Econ lecture - Supply

The supply curve is upward sloping in this course: price increases incentivize producers to supply more. This reflects higher costs for extra inputs as output rises, meaning there's an opportunity cost to producing additional units as labor/resources could be used elsewhere.
Producers compare the benefit (price received) and cost (inputs required) for each additional unit.

Core concepts: costs, revenues, and net benefits

• Total expenditure (for buyers) = price imes quantity sold. $TE = P \times Q$

• Total revenue (for sellers) = price imes quantity sold. $TR = P \times Q$

• Total value (for consumers) = sum of willingness to pay for all units consumed; conceptually, the area under the demand curve up to quantity Q.

• Marginal value (demand side) = value from the next unit consumed; typically falls as quantity rises due to diminishing marginal value.

• Total cost (for producers) = fixed cost + variable cost. $TC = FC + VC$

• Variable cost (VC) = portion of cost that varies with output; Example: Raw materials, hourly labor.

• Fixed cost (FC) = cost incurred before production begins; Example: Building a factory, machinery. FC is a sunk cost for short-run decisions.

• Marginal cost (MC) = the additional cost to produce one more unit. $MC = \frac{\text{d}TC}{\text{d}Q} \text{ (discrete: } MC_n = TC(n) - TC(n-1) \text{)}$

• Producer surplus (PS) = total revenue minus the variable cost ($PS = TR - VC$); it's the producer-side counterpart to consumer surplus. PS approximates profit when fixed costs are ignored.

• Profit = total revenue

− total cost ($Profit = TR - TC$). PS differs from profit by excluding fixed costs.

The production decision: how much to produce

• The producer decision rule (short run): produce as long as price ($P$) is greater than or equal to marginal cost ($MC$) for the next unit; stop when price equals marginal cost ($P = MC$).

  • Example: If the market price for a product is $15 and the marginal cost of producing the next unit is $12, the firm will produce it. If the marginal cost of the unit after that rises to $18, the firm will stop at the previous unit.

• This rule ignores fixed costs because they are sunk from the perspective of deciding how many units to produce in the short run.

From individual firms to the market

• Individual (firms

’) supply curve: each firm has its own marginal cost curve, determining its quantity supplied at a given price.

• Market supply curve: horizontally sum the individual supply curves of all firms in the market.

  • Example: If four firms A, B, C, D supply at price P=3 units A=8, B=15, C=8, D=6, then total market supply at P=3 is 8+15+8+6 = 37 units.

Quantity supplied vs. supply (definitions and distinctions)

• Quantity supplied: The actual amount of a good that suppliers are willing to sell at a specific price; represented by a point on a given supply curve.

• Change in quantity supplied: A movement along the same supply curve in response to a change in price (holding other factors constant).

• Change in supply: A shift of the entire supply curve due to a change in some factor other than price.

• Supply (the curve): The total set of quantities suppliers are willing to sell at every price; a shift indicates a change in supply conditions.

Shifts of the supply curve: what shifts supply?

The supply curve can shift due to several factors that affect marginal cost or overall production ability:

• Changes in input prices and availability: Higher labor costs or raw material prices (e.g., increased steel costs for car manufacturers) raise marginal cost and shift supply left/up. Lower input costs shift it right/down.

• Changes in technology or production processes: Improved technology (e.g., a more efficient assembly line) lowers marginal cost and shifts supply to the right.

• Changes in the number of sellers in the market: Entry of new firms shifts market supply to the right; firms exiting shift it to the left.

• Changes in expectations about future prices: If producers expect higher prices in the future, they might reduce current supply to hold inventory, shifting current supply left.

• Changes in regulation or taxes/subsidies: A new tax on production increases costs, shifting supply left. A government subsidy decreases costs, shifting supply right.

The demand-supply analogy and the graphical interpretation

• The demand curve shows marginal value/willingness to pay; the area under it is total value and consumer surplus.

• The supply curve corresponds to marginal cost; the area under it (up to Q) is the total variable cost, and the area above it (below price) is producer surplus.

• Consumer surplus (CS) is the area under the demand curve above the price.

• Producer surplus (PS) is the area above the marginal cost (or below the price).

• Total revenue (TR) = price imes quantity (a rectangle on the graph).

• Graphically, $PS = TR - VC$.

Worked example (conceptual)

• To illustrate market supply, consider four firms (A, B, C, D) with individual supply schedules. To find the total market supply at a given price (e.g., $40), you sum the quantity each firm is willing to supply at that price (e.g., Firm D might supply 18 units at $40). This demonstrates that firms have different cost structures and respond to the same price by supplying different quantities.

Key formulas to memorize (LaTeX)

• Total expenditure: $TE = P \times Q$

• Total revenue: $TR = P \times Q$

• Total cost: $TC = FC + VC$

• Marginal cost: $MC = \frac{\text{d}TC}{\text{d}Q} \text{ (discrete: } MC_n = TC(n) - TC(n-1) \text{)}$

• Producer surplus: $PS = TR - VC$

• Profit: $Profit = TR - TC$

• Supply decision rule: Produce while $P \ge MC$; stop where $P = MC$

• Market supply: $Qs^{\text{market}} = \sum<em>i Qs</em>i$ across all firms at each price

Quick recap of major ideas

• The supply curve illustrates the quantity firms are willing to produce at each price, primarily driven by rising marginal costs as output increases.

• The short-run production decision for firms involves comparing the market price to the marginal cost of producing additional units; fixed costs are considered sunk for this decision.

• The market supply curve is formed by horizontally adding up the individual supply curves of all firms operating in that market.

• It's crucial to differentiate between a change in quantity supplied (a movement along the curve due to price changes) and a change in supply (a shift of the entire curve due to non-price factors like technology or input costs).

• Concepts on the demand side (marginal value, consumer surplus) have direct parallels on the supply side (marginal cost, producer surplus), helping in graphical interpretation of market outcomes.

Note: The lecture emphasizes that while demand curves are consistently downward sloping (law of demand), supply curves are upward sloping in this framework, but it is not a universal “law” of economics that supply must always rise with price; in some contexts, supply may behave differently due to capacity, technology, or other constraints. The course treats the upward-sloping supply curve as the standard, especially for the material covered in these notes.