Supply and Demand: Core Concepts and Applications

Supply and Demand

Definitions and Core Concepts

• Demand: The quantity of a good or service that consumers are willing and able to purchase at various prices during a given period.

• Quantity Demanded (QD): The specific amount of a good consumers are willing to buy at a particular price.

• Supply: The quantity of a good or service that producers are willing and able to sell at various prices during a given period.

• Quantity Supplied (QS): The specific amount of a good producers are willing to sell at a particular price.

Demand vs. Quantity Demanded

• Demand: Refers to the entire relationship between price and quantity demanded, represented by the demand curve.

• Quantity Demanded: Refers to a single point on the demand curve at a specific price.

• Graphical Representation: Demand is the whole curve; quantity demanded is a point on the curve.

Creating a Market Demand Curve

• Add the quantity demanded by all consumers at each price to obtain the market demand curve (horizontal summation).

• Example: If at $10, Alice demands 2 units and Bob demands 3 units, market demand at $10 is 5 units.

Law of Demand

• States that, ceteris paribus (all else equal), as the price of a good falls, the quantity demanded rises, and as the price rises, the quantity demanded falls.

Why the Demand Curve Slopes Downward

• Substitution Effect: As price falls, the good becomes relatively cheaper compared to substitutes, increasing quantity demanded.

• Income Effect: As price falls, consumers' purchasing power increases, allowing them to buy more.

Movements vs. Shifts in Demand

• Movement Along the Curve: Caused by a change in the good's own price (results in a change in quantity demanded).

• Shift of the Curve: Caused by changes in non-price determinants (results in a change in demand).

Change in Demand vs. Change in Quantity Demanded

• Change in Demand: The entire demand curve shifts due to non-price factors.

• Change in Quantity Demanded: Movement along the demand curve due to a change in the good's price.

Causes of Movement Along the Demand Curve

• Only a change in the price of the good itself causes movement along the demand curve.

• Upward movement (higher price): Decrease in quantity demanded.

• Downward movement (lower price): Increase in quantity demanded.

Causes of Shifts in the Demand Curve

• Changes in non-price determinants cause the demand curve to shift.

• Rightward shift: Increase in demand.

• Leftward shift: Decrease in demand.

Determinants of Demand

• Income: Higher income increases demand for normal goods, decreases for inferior goods.

• Prices of Related Goods: Substitutes and complements affect demand.

• Tastes and Preferences: Changes can increase or decrease demand.

• Expectations: Future price or income expectations can shift demand.

• Number of Buyers: More buyers increase market demand.

Normal vs. Inferior Goods

• Normal Goods: Demand increases as income rises (e.g., organic food).

• Inferior Goods: Demand decreases as income rises (e.g., instant noodles).

Substitutes vs. Complements

• Substitutes: Goods used in place of each other (e.g., tea and coffee). An increase in the price of one increases demand for the other.

• Complements: Goods used together (e.g., printers and ink). An increase in the price of one decreases demand for the other.

Supply vs. Quantity Supplied

• Supply: The entire relationship between price and quantity supplied, represented by the supply curve.

• Quantity Supplied: The amount supplied at a specific price (a point on the supply curve).

Creating a Market Supply Curve

• Add the quantity supplied by all producers at each price (horizontal summation).

Law of Supply

• States that, ceteris paribus, as the price of a good rises, the quantity supplied rises; as the price falls, the quantity supplied falls.

Why the Supply Curve Slopes Upward

• Higher prices provide an incentive for producers to supply more due to higher potential profits.

Movements vs. Shifts in Supply

• Movement Along the Curve: Caused by a change in the good's own price (change in quantity supplied).

• Shift of the Curve: Caused by changes in non-price determinants (change in supply).

Change in Supply vs. Change in Quantity Supplied

• Change in Supply: The entire supply curve shifts due to non-price factors.

• Change in Quantity Supplied: Movement along the supply curve due to a change in the good's price.

Causes of Movement Along the Supply Curve

• Only a change in the price of the good itself causes movement along the supply curve.

• Upward movement (higher price): Increase in quantity supplied.

• Downward movement (lower price): Decrease in quantity supplied.

Causes of Shifts in the Supply Curve

• Changes in non-price determinants cause the supply curve to shift.

• Rightward shift: Increase in supply.

• Leftward shift: Decrease in supply.

Determinants of Supply

• Input Prices: Higher input costs decrease supply; lower input costs increase supply.

• Technology: Technological improvements increase supply.

• Expectations: Expectations of future prices can affect current supply.

• Number of Sellers: More sellers increase market supply.

• Prices of Related Goods: If the price of a substitute in production rises, supply of the good may decrease.

Supply and Demand Together

• Both curves are plotted on the same graph: price on the vertical axis, quantity on the horizontal axis.

Equilibrium

• Equilibrium: The point where the supply and demand curves intersect; quantity supplied equals quantity demanded.

• Equilibrium Price (Pe): The price at which the market clears.

• Equilibrium Quantity (Qe): The quantity bought and sold at equilibrium price.

Determining Equilibrium Price and Quantity

• Graphically: Intersection of supply and demand curves.

• Mathematically: Set quantity supplied equal to quantity demanded and solve for price and quantity.

Example: If $Q_D = 100 - 2P$ and $Q_S = 20 + 3P$, set $Q_D = Q_S$:

$100 - 2P = 20 + 3P$

$80 = 5P$

$P = 16$

Substitute $P$ back to find $Q$:

$Q = 100 - 2(16) = 68$

Shortage and Surplus

• Shortage: Quantity demanded exceeds quantity supplied at a given price (below equilibrium).

• Surplus: Quantity supplied exceeds quantity demanded at a given price (above equilibrium).

• Graphical Representation: Shortage is below equilibrium; surplus is above equilibrium.

• Calculation: Shortage = QD - QS; Surplus = QS - QD at a given price.

Effects of Shifts in Supply and Demand on Equilibrium

• Supply Shifts: Rightward shift lowers price, increases quantity; leftward shift raises price, decreases quantity.

• Demand Shifts: Rightward shift raises price and quantity; leftward shift lowers price and quantity.

• Both Shift in Same Direction: Quantity changes, but price effect is ambiguous.

• Both Shift in Opposite Directions: Price changes, but quantity effect is ambiguous.

Summary Table: Movements vs. Shifts

Concept	Movement Along Curve	Shift of Curve
Demand	Change in price of the good	Change in non-price determinants (income, tastes, etc.)
Supply	Change in price of the good	Change in non-price determinants (input prices, technology, etc.)

Additional info: This guide expands on the listed points with academic definitions, examples, and equations for clarity and exam preparation.