Competitive market equilibrium

The point where the quantity demanded by consumers equals the quantity supplied by producers (Qd = Qs). It occurs where the demand and supply curves intersect. At this point the market clears — no surplus and no shortage.

P axis, Q axis, demand curve (D), supply curve (S), equilibrium point (E), equilibrium price (Pₑ), and equilibrium quantity (Qₑ). A title (e.g., "Market for coffee") is also expected. Missing labels lose marks.

There is excess supply (a surplus). Quantity supplied exceeds quantity demanded (Qs > Qd). Producers have unsold stock, so they cut prices to attract buyers. Price falls back towards equilibrium.

Pₑ is the price at which Qd = Qs — the market-clearing price. Qₑ is the quantity actually bought and sold at that price. At equilibrium, every willing buyer finds a seller and every willing seller finds a buyer.

There is excess demand (a shortage). Quantity demanded exceeds quantity supplied (Qd > Qs). Buyers compete for limited stock, so firms raise prices. Price rises back towards equilibrium.

1. Draw axes (P on Y, Q on X). 2. Draw downward-sloping D curve. 3. Draw upward-sloping S curve. 4. Mark intersection as E. 5. Draw dashed lines to both axes. 6. Label Pₑ and Qₑ. 7. Add a title.

The process by which market forces of supply and demand interact to determine and adjust prices. Surpluses push prices down and shortages push prices up, naturally guiding the market towards equilibrium. It is the self-correcting nature of free markets.

Because almost every microeconomic analysis uses it: price changes, government intervention (tax, subsidies, price controls), market failure, surplus analysis, and elasticity. Mastering this diagram is essential for every exam paper.

The market clears when there is no excess supply (surplus) and no excess demand (shortage). All goods produced at the equilibrium price are sold, and all consumers willing to pay that price can buy.

Because surpluses create downward pressure on price (firms cut prices to clear unsold stock) and shortages create upward pressure (firms raise prices as buyers compete). These forces automatically push the market towards the equilibrium point.

Mark the intersection of D and S with a dot labelled "E". Draw dashed lines from E down to the X-axis (labelled Qₑ) and across to the Y-axis (labelled Pₑ). Always label both values clearly.

Either is acceptable, but slightly curved lines look more professional. What matters most is: D slopes downward, S slopes upward, both are clearly labelled, and they intersect at a clearly marked point.

It is the natural resting point of a free market — the price and quantity the market tends towards. It represents balance between buyers and sellers and is the starting point for analysing any market change.

Draw a horizontal line above Pₑ at price P₁. At P₁, read Qs (on the supply curve) and Qd (on the demand curve). The gap between Qs and Qd is the surplus. Label it clearly with a double-headed arrow.

Show the original equilibrium (E₁) with Pₑ₁ and Qₑ₁. Then show the curve shift, labelling the shifted curve. Mark the new equilibrium (E₂) with Pₑ₂ and Qₑ₂. Use arrows to show the shift direction.

One variable has a definite outcome, but the other is indeterminate (depends on which shift is bigger). You can predict what happens to one variable but must state the other is uncertain.

Demand increases → at the old price there is a shortage (Qd > Qs) → price is bid up → new equilibrium has higher price AND higher quantity (P↑ Q↑).

Supply increases → at the old price there is a surplus → price falls → new equilibrium has lower price and higher quantity (P↓ Q↑).

Supply decreases → at the old price there is a shortage → price rises → new equilibrium has higher price and lower quantity (P↑ Q↓).

Quantity definitely increases (both shifts push Q up). But price is indeterminate — demand pushes P up while supply pushes P down. The net effect on price depends on which shift is larger.

Demand decreases → at the old price there is a surplus (Qs > Qd) → price falls → new equilibrium has lower price AND lower quantity (P↓ Q↓).

Price and quantity move in OPPOSITE directions. Supply right → P↓ Q↑. Supply left → P↑ Q↓. This opposite-direction pattern identifies a supply shift.

Price definitely increases (both shifts push P up). But quantity is indeterminate — demand pushes Q up while the supply decrease pushes Q down. The net effect on quantity depends on which shift is larger.

Price and quantity move in the SAME direction. Demand right → P↑ Q↑. Demand left → P↓ Q↓. This pattern helps identify that a demand shift (not supply) caused the change.

Bad weather destroys part of the wheat crop → supply shifts left. At the old price there is a shortage. Price rises to a new equilibrium with less wheat sold at a higher price (P↑ Q↓).

Analyse each shift separately: identify the cause, determine which curve it affects and in which direction. Then combine: state the definite outcome for one variable and explain that the other is indeterminate without knowing the relative size of each shift.

Higher consumer income (for a normal good like restaurant meals) shifts demand right. At the old price there is a shortage. Price rises to a new equilibrium with more meals sold at a higher price (P↑ Q↑).

