Keep short-run adjustment separate from long-run adjustment

The same demand increase can have two different price effects over time.

Immediately after demand rises, the number of firms is fixed. Market price rises, existing firms move along MC, and economic profit appears. Over time, entry increases market supply. In a constant-cost industry, price returns to the original minimum-ATC level even though total industry output remains higher and more firms operate. The short-run price increase was real but temporary.

Build a timeline rather than placing all curves on one undifferentiated graph:

If demand instead falls, the mirror image occurs: price drops, firms contract and may incur losses, then exit shifts supply left. A surviving representative firm returns to zero economic profit. Normal profit is included in economic cost, so “zero economic profit” does not mean owners work for nothing or receive no accounting income.

Short-run and long-run answers differ

Demand for a competitive constant-cost product increases. If asked “immediately,” answer higher price, greater output per existing firm, and positive profit. If asked “after all adjustment,” answer original price, zero economic profit, more firms, and greater market output. Both answers describe the same event at different horizons.

For an increasing-cost industry, entry only partially reverses the price rise because industry expansion raises costs. For a decreasing-cost industry, long-run price can fall below the original level. The timing logic remains, while the cost condition changes the final price.

Do not say entry shifts one firm’s supply curve right in the constant-cost model. Entry shifts market supply. The incumbent moves along its unchanged MC as price changes. If input prices change, then cost curves may shift, but that requires the industry-cost condition.

Similarly, do not report zero short-run profit merely because long-run competition eliminates it. Temporary profit is the incentive that produces entry. Erasing it removes the mechanism of adjustment.

Distinguish zero profit from zero accounting profit

Long-run competitive firms cover wages, rent, materials, forgone owner salary, and the normal return on invested resources. Economic profit is zero because no excess return remains to attract entry.

The temporary profit or loss is not a mistake in the model. It is the signal that causes entry or exit. Erasing it because long-run profit is zero removes the adjustment mechanism. “Immediately” and “after all firms can enter or exit” can therefore produce different correct answers to the same demand change.

For a cost-changing event, the long-run endpoint may also use a new minimum ATC. Do not automatically return to the old price unless the industry is explicitly constant cost and firm technology is unchanged.

A disciplined timeline names three stages: the initial equilibrium, the short-run firm response with seller count fixed, and the long-run entry or exit response. If the stem stops at one stage, do not import conclusions from the next. This sequencing is more reliable than memorizing that competitive profit “always becomes zero” without asking when.