Topic 1.6 Market Equilibrium, Disequilibrium, and Changes in Equilibrium: determine equilibrium price and quantity, analyze surpluses and shortages, and predict the new equilibrium when supply or demand shifts.

What this topic is asking

Topic 1.6 combines demand and supply into a working market. The College Board wants you to find the equilibrium price and quantity, to analyze disequilibrium (surpluses and shortages) and how markets self-correct, and to predict the new equilibrium when supply or demand shifts, including the tricky double-shift cases where one of price or quantity is indeterminate. This is the capstone of Unit 1 and the model you reuse throughout the course.

Market equilibrium

At the equilibrium price, the market clears: every buyer willing to pay the equilibrium price finds a seller, and every seller willing to sell at that price finds a buyer. There is no surplus and no shortage.

Disequilibrium: surpluses and shortages

When price is not at equilibrium, market forces push it back:

This automatic adjustment is the market mechanism at work: prices act as signals that eliminate surpluses and shortages without any central coordinator, which is why competitive markets tend toward equilibrium.

Single shifts: predictable outcomes

When only one curve shifts, both the new equilibrium price and quantity are determinate:

• Demand increases (rightward): price rises, quantity rises.
• Demand decreases (leftward): price falls, quantity falls.
• Supply increases (rightward): price falls, quantity rises.
• Supply decreases (leftward): price rises, quantity falls.

A reliable method is to redraw the graph, shift the one curve, and read off the new intersection. Notice that demand shifts move price and quantity in the same direction, while supply shifts move them in opposite directions.

Double shifts: one effect is indeterminate

For example, if demand and supply both increase (both shift right), quantity definitely rises (both push quantity up), but price is indeterminate, because the rightward demand shift pushes price up while the rightward supply shift pushes price down. Whichever shift is larger decides the net price change. The four double-shift cases are worth memorizing, but the underlying logic, comparing the directions each shift pushes price and quantity, lets you reason through any of them.

Why this model matters

The supply-and-demand model is the workhorse of the entire course, so getting fluent with equilibrium analysis here pays dividends in every later unit. The disciplined approach is always the same: start at the original equilibrium, identify which curve (or curves) the event shifts and in which direction, redraw the graph with clearly labelled curves (S1, S2, D1, D2) and a new intersection, and then read the new equilibrium price and quantity. For single shifts, both outcomes are clear; for double shifts, find the variable both shifts push the same way (determinate) and flag the other as indeterminate, noting that it depends on relative shift magnitudes. This same logic scales up to the aggregate demand and aggregate supply model in later units, where shifts in AD and AS change the price level and real output exactly as shifts in micro demand and supply change price and quantity here. The indeterminacy idea also reappears: when a recessionary policy and a supply shock hit at once, you reason about which effect dominates just as you do with a double shift. Building the habit of narrating each step, name the shift, state its direction, redraw, read the new equilibrium, is precisely what free-response graders reward, and it prevents the careless errors that come from guessing at the answer instead of working the graph.

Try this

Q1. Describe what happens in a market when the price is below equilibrium. [2 points]

• Cue. Quantity demanded exceeds quantity supplied, creating a shortage; buyers bid the price up until it returns to equilibrium.

Q2. Both demand and supply decrease (both shift left). State which of price and quantity is determinate and which is indeterminate. [2 points]

• Cue. Quantity definitely falls (both shifts reduce it), so quantity is determinate; price is indeterminate, because a leftward demand shift lowers price while a leftward supply shift raises it.