基本经济概念

Economics 经济学 begins with scarcity 稀缺性: people's wants are unlimited, but the resources 资源 to satisfy them are limited. The resources – the factors of production 生产要素 – are land, labor 劳动, capital 资本, and entrepreneurship 企业家才能. Scarcity forces every society to choose what, how, and for whom to produce, and it makes trade-offs 权衡 unavoidable.

Every choice has an opportunity cost 机会成本 – the value of the next-best alternative given up. The production possibilities curve (PPC) 生产可能性曲线 shows the maximum combinations of two goods an economy can make when resources are fully and efficiently used.

• Points on the curve are efficient; inside, inefficient (unemployed resources); outside, unattainable.
• The downward slope shows opportunity cost; a bowed-out curve shows increasing opportunity cost.
• More resources or better technology cause economic growth 经济增长, shifting the PPC outward.

A producer has an absolute advantage 绝对优势 in a good if it can make more of it. It has a comparative advantage 比较优势 if it has the lower opportunity cost. Nations should specialize in their comparative-advantage goods and trade – both gain if the terms of trade 贸易条件 lie between the two opportunity costs. Trade lets a country consume beyond its own PPC.

Worked example. Country A can make $10$ wheat or $5$ cloth per day; Country B can make $8$ wheat or $8$ cloth. For A, one cloth costs $\tfrac{10}{5}=2$ wheat; for B it costs $\tfrac{8}{8}=1$ wheat. B's opportunity cost of cloth is lower, so B specializes in cloth. Checking wheat: A gives up $0.5$ cloth per wheat and B gives up $1$, so A specializes in wheat. Trading at any rate between the two costs, both consume beyond their own PPC.

Check the gains with numbers. Before trade, each country splits its day in half: A makes 5 wheat and 2.5 cloth, B makes 4 wheat and 4 cloth. After specializing, A makes 10 wheat and B makes 8 cloth. They trade 4 wheat for 3 cloth, a price of $\tfrac43 \approx 1.33$ wheat per cloth, which lies between B's cost (1) and A's cost (2). A now has 6 wheat and 3 cloth; B has 4 wheat and 5 cloth. Both have more than before, so both points lie outside their own PPCs.

Output or input data? AP questions give one of two kinds of table, and the method differs.

• Output (units made with the same resources), as above: the opportunity cost of good X is $\dfrac{\text{units of Y}}{\text{units of X}}$ ("other over").
• Input (hours or workers needed to make one unit): the opportunity cost of X is $\dfrac{\text{hours per X}}{\text{hours per Y}}$ ("same over").

Input example. Country C needs 2 hours for a shirt and 4 hours for a bike; Country D needs 3 hours for a shirt and 9 hours for a bike. C has the absolute advantage in both goods (fewer hours each). One shirt costs C $\tfrac24 = 0.5$ bike but costs D only $\tfrac39 \approx 0.33$ bike, so D has the comparative advantage in shirts. One bike costs C $\tfrac42 = 2$ shirts but costs D $\tfrac93 = 3$ shirts, so C has the comparative advantage in bikes.

Demand 需求 relates price to the quantity buyers will buy. The law of demand 需求定律 makes the curve slope downward. A change in the good's own price is a movement along the curve; the determinants (income, tastes, prices of substitutes 替代品 and complements 互补品, number of buyers, expectations) shift the whole curve.

Supply 供给 relates price to the quantity sellers will offer; the law of supply 供给定律 makes it slope upward. Its determinants – input prices, technology, taxes and subsidies 补贴, number of sellers, expectations – shift the curve, while the good's own price moves along it.

Equilibrium 均衡 is where supply meets demand, clearing the market. Above the equilibrium price a surplus 过剩 pushes price down; below it a shortage 短缺 pushes price up. When a determinant shifts one curve, price and quantity move predictably; when both shift, one of them is indeterminate.

Worked example. A new technology lowers producers' costs, shifting supply right. Along the unchanged demand curve, the equilibrium price falls and the equilibrium quantity rises. If instead a rise in incomes shifts demand right at the same time, quantity clearly rises but the effect on price depends on which shift is larger – the "both shift" indeterminate case.

When the two shifts move one variable in the same direction, that variable's change is certain; the other depends on which shift is larger.

A real case. An outbreak of bird flu in 2022 killed tens of millions of US egg-laying hens. Supply shifted left, and because people keep buying eggs when prices rise, the price rose sharply: the average US price of a dozen large eggs reached $4.82 in January 2023, more than double its level a year earlier.

Exam skill: macro builds on these micro tools – you must draw and shift supply-and-demand and PPC graphs correctly, because the same shift logic drives the aggregate (whole-economy) models later in the course.

• Opportunity cost is what you give up — read it off a PPC as the slope; a bowed-out PPC shows increasing opportunity cost.
• Comparative advantage (lower opportunity cost), not absolute advantage, decides who should specialise and trade.
• Points inside the PPC are inefficient, outside unattainable; more resources/technology shift it out (growth).
• Check whether a trade table gives output (units made: "other over") or input (hours per unit: "same over") before computing opportunity cost.
• Distinguish a movement along a demand/supply curve (own price) from a shift (a determinant).
• These micro tools drive the aggregate models later — draw and shift the correct curve in the correct direction.