The key difference between the demand curve in a perfect competition and the demand curve in a monopoly is their movement. A firm in a perfect competition can sell an unlimited number of goods on a fixed price because the price of the output is determined by the market and the market conditions in which the firm operates. In other words, the firm accepts the preexisting price of the output that has already been established on the market. As a result, the demand curve in a perfect competition is a straight, horizontal line. In fact, because the competitive firm sells an article that has a lot of perfect substitutes, it faces a perfectly elastic demand curve.
On the other hand, because the monopoly is the only manufacturer on the market, its individual demand curve is also the market’s demand curve. This means that it faces a downward-sloping demand curve. Monopolists can change the price of their output depending on its quantity in the market. If they increase the price, naturally, the costumers will buy less of their goods. Furthermore, if they lower the quantity of their output, the price will go up.