I get intuitively how if there are a lot of firms selling identical products the firm demand curve would be perfectly elastic, and I also get the definition of economic profit.
Say a firm is making $50000 in accounting profit. Economic profit is accounting profit - opportunity cost. Why, in the long run, would the economic profit go to 0 but in the short run it would be positive. Wouldn't the opportunity cost be the same in short run and long run?
Secondly, I don't really intuitively get the link between perfect competition and no entry or exit costs. I thought it would be because if you are entering you are producing identical goods to other existing firms so there are no additional cost in research and whatnot, but then production should still cost money as in if you are entering you are going to have to buy a machine to produce this good. In that case wouldn't that be entry cost? And when you leave the market, you might have to demolish a factory or something, so wouldn't that be an exit cost?