Perfect competition in the short run

The short-run market supply curve shows the quantity supplied by all $\,$ the firms in the market as price varies.

The firms will do one of 3 things in the supply curve:

1. At the shutdown price, firms will choose to either choose to shutdown, or produce the shutdown quantity.
2. When the price is below the shutdown price, firms will shutdown and not produce.
3. When the price is above the shutdown price, firms will produce at the given output.

The short-run supply curve and the market demand curve determines the equilibrium price and quantity.

Note: The equilibrium is found at the intersection.

Recall that the market demand curve can change in 2 ways.

Case 1: The demand increases, causing the curve to shift rightward. The result would be an increase to both the market price and the output.

Case 2: The demand decreases, causing the curve to shift leftward. The result would be a decrease to both the market price and the output.

There are 3 possible outcomes in the short run for firms who are perfectly competitive.

Case 1: Suppose the demand curve is in $D_1$. Then the firm breaks even and does not gain any profit or loss. This is because p = ATC $\,$ at the profit-maximizing output.

Case 2: Suppose the demand curve is in $D_2$. Then the firm gains economic profit. This is because p > ATC $\,$ at the profit-maximizing output.

Case 3: Suppose the demand curve is in $D_3$. Then the firm has economic loss. This is because p < ATC $\,$ at the profit-maximizing output.