Fluid Mechanics · Unit 1 · Lesson 10 of 13
Where Does Your System End?
A pressure cooker is a closed system while it builds up pressure, and an open system the moment it whistles. How can one cooker be both?
1. Real-World Problem
An engineer is asked how much energy a building's water heater uses each day. Before any calculation, she must decide exactly what she is analysing: the heater alone, the heater and its pipes, or the whole bathroom. The answer depends on where she draws the line.
2. Predict
Make your guess before you try the experiment. You will see the answer after the next step.
Predict
3. Experiment
Sort the 8 cards into Open, Closed and Isolated (drag a card, or tap a card and then a bin). Each card flips to show what crosses its boundary: mass, heat or work.
Then open Move the boundary: draw it around the rooftop tank alone, then around the tank, pipes, taps and buckets together, and see how the change in mass inside changes.
4. Understand
- System: the part of the universe chosen for study. It can be a fixed quantity of matter or a region in space.
- Surroundings: everything outside the system that can interact with it.
- Boundary: the real or imaginary surface separating the two. It can be fixed or moving, rigid or flexible.
| Type | Mass crosses? | Energy crosses? | Example |
|---|---|---|---|
| Open system | Yes | Yes | Pump, nozzle, radiator |
| Closed system | No | Yes | Sealed piston and cylinder |
| Isolated system | No | No | Ideal thermos; the universe as a whole |
A boundary that lets no heat through is called adiabatic; you will use this word in Thermodynamics. In the next lesson you meet the two tools engineers use to analyse these systems: the control mass and the control volume.
5. Derive
Step 1. Everything is either inside the boundary or outside it:
\text{Universe} = \text{System} + \text{Surroundings}Step 2. Mass is conserved, so whatever crosses the boundary changes the system's mass by exactly that amount:
\Delta m_{system} = m_{in} - m_{out}Step 3. For a closed system m_in = m_out = 0, so its mass never changes, even if heat and work cross the boundary.
6. Calculate
A boundary is drawn around a rooftop tank holding 500 kg of water. In 10 minutes the pump delivers 300 kg and the taps draw 120 kg.
- Δm = 300 − 120 = 180 kg; the tank ends with 680 kg.
- Mass crosses the boundary, so with this boundary the tank is an open system.
- Redraw the boundary around the tank, the pipes and the taps' buckets together: the 120 kg drawn now stays inside, and only the 300 kg from the pump crosses in. Same water, different boundary, different bookkeeping.
7. Apply
- A refrigerator's cabinet is drawn as a system to count how much heat leaks in.
- A car engine is drawn as an open system: air and fuel in, exhaust out, work out at the shaft.
- A building's water supply: the boundary is drawn around the overhead tank.
- A battery-powered phone: a closed system (no mass crosses, energy leaves as heat).
8. Validate
In Move the boundary mode, set the boundary around the tank alone with the default flows (300 kg in, 120 kg drawn): the tool must show +180 kg inside. Move the boundary to include the buckets: +300 kg. The physics is unchanged; only what you chose to count changed. Check: what the system gains, the surroundings lose, so the universe's total mass stays the same.
Easy To Mix Up
- The boundary does not have to be a real wall. In a river you can draw an imaginary box in the middle of the water.
- Closed does not mean nothing crosses. Heat and work still can; only mass cannot.
- Truly isolated systems are an idealisation; real ones are only close.
9. Quiz
Four quick questions: a concept, a calculation and an interpretation. Each answer comes with an explanation. Score 3 or more to mark this lesson completed.
Check Your Understanding
Four questions: a concept, a calculation and an interpretation. Every answer comes with an explanation. Score 3 of 4 to mark the lesson completed.
You have drawn the boundary. Should it travel with the fluid, or stay put while fluid flows through it?
Go to Lesson 11: Follow The Fluid Or Watch A Fixed Box? →Read More
- Cengel and Cimbala, "Introduction and Basic Concepts" (system and control volume); Cengel and Boles, Thermodynamics: An Engineering Approach, chapter 1 (systems).
- NPTEL, Introduction to Fluid Mechanics (Prof. Suman Chakraborty, IIT Kharagpur): Week 1.