World Price, Imports, and Exports

Before trade, a domestic market settles at its own equilibrium price, where domestic quantity supplied equals domestic quantity demanded. Opening the border introduces a world price, set by the global market, which the country is too small to influence. If the world price sits below the domestic equilibrium price, domestic consumers can now buy for less than they used to pay, so they demand more than domestic producers are willing to supply at that low price. The gap between quantity demanded and quantity supplied, both measured at the world price, is imports. If the world price sits above the domestic equilibrium price, the logic flips: domestic producers want to sell more than domestic consumers want to buy, and the gap becomes exports.

The surplus story explains the politics. With imports, the lower price helps every consumer and hurts every domestic producer, but consumer surplus grows by more than producer surplus shrinks, so total surplus rises: trade creates a net gain, even though it creates losers. With exports, the higher world price helps producers and hurts consumers, but again total surplus rises overall. Either direction, opening trade moves the price toward the world price and expands the combined surplus beyond what the closed economy could reach, because trade lets the country specialize in whichever side of the market it does better.

This sits right after taxes and price controls in the unit because the tools are identical: find the new price, read quantities off each curve, and compare the surplus triangles before and after. World price just replaces the tax or the ceiling as the thing that moves price away from equilibrium, and the same consumer surplus and producer surplus regions do the explaining.