Storages and flows
Big idea (learn this first): All systems have storages, flows, and system boundaries. If you can name these three, you can answer most IB questions.
[Diagram: storage-flow-basic] - Available in full study mode
What is a storage?
A storage (also called a stock) is a place where matter, energy, or information builds up over time.
Easy rule: if it can increase or decrease, it is a storage.
- 💧 Water in a reservoir
- 🌫️ CO₂ in the atmosphere
- 🧊 Ice in a glacier
- 🛢️ Oil underground
- 💰 Money in a bank account
In system diagrams, storages are shown as boxes 📦. A bigger box means a larger amount stored.
What is a flow?
A flow is the movement of matter, energy, or information into or out of a storage.
Flows change storages. No flow = no change.
| Type | What it does | Arrow direction |
|---|---|---|
| Inflow | Increases the storage | → INTO the box |
| Outflow | Decreases the storage | ← OUT OF the box |
- 🌧️ Rain into a reservoir → inflow
- 🚿 Water released from a dam → outflow
- 🏭 CO₂ emissions → inflow to the atmosphere
- 🌲 CO₂ absorbed by forests → outflow from the atmosphere
In diagrams, flows are shown as arrows →. A thicker arrow means a larger flow.
See it in action: Reservoir example
[Diagram: reservoir-system] - Available in full study mode
⚠️ Inputs vs inflows (IB exam trap!)
Students often mix these up. Here's the simple difference:
| Term | What it means | Example |
|---|---|---|
| Inputs / Outputs | The THINGS that move | Water, oil, money, pollution |
| Inflows / Outflows | The PROCESSES that move them | Rainfall, evaporation, spending |
In exams, say "rainfall is an inflow", not "rainfall is an input". The water itself is the input; rainfall is the process (inflow).
System boundaries
A system boundary shows what is included in the system and what is left out.
- 🔍 Too small → important influences are missed
- 🌍 Too large → the system becomes too complex to analyse
Good models choose a useful boundary, not a perfect one. There is no single "correct" boundary.
The 5 key rules (VERY exam-important)
- Storage increases when inflows > outflows (Example: more rain than evaporation → reservoir fills up)
- Storage decreases when outflows > inflows (Example: more evaporation than rain → reservoir empties)
- Dynamic equilibrium happens when inflows = outflows (storage stays the same, but things are still moving)
- Storages change slowly — flows can change quickly, but storages take time to respond
- Storages act as buffers — they slow down change and create time delays
[Diagram: equilibrium-diagram] - Available in full study mode
Real-world example: The carbon cycle
Let's apply storages and flows to something you'll see in exams:
- Storages: atmosphere, oceans, forests, fossil fuels, soil
- Inflows to atmosphere: burning fossil fuels, respiration, decomposition
- Outflows from atmosphere: photosynthesis, ocean absorption
- Human impact: We've increased inflows (burning) faster than outflows can remove CO₂
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Transfers and Transformations
Flows move matter or energy through a system. There are two main types you need to know for the IB.
Transfers
A transfer happens when something moves location but stays the same.
- Water flowing in a river
- Wind moving air
- Animals migrating
- Heat moving by ocean currents
Key idea: Same substance, new location.
Transformations
A transformation happens when something changes into something different.
- Water evaporating (liquid → gas)
- Photosynthesis (light energy → chemical energy)
- Combustion of fossil fuels
- Digestion of food
Key idea: Something new is created.