AP Microeconomics: Market Equilibrium, Surplus, and Shortage

AP Microeconomics – Market Equilibrium, Surplus, and Shortage

What This Is

Market equilibrium is the point where the quantity of a good that buyers want to purchase (quantity demanded) exactly equals the quantity that sellers are willing to produce (quantity supplied). At this price, there is no surplus (extra goods left unsold) and no shortage (unmet demand). Understanding how the equilibrium moves when the government imposes a tax on sugary drinks, or when a hurricane destroys a wheat harvest, is a staple of the AP?Micro exam because every multiple?choice and free?response question about “price, quantity, and the market” starts here.

Key Terms & Formulas

• Supply Curve (S) – Graph with price (P) on the vertical axis and quantity supplied (Qs) on the horizontal axis; upward?sloping because higher prices motivate producers to supply more.
• Demand Curve (D) – Same axes as the supply curve; downward?sloping because higher prices discourage buyers, so quantity demanded (Qd) falls.
• Market Equilibrium – The intersection of S and D; the equilibrium price (Pe) and equilibrium quantity (Qe) satisfy Qd = Qs.
• Surplus = Qs – Qd – Occurs when P > Pe; the vertical distance between S and D at that price shows the excess units.
• Shortage = Qd – Qs – Occurs when P < Pe; the vertical distance between D and S at that price shows the unmet demand.
• Equilibrium Condition: Qd(Pe) = Qs(Pe) – Algebraically, set the demand function equal to the supply function and solve for Pe.
• Consumer Surplus (CS) = ½ × (Qe) × (Pmax – Pe) – The triangular area above the price paid and below the demand curve; Pmax is the price at which quantity demanded would fall to zero.
• Producer Surplus (PS) = ½ × (Qe) × (Pe – Pmin) – The triangular area below the price received and above the supply curve; Pmin is the price at which quantity supplied would fall to zero.
• Shift vs. Movement – A shift of the entire curve (right = increase, left = decrease) is caused by a non?price factor; a movement along a curve is caused by a change in the price of the good itself.
• Price Ceiling – A legally imposed maximum price below the equilibrium price; creates a shortage.
• Price Floor – A legally imposed minimum price above the equilibrium price; creates a surplus.

Step?by?Step / Process Flow

1. Read the prompt and identify the initial market (e.g., “the market for gasoline”).
2. Write the demand and supply equations (or sketch the curves) using the given numbers.
3. Set Qd = Qs and solve for the equilibrium price (Pe) and quantity (Qe).
4. Determine the policy/shock (tax, subsidy, price ceiling, etc.). Decide whether it shifts the demand curve, the supply curve, or both.
5. Redraw the graph with the new curve(s). Locate the new intersection-new price (P?) and quantity (Q?).
6. Label surplus or shortage if the policy price is above or below the new equilibrium, and calculate the size (?Q = |Qs – Qd| at the policy price).

Common Mistakes

• Mistake: Saying a “price increase” shifts the demand curve.
  Correction: A price change causes a movement along the demand curve; only non?price factors (income, tastes, etc.) shift the curve.

• Mistake: Confusing “surplus” with “profit.”
  Correction: Surplus is a quantity excess (Qs?>?Qd) at a given price; profit is revenue minus cost and is unrelated to the market?wide surplus.

• Mistake: Forgetting that a price floor creates a surplus only when it is set above the equilibrium price.
  Correction: If the floor is below equilibrium, the market simply ignores it—no surplus forms.

• Mistake: Using the original equilibrium price to compute consumer surplus after a tax is imposed.
  Correction: Re?calculate CS with the new price that buyers actually pay (including the tax).

• Mistake: Mixing up the direction of a supply shift after a tax on producers (thinking it moves the curve left).
  Correction: A per?unit tax on producers shifts the supply curve upward (or left) because at each quantity the price received must be higher to cover the tax.

AP Exam Insights

1. “Change in demand vs. change in quantity demanded” – The exam loves to test whether you recognize a shift (change in demand) versus a movement along the curve (change in quantity demanded). Look for words like “income rises” or “consumer tastes change.”
2. Graphing requirements – You will be asked to draw a price?quantity diagram, label the original equilibrium, then add a price ceiling or tax and shade the resulting shortage or surplus. Remember to label axes, curves, and the new equilibrium points.
3. Free?Response Prompt pattern – FRQs often give a market, impose a policy (e.g., “the government sets a price floor”), and ask you to (a) illustrate the effect, (b) explain the resulting surplus, and (c) discuss the impact on consumer and producer surplus. Practice the three?part answer: graph-label-explain.
4. Multiple?choice traps – Look for answer choices that mistakenly treat a “tax on buyers” as a shift of the demand curve; the correct shift is downward (buyers are willing to pay less at each quantity).

Quick Check Questions

1. MC: A price ceiling is set below the market equilibrium price. Which of the following will occur?
2. A) Surplus, price above equilibrium
3. B) Shortage, price below equilibrium
4. C) No change, because the ceiling is non?binding

5. D) Surplus, price below equilibrium
   Answer: B – The ceiling forces the price down, creating excess demand (shortage).

6. FRQ?style: The market for wheat is in equilibrium at $5 per bushel and 10?million bushels. A disease reduces wheat yields, shifting supply left. Sketch the new supply curve, label the new equilibrium, and state whether consumer surplus increases or decreases.
   Answer: The new supply curve shifts left; the new equilibrium price rises above $5 and quantity falls below 10?million. Consumer surplus falls because buyers pay a higher price and buy less.

7. MC: If a $2 per?unit tax is imposed on sellers, the supply curve will:

8. A) Shift right by $2
9. B) Shift left by $2
10. C) Remain unchanged; only the price received changes
11. D) Shift upward (or left) by the amount of the tax.
    Answer: D – The tax raises the price sellers need to receive, shifting the supply curve upward (left).

Last?Minute Cram Sheet

1. Equilibrium: Set Qd = Qs-solve for Pe and Qe.
2. Axes: Price (vertical) vs. Quantity (horizontal).
3. Supply-? right shift; Supply-? left shift.
4. Demand-? right shift; Demand-? left shift.
5. Surplus = Qs – Qd when P > Pe.
6. Shortage = Qd – Qs when P < Pe.
7. Price ceiling (max price) below Pe-shortage.
8. Price floor (min price) above Pe-surplus.
9. Tax on sellers-supply curve shifts up/left by the tax amount.
10. Supply increases means the curve shifts right, not “moves up”; a movement up the curve is a price increase, not a shift.