The discussion on long run can be confusing. There's long run equilibrium and a long run supply curve . What's the difference? Then you also have constant cost , increasing , and decreasing cost industries which demonstrate the long run supply curve. Most AP economics books don't discuss the long run supply curve because constant , increasing, and decreasing cost industries are rarely discussed. The AP assumes constant cost industries and I am positive there never has been an increasing or decreasing cost industries FRQ .
Constant cost industry-The long run supply curve is horizontal at the long run equilibrium price. The assumptions are the following: the marginal and total costs don't change when more supply enters the market. Because the the long run equilibrium price does not change, the ATC will remain the same. Here is an example: assume the economy is at long run equilibrium initially and then the income of the population doubles. As a result the demand for the market will shift rightward lifting price and quantity. Next, the price taking firm will experience a higher AR, P, MR, and D (Mr.Darp)which will lead to movement along the MC curve for the firm and a higher price and quantity. As a result of profits, new entrants will enter and thus shift the supply of the short run industry , which lowers price and increases output. This creates a dichotomy for output between the firm and short run market supply curve. The new entrants increase supply more than the existing firms decrease output. For the firm, the decreased price from the new suppliers will change the output for the individual firm. Mr.Darp falls and thus movement, downward and to the left on the MC . This creates a decrease in output and thus affirming the aforementioned dichotomy.