In short-run, the firm in the perfect competition act as a “price taker”, and has to accept whatever price is set in the market. The firm faces a perfectly elastic demand curve for its product, as shown in Figure 1. In this figure P1 is the price set in the market, and the firm cannot sell at any other price. If the firm tried to set above P1 it will sell nothing, as buyers know that there is no quality difference between the product as produced by other firms in the markets. Also, there is no incentive for any firm to set a price below P1.
As the firm choose to produce at the profit maximum point, the firm needs to set output at a level where marginal revenue is equal to marginal cost. Figure 2 illustrates this by adding the short-run cost curves to the demand curves. The firm faces constant average revenue and marginal revenue, as the demand curve is horizontal and will choose output at q1,