Equilibrium and Market Dynamics

Markets and Organization

• Definition of a Market
  • A setting that brings together potential buyers (demanders) and sellers (suppliers).
  • Participation examples:
  • Buying a coffee ➔ you are a buyer in the coffee market.
  • Owning a coffee shop ➔ you are a seller in the coffee market.
  • Booking a room on Airbnb ➔ buyer in the online‐rental‐lodging market.
  • Listing an apartment on Airbnb ➔ supplier in that same market.
  • Shopping on Etsy ➔ buyer in the hand-made-crafts market.
  • Running an Etsy shop ➔ producer/supplier of hand-made goods.
• Non-monetary (implicit) markets
  • The marriage market: daters compare potential partners’ characteristics (personality, finances, etc.) and decide whether to “buy” (marry).
  • Some countries formalize this with literal matchmaking marketplaces (e.g., Indian newspaper ads).
• Two broad ways societies organize production & allocation:
  • Planned (command) economy: Central authority decides what is produced, how, by whom, and who receives output.
  • Market economy: Individuals make their own production & consumption choices, coordinating through markets.

Equilibrium: Where Supply Meets Demand

• Key definition
  • Equilibrium: $Q<em>s = Q</em>d$ — the quantity supplied equals the quantity demanded, so there is no tendency for change.
• Terminology
  • Equilibrium Price ($P^*$): the price at which equilibrium occurs.
  • Equilibrium Quantity ($Q^<em>$): the quantity traded at $P^</em>$.
• Intuition
  • Every willing seller finds a buyer.
  • Every willing buyer finds a seller.
  • Because neither side is frustrated, price has no reason to rise or fall.

Shortage and Surplus Mechanics

• Shortage (Excess Demand)
  • Condition: $Q<em>d > Q</em>s$.
  • Always arises when price is below $P^*$.
  • Adjustment story: Buyers bid prices up; sellers realize they can raise price and still sell out ➔ price rises until shortage disappears.
• Surplus (Excess Supply)
  • Condition: $Q<em>d < Q</em>s$.
  • Always arises when price is above $P^*$.
  • Adjustment story: Sellers cut prices to clear unsold stock; lower prices stimulate demand and reduce supply ➔ price falls until surplus disappears.

Numerical Gasoline Example (U.S.)

• Graphically the curves intersect at $(P^<em>,Q^</em>) = (\$3, 2\text{ billion gal/wk})$.
• Surplus and shortage magnitudes are identical here ( $0.9\text{ billion gal}$ ) but occur on opposite sides of $P^*$.

Price Adjustment Stories (Graph Insights)

• Shortage at $2$: Too many buyers for limited gas.
  • Suppliers repeatedly raise price and still sell out.
  • Buyers compete, offering to pay a premium to secure gas.
• Surplus at $4$: Unsold inventory forces sellers to discount.
  • Falling price raises $Q<em>d$ (law of demand) and lowers $Q</em>s$ (law of supply) until equilibrium is restored.

Demand Shifters (Six)

1. Income
   • Normal goods: higher income ➔ higher demand.
   • Inferior goods: higher income ➔ lower demand.
2. Preferences / Tastes
3. Prices of related goods
   • Substitutes: price of tea $\uparrow$ ➔ demand for coffee $\uparrow$.
   • Complements: price of cream $\uparrow$ ➔ demand for coffee $\downarrow$.
4. Expectations about the future (prices, income, product availability).
5. Congestion & network effects
   • Congestion: more users reduce usefulness (e.g., traffic routes).
   • Network: more users increase usefulness (e.g., social media).
6. Type and number of buyers (market size, demographics).

(Changing price itself moves along the curve; it does not shift demand.)

Effects of Demand Shifts

• Increase in Demand ➔ curve shifts right.
  • At old $P$ a shortage arises, pushing price up.
  • New equilibrium: $P^<em>$ up, $Q^</em>$ up.
• Decrease in Demand ➔ curve shifts left.
  • At old $P$ a surplus arises, pushing price down.
  • New equilibrium: $P^<em>$ down, $Q^</em>$ down.

