Physic Labs

Thermal physics

Heat transfer and the direction of entropy change

Observe spontaneous heat flow from hot to cold and the increase of total entropy in an irreversible process; examine the reversed exchange control. Record the reservoirs’ temperatures and total entropy during heat flow, and verify ΔStotal≥0ΔS_{total} ≥ 0.

Undergraduate

⚠ The thermal simulation is virtual; in real experiments, handle hot reservoirs and thermal equipment only with appropriate supervision.

Equipment

  • Hot and cold thermal reservoirs exchanging heat through a wall
  • Initial-temperature and thermal-conductance sliders; temperature and entropy plot

Procedure

  1. Follow spontaneous heat exchange

    Use the two sliders to set the hot reservoir temperature T_h above the cold temperature T_c, keep κ/C fixed, and watch the plot: T_h falls and T_c rises toward one another. Read total entropy and check that it does not decrease. Increase wall conductance to make the exchange faster. Press Reverse direction to inspect the simulated reverse heat transfer and its entropy change, then reset to restore the default process. Compare the observed quantities with ΔStotal≥0ΔS_total ≥ 0.

  2. Read the entropy changes

    Record both reservoirs' initial and final temperatures from the plot, then inspect total entropy. For heat Q transferred from hot to cold, check ΔStotal=−Q/Th+Q/Tc>0\Delta S_{total} = -Q/T_h + Q/T_c > 0 when Th>TcT_h > T_c.

  3. Vary the temperature difference

    Keep wall conductance fixed and increase, then reduce, the initial temperature difference. Compare equilibration rates and total-entropy increases; then use the reverse control to recognize that the reverse process is not spontaneous. Compare the observed quantities with ΔStotal≥0ΔS_total ≥ 0.

Simulation

Experiment history

The second law of thermodynamics developed during the nineteenth century from questions about why heat cannot be converted wholly into useful work and why natural processes have a preferred direction. Sadi Carnot analyzed heat-engine efficiency in 1824; Rudolf Clausius and William Thomson (Lord Kelvin) later clarified limits on heat conversion and formulated equivalent statements about the direction of heat transfer. Clausius introduced entropy into thermodynamics in 1865 and stated that the entropy of an isolated system does not decrease. When bodies at different temperatures are placed in contact, heat flows spontaneously from hot to cold; equilibration raises the total entropy, whereas the reverse does not occur spontaneously. This idealized account requires considering the entropy of both bodies, not just one.

Related physicists

Sadi CarnotRudolf ClausiusWilliam Thomson (Lord Kelvin)

Related library topics

The second law of thermodynamicsEntropy and the generalized second law