Welcome to the Heart of Economics: Supply and Demand!
Welcome to Unit 2! If Microeconomics were a language, Supply and Demand would be the alphabet. This unit is the foundation for everything else you will learn in this course. We are going to explore how buyers and sellers interact in a market to determine prices and quantities. Don't worry if it feels like a lot of graphs at first—once you see the patterns, it becomes like a puzzle that always fits together perfectly.
2.1 Demand
Let's start with you! As a consumer, you represent Demand. Demand is the quantity of a good or service that consumers are willing and able to purchase at various prices.
The Law of Demand
The Law of Demand states that there is an inverse (negative) relationship between price and quantity demanded.
- When Price (\(P\)) goes up, Quantity Demanded (\(Q_D\)) goes down.
- When Price (\(P\)) goes down, Quantity Demanded (\(Q_D\)) goes up.
Think about it: If your favorite pizza place doubles its prices, you'll probably buy fewer slices. If they have a "half-off" sale, you’ll likely buy more! This creates a downward-sloping line on a graph.
Movement vs. Shift
This is the most common place students lose points, so pay close attention!
- Movement: A change in Price causes a movement along the existing demand curve. We call this a "change in quantity demanded."
- Shift: A change in something other than price causes the entire curve to move left or right. We call this a "change in demand."
Determinants of Demand (The "TRIBE" Mnemonic)
What makes a demand curve shift? Remember TRIBE:
- Tastes and Preferences: If a celebrity starts wearing a certain brand, demand shifts right.
- Related Goods: Substitutes (Coke vs. Pepsi) and Complements (Hot dogs and buns). If the price of hot dogs rises, the demand for buns falls.
- Income: For Normal Goods, more money means more demand. For Inferior Goods (like instant ramen), more money means less demand.
- Buyers: More people in the market means more demand.
- Expectations: If you think the price of gas will double tomorrow, you’ll go buy gas today (demand shifts right).
Quick Review:
Left is Less, Right is Right! If demand increases, the curve shifts to the right. If demand decreases, it shifts to the left.
2.2 Supply
Now, put on your "Business Owner" hat. You are now the producer! Supply is the quantity of a good that firms are willing and able to produce and sell.
The Law of Supply
The Law of Supply states that there is a direct (positive) relationship between price and quantity supplied.
- When Price (\(P\)) goes up, Quantity Supplied (\(Q_S\)) goes up.
- When Price (\(P\)) goes down, Quantity Supplied (\(Q_S\)) goes down.
Analogy: If you mow lawns and people start offering you \(\$100\) per lawn instead of \(\$20\), you'll be much more willing to work extra hours and mow more lawns!
Determinants of Supply (The "ROTTEN" Mnemonic)
What makes a supply curve shift? Remember ROTTEN:
- Resource Costs: If the price of cheese goes up, the supply of pizza shifts left (it's more expensive to make).
- Other goods' prices: If a farmer can grow corn or wheat, and the price of wheat goes up, they will supply less corn.
- Taxes and Subsidies: Taxes make things more expensive (Shift Left). Subsidies are like "free money" from the government (Shift Right).
- Technology: Better tech makes production faster and cheaper (Shift Right).
- Expectations: If producers think prices will rise in the future, they might hold back supply now.
- Number of Sellers: More businesses entering the market shifts supply to the right.
Key Takeaway: Price changes never shift the curve; they only move you along the curve.
2.3 & 2.4 Elasticity
Elasticity measures "responsiveness." It tells us how much consumers or producers change their behavior when prices change.
Price Elasticity of Demand (PED)
The formula is: \( \text{PED} = \frac{\%\Delta Q_D}{\%\Delta P} \)
- Inelastic (\( < 1 \)): Consumers aren't very sensitive to price changes (e.g., life-saving medicine or gasoline). The curve is steep (looks like an "I" for Inelastic).
- Elastic (\( > 1 \)): Consumers are very sensitive (e.g., a specific brand of candy). The curve is flatter.
- Unit Elastic (\( = 1 \)): The percentage change in quantity is exactly equal to the percentage change in price.
The Total Revenue Test
This is a favorite on AP exams! Total Revenue (TR) is \( P \times Q \).
- If Price and TR move in opposite directions, demand is Elastic.
- If Price and TR move in the same direction, demand is Inelastic.
Did you know? Businesses use this to decide whether to have a sale! If demand is elastic, lowering the price will actually increase their total money earned.
2.5 Other Elasticities
We can measure responsiveness to things other than the price of the good itself.
Income Elasticity of Demand (\(E_I\))
Formula: \( \frac{\%\Delta Q}{\%\Delta \text{Income}} \)
- Positive: The good is a Normal Good.
- Negative: The good is an Inferior Good.
Cross-Price Elasticity of Demand (\(E_{xy}\))
Formula: \( \frac{\%\Delta Q \text{ of Good X}}{\%\Delta P \text{ of Good Y}} \)
- Positive: The goods are Substitutes (Price of Coke goes up, demand for Pepsi goes up).
- Negative: The goods are Complements (Price of hot dogs goes up, demand for buns goes down).
2.6 & 2.7 Market Equilibrium and Surplus
Equilibrium is the "sweet spot" where the Supply and Demand curves intersect. At this point, \( Q_D = Q_S \).
Surplus and Shortage
- Shortage: When the price is below equilibrium. Consumers want more than producers are selling.
- Surplus: When the price is above equilibrium. Producers have too much stock sitting on shelves.
Consumer and Producer Surplus
- Consumer Surplus (CS): The difference between what you were willing to pay and what you actually paid. (It's the area below the demand curve and above the price).
- Producer Surplus (PS): The difference between the price the seller received and the minimum they were willing to accept. (It's the area above the supply curve and below the price).
- Total Surplus: \( CS + PS \). Markets are most efficient at equilibrium because Total Surplus is maximized.
Key Takeaway: When Supply or Demand shifts, the equilibrium Price and Quantity will change. Pro-tip: Always draw the graph! It’s the easiest way to see what happens to \(P\) and \(Q\).
2.8 Government Intervention
Sometimes the government thinks the market price is "unfair" and steps in.
Price Ceilings
A Price Ceiling is a legal maximum price. To be "binding" (effective), it must be placed below the equilibrium price.
- Result: It causes a permanent Shortage.
- Memory Trick: You can't go above a ceiling! If the ceiling is in the basement (below equilibrium), you're stuck down there.
Price Floors
A Price Floor is a legal minimum price (like Minimum Wage). To be binding, it must be above equilibrium.
- Result: It causes a permanent Surplus.
- Memory Trick: You can't go below a floor! If the floor is in the attic (above equilibrium), you're stuck up there.
Deadweight Loss (DWL)
When the government intervenes with floors, ceilings, or taxes, the market no longer produces the efficient quantity. The lost Total Surplus that is not captured by anyone is called Deadweight Loss. It represents "wasted" opportunity for trade.
2.9 International Trade
Markets aren't just local; they are global!
- World Price: The price of a good on the global market.
- Exports: If the World Price is higher than the domestic price, a country will export the good (sellers are happy!).
- Imports: If the World Price is lower than the domestic price, a country will import the good (consumers are happy!).
Tariffs
A Tariff is a tax on imported goods. It raises the price of the import, helps domestic producers, but hurts domestic consumers and creates Deadweight Loss.
Don't worry if this seems tricky at first! Just remember that every time the government adds a tax or a tariff, it "wedges" itself into the market, usually reducing the total amount of buying and selling happening.
Quick Review:
1. Demand = Consumers. Supply = Producers.
2. Price only moves you along the curve.
3. Equilibrium is where they meet.
4. Elasticity is how much people react to price changes.