Economics

Supply and demand is the foundational model economists use to explain how prices and quantities are determined in competitive markets. This lesson builds a working understanding of the two forces that drive every market transaction: how much buyers want to purchase, and how much sellers are willing to produce, at any given price.

The lesson starts with the law of demand, which states that as the price of a good rises, the quantity consumers are willing to buy falls, all else being equal — an inverse relationship visualized as a downward-sloping demand curve. Its mirror image, the law of supply, describes a direct relationship: as price rises, producers are willing to supply more, giving an upward-sloping supply curve. A recurring distinction the quiz reinforces is the difference between a movement along a curve, caused only by a change in the good's own price, and a shift of the entire curve, caused by non-price determinants.

On the demand side, those determinants include consumer income (distinguishing normal goods, whose demand rises with income, from inferior goods, whose demand falls), consumer preferences, the number of buyers in the market, and the prices of related goods — substitutes, where a price increase for one raises demand for the other, and complements, where a price increase for one lowers demand for the other. On the supply side, the determinants are production-side factors: input prices, technological change, the number of sellers, and producers' expectations about future prices.

Where the two curves intersect defines market equilibrium — the price and quantity at which the amount buyers want to purchase exactly equals the amount sellers want to sell, leaving no natural pressure for price to move further. The lesson also examines what happens away from that point: when price is pushed above equilibrium, quantity supplied exceeds quantity demanded and a surplus results, pushing price back down; when price sits below equilibrium, quantity demanded exceeds quantity supplied and a shortage results, pushing price back up. Together these self-correcting pressures explain why markets tend to gravitate toward their equilibrium price.

This example set includes 10 multiple-choice questions (some with more than one correct option) plus a companion 12-card flashcard deck covering the same vocabulary — law of demand, law of supply, equilibrium, surplus, shortage, substitute and complementary goods, and normal versus inferior goods. Zestly creates quizzes, exams, and flashcards like this one automatically from any topic or uploaded document, and you can generate your own personalized version — adjusting difficulty, question count, and format — in seconds.

• State the law of demand and the law of supply and explain why each curve has the slope it does
• Distinguish a movement along a demand or supply curve from a shift of the entire curve
• List the key non-price determinants that shift demand (income, preferences, number of buyers, related goods) and supply (input prices, technology, number of sellers, expectations)
• Differentiate normal goods from inferior goods based on how their demand responds to income changes
• Explain how substitute and complementary goods affect each other's demand curves when their prices change
• Define market equilibrium and identify the equilibrium price and quantity in a simple market
• Explain how a surplus arises when price is set above equilibrium and how a shortage arises when price is set below equilibrium
• Describe the self-correcting pressure that pushes market price back toward equilibrium after a shortage or surplus

Supply and demand is the core model of price theory in microeconomics. The law of demand holds that, all else equal, the quantity of a good consumers are willing to buy falls as its price rises, producing a downward-sloping demand curve. The law of supply holds the opposite: as price rises, the quantity producers are willing to sell increases, producing an upward-sloping supply curve. A change in the good's own price causes movement along a curve, while a change in a non-price determinant — such as income, consumer preferences, the number of market participants, input costs, or technology — shifts the curve itself. The point where the demand and supply curves intersect is the market equilibrium, defined by an equilibrium price and equilibrium quantity at which the amount buyers wish to purchase equals the amount sellers wish to sell. When the market price is set above this equilibrium, quantity supplied exceeds quantity demanded, creating a surplus; when it is set below equilibrium, quantity demanded exceeds quantity supplied, creating a shortage. Economists treat both surpluses and shortages as temporary states that create pressure — falling prices in the case of a surplus, rising prices in the case of a shortage — pushing the market back toward its equilibrium.