Learning objectives
Explain scarcity and the choices of what, how and for whom to produce
Define opportunity cost and apply it to consumers, producers and governments
Identify the four factors of production and the payment each earns
Draw and interpret a PPC to show scarcity, choice, opportunity cost, unemployment, efficiency and actual versus potential growth
Use the circular flow model with leakages and injections, and outline the nine key concepts
Key definitions
Scarcity
The condition in which resources are finite while wants are unlimited, so that not every want can be satisfied and choices must be made.
Opportunity cost
The value of the next best alternative forgone when a choice is made.
Production possibility curve
A curve showing the maximum combinations of two goods an economy can produce when all resources are fully and efficiently employed with existing technology.
Explanation
The economic problem. Resources are finite but wants are unlimited, so every society must choose what to produce, how to produce it and for whom. Every choice carries an opportunity cost — the next best alternative forgone. A student who spends $60 on a concert ticket gives up the textbook; a farmer who plants rice forgoes the maize the same field could grow; a government that builds a hospital gives up a school. The resources are the four factors of production: land (natural resources, earning rent), labour (human effort, earning wages), capital (man-made aids to production such as machines, earning interest) and entrepreneurship (organising and risk-taking, earning profit). Free goods such as air have no opportunity cost; economic goods always do.
The production possibility curve plots the maximum combinations of two goods an economy can make with its resources and technology. Points on the curve are productively efficient; points inside show unemployed or inefficiently used resources; points outside are unattainable for now. Moving along the curve shows choice and opportunity cost: more capital goods means fewer consumer goods. The curve is bowed outwards because resources are not equally suited to both goods, so opportunity cost rises. Actual growth is a movement from inside the curve towards it as idle resources are put to work, as in a recovery. Potential growth is an outward shift of the whole curve, caused by more or better resources — investment, population growth, education or new technology.
The circular flow of income. Households sell factor services to firms in return for wages, rent, interest and profit, and spend those incomes on firms' output. Three further sectors extend the model. The financial sector receives savings (S) and lends to firms for investment (I); the government collects taxes (T) and returns government spending (G); the foreign sector takes import spending (M) and sends export revenue (X). Savings, taxes and imports are leakages that remove income from the flow; investment, government spending and exports are injections. When injections exceed leakages national income rises; when leakages exceed injections it falls; the flow is in equilibrium when J = W. The model shows interdependence: lower household spending cuts firms' revenue, employment and, in turn, household income.
Nine key concepts and economics as a social science. The course is built around scarcity, choice, efficiency (the most from scarce resources), equity (fairness), economic well-being (living standards and quality of life), sustainability (meeting present needs without compromising future generations), change, interdependence and intervention (governments acting where markets fail). Economics is a social science because it studies human choices under scarcity using the scientific method — models, hypotheses and evidence — while recognising that people, unlike molecules, learn, change their behaviour and cannot easily be placed in controlled experiments. That is why economists rely on the ceteris paribus assumption and why their predictions are probabilistic rather than exact.
Formulae
Opportunity cost of one unit of X = quantity of Y forgone ÷ quantity of X gained
Injections J = I + G + X; leakages (withdrawals) W = S + T + M
National income rises when J > W, falls when W > J, is stable when J = W
Worked examples
q: An economy with a straight-line PPC can produce a maximum of 400 tonnes of rice or 100 tractors. It currently produces 200 tonnes of rice and 40 tractors. Calculate the opportunity cost of one tractor and explain what the current position shows.
steps: Straight-line PPC, so opportunity cost is constant: 400 ÷ 100 = 4 tonnes of rice per tractor.
40 tractors use capacity worth 40 × 4 = 160 tonnes of rice, so the maximum rice alongside 40 tractors is 400 − 160 = 240 tonnes.
The economy produces only 200 tonnes, so it is inside the PPC: 40 tonnes of rice capacity is lost to unemployed or inefficiently used resources.
Moving to the frontier would be actual growth; producing beyond it would need potential growth (an outward shift). Answer: 4 tonnes of rice per tractor.
In context
In the 2020 pandemic lockdowns most economies moved well inside their PPCs as workers and machines stood idle; the 2021 recovery was mainly actual growth back towards the frontier.
Vietnam's investment in ports, roads and schooling has shifted its PPC outwards, with real GDP growth averaging around 6–7% a year for two decades.
Common misconceptions
Treating money as a factor of production — capital means man-made aids to production such as machines, not finance.
Confusing actual growth (moving towards the curve) with potential growth (the curve shifting outwards).
Describing opportunity cost as everything forgone rather than the single next best alternative.
Study tips
When you draw a PPC, mark a point inside, on and outside the curve and state what each shows — the labels earn the marks.
For opportunity cost calculations write "what is given up ÷ what is gained" before substituting numbers.
Evaluate the idea
The PPC is a deliberate simplification — two goods, fixed technology, one period — so it illustrates scarcity and choice clearly but cannot show how today's choice of capital goods changes tomorrow's frontier without a second diagram.
The circular flow shows interdependence between sectors but not why leakages and injections change; that needs the macroeconomic models met later.
Reasoning
Scarce resources → every choice has an opportunity cost → society must decide what, how and for whom → the position on the PPC and the flows in the circular flow model show the outcome
Summary
Scarcity forces choice; every choice has an opportunity cost — the next best alternative forgone.
Factors: land (rent), labour (wages), capital (interest), entrepreneurship (profit). PPC: inside = unemployment, on = efficiency, outward shift = potential growth.
Circular flow: income rises when injections (I + G + X) exceed leakages (S + T + M).
Check your understanding
Question 1: A movement from a point inside the PPC to a point on it illustrates…
1. potential growth
2. a rise in opportunity cost
3. actual growth
Question 2: Which of these is an injection into the circular flow?
1. imports
2. savings
3. exports