11/12.2 First Law of Thermodynamics | General Physics

Chad provides a lesson on the First Law of Thermodynamics which begins with the relationship between the internal energy change, heat, and work (Delta U = q + w). He explains how to determine when q and w are positive and negative:
1.Heat flows from the surroundings to the system (q is positive).
2. Heat flows from the system to the surroundings (q is negative).
3. Work is done on the system by the surroundings (w is positive).
4. Work is done by the system on the surroundings (w is negative).
A description of PV work (pressure-volume work) follows and how the work is equal to the area under the curve on a graph of pressure vs volume. If a gas expands, work is done by the system (w is negative), whereas if a gas is compressed work is done on the system (w is positive). The equations for PV work for an isobaric expansion and an isothermal expansion are then derived and presented.

Chad then explains how to calculate heat using molar heat capacity. He begins with a review of the internal energy of an ideal gas. He explains that difference between the constant volume heat capacity and the constant pressure heat capacity (Cv and Cp) and how and when each can be used to calculate heat. Equations for Cv and Cp of an ideal gas are introduced with how they relate to the degrees of freedom. Chad explains how to determine the number of degrees of freedom (translational and rotational) for a monatomic ideal gas, a diatomic (linear) ideal gas, and for a non-linear ideal gas, and thereby the corresponding values for Cv and Cp.

Finally, Chad summarizes how to calculate the internal energy change, the heat, and the work for 4 different types of processes:
1. An Isobaric process (constant pressure)
2. An Isochoric process (constant volume)
3. An Adiabatic process (no heat transfer)
4. An Isothermal process (constant temperature)

Chad concludes the lesson by solving two First Law of Thermodynamics physics problems, the internal energy change, heat, and work for an isobaric expansion and for an isothermal expansion of an ideal gas.

00:00 Lesson Introduction
00:53 Delta U = q + w
05:06 PV Work of an Ideal Gas
13:35 Heat and Heat Capacity (Cv and Cp)
25:37 Thermodynamic Calculations for Isobaric, Isochoric, Adiabatic, and Isothermal Processes
31:53 First Law Calculations for Isobaric Expansion of an Ideal Gas
38:31 First Law Calculations for Isothermal Expansion of an Ideal Gas

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