Physic Labs

Thermal physics

Heat, work, and internal energy

Test the first law ΔU = Q − W, with W defined as work done by the gas, in isobaric and isochoric processes. Measure heat, gas work, and internal-energy change, then verify ΔU=Q−WΔU = Q − W.

Undergraduate

⚠ The gas and heat-exchange model is virtual; real equipment can be hot and pressurized and requires supervision.

Equipment

  • Ideal-gas cylinder with a free or locked piston
  • Volume V and heat-per-step Q sliders; Q, W, and ΔU plot

Procedure

  1. Compare isobaric and isochoric processes

    Select Isobaric (free piston), set the initial volume with the V slider, and add heat using Q; watch the piston move and follow the accumulated quantities on the plot. Reset, choose Isochoric (locked piston), and supply the same Q: volume stays fixed, so the gas does no boundary work and the heat increases its internal energy. Compare the observed quantities with ΔU=Q−WΔU = Q − W.

  2. Calculate the internal-energy change

    Record Q and work W on the isobaric plot and check ΔU=Q−W\Delta U = Q - W. Repeat in the isochoric mode, where W = 0, to compare how the supplied heat changes internal energy.

  3. Compare accumulated quantities

    Reset before each run, change only the process mode, and keep Q the same. Read the volume change, work, and internal energy; explain the difference by whether the piston moves or is locked. Compare the observed quantities with ΔU=Q−WΔU = Q − W.

Simulation

Experiment history

The first law of thermodynamics took shape in the nineteenth century as scientists sought one conservation principle for heat and work. James Prescott Joule stirred water with falling weights, measured its temperature rise, and showed that mechanical work could be converted into heat in a definite proportion. His experiments helped displace the idea that heat was a separate conserved substance. Rudolf Clausius systematized the principle in 1850, describing heat and work as ways of changing a system's internal energy. With the convention used here, Q is positive when the gas receives heat and W is positive when the gas does work, giving ΔU = Q − W. Work-sign conventions vary, so stating the convention is essential when comparing isobaric and isochoric processes.

Related physicists

James Prescott JouleRudolf Clausius

Related library topics

The first law of thermodynamicsInternal energy and its change