, PV is constant. So the internal energy of the system does not change. As a result, there has to be heat flow to or from the system when work is done by or to the system (if it is compressed, heat is lost to the system; if it expands isothermally, it absorbs heat). The heat flow is: \Delta Q = \Delta U + P\Delta V = P\Delta V.

And also, for a PV graph, if the state of the system loops back onto itself, as in it starts at a and ends at a, is the total change in internal energy always 0?
Yes. If PV is the same, T is the same (PV=nRT). Since U = nC_vT, if the temperature is the same and n does not change, internal energy is the same.

AM
 
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