Firms in Competitive Markets: Perfect Competition and Supply Decisions

Firms in Competitive Markets

Characteristics of a Competitive Market

A competitive market is defined by several key features that distinguish it from other market structures. Understanding these characteristics is essential for analyzing firm behavior and market outcomes.

• Many buyers and sellers: No single buyer or seller can influence the market price; all are price takers.

• Identical products: Goods offered by different sellers are homogeneous, making them perfect substitutes.

• Free entry and exit: Firms can enter or leave the market without significant barriers.

Example: Agricultural markets, such as wheat or corn, often exhibit these characteristics.

Revenue Concepts for Competitive Firms

Competitive firms aim to maximize profit by analyzing their revenues and costs. Key revenue concepts include:

• Average Revenue (AR): Total revenue divided by quantity sold. For all firms, $AR = \frac{TR}{Q}$.

• Marginal Revenue (MR): The change in total revenue from selling one more unit. For competitive firms, $MR = P$.

• Total Revenue (TR): The product of price and quantity, $TR = P \times Q$.

Example: If a firm sells 10 units at $5 each,$TR = 5 \times 10 = 50$.

Table: Total, Average, and Marginal Revenue for a Competitive Firm

Quantity (Q)	Total Revenue (TR)	Average Revenue (AR)	Marginal Revenue (MR)
1	$P \times 1$	$P$	$P$
2	$P \times 2$	$P$	$P$
...	...	...	...

Additional info: In a competitive market, AR and MR always equal the market price.

Profit Maximization and the Competitive Firm’s Supply Curve

Rules for Profit Maximization

Firms maximize profit by comparing marginal revenue and marginal cost for each unit produced.

• Profit: $\text{Profit} = TR - TC$

• Marginal Cost (MC): The increase in total cost from producing one more unit.

• Decision Rule:

  • If $MR > MC$, increase output.

  • If $MC > MR$, decrease output.

  • Profit is maximized where $MR = MC$.

Example: If $MR = 10$ and $MC = 8$, the firm should produce more. If $MR = 10$ and $MC = 12$, the firm should produce less.

Table: Profit Maximization Example

Quantity (Q)	Marginal Revenue (MR)	Marginal Cost (MC)	Decision
10	10	8	Increase output
20	10	10	Profit maximized
30	10	12	Decrease output

Additional info: The MC curve is the firm’s supply curve above relevant cost thresholds.

The Marginal-Cost Curve and Supply Decision

The marginal-cost curve determines the quantity a firm is willing to supply at any price. It is upward sloping and crosses the average-total-cost curve at its minimum.

• ATC curve: U-shaped due to economies and diseconomies of scale.

• MC curve: Upward sloping, reflecting increasing marginal costs.

• Supply curve: The portion of the MC curve above relevant cost curves (AVC or ATC).

Short-Run Decision to Shut Down

In the short run, a firm may decide to shut down if it cannot cover its variable costs.

• Shutdown: Temporary cessation of production; fixed costs must still be paid.

• Exit: Permanent departure from the market; all costs cease.

• Shutdown Rule: Shut down if $TR < VC$ or $P < AVC$.

• Short-run supply curve: Portion of MC curve above AVC.

Example: If price falls below average variable cost, the firm should shut down temporarily.

Sunk Costs

Sunk costs are costs that have already been incurred and cannot be recovered. They should not affect current decisions.

• In the short run, fixed costs are sunk and irrelevant to shutdown decisions.

Long-Run Decision to Exit or Enter

In the long run, firms decide to exit or enter based on total costs and revenues.

• Exit Rule: Exit if $TR < TC$ or $P < ATC$.

• Entry Rule: Enter if $TR > TC$ or $P > ATC$.

• Long-run supply curve: Portion of MC curve above ATC.

Table: Profit-Maximizing Rules for a Competitive Firm

Condition	Short-Run Action	Long-Run Action
$P < AVC$	Shut down	Exit
$AVC < P < ATC$	Operate	Exit
$P > ATC$	Operate	Stay

Measuring Profit and Loss

Profit or loss can be visualized as the area between price and average total cost on a graph.

• Profit: If $P > ATC$, $\text{Profit} = (P - ATC) \times Q$

• Loss: If $P < ATC$, $\text{Loss} = (ATC - P) \times Q$

Example: If $P = 6$, $ATC = 5$, $Q = 30$, then $\text{Profit} = (6 - 5) \times 30 = 30$.

The Supply Curve in a Competitive Market

Short-Run Market Supply

In the short run, the number of firms is fixed. The market supply curve is the sum of individual firms’ supply curves.

• Each firm produces where $MR = MC$ and $P > AVC$.

• Market supply is the horizontal sum of all firms’ MC curves above AVC.

Example: If 1,000 identical firms each supply 10 units at a given price, market supply is 10,000 units.

Long-Run Market Supply with Entry and Exit

In the long run, firms can freely enter or exit the market, adjusting the number of firms and market supply.

• If $P > ATC$, new firms enter, increasing supply.

• If $P < ATC$, firms exit, decreasing supply.

• Long-run equilibrium occurs when $P = min\ ATC$ and firms earn zero economic profit.

Example: After an increase in demand, price rises and firms earn profit. Over time, entry increases supply and price returns to $min\ ATC$.

Zero-Profit Equilibrium

In long-run equilibrium, firms earn zero economic profit, but accounting profit may be positive.

• Economic profit: Includes opportunity costs.

• Accounting profit: Excludes opportunity costs.

Elasticity of Supply Curves

The long-run supply curve is typically more elastic than the short-run supply curve because firms can adjust their production and enter or exit more easily.

• Short-run supply: Less elastic due to fixed number of firms.

• Long-run supply: More elastic due to free entry and exit.

Why the Long-Run Supply Curve Might Slope Upward

While the long-run supply curve is often horizontal, it may slope upward if:

• Firms have different costs: Higher-cost firms enter only at higher prices.

• Costs rise as firms enter: Entry increases demand for inputs, raising input prices and firm costs.

Example: In farming, limited land means entry increases land prices, raising costs for all firms.

Conclusion: Behind the Supply Curve

The analysis of competitive markets provides foundational tools for understanding firm behavior and market outcomes. Marginal analysis is central to these decisions and will be applied to other market structures in subsequent chapters.

Application Example: Walmart’s Decision to Stay Open All Night

• Fixed costs (rent, equipment, management salaries) are sunk in the short run and irrelevant to shutdown decisions.

• Variable costs (hourly wages, utilities) are relevant for deciding whether to stay open during low-demand periods.

• If revenue from night customers covers variable costs, Walmart stays open; otherwise, it may shut down temporarily.

Self-Assessment

• Market supply curves are typically more elastic in the long run because firms can enter or exit the market, adjusting supply more responsively to price changes.