Draw original D₁ and S with equilibrium E₁ (Pₑ₁, Qₑ₁). Draw new D₂ to the right or left. Mark new equilibrium E₂ where D₂ intersects S. Label Pₑ₂ and Qₑ₂. Add an arrow showing the shift direction.

Population growth increases demand for coffee (D right). At the same time, new farming technology increases supply (S right). Quantity definitely rises (both shifts push Q up), but price could rise, fall, or stay the same — it depends on which shift is bigger.

If P and Q moved in the same direction (both up or both down) → demand shifted. If P and Q moved in opposite directions (one up, one down) → supply shifted. This pattern is a key exam diagnostic tool.

Community surplus = consumer surplus + producer surplus. It represents the total net benefit to society from trading in the market. At the free-market equilibrium, community surplus is maximised.

The difference between the maximum price a consumer is willing to pay and the actual market price they pay. It is the "bonus" buyers get when they pay less than they were prepared to.

The difference between the actual market price received and the minimum price a producer was willing to accept. It is the "bonus" sellers get when they sell for more than their minimum acceptable price.

The triangle above the equilibrium price line and below the demand curve, from Q = 0 to Qₑ. The demand curve shows willingness to pay; the market price is what consumers actually pay. The gap is their surplus.

When resources are allocated so that the net benefit to society is maximised, occurring where price equals marginal cost (P = MC). This happens at the free-market equilibrium where community surplus is maximised.

The triangle below the equilibrium price line and above the supply curve, from Q = 0 to Qₑ. The supply curve shows the minimum price producers need; the market price is what they actually receive. The gap is their surplus.

A loss of total surplus that occurs when the quantity traded is not at the efficient level. Government intervention (price controls, taxes) can prevent the market from reaching equilibrium, creating a triangle of lost welfare.

$6. Consumer surplus = willingness to pay ($10) minus actual price ($4) = $6. On the diagram this $6 is part of the triangle above the price line.

Consumer surplus is ABOVE the price line, BELOW the demand curve (consumers are "on top"). Producer surplus is BELOW the price line, ABOVE the supply curve (producers are "below"). Together they form the total welfare area.

Consumer surplus decreases (buyers pay more, fewer can afford it). Producer surplus increases (sellers receive more per unit). Some surplus transfers from consumers to producers. Total community surplus may or may not change depending on the cause.

Because the first consumers value the good most highly (top of the demand curve). They would have paid much more than the market price. The last unit at Qₑ has zero surplus — the buyer's willingness to pay equals the market price.

Because the first units are cheapest to produce (bottom of the supply curve). The gap between the market price and the low production cost is large. The last unit at Qₑ has zero surplus — the cost of production equals the market price.

Consumer surplus increases. The price line drops, enlarging the triangle between the demand curve and the price. More consumers can now buy, and existing buyers pay less — both effects increase total consumer surplus.

Producer surplus increases. The price line rises, enlarging the triangle between the price and the supply curve. Existing producers earn more per unit, and higher-cost producers can now enter profitably — both effects increase total PS.

Interventions like price controls and taxes move the market away from the equilibrium quantity. When Q traded ≠ Qₑ, some mutually beneficial trades do not happen. The surplus those trades would have generated is lost — that is deadweight loss.

Set Qd = Qs. If Qd = a − bP and Qs = c + dP: a − bP = c + dP → a − c = bP + dP → P* = (a − c)/(b + d). Then substitute P* back into either equation to find Q*: Q* = a − bP* or Q* = c + dP*.

Set equal: 200 − 4P = −20 + 3P → 220 = 7P → P* = $31.43 (approx). Q* = 200 − 4(31.43) = 200 − 125.71 = 74.29 ≈ 74 units. Check: Qs = −20 + 3(31.43) = −20 + 94.29 = 74.29 ✓

P* depends on: (a − c) = the gap between demand intercept and supply intercept — a bigger gap means higher equilibrium P. (b + d) = the combined responsiveness of demand and supply — higher combined responsiveness means lower P* (more elastic → price adjusts less).

New P*: 260 − 4P = −20 + 3P → 280 = 7P → P* = $40. New Q* = 260 − 4(40) = 100 units. Compare to original: P* rose from ≈$31.43 to $40, Q* rose from ≈74 to 100. Demand shift right → higher P* and Q*.

If the price floor (Pf) is above P*: excess supply = Qs(Pf) − Qd(Pf). Calculate by substituting Pf into both equations. If Pf is below P*, it is NON-BINDING (market stays at P*). Excess supply = (c + dPf) − (a − bPf) = (b + d)Pf − (a − c).

CS = area of triangle ABOVE P* and BELOW demand curve = ½ × Q* × (P-intercept − P*) = ½ × Q* × (a/b − P*). PS = area of triangle BELOW P* and ABOVE supply curve = ½ × Q* × (P* − supply P-intercept) = ½ × Q* × (P* − (−c/d)). Total surplus = CS + PS.