What are the differences in the conclusions between the industrial organizations of perfect competition and monopoly?

In both a monopoly and in perfect competition, firms can earn positive profits in the short run. In monopolies, firms can also earn positive profits in the long run because the strong barriers to entry prevent new firms from entering the market and eating away at their profits. In perfect competition, however, this is not the case. Because of the lack of barriers to entry in perfect competition, if firms are earning positive profits, new firms will enter the market, which will drive down the profits of the existing firms. Therefore, in the long run, firms in perfect competition earn zero economic profits.