Chapter 2 · Competitive markets: Demand and supply
Market clearing: surplus and shortage
Compare quantity demanded and quantity supplied above and below equilibrium. Show the price pressure that clears a surplus or shortage.
The economic idea
Market equilibrium occurs at the price at which quantity demanded equals quantity supplied. The equilibrium price is the market-clearing price: buyers’ and sellers’ plans are compatible, so there is no shortage or surplus.
Excess supply, also called a market surplus, occurs when quantity supplied exceeds quantity demanded at a given price. Excess demand, also called a shortage, occurs when quantity demanded exceeds quantity supplied at a given price.
The price mechanism coordinates these plans through signals and incentives. A shortage puts upward pressure on price; a surplus puts downward pressure on price. The model assumes that prices can adjust and that non-price determinants remain unchanged, ceteris paribus.
A market surplus is a quantity of excess supply. It is not producer surplus, which is the difference between the price producers receive and the minimum price they would accept for units sold, represented by a welfare area.
Excess supply: a market surplus
The horizontal axis measures the quantity of the good per stated time period. Quantities demanded and supplied must refer to the same time period.
The vertical axis measures the price per unit of the good, not the general price level.
“D” shows the quantities consumers are willing and able to buy at different prices, ceteris paribus.
“S” shows the quantities producers are willing and able to offer for sale at different prices, ceteris paribus. It does not establish how much is actually sold.
Compare quantities at the same price
The current market price “P” is above the equilibrium price “Pₑ”. At this same price, quantity supplied “Qs” exceeds quantity demanded “Qd”. This difference is excess supply, or a market surplus.
Read the pressure on price
Sellers cannot sell all the quantity they are willing and able to offer at this price. Competition for buyers gives sellers an incentive to lower price, creating downward pressure on price.
The price-pressure arrow points downward, towards “Pₑ”. It shows the direction of adjustment, not the speed at which it happens.
Separate the imbalance from the adjustment
A shortage or surplus is the difference between planned quantities at one price. Price pressure describes the incentive to change that price. A downward arrow does not mean demand shifts left, and an upward arrow does not mean supply shifts right.
A market surplus means excess supply, measured in units of the good over a period. Consumer and producer surplus are welfare areas, measured in money. Removing excess supply is market clearing; it does not mean eliminating the gains consumers and producers receive from trade.
Explain how the market clears
As price falls, quantity demanded increases along “D” and quantity supplied decreases along “S”. These movements along the existing curves narrow the surplus until quantity demanded equals quantity supplied at “E”.
What the current state establishes
The market-clearing position is “E” at “Pₑ” and “Qₑ”. The price mechanism tends to move the current price towards this position if prices can adjust.
Common mistakes to avoid
- Compare quantity demanded with quantity supplied at the same price. Write “quantity demanded equals quantity supplied”, not “demand equals supply”.
- A change in the good’s own price causes movements along the demand and supply curves, not shifts of those curves.
- Excess supply is a quantity difference. Producer surplus is a welfare area; the two meanings of “surplus” are different.
- The horizontal price line here is a trial market price, not a price floor or price ceiling. A legal control could prevent the adjustment shown in this lesson.
How to draw this diagram
- Title the diagram “Market clearing”. Label the vertical axis Price and the horizontal axis Quantity, with units and a time period if supplied.
- Draw and label downward-sloping demand D.
- Draw and label upward-sloping supply S.
- Mark the intersection E and use dashed guides to label the equilibrium price Pₑ and quantity Qₑ.
- Draw a horizontal current-price line above Pₑ for a surplus. Project its intersections with D and S to the quantity axis as Qd and Qs, so Qs > Qd.
- Mark the gap Qs − Qd as excess supply and add a downward price-pressure arrow pointing towards Pₑ.
- For a shortage, use a separate diagram or move the price line below Pₑ. Label Qd > Qs, mark the gap Qd − Qs as excess demand and show upward pressure towards Pₑ.
- At Pₑ, show Qd = Qs and no directional price-pressure arrow. Keep D and S unchanged throughout the adjustment.
Assumptions and limitations
- The model assumes competitive behaviour, unchanged non-price determinants and a price that can adjust. Contracts, search costs, incomplete information or a legal price control can slow or prevent that adjustment.
- The arrow shows the direction of price pressure, not an immediate change or a measured speed. Price elasticities and the time available affect how quantities respond.
- Quantity supplied is an intended offer for sale, not proof of actual production or sales. The diagram does not determine inventories, rationing rules or the quantity actually traded at a disequilibrium price.
- This is a market-clearing lesson, not a complete welfare calculation. A temporary shortage or surplus alone does not establish a permanent deadweight loss or prove that the equilibrium is equitable.