Illustrated Gas Examples

• Demand increase moved $P^<em>$ from 3\$ to 4\ and $Q^</em>$ from $2$ to $2.5$ billion gal/wk.
• Demand decrease lowered $P^<em>$ to 2\ and $Q^</em>$ to $1.5$ billion gal/wk.

Supply Shifters (Five)

1. Input prices (wages, raw materials).
2. Productivity & technology.
3. Prices of related outputs / alternative opportunities.
4. Expectations about future prices.
5. Type and number of sellers (market entry/exit).

(Price changes again move along, not shift, the supply curve.)

Effects of Supply Shifts

• Increase in Supply ➔ curve shifts right.
  • Surplus at original $P$ pushes price down.
  • New equilibrium: $P^<em>$ down, $Q^</em>$ up.
• Decrease in Supply ➔ curve shifts left.
  • Shortage at original $P$ pushes price up.
  • New equilibrium: $P^<em>$ up, $Q^</em>$ down.

Illustrated Gas Examples

• Supply increase: $P^<em>$ down to 2\, $Q^</em>$ up to $2.4$ billion gal/wk.
• Supply decrease: $P^<em>$ up to 4\, $Q^</em>$ down to $1.6$ billion gal/wk.

Simultaneous Shifts ("It Depends")

• When both curves move, price or quantity effects can be ambiguous.
• Four classic combos:
  1. Demand $\uparrow$ & Supply $\uparrow$
  2. Demand $\uparrow$ & Supply $\downarrow$
  3. Demand $\downarrow$ & Supply $\uparrow$
  4. Demand $\downarrow$ & Supply $\downarrow$
• Direction of $P^<em>$ or $Q^</em>$ hinges on which curve shifts more.

Graphical Scenarios (Summary)

Three-Step Method for Predicting Outcomes

1. Determine whether supply, demand, or both shift.
2. Decide if the shift is an increase (right) or decrease (left).
3. Draw/visualize to see new $P^<em>$ and $Q^</em>$.

Worked Market-Change Examples

1. EV Charging Stations (Demand ↑)
   • More convenient ownership ➔ $D_{EV}\uparrow$ (preferences).
   • Result: $P<em>{EV}$ ↑, $Q</em>{EV}$ ↑.
2. California Drought & Almonds (Supply ↓)
   • Higher water cost raises marginal cost ➔ $S_{almonds}\downarrow$ (input price).
   • Result: $P<em>{almonds}$ ↑, $Q</em>{almonds}$ ↓.
3. 30-Minute Delivery Competitor (Demand ↓ for local stores)
   • Preference shifts to fast delivery ➔ $D_{local}\downarrow$.
   • Result: $P<em>{local}$ ↓, $Q</em>{local}$ ↓.
4. Cheaper EV Batteries via Research Funding (Supply ↑)
   • Lower input cost ➔ $S_{EV}\uparrow$.
   • Result: $P<em>{EV}$ ↓, $Q</em>{EV}$ ↑.

Interpreting Market Data: Two Diagnostic Rules

• Rule 1: If $P$ and $Q$ move in the same direction, demand definitely shifted (supply may also have moved).
• Rule 2: If $P$ and $Q$ move in opposite directions, supply definitely shifted (demand may also have moved).

Diagnostic Examples

• Scenario 1 (E-Books): Book quantity ↑, price ↓ ➔ supply increased (production & distribution cheaper).
• Scenario 2 (Valentine’s Roses): Price ↑, quantity ↑ ➔ demand increased (holiday preference spike).

Grand Recap / Key Takeaways

• Equilibrium occurs at $Q<em>s = Q</em>d$ with no pressure to change price.
• Shortage (below $P^<em>$) drives price up; surplus (above $P^</em>$) drives price down.
• Six demand shifters & five supply shifters — price itself never shifts the curves.
• Demand shifts ↔ $P^<em>$ & $Q^</em>$ move together.
• Supply shifts ↔ $P^<em>$ & $Q^</em>$ move oppositely.
• Simultaneous shifts can yield ambiguous outcomes; analyze relative magnitudes.
• Apply Rules 1–2 to infer which curve moved when observing real-world data.