Demand and supply

Fundamentals of Demand

Demand represents the amount of a good or service that consumers are both willing and able to purchase at various price points. Willingness to pay (or willingness to buy) establishes the relationship between price and the specific quantity a consumer will acquire.

• Price ($P$): The monetary value a buyer pays per unit of a specific good or service.

• Quantity Demanded ($Q_d$): The total number of units of a good or service that consumers are willing and able to buy at a specific given price.

• Law of Demand: All else being equal (ceteris paribus), as the price of a good increases, the quantity demanded decreases ($P \rightarrow \text{up}, Q_d \rightarrow \text{down}$ ). Conversely, as the price decreases, the quantity demanded increases ($P \rightarrow \text{down}, Q_d \rightarrow \text{up}$). This reflects an inverse or negative relationship between price and quantity demanded.

• Ceteris Paribus Requirement: The Law of Demand holds strictly under the assumption of ceteris paribus (Latin for "all else equal" or "all other variables held constant"). If other factors change simultaneously, the law's predictive accuracy breaks down.

• Demand Schedule: A structured table displaying a range of prices alongside the corresponding quantity demanded at each price point (e.g., price per gallon of gasoline mapped against quantity demanded in millions of gallons).

• Demand Curve: A graphical representation of the demand schedule, plotting price on the vertical axis ($Y\text{-axis}$) and quantity on the horizontal axis ($X\text{-axis}$).

  • Slopes downward from left to right for all normal goods due to the negative relationship defined by the Law of Demand.

  • Every point along the demand curve shows the exact quantity a consumer is willing and able to purchase at that exact price (e.g., at a price of $\$5$, a consumer buys $15$ units; if price increases to $\$6$, quantity demanded drops to $10$ units).

  • Proper graphing standards require explicitly labeling the vertical axis as Price ($P$), the horizontal axis as Quantity ($Q$), and the line itself as Demand ($D$).

Fundamentals of Supply

Supply represents the amount of a good or service that a producer is willing and able to offer for sale at various price points. While demand reflects the consumer side of the market, supply reflects the producer side.

• Quantity Supplied ($Q_s$): The total number of units of a good or service that producers are willing and able to sell at a specified price.

• Law of Supply: All else being equal (ceteris paribus), as the price of a good increases, the quantity supplied increases ($P \rightarrow \text{up}, Q_s \rightarrow \text{up}$). Conversely, as the price decreases, the quantity supplied decreases ($P \rightarrow \text{down}, Q_s \rightarrow \text{down}$). Higher prices incentivize producers to offer more units to maximize potential profit.

• Supply Schedule: A tabular listing of prices alongside the corresponding quantity supplied at each price.

• Supply Curve: A graphical plot of the supply schedule showing the relationship between price on the vertical axis ($P$) and quantity on the horizontal axis ($Q$).

  • Slopes upward from left to right, reflecting a direct, positive relationship between price and quantity supplied.

  • Proper graphing standards require labeling axes as Price ($P$) and Quantity ($Q$), and the curve as Supply ($S$).

Market Equilibrium, Surpluses, and Shortages

Market equilibrium occurs at the exact intersection of the demand curve ($D$) and the supply curve ($S$).

• Equilibrium Point: The coordinate on a market graph where quantity demanded equals quantity supplied ($Q_d = Q_s$).

• Equilibrium Price ($P_e$): The market-clearing price determined by the intersection of supply and demand. It is the real-world price consumers pay when buying goods or services in an undistorted market.

• Equilibrium Quantity ($Q_e$): The quantity of a good bought and sold at the equilibrium price.

• Geographic Price Differences: Market prices differ between regions or states because local consumer demographics alter the demand curve, while local resource availability and shipping costs alter the supply curve.

Market Imbalances

When a firm sets a price higher or lower than the equilibrium price ($P_e$), market imbalances occur:

• Surplus (Excess Supply):

  • Occurs when price is set above the equilibrium price ($P > P_e$).

  • At this elevated price, quantity supplied exceeds quantity demanded ($Q_s > Q_d$).

  • $\text{Surplus} = Q_s - Q_d$

  • Numerical Example: If a firm supplies $120$ units at a high price, but consumers only demand $50$ units, the excess supply is $120 - 50 = 70$ units.

  • Producers bear overhead, storage, and inventory costs for unsold units. Perishable goods risk complete spoilage. To eliminate inventory, producers cut prices, exerting downward pressure on price until equilibrium ($P_e$) is restored.

• Shortage (Excess Demand):

  • Occurs when price is set below the equilibrium price ($P < P_e$).

  • At this low price, quantity demanded exceeds quantity supplied ($Q_d > Q_s$).

  • $\text{Shortage} = Q_d - Q_s$

  • Consumers compete for limited inventory. Producers recognize high demand and raise prices, pushing the price upward until equilibrium ($P_e$) is restored.

Determinants of Demand and Curve Shifts

A critical economic distinction exists between a movement along a demand curve and a shift of a demand curve.

• Change in Quantity Demanded: A movement from one point to another along an existing demand curve. Caused only by a change in the price of the good itself ($P$).

• Change in Demand: A shift of the entire demand curve to the right (increase in demand) or to the left (decrease in decrease). Caused by changes in non-price determinants.

