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Market Equilibrium, Excess Demand/Supply & Free Entry

🎓 Class 12 Economics CBSE Theory Chapter 5 — Market Equilibrium ⏱ ~28 min
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Class 12 · Introductory Microeconomics · Chapter 5 · Part 1

Market Equilibrium: Excess Demand, Excess Supply & Free Entry-Exit

Chapter 2 gave us the consumer's demand curve. Chapter 4 gave us the firm's supply curve. Chapter 5 brings them together. The point at which the two curves cross is where every consumer's plan meshes with every firm's plan — there is nobody left wanting to buy who cannot, and nobody left wanting to sell who cannot. This is market equilibrium. In this part we define the equilibrium price p* and quantity q* with a fixed number of firms, watch the Walrasian "Invisible Hand" eliminate excess demand and excess supply, work through a numerical example with the wheat-market schedules qD = 200 − p and qS = 120 + p, and then drop the fixed-firms assumption to derive the long-run free entry-exit result p = min AC.

5.1 Equilibrium, Excess Demand, Excess Supply

A perfectly competitive market is built on two self-interested groups. Consumers want to maximise their satisfaction; firms want to maximise their profits. Recall from Chapters 2 and 4 that both groups take the market price as given. The remarkable claim of demand-supply analysis is that, despite each side acting purely on its own behalf, there is one price at which the two sides' plans match. That price is the equilibrium price?.

📖 Definition — Market Equilibrium

An equilibrium is a situation where the plans of all consumers and firms in the market match and the market clears. The aggregate quantity that all firms wish to sell at the equilibrium price equals the aggregate quantity that all consumers wish to buy at that price — market supply equals market demand. The price at which this happens is the equilibrium price (p*) and the quantity bought and sold at this price is the equilibrium quantity (q*).

Equilibrium condition:  qD(p*) = qS(p*)

Here p* is the equilibrium price; qD(p) is the market demand at price p; and qS(p) is the market supply at price p.

5.1.1 What if the Market is Not in Equilibrium?

Suppose the price prevailing in the market is not p*. Two things can go wrong, and each has a name.

📉
Excess Demand
If at some price market demand exceeds market supply, that price has excess demand?. Some buyers are unable to obtain the good, or obtain too little of it. Frustrated buyers offer a higher price. Price tends to rise.
📈
Excess Supply
If at some price market supply exceeds market demand, that price has excess supply?. Some firms cannot sell what they want to sell. They lower their price to clear the unsold stock. Price tends to fall.
⚖️
Equilibrium = Zero ED = Zero ES
Equilibrium can be defined alternatively as the price at which both excess demand and excess supply are zero. Whenever supply ≠ demand, there is a tendency for price to change.

5.1.2 Out-of-Equilibrium Behaviour — the Walrasian "Invisible Hand"

Adam Smith (1723–1790) argued that in a perfectly competitive market an Invisible Hand? moves prices whenever the market is out of balance. Intuition agrees: the Invisible Hand should push prices up when there is excess demand, and push prices down when there is excess supply. Throughout this chapter we accept this assumption — and accept further that the Invisible Hand always succeeds in driving the price to p*. This price-adjustment story is also called Walrasian adjustment, after the French economist Léon Walras.

5.1.3 Market Equilibrium with a Fixed Number of Firms

For now, hold the number of firms fixed. The market demand curve DD slopes downward (consumers buy more at lower prices). The market supply curve SS slopes upward (firms supply more at higher prices). The equilibrium is the point where the two curves cross — graphically, the unique point at which qD(p) = qS(p).

Quantity → Price → O SS DD E q* p* p₂ EXCESS SUPPLY p₁ EXCESS DEMAND price falls ↓ price rises ↑
Figure 5.1 — Equilibrium occurs at point E where DD intersects SS, giving equilibrium price p* and quantity q*. At any p > p* there is excess supply; at any p < p* there is excess demand. The Invisible Hand drives the price back to p*.

5.1.4 Reading the Diagram Carefully

Two cases emerge once we let the price wander away from p*.

  • If the prevailing price is p₁ (below p*). At this price the market demands a quantity q₁, but the firms supply only q₁′, where q₁′ < q₁. There is excess demand of (q₁ − q₁′). Some consumers are unable to buy at all — others can buy only an insufficient amount. They offer a higher price. As price rises, quantity demanded falls and quantity supplied rises, until the two are equal at p*.
  • If the prevailing price is p₂ (above p*). At this price firms wish to supply q₂ but consumers want to buy only q₂′, where q₂′ < q₂. There is excess supply of (q₂ − q₂′). Some firms cannot sell their unsold stock and lower their price. As price falls, quantity supplied falls and quantity demanded rises, until both equal q* at p*.

In both cases the market self-corrects toward (p*, q*). This is what is meant by saying the equilibrium is stable.

5.1.5 Worked Example 5.1 — Solve for p* and q* Algebraically

Consider a market consisting of identical wheat farms (identical means same cost structure). Suppose the market demand and market supply curves for wheat are:

qD = 200 − p   for 0 ≤ p ≤ 200;   qD = 0 for p > 200
qS = 120 + p   for p ≥ 10;   qS = 0 for 0 ≤ p < 10

where qD and qS are in kg and p is the price of wheat per kg in rupees.

Step 1 — Set qD = qS and solve for p*.

