By the end of this lesson
- Solve a simple linear demand-and-supply model.
- Distinguish a price movement along a curve from a shift of the curve.
- Interpret equilibrium without treating it as a forecast or a fairness judgment.
Two schedules, one price
A demand schedule relates the quantity buyers wish to buy to the price, holding other relevant conditions constant. A supply schedule relates the quantity sellers wish to offer to the price under its own fixed conditions. Equilibrium occurs where the two quantities are equal.
The phrase 'holding other things constant' matters. A demand curve is not just a line joining every observed price and quantity over time. Income, expectations, substitutes, and many other conditions may have changed between observations. The model separates the relationship with price from changes in those other conditions.
Work through a numerical example
Our illustrative model is Qd = A − 2P and Qs = 10 + P. Q is units per week and P is euros per unit. With A = 100, set 100 − 2P equal to 10 + P. Solving gives P = 30 and Q = 40. These are calculated values from the assumptions, not observed market data.
At P = 20, buyers demand 60 units and sellers supply 30, leaving excess demand of 30 units in this model. At P = 40, buyers demand 20 units and sellers supply 50, creating excess supply. The diagram helps identify the direction of pressure, but it does not specify how fast actual prices will adjust.
Movement and shift are different claims
A change in the product's own price produces a movement along a given demand curve. An increase in demand means that buyers want more at the same price, represented by a shift of the curve. OpenStax uses this distinction to organize analysis of market changes.
Move the demand intercept in the tool. Increasing A shifts demand outward while leaving the supply schedule fixed. The new equilibrium has a higher price and quantity. If both supply and demand moved, one outcome could become ambiguous without knowing their relative magnitudes.
An important diagnostic is to name the event before moving a curve. A cheaper production input changes suppliers' conditions; it is not simply a movement along the old supply curve. A fall in the product's selling price, holding input costs fixed, is a movement along that curve.
Use the model within its boundaries
Illustrative case: a market receives more buyers while production capacity initially stays the same. The model suggests a higher clearing price. A real market may instead show queues, rationing, contracts, delayed supply, or a regulated price. Those institutions are additional assumptions, not details the diagram already captures.
Equilibrium is a consistency condition within a model. It does not mean that the outcome is fair, stable under every disturbance, or socially optimal in the presence of externalities or market power. Before applying the result, ask which competitive conditions, time horizon, and institutional arrangements the model assumes.
To analyze a new event, describe the starting point, identify which schedule changes, state the direction of the shift, and compare the old and new intersections. If you cannot identify the changed condition, a confident diagram may be hiding an unsupported premise.
Explore the model
Enable JavaScript to use this interactive model. The surrounding lesson explains its assumptions.
Keep the terms precise
- Equilibrium
- A price and quantity at which modeled quantity demanded equals quantity supplied.
- Excess demand
- Quantity demanded minus quantity supplied when demand exceeds supply at a given price.
- Demand shift
- A change in quantity demanded at each price because a non-price condition changes.
- Ceteris paribus
- Holding other relevant conditions constant.
Go deeper: assumptions and boundaries
With Qd = A − bP and Qs = C + dP, equilibrium price is (A−C)/(b+d) when b+d is positive. Check that the resulting price and quantity lie in the economically meaningful domain. A linear curve is a simplifying approximation and should not be extrapolated to negative quantities.
A question worth thinking through
If observed price and quantity both rise, what different changes could explain that pattern? What evidence would distinguish them?
Check your understanding
Check each answer and read the explanation. You can try again. Complete all 2 assessed questions to record the lesson as finished.
What to carry forward
- Equilibrium equates two modeled quantities.
- A curve shift changes the conditions behind the schedule.
- The diagram is conditional reasoning, not a standalone market forecast.
Sources & evidence
Original instructional explanation with hypothetical examples. Sources support the named concepts; they do not validate the illustrative scenarios.