Thermodynamics · Equilibrium states, work, heat, and the first law
A sealed, rigid 0.080 m³ pharmaceutical autoclave is loaded with a spatially uniform…
Problem
A sealed, rigid 0.080 m³ pharmaceutical autoclave is loaded with a spatially uniform water charge and then isolated from every subsequent mass transfer. During a 12 min sterilisation interval the vessel walls remain rigid, an internal electric heater transfers 18.6 kJ of electrical energy into the water, and 5.1 kJ of heat leaves through the outer surface into the laboratory air. Treat the water inside the autoclave as the system. Adopt the closed-system convention ΔU = Q − W, with Q positive when heat is transferred to the system and W positive when the system does work on the surroundings; treat electrical energy supplied to the system as negative work. (a) Classify the system as open, closed, or isolated, and identify the system boundary and the surroundings. (b) List every energy interaction that crosses the boundary during the 12 min interval and state its direction. (c) Neglecting kinetic and potential energy, determine the change in internal energy of the water, ΔU, in kJ. (d) After the heater is switched off the valve remains shut and the walls remain rigid. Classify the system during this later period as open, closed, or isolated if heat loss to the room continues, and again if the outer surface is subsequently perfectly insulated.
Hint
Isolation forbids *all* interactions; a sealed rigid tank with heat or electrical work is only closed.
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