Explain how supply, demand and competition determine market price and market quantity, and how price rations goods and sets the quantities of productive resources used

Learning goal: Explain how supply, demand and competition determine market price; how the price of a good or service helps determine which consumers receive the limited goods and services produced with scarce productive resources; and how supply and demand interactions determine the market quantity of a good or service and the quantities of productive resources used. This lesson teaches Ohio's 2026 social studies standards for Grade 7, published in July 2026. The Ohio Department of Education and Workforce anticipates that districts will begin implementing the 2026 standards in the 2027-28 school year.

Demand. Demand is the quantity of a good or service that buyers are willing and able to buy at each possible price during a period of time. Wanting something is not enough; buyers must be able to pay. The law of demand says that, other things staying the same, when the price rises, the quantity demanded falls, and when the price falls, the quantity demanded rises. A demand schedule lists the quantity demanded at each price.

Supply. Supply is the quantity sellers are willing and able to offer for sale at each possible price during a period of time. The law of supply says that, other things staying the same, when the price rises, the quantity supplied rises, because producing more becomes worthwhile, and when the price falls, the quantity supplied falls. A supply schedule lists the quantity supplied at each price.

A market schedule (hypothetical data). A weekly town market sells handmade clay pots. At each price, buyers and sellers would trade these quantities:

Market price and market quantity. At $3, buyers want 60 pots and sellers offer 60 pots. This is the equilibrium, or market-clearing, price: the price at which quantity demanded equals quantity supplied. The market quantity is 60 pots. Supply and demand interactions therefore determine two things together: the price and the quantity.

Why the price moves toward $3. At $4, sellers offer 80 but buyers want only 40, a surplus of 40 pots. Unsold pots pile up, so sellers compete for buyers by cutting prices. At $2, buyers want 80 but sellers offer only 40, a shortage of 40 pots. Buyers compete for the scarce pots, some offering more, and sellers see they can raise prices. Either way, pressure pushes the price toward $3.

Competition. Competition is rivalry among sellers for buyers and among buyers for goods. When more sellers enter a market, supply increases, and competition among them pushes the price down. Suppose new potters arrive and the quantity supplied rises by 40 at every price (60 at $1, 80 at $2, 100 at $3, 120 at $4, 140 at $5). Now at $3 there is a surplus (60 demanded, 100 supplied), and the new equilibrium is $2, where 80 pots are demanded and 80 supplied. More competition among sellers led to a lower price and a larger market quantity. When many buyers compete for a scarce good, competition among buyers pushes the price up.

Price decides who receives scarce goods. Pots are made with scarce productive resources: clay, fuel for kilns, potters' labor and skill, and wheels and kilns (capital). Because resources are scarce, not everyone who wants a pot can have one. The market price acts as a rationing device. At the equilibrium price of $3, the pots go to the 60 buyers who are willing and able to pay at least $3 (in this model each buyer wants one pot). Buyers who value a pot at less than $3, or who cannot afford $3, do not receive one. Other systems ration differently — first come, first served, lottery, or a ruler's decision — but in a market the price determines which consumers receive the limited goods and services.

Market quantity determines the productive resources used. The quantity produced also decides how many productive resources the market uses. Suppose each pot requires 2 hours of a potter's labor and 3 kilograms of clay. At the market quantity of 60 pots, potters work 60 × 2 = 120 hours and use 60 × 3 = 180 kilograms of clay per week. After competition raises the market quantity to 80 pots, the market uses 160 hours of labor and 240 kilograms of clay. Supply and demand in the market for pots thus determine the quantities of labor, clay and other productive resources used, and those resources are then not available for other products. In the same way, the demand for pots creates demand in the markets for clay and for potters' labor.

Historical connection. In medieval and early modern towns, market days brought many buyers and sellers together. Where a single guild or a single merchant controlled a product, competition was limited, and prices could be kept higher than they would have been if many sellers had competed; how much higher varied by town and product.

The misconception. Students often think sellers simply choose any price they like. In a competitive market like the lesson's, where many sellers offer the same kind of pot and buyers can compare prices, a seller can post any price, but if it is above the market price, buyers go to competitors and goods go unsold; the interaction of supply, demand and competition determines the price at which goods actually sell.

Worked example 1 — find the equilibrium from the schedule: Compare quantity demanded and quantity supplied at each price. $1: 100 vs 20, shortage of 80. $2: 80 vs 40, shortage of 40. $3: 60 vs 60, equal. $4: 40 vs 80, surplus of 40. $5: 20 vs 100, surplus of 80. The market price is $3 and the market quantity is 60 pots. Total spending at equilibrium is 60 × $3 = $180 a week.

Worked example 2 — competition shifts supply: New potters add 40 pots at every price, so the new supply is 60, 80, 100, 120 and 140 at prices $1 to $5. Demand is unchanged. At $3, demanded 60 but supplied 100, a surplus of 40, so sellers compete by lowering prices. At $2, demanded 80 and supplied 80, so $2 is the new market price and 80 the new market quantity. More competition among sellers lowered the price by $1 and raised the quantity by 20. Twenty more buyers now receive pots, those willing and able to pay at least $2 but not $3.

Worked example 3 — productive resources used: Each pot takes 2 labor hours and 3 kilograms of clay. Before the new potters arrived: 60 pots × 2 = 120 hours and 60 × 3 = 180 kg of clay. After: 80 pots × 2 = 160 hours and 80 × 3 = 240 kg of clay. The increase is 40 labor hours and 60 kg of clay a week. Because the market quantity rose, the market draws more scarce resources away from other uses; the opportunity cost is whatever else that labor and clay could have produced.

Graph task (representation): On grid paper, graph the demand and supply schedules with price on the vertical axis ($0 to $6) and quantity on the horizontal axis (0 to 140). Label the demand curve D and the supply curve S, mark the equilibrium point, and draw the new supply curve S2 after the new potters arrive. Label the old and new market price and quantity, and shade the shortage at $2 on the original supply curve.

Independent performance: Create your own hypothetical market for a good sold in a medieval or modern marketplace, with a five-price demand and supply schedule. Write 250–350 words. Success criteria: (1) your schedules follow the law of demand and the law of supply; (2) you identify the market price and market quantity and show a surplus and a shortage with calculations; (3) you explain how competition among sellers or buyers would change the price; (4) you explain which consumers receive the good at the market price and which do not; (5) you choose two productive resources, state how much each unit needs, and calculate the quantities used at the market quantity.