Competitive Firm's have different costs
As P rises, firms with lower costs enter the market before those with higher costs.
Further increases in P make it worthwhile for higher-cost firms to enter the market, which increases market quantity supplied.
Hence, LR market supply curve slopes upward.
At any P,
- For the marginal firm, P = minimum ATC and
profit = 0.
- For lower-cost firms, profit > 0