Notes on Demand, Supply, and Equilibrium: Key Concepts and Examples

Determinants of supply

• Costs of inputs and materials (the cost of the materials, whatever). Affects the cost of producing the good.
• Number of sellers in the market. More sellers means more production capacity and shifts the supply curve to the right.
• Expectations about the future. A catchall for anything that might affect the position of the supply curve (e.g., producers’ expectations about future demand, entry/exit of sellers).
• Other non-price factors can shift the supply curve; when these change, the entire supply curve shifts (not just a movement along it).
• Example from transcript: in Cambridge, the population is very concentrated in students (two thirds). In the summer the town virtually empties; on September 1, demand for moving services spikes because everyone moves in. This illustrates expectations and non-price factors shifting the supply/demand environment (anticipation of higher demand may prompt new movers or movers to enter the market).

Demand vs supply shifts: movements along vs shifts

• Price change of the good itself -> movement along the curve (no shift in the curve).
• Any non-price change (income, costs, number of sellers, expectations, prices of inputs, etc.) -> the entire curve shifts.
• When you hear a list of factors that affect demand, identify whether a factor affects the demand curve (shift) or the supply curve (shift).
• Concept: Whichever curve is affected determines the new equilibrium; the other curve may be held or adjusted along its path.

Equilibrium: definition and key idea

• Equilibrium is the price at which the quantity demanded equals the quantity supplied.
• Mathematically: find the price P^ such that Qd(P^) = Qs(P^*).
• Equilibrium quantity is Q^* = Qd(P^) = Qs(P^).
• At equilibrium, there is no inherent incentive for either buyers or sellers to change their behavior.
• Intuition: it’s like a balance where demand and supply meet; if the price is too high, sellers don’t sell enough (surplus); if too low, buyers can’t get enough (shortage).

Class example: moving services in a college town

• Equilibrium price: ≈ P^* = 200\$
• Equilibrium quantity: Q^* = 16 (16 moves booked or performed)
• How the equilibrium arises: intersection of the demand curve (buyers’ willingness to pay) and the supply curve (sellers’ willingness to supply) determines the equilibrium.
• If you at first think the equilibrium price is much higher (e.g., P = 1000\$), you may hire more workers and rent more trucks, hoping to move more people, but the actual market demand is only about 5 moves at that price.
• Consequences of overpricing (P > P^*):
• Quantity supplied exceeds quantity demanded (surplus).
• Example in transcript: at P = 1000\$, Qs ≈ 20, Qd ≈ 5; revenue may not cover costs; you’d realize you overcharged.
• Adjustments when price is too high: sellers would lower price to attract more customers and move toward equilibrium.
• Consequences of underpricing (P < P^*):
• Quantity demanded exceeds quantity supplied (shortage).
• Example: at a price well below equilibrium, demand outstrips supply; some customers will be left waiting while producers may run out of capacity.
• The adjustment process: price movement toward P^* and quantity movement toward Q^* until both sides are satisfied.
• Metaphor for equilibrium: finding the right price is like feeling your way in a dark room until you find the right spot; once there, neither buyers nor sellers want to change.

How to use the idea of shifts to analyze real changes

• If demand shifts to the right (e.g., income increases for a normal good):
• New equilibrium has higher price and higher quantity sold: P^2 > P^1 and Q^2 > Q^1.
• Reason: increased willingness to pay raises the overall demand at each price; producers reallocate along their supply curve to the higher equilibrium price.
• If production costs increase (supply shifts left):
• New equilibrium has higher price and lower quantity: P^2 > P^1 and Q^2 < Q^1.
• Reason: higher costs make production less attractive at any given price; fewer units supplied at each price.
• Memory aid: leftward shift = bad for sellers; rightward shift = good for sellers.

Subsidies and other policy tools (illustrated via the transcript)

• A subsidy to producers is the opposite of a tax: it reduces the production cost for sellers.
• Effect on supply: subsidy shifts the supply curve to the right (lower effective cost, more supply at each price).
• Resulting changes in equilibrium: typically higher quantity and lower price (relative to the pre-subsidy equilibrium), assuming demand remains unchanged.
• Practical implication: subsidies can help expand production or reduce prices, but they have budgetary and distributional consequences.

Quick quiz review (key ideas and formulations used in the class)

• Equilibrium concept from a provided table: equilibrium occurs where quantity demanded equals quantity supplied.
• Example values (from the transcript): one version yields P^* = 11.70\$ with Q^* = 360. Note that different iterations can yield slightly different numbers, but the principle remains: set Qd(P) = Qs(P) to find the equilibrium.
• Surplus at a given price: occurs when the price is above the equilibrium price.
• Example from the transcript: if the equilibrium price is P^* = 4\$ and the price is P = 7\$,, the surplus magnitude equals the horizontal distance between the two quantities on the supply and demand sides (in the example, the distance from 2 to 8), yielding a surplus of 6,000,000\text{ bags} (units may be millions depending on the table).
• Shortage at a price below equilibrium: occurs when demand exceeds supply; the price should rise toward equilibrium to reduce the shortage.
• Notation recap:
• Demand curve: Q_d = f(P)
• Supply curve: Q_s = g(P)
• Equilibrium: Qd(P^) = Qs(P^) \Rightarrow Q^* = Qd(P^) = Qs(P^)

Connections to broader concepts

• Comparative statics: the study of how equilibrium changes when exogenous factors (other than the price) change; this is precisely what shifts the curves and moves the equilibrium point.
• Real-world relevance: markets rarely stay fixed; non-price information (income, expectations, costs, policy) constantly reshapes supply and demand; businesses adjust pricing and capacity in response to these shifts.
• Practical takeaway: to analyze a market, identify whether a factor shifts demand or supply, and then determine the new equilibrium and potential shortages or surpluses in the short run.

Practical implications and reflections

• Markets tend toward an equilibrium where incentives align; deviations create pressures (surplus or shortage) that push the price toward the equilibrium.
• Policymaking (like subsidies or taxes) can deliberately shift supply or demand, but these interventions have effects on prices and quantities that must be weighed against costs and benefits.
• Ethical/philosophical dimension (implicit in the lecture): the balance between efficiency (allocation at equilibrium) and equity (who benefits from shifts and subsidies) is a real-world consideration policymakers face; while not argued explicitly in the transcript, the framework helps analyze such trade-offs.