A sealed gas can gain energy. An insulated pump can exchange mass and shaft work. The words closed, open, and isolated describe exchanges across your chosen boundary.
Follow what crosses
What you’ll be able to do
Classify systems by actual boundary crossings.
Recognize a moving closed-system boundary.
Distinguish steady flow from no flow.
Before you touch the controls
A sealed cylinder is insulated, but you push its piston inward. Is the gas isolated?
Explore the modelChange a control. Follow the response.
MassNo crossing
HeatNo crossing in this model
WorkCrosses at the piston
ClassificationClosed+adiabatic
ΔU=40J if changes in bulk motion and elevation are negligible. The drawn piston displacement is schematic.
01
Closed systems follow a fixed mass.
No mass crosses a closed-system boundary. The boundary may still move or deform, as in a sealed piston-cylinder. Heat and work can cross. Closed says nothing by itself about thermal insulation.
02
Control volumes account for flowing material.
A control volume is a chosen region used to account for mass and energy crossing its surface. A fixed region around a pump is convenient for steady operation. Equal inflow and outflow mean zero mass accumulation, not zero boundary crossing.
03
Isolation forbids both mass and energy exchange.
An isolated system exchanges neither mass nor energy. Adiabatic means no heat transfer, a weaker condition. Insulation does not prevent shaft work, electrical work, or moving-boundary work. A rigid boundary prevents volume-change work, but other work mechanisms can remain.
A worked example
A heated, sealed, rigid tank.
No mass crosses: the tank contents form a closed system.
Heat crosses the wall: it is not isolated. Its rigid wall does no volume-change work.
If there is no other work and negligible motion or elevation change, added heat increases internal energy.
Make the idea your own
Predict. Calculate. Question the model.
Try each problem before opening its solution. Then change a value to test whether the reasoning still holds.
01 · Reason
Can insulation make a gas isolated?
A sealed, insulated piston-cylinder is compressed by an external force. No gas escapes. Classify the gas and identify the energy transfer.
A small hint
Insulation blocks heat, not a force acting through a moving boundary.
Explore the visual solution
Adiabatic closed-system energy changeΔU = +30 J
Q = 0; W = −30 J. ΔU = Q − W. The piston position is schematic.
What if you change it?
The visual responds to your control. The worked steps below use the original question.
Original question · Step 1 of 3
The system is closed because its mass is fixed.
Carry this forward.A moving boundary can transfer energy while remaining impermeable to mass.
02 · Calculate
A steady stream through a device.
A tank receives and releases 2 kg/s. How much mass crosses its inlet in 30 s? How much mass accumulates?
A small hint
Gross transfer through one port is not the same as net accumulation.
Explore the visual solution
Mass entering60 kg
Mass leaving60 kg
Mass accumulated0 kg
What if you change it?
The visual responds to your control. The worked steps below use the original question.
Original question · Step 1 of 3
Inlet mass=2×30=60kg; outlet mass is also 60 kg.
Carry this forward.Steady behavior permits continuous throughput.
03 · Challenge
Move the boundary around a heater.
A resistor immersed in water receives 200 J of electrical energy. Compare the water alone with water plus resistor. Neglect losses and assume the resistor ends at its initial internal energy.
A small hint
The same energy may cross different chosen boundaries through different mechanisms.
Explore the visual solution
Transfer across the chosen boundaryHeat
For the original process, water gains 200 J. Changing the boundary changes the transfer label, not the energy result.
What if you change it?
The visual responds to your control. The worked steps below use the original question.
Original question · Step 1 of 3
For water alone, energy crosses the resistor-water interface because of a temperature difference: heat transfer.
Carry this forward.Heat and work labels depend on the mechanism at the selected boundary.
Your learning path
Ready to carry this forward?
Mark this lesson complete when you can explain the main idea and work through its practice. Completion is your choice.
