Revise Market equilibrium and the price mechanism for IB Economics (2.3)

In any market, buyers and sellers pull in opposite directions: buyers prefer low prices, sellers prefer high ones. Market equilibrium is the compromise between them. It is the price at which the quantity consumers are willing and able to buy exactly equals the quantity producers are willing and able to sell. The market is then said to 'clear' — every unit offered for sale is bought, and every buyer willing to pay the going price finds a seller. With both sides satisfied, there is no internal pressure for price to move; it will only change if an outside factor shifts demand or supply.

When the ruling price differs from $P^*$ the market is in disequilibrium, showing up as either excess supply or excess demand. The key insight is that neither can persist in a free market — the price itself moves to remove the imbalance.

Excess supply (a surplus) occurs when price is set above $P^*$. The high price encourages producers to offer a lot but discourages buyers, so $Q_s > Q_d$. Left with unsold stock, sellers cut their prices to shift it. As price falls, quantity supplied contracts (a movement down along the supply curve) and quantity demanded expands (a movement down along the demand curve). The gap narrows until, at $P^*$, it closes entirely.

Excess demand (a shortage) occurs when price is set below $P^*$. The low price attracts many buyers but makes supplying unprofitable, so $Q_d > Q_s$. With too many buyers chasing too few goods, sellers realise they can charge more and buyers bid against one another. As price rises, quantity demanded contracts and quantity supplied expands — both movements along their respective curves — until the shortage is eliminated at $P^*$. Notice that the curves themselves do NOT move here; only the price adjusts along fixed curves.

Equilibrium only changes when a non-price determinant shifts a curve. A useful three-step routine handles every case: (1) state the starting equilibrium; (2) identify the determinant that changed, which curve shifts and in which direction; (3) read off the new equilibrium and compare price and quantity. Because buyers and sellers are interdependent, a change originating on one side of the market always works its way through to the other via the price.

No official decides what an economy should produce, how, and for whom. In a market economy those decisions emerge from the price mechanism — Adam Smith's 'invisible hand'. Through constantly changing prices, the independent choices of millions of buyers and sellers are coordinated. It is worth seeing the three functions as one continuous story rather than three separate ideas: a scarcity shows up as a rising price, which simultaneously signals, incentivises and rations.

Example in action: a bad harvest makes wheat scarce. The rising wheat price SIGNALS the scarcity, gives farmers an INCENTIVE to plant more next season and buyers an incentive to switch to substitutes, and RATIONS this year's smaller crop to the buyers who value it most. Resources are reallocated — all without a central planner.

The demand curve shows the maximum price buyers are willing to pay for each unit; the supply curve shows the minimum price sellers would accept. Because everyone trades at the single equilibrium price, most buyers and sellers do better than their limit — and the two surpluses measure that gain.

At the competitive equilibrium, social surplus is as large as it can be — this is allocative efficiency. Here the price on the demand curve (society's marginal benefit from the last unit) equals the price on the supply curve (the marginal cost of producing it), so the 'right' amount is produced: neither too little nor too much. Produce below $Q^*$ and there are willing buyers whose benefit exceeds the cost of supplying them, so a mutually beneficial trade is being missed. Produce above $Q^*$ and the cost of the extra units exceeds the benefit. Only at $Q^*$, where marginal benefit = marginal cost, is community surplus maximised and resources allocated efficiently — the deeper reason economists care about equilibrium.

Equilibrium and the price mechanism are the benchmark for everything that follows. When later topics ask what a price ceiling, a price floor, a tax or a subsidy 'does', the question is always the same: how does it push the market away from the efficient equilibrium, and what happens to consumer and producer surplus as a result? Master the balancing act here and market failure and government intervention become variations on one familiar diagram.