Unit 20: The First Law of Thermodynamics
Heat, work, and internal energy tracked through PV diagrams across the four special thermodynamic processes.
5 topics · 16 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
Thermodynamic Systems and Internal Energy
- How it worksHow does internal energy account for all the microscopic kinetic and potential energy hidden inside a system?
- Common mistakeWhy isn't internal energy the same thing as temperature, even though the two are closely related?
- Going furtherWhat happens to a gas's internal energy when work is done on it without letting any heat escape?
Your turn1 of 2
Two containers hold different amounts of the same ideal gas at the exact same temperature: one holds 1 mole, the other holds 10 moles. Do they have the same internal energy?Heat, Work, and the First Law
- How it worksHow does the first law of thermodynamics track energy as it moves between heat, work, and internal energy?
- Worked exampleFind the change in internal energy of a gas that absorbs 500 J of heat while doing 200 J of work on its surroundings.
- Common mistakeWhy doesn't doing work on a gas always mean pumping heat into it?
Your turn1 of 2
A gas absorbs heat Q while doing work W on its surroundings. Write an expression, in terms of Q and W, for the change in its internal energy.PV Diagrams and Thermodynamic Processes
- How it worksHow does the area under a curve on a PV diagram represent the work done by or on a gas?
- Worked exampleFind the work done by a gas expanding at a constant pressure of 2e5 Pa from 0.01 m3 to 0.03 m3.
- Common mistakeWhy doesn't the work done during a thermodynamic process depend only on its initial and final states?
- Going furtherWhat happens to the work done by a gas if it's compressed instead of expanded along that same path on a PV diagram?
Your turn1 of 2
A gas expands at a constant pressure of 239283 Pa from 0.0147 cubic meters to 0.058 cubic meters. How much work does it do, in joules?Isothermal, Isobaric, Isochoric, and Adiabatic Processes
- How it worksHow does each of the four special thermodynamic processes hold a different variable fixed while the others change?
- Worked exampleFind the work done by an ideal gas expanding isothermally at 300 K from 0.01 to 0.02 m3, for 1 mole using W = nRT ln(V2/V1).
- Common mistakeWhy doesn't an adiabatic process mean nothing at all is happening to the gas's temperature?
Your turn1 of 2
1 mole of an ideal gas expands isothermally at 354 K from 0.0115 cubic meters to 0.0386 cubic meters. Using W = nRT ln(V2/V1), how much work does it do, in joules?Molar Specific Heats of an Ideal Gas
- How it worksHow does some of the added heat go into expansion work at constant pressure instead of purely raising internal energy?
- Worked exampleFind the heat needed to raise the temperature of 2 moles of an ideal monatomic gas by 50 K at constant volume, using Cv = (3/2)R.
- Ties togetherHow does the difference between Cp and Cv for an ideal gas connect back to the work term in the first law?
Your turn1 of 2
How much heat is needed to raise the temperature of 4 moles of an ideal monatomic gas by 55 K at constant volume, using Cv = (3/2)R?