Market supply adds quantities at each price

A change in seller count shifts the market relationship.

Market supply is the horizontal sum of individual supply curves. If one firm offers 20 units and another 30 at $8, the market offers 50. Entry adds quantities at each price and shifts market supply right. Exit shifts it left.

Build the sum price by price. At $8, firms may supply 20 and 30. At $10, they may supply 27 and 41. Market quantities are 50 and 68, respectively. Adding one firm’s $8 quantity to another firm’s $10 quantity would not describe a possible market outcome because all sellers face the same market price in the competitive model.

Entry is different from movement along market supply. If the product price rises, existing sellers move to higher quantities on their curves and the market moves along its curve. If new firms enter, more quantity is offered at every price, so market supply shifts right. In the long run, price can influence entry, but the graph still separates the initial price change from the resulting change in seller count.

Joint products and production substitutes create related-good effects on supply. If beef production also produces hides, a higher beef price can increase cattle processing and shift hide supply right. If farmland can grow corn or soybeans, a higher soybean price can shift corn supply left because land moves toward soybeans.

The two cases differ because joint products share a production process while production substitutes share a scarce input. In the first case, expanding the primary output automatically creates more of the associated output. In the second, expanding one output pulls land, labor, or equipment away from the other. Name the physical relationship before choosing the shift direction.

Seller count and market quantity

At $12, each of 40 identical firms supplies 25 units, so market quantity supplied is 40×25=1,000. Ten firms enter while each firm’s schedule is unchanged. At $12, market supply becomes 50×25=1,250. This is a rightward market-supply shift caused by entry, not movement along one firm’s supply curve.

Market supply can shift even if each incumbent firm’s costs are unchanged, because entry or exit changes the number of quantities being added. Conversely, an input-price change can shift every incumbent’s curve and therefore market supply even if seller count remains fixed. Exam choices sometimes describe one correct source while the stem gives the other. Match the mechanism.

The horizontal-sum rule assumes firms’ quantities can be added without one firm’s output directly changing another’s cost. If an industry shares a scarce specialized input, expansion can bid up its price and alter firms’ costs. Introductory questions generally hold such feedback constant unless they describe an increasing-cost industry. Use the stated horizon and assumptions.

After finding a market-supply shift, move to the new equilibrium. Entry shifts supply right, lowering price and increasing total market quantity under ordinary demand. Each incumbent may produce less at the lower price even while market output rises because more firms now share the market. This is why firm and market quantities must remain separate.

Ask whether the event changes the next unit’s cost

A wage increase, input tax, or productivity improvement generally changes marginal cost. Rent on an already leased building is fixed in the short run. This single question separates many valid supply shifts from fixed-cost distractors.