Final answer:
In the scenario of a steel rail expanding and displacing its restraints, the work (W) would be negative as the rail is doing work on its surroundings while the heat transfer (Q) is positive because the rail is absorbing heat. The change in internal energy (ΔU) cannot be determined without additional data.
Step-by-step explanation:
The student is asking which thermodynamic values (U, Q, or W) would be negative for a steel rail system undergoing thermal expansion on a hot day that displaces the spikes and ties holding it in place. In thermodynamics, Q represents the heat transfer in or out of the system, W represents the work done by or on the system, and ΔU represents the change in internal energy of the system. According to the first law of thermodynamics, expressed as ΔU = Q - W, the internal energy change ΔU of a system is equal to the heat added to the system (Q) minus the work done by the system (W).
For a steel rail undergoing slow thermal expansion, W would be negative because the steel rail is doing work on the surroundings (the spikes and ties) as it expands. The heat transfer Q would be positive because the rail is absorbing heat from the environment due to the high temperature. The change in internal energy ΔU could be positive or negative depending on the magnitude of Q and W, but without specific values of heat and work, it cannot be determined from the given information alone.