Non-Price Determinants of Demand

1. Buyer Income ($I$):

   • Normal Goods: Products whose demand increases ($D \rightarrow \text{shift right}$) when buyer income rises, and decreases ($D \rightarrow \text{shift left}$) when buyer income falls.

   • Inferior Goods: Products whose demand decreases ($D \rightarrow \text{shift left}$) when buyer income rises. Examples include public transit/bus tickets, canned goods, rent (versus home ownership), and government assistance.

2. Population and Buyer Demographics: An increase in total population or household size increases overall market demand ($D \rightarrow \text{shift right}$).

3. Prices of Related Goods:

   • Substitutes: Goods used in place of one another (e.g., Pepsi vs. Coke). An increase in the price of a substitute ($P_{\text{sub}} \rightarrow \text{up}$) causes a decrease in quantity demanded for that substitute, driving consumers to buy the primary product instead ($D_{\text{primary}} \rightarrow \text{shift right}$).

   • Complements: Goods used together in consumption (e.g., coffee and sugar, bread and lunch meat, smartphones and chargers). An increase in the price of a complement ($P_{\text{comp}} \rightarrow \text{up}$) reduces quantity demanded for that complement, causing a decrease in demand for the primary product ($D_{\text{primary}} \rightarrow \text{shift left}$).

4. Tastes and Preferences: Changes in consumer preference, fashion trends, or incentives (e.g., limited-edition marketing) increase desirability, shifting demand to the right ($D \rightarrow \text{shift right}$).

5. Future Price Expectations: Speculation regarding future price movements alters current behavior:

   • If consumers expect prices to rise in the future ($P_{\text{future}} \rightarrow \text{up}$), current demand spikes ($D_{\text{current}} \rightarrow \text{shift right}$), which can induce market shortages (e.g., panicking to buy protective masks during health crises).

   • If consumers expect prices to fall in the future ($P_{\text{future}} \rightarrow \text{down}$), current demand decreases ($D_{\text{current}} \rightarrow \text{shift left}$).

Determinants of Supply and Curve Shifts

A movement along a supply curve differs fundamentally from a shift of the supply curve.

• Change in Quantity Supplied: Movement along an existing supply curve caused solely by a change in the price of the good itself ($P$).

• Change in Supply: A shift of the entire supply curve to the right (increase in supply) or left (decrease in supply) driven by underlying production costs or external environment factors.

Determinants of Supply Shifts

1. Price of Inputs / Cost of Production: Inputs are factors of production (labor, raw materials, capital, land). Supply curves reflect unit production costs plus desired profit.

   • If input prices rise, production costs increase. For a fixed budget, fewer materials can be acquired, reducing output ($S \rightarrow \text{shift left}$).

   • Numerical Example: A firm has a $\$2000$ budget. At an input cost of $\$100$ per unit, the firm buys $20$ input units. If input costs double to $\$200$ per unit, the firm can only purchase $10$ input units, reducing overall supply.

   • If input prices fall, production costs decrease, allowing firms to supply more at every price point ($S \rightarrow \text{shift right}$).

2. Natural Conditions: Weather, climate events, and agricultural conditions.

   • Favorable weather conditions boost yields ($S \rightarrow \text{shift right}$).

   • Natural disasters, severe frosts, or severe droughts destroy output ($S \rightarrow \text{shift left}$).

3. Technology: Technological advancements that streamline production lower per-unit costs and expand output capacity ($S \rightarrow \text{shift right}$).

4. Government Policies: Government interventions alter production costs.

   • Taxes on production increase total unit costs ($S \rightarrow \text{shift left}$).

   • Price controls (price ceilings and price floors) distort standard equilibrium price signals.

Four-Step Process for Analyzing Equilibrium Changes

To determine how any economic event alters market price and quantity, use the following four-step process:

1. Step 1: Draw Initial Market State: Graph the initial demand curve ($D_1$) and supply curve ($S_1$). Label the vertical axis Price ($P$) and the horizontal axis Quantity ($Q$). Identify the initial equilibrium price ($P_1$ or $P_{e1}$) and equilibrium quantity ($Q_1$ or $Q_{e1}$).

2. Step 2: Determine Curve Affected: Evaluate whether the economic event alters demand factors (income, population, preferences, related goods, expectations) or supply factors (input prices, weather, technology, policy).

3. Step 3: Determine Shift Direction: Decide if the event increases the curve (shift right) or decreases the curve (shift left), and draw the shifted parallel linear curve ($D_2$ or $S_2$).

4. Step 4: Identify and Compare New Equilibrium: Locate the new intersection point between the shifted curve and the unshifted curve. Project down to axes to determine the new equilibrium price ($P_2$ or $P_{e2}$) and quantity ($Q_2$ or $Q_{e2}$). Compare the new values to original baseline values.

Standard Shift Scenarios

• Scenario A: Increase in Income (Normal Good)

  • Effect: Demand shifts right ($D_1 \rightarrow D_2$).

  • Result: Equilibrium price increases ($P_e \rightarrow \text{up}$) and equilibrium quantity increases ($Q_e \rightarrow \text{up}$).

• Scenario B: Excellent Weather Conditions (Agricultural Good)

  • Effect: Supply shifts right ($S_1 \rightarrow S_2$).

  • Result: Equilibrium price decreases ($P_e \rightarrow \text{down}$) and equilibrium quantity increases ($Q_e \rightarrow \text{up}$).