Here is a brief summary of what we've learned about how perfectly competitive industries adjust to long run equilibrium.

A change in demand alone doesn't change long run price.
If demand increases or decreases the number of firms in the industry adjust in the long run to keep price at the minimum of ATC, so that profits are zero.
Industry supply only shifts in the long run due to entry or exit.
Industry supply is the sum of the firms supplies. Only in the long run is there time for firms to enter or exit the industry.
A change in variable costs is the only way in which the industry supply curve can shift in the short run.
In the short run the MC curve can shift due to a change in variable costs . This shifts industry supply, since industry is the sum of the firms and the MC curve (above AVC) is the firm's supply curve.

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