Law of Supply and the Supply Curve

Law of Supply and the Supply Curve: If demand is the buyers’ side, then supply is the sellers’ side. Understanding supply is necessary for a complete picture of the market: how price is formed, what forces determine the quantity of goods on the shelves, how firms react to changes in conditions.

What is supply

Supply — the willingness and ability of producers to sell a product at a certain price. Like demand, it is a relationship between price and quantity. Quantity supplied ($Q_s$) — the amount of goods that producers are willing and able to sell at a given price over a given period. Important: supply is not what is produced or in stock, but what producers want to sell at different prices. It is a potential that depends on price.

Law of supply

Law of Supply: ceteris paribus, the higher the price of a good, the greater the quantity supplied, and vice versa. There is a direct relationship between price and quantity.

Why does the law work?

Profit motive: higher price means higher profit, which is a greater incentive to produce.
Increasing marginal costs: to increase production, less efficient resources must be used — costs rise. Producers are ready to do this only at a higher price.
Entry of new producers: high price attracts new participants, increasing supply.

Supply Curve

Supply curve — a graphical representation of the law of supply. On the horizontal axis — quantity ($Q$), on the vertical axis — price ($P$). The curve slopes upward from left to right, reflecting the direct relationship. Each point on the curve shows the quantity supplied at a given price. Movement along the curve is a change in $Q_s$ due to a change in price.

Linear supply function: $Q_s = c + dP$, where $c$ can be negative (minimum price for production to start), $d$ — sensitivity to price.

The shape of the curve depends on costs. With increasing $MC$, the curve bends upwards more steeply. With constant $MC$ — it is linear. With economies of scale (declining $AC$), “strange” dynamics are possible.

Individual and Market Supply

Individual supply — supply of a single firm. Determined by costs, technology, capacity.

Market supply — the sum of all individual supplies of firms in the market. At each price, we add up the quantities from all producers. Horizontal summation: similar to demand.

For $P = $10$, firm A is ready to sell 100, B — 150, C — 50. Market $Q_s = 300$.

Factors Shifting the Supply Curve

As with demand, supply can shift:

Shift right — increase in supply: at every price, producers are willing to sell more.
Shift left — decrease in supply: at every price, willing to sell less.

Factors shifting supply:

Resource prices: increase in prices of raw materials, labor, energy → costs higher → shift left.
Technologies: improvement in technology → costs lower → shift right.
Prices of alternative goods: if a firm can produce $X$ or $Y$, increase in price of $Y$ → part of capacity switches → supply of $X$ shifts left.
Expectations: expectation of price increase → hold back goods today → shift left.
Number of sellers: entry of new firms → shift right, exit → shift left.
Government: subsidies → right, taxes → left, regulation affects variably.
Weather and natural factors: for agricultural products — a critical factor.

Short-run and Long-run Supply

Time horizon is critical for supply:

Immediate period: supply is fixed — what is in stock. The curve is vertical.
Short-run period: firms can change output, but capital (factories, equipment) is fixed. The curve is relatively steep.
Long-run period: firms can change everything — build new factories, enter or exit the industry. The curve is more flat (elastic).

Consequence: short-term response to a price increase — small rise in output. Long-term — substantial increase (new capacity, new firms).

Supply and Costs

A firm's supply curve is closely related to its cost curve:

In the short run, the supply curve of a competitive firm is the portion of the $MC$ curve above the $AVC$ (average variable cost) point. If the price is below $AVC$, the firm does not produce; it is better to shut down.
In the long run, the supply curve is the part of the $MC$ curve above the $ATC$ (average total cost) point. If the price is below $ATC$, the firm incurs losses and exits the industry.

Understanding costs is key to understanding supply.

Special Cases

Absolutely inelastic supply: vertical curve. Quantity is fixed regardless of price. Example: land in the city center, unique artwork.
Absolutely elastic supply: horizontal curve. Firms are ready to supply any quantity at the given price. Possible with constant $MC$ and free entry.
“Backward-bending” curve: labor supply may decrease at very high wages (income effect exceeds substitution effect — people prefer leisure).

For the Investor

Supply analysis helps understand:

Pricing power: if supply is inelastic (limited capacity, rare resources), producers have pricing power.
Entry barriers: if supply is easy to expand (low barriers), high profits attract competitors and fall.
Cyclicality: industries with long investment cycles (oil, mining) have inelastic short-term supply — prices are volatile.
Costs: growth of resource prices (oil, metals, labor) shifts the supply curve left — higher prices, lower profits.