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Monopoly vs Monopolistic Competition: SR→LR Interactive Chart | EasyCFA | Wall Street Uni

Economics

Monopoly vs Monopolistic Competition: SR → LR Dynamic Adjustment

Interactive visualization of the short-run to long-run adjustment under monopoly and monopolistic competition. Watch entry and exit shift the demand curve in monopolistic competition until P=ATC, while monopoly maintains profit behind barriers to entry. Apply MR=MC to find Q*, track the profit rectangle, and identify the LR tangency and excess capacity.

Subtopic: Market Structures
Level: Fundamental
Status: Featured
Views: 387

Parameters

Market Structure

Initial Scenario

tPhaseTimeline SR → LR (%)%

0100

elasticityDemand Elasticity

fixedCostsFixed Costs F ($)

mcBaseMC Base Level ($)

Formula

\begin{aligned} MR &= MC \Rightarrow Q^* \quad ; \quad \pi = (P^* - ATC^*) \times Q^* \\ MR &= (58.1 - 42.9) \times 8.02 \\ MR &= \$121.97 \end{aligned}

Interactive Chart

MONOPOLISTIC COMPETITIONSHORT RUN

ECONOMIC PROFIT — π = +$122.0

Quantity Q*

8.02

Price P*

$58.15

Economic Profit π

+$122.0

Markup (P − MC)

$32.10

ENTRY/EXIT MECHANISM

• Economic profit → new firms enter • Entry shifts individual demand curve LEFT • Demand becomes more elastic with each entrant • Adjustment continues until P = ATC

CFA KEY RELATIONSHIPS

• MR = MC determines Q* — never P* • LR: P = ATC (zero economic profit) • LR: P > MC (markup = $32.10 persists) • LR: production NOT at minimum ATC • Excess capacity = productively inefficient

STUDENT TASKS

1. Set marketStr = 1 (MC), scenario = 0 (SR Profit). Drag tPhase from 0 → 100. Watch demand shift left and the profit rectangle shrink to zero. 2. Switch to marketStr = 0 (Monopoly) with the same SR profit. Advance tPhase. Explain why the profit rectangle does NOT disappear. 3. At tPhase = 100 under MC: verify that the demand curve is exactly tangent to ATC and that MR = MC holds simultaneously. 4. Increase elasticity to 5: observe how the markup (P − MC) decreases as the demand curve flattens. 5. At LR for MC: identify the excess capacity — the distance between Q* and the minimum-ATC output (orange dot).

Economic profit attracts new firms entering the market

PROFIT MAXIMIZATION RULE

MR = MC determines Q* in every market structure — it never directly determines price.

Once Q* is found, the firm reads P* off the demand curve at Q*:

•

P* = a − b × Q* (where a is demand intercept, b is slope)

Economic profit per period:

•

π = (P* − ATC*) × Q*

•

Positive when P* > ATC* — green rectangle

•

Zero when P* = ATC* — break-even

•

Negative when P* < ATC* — red rectangle (loss)

MONOPOLY LONG-RUN LOGIC

Barriers to entry prevent new competition:

•

Demand curve is stable — no leftward shift from entry

•

If π > 0 in SR, it persists in LR (no adjustment mechanism)

•

If π < 0 in SR, firm considers shutdown vs. continue at a loss

•

Shutdown rule: continue if P ≥ AVC (covers variable costs)

•

No LR tangency condition — ATC does not have to touch demand

MONOPOLISTIC COMPETITION LONG-RUN LOGIC

Free entry and exit act as the equilibrating force:

•

SR profit (P > ATC) → new firms enter → individual demand shifts LEFT → demand becomes more elastic

•

SR loss (P < ATC) → firms exit → remaining demand shifts RIGHT

•

Adjustment continues until:

•

P = ATC (zero economic profit)

•

Demand curve is exactly tangent to ATC

•

MR = MC still holds at the same Q*

TANGENCY CONDITION

At LR equilibrium for monopolistic competition:

•

The slope of demand = slope of ATC (tangency, not intersection)

•

This guarantees P = ATC and MR = MC simultaneously

•

Q_LR < Q at minimum ATC → excess capacity exists

EXCESS CAPACITY

Minimum ATC output (Q_min) is where d(ATC)/dQ = 0:

•

Q_min = sqrt(Fixed Costs / atc_slope)

•

LR Q* for MC is always below Q_min

•

Excess capacity = Q_min − Q*_LR

•

Society pays a higher price for variety (product differentiation)

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Study aids

Working with this chart

The errors candidates make on this topic, and what to carry into the exam.

Common mistakes

COMMON MISTAKES IN MARKET STRUCTURE ANALYSIS
Reading price from MR=MC directly: MR=MC gives Q*. Price is always found from the demand curve at Q*. Writing P = MC (or P = MR) is the profit-maximization error.
Believing monopolistic competition reaches minimum ATC in LR: The LR equilibrium requires demand to be tangent to ATC, which occurs on the downward-sloping portion of ATC — always to the left of the minimum. Excess capacity is a structural feature, not an accident.
Assuming zero profit means P = MC: In monopolistic competition, P = ATC in LR (zero economic profit), but P > MC (markup still exists). These are two different conditions.
Ignoring the difference between shutdown and exit: In the short run, a firm shuts down if P < AVC. Exit from the industry is a long-run decision when economic profit is persistently negative.
Treating the MR curve as linear when demand is non-linear: For linear demand P = a − bQ, MR = a − 2bQ. The MR curve has the same intercept but twice the slope — a common calculation error.
Applying monopolistic competition LR condition to monopoly: The P = ATC tangency is ONLY the LR equilibrium condition for free-entry markets. Monopoly with barriers to entry has no equivalent automatic adjustment.

Exam tips

EXAM TIPS FOR MARKET STRUCTURE QUESTIONS
Always identify the market structure first: the number of firms, product differentiation, and barriers to entry determine which framework applies.
For any profit-maximization question, start with MR = MC → Q* → read P* from demand → compare P* to ATC* for profit status.
In monopolistic competition, the LR equilibrium has three simultaneous conditions:
1. MR = MC (profit maximization)
2. P = ATC (zero economic profit from entry)
3. Demand is tangent to ATC (graphical condition)
Excess capacity is always present in LR monopolistic competition. If an exam question asks about productive efficiency, the answer is: monopolistic competition is NOT productively efficient.
For monopoly questions on LR profit: profit persists if and only if barriers to entry persist. Regulatory, patent, or natural barriers are the typical CFA contexts.