200 − p* = 120 + p*
⇒ 2p* = 80
⇒ p* = Rs 40 per kg

Step 2 — Substitute back to find q*.

q* = qD(40) = 200 − 40 = 160 kg
q* = qS(40) = 120 + 40 = 160 kg ✓

Both demand and supply schedules give the same answer at p* — that is precisely what equilibrium means.

Step 3 — Verify excess demand below p*. Take p₁ = 25:

qD = 200 − 25 = 175 kg;   qS = 120 + 25 = 145 kg
ED(25) = qD − qS = 175 − 145 = 30 kg > 0

Algebraically, the excess demand function is

ED(p) = qD − qS = (200 − p) − (120 + p) = 80 − 2p

which is positive whenever p < 40, zero at p = 40, and negative whenever p > 40.

Step 4 — Verify excess supply above p*. Take p₂ = 45:

qD = 200 − 45 = 155 kg;   qS = 120 + 45 = 165 kg
ES(45) = qS − qD = 2p − 80 = 90 − 80 = 10 kg > 0

The complete schedule looks like this:

Table 5.1 — Demand & supply schedules for wheat (qD = 200 − p, qS = 120 + p)
Price p (Rs/kg)qD (kg)qS (kg)ED = qD − qSStatus
20180140+40Excess demand → price ↑
30170150+20Excess demand → price ↑
35165155+10Excess demand → price ↑
401601600EQUILIBRIUM
45155165−10Excess supply → price ↓
50150170−20Excess supply → price ↓
60140180−40Excess supply → price ↓
Figure 5.2 — Plotting both schedules: qD = 200 − p (downward) and qS = 120 + p (upward) cross exactly at (q*, p*) = (160 kg, Rs 40).
⚙️ Stability of Equilibrium

In this textbook example the equilibrium is stable: any disturbance — a price below or above Rs 40 — generates a corrective force (excess demand or excess supply) that pushes the price back to Rs 40. Stability is what licenses us to call p* the price the market actually reaches, not just a price that solves an algebraic equation.

Activity 5.1 — Market-Clearing Detective
  1. Visit your local sabzi mandi early in the morning and again in the late evening.
  2. Note the price of one perishable vegetable (say, tomatoes) at both times. Also note whether vendors look "sold out" or whether large unsold heaps remain.
  3. Use the language of excess demand and excess supply to explain why the evening price is typically different from the morning price.

Sample observation: Morning prices are usually higher because demand is large and supply is constrained to that day's harvest — small excess demand is cleared by raising the price slightly. Late evening, vendors face excess supply: stock is perishable and they cannot carry it home, so they cut the price aggressively to clear it. The Walrasian Invisible Hand can be observed in real time on the mandi floor.

5.2 Market Equilibrium with Free Entry & Exit (Long Run)

The fixed-firms analysis above was a short-run story — the number of firms could not adjust. But what makes perfect competition different in the long run is that firms can enter and exit freely?. The implication is dramatic.

📖 The Long-Run Equilibrium Result

In a perfectly competitive market with identical firms and free entry-exit, no firm can earn supernormal profit and no firm can incur a loss in the long run. The equilibrium price will equal the minimum point of the average cost curve:

p = min AC

5.2.1 Why does p settle at min AC?

Recall from Chapter 4 that a firm earns supernormal profit when p > min AC and incurs a loss when p < min AC. Now combine that with free entry and exit:

⬆️
Case 1 — p > min AC
Existing firms earn supernormal profit. Outside firms see this and enter. Market supply curve shifts right. Demand unchanged. Price falls. Entry continues until supernormal profit is wiped out — i.e. price has fallen back to min AC.
⬇️
Case 2 — p < min AC
Existing firms earn less than normal profit (loss). Some firms exit. Market supply curve shifts left. Demand unchanged. Price rises. Exit continues until losses vanish — i.e. price has risen back to min AC.
🎯
Case 3 — p = min AC
Each firm earns exactly normal profit. No new firm wants to enter (no supernormal profit on offer) and no existing firm wants to exit (no loss). The market is in long-run equilibrium.

So the long-run equilibrium price is pinned down entirely by the cost side — by the minimum of the firm's average cost curve. The demand curve plays no role in determining price in the long run; demand only fixes the quantity traded and (therefore) the number of firms.

Quantity → Price → O p₀ = min AC p₀ DD E q₀ Long-run market equilibrium: price fixed by min AC; quantity by DD.
Figure 5.3 — Long-run equilibrium under free entry-exit. The horizontal "p₀ = min AC" line plays the role of the supply curve. The market clears at point E where DD intersects this line, giving equilibrium quantity q₀.

5.2.2 The Equilibrium Number of Firms

At p₀ = min AC, each identical firm produces the same output, call it q0f (the firm's individual output at minimum AC). The total market quantity q₀ is supplied by the entire industry. The equilibrium number of firms n₀ is therefore

n₀ = q₀ / q0f

5.2.3 Worked Example 5.2 — Solve for p₀, q₀ and n₀

Consider a market for wheat with demand

qD = 200 − p   for 0 ≤ p ≤ 200;   qD = 0 for p > 200

and identical firms whose individual supply curve is

qfS = 10 + p   for p ≥ 20;   qfS = 0 for 0 ≤ p < 20

The kink at p = 20 is where the firm's supply curve begins — at any price below this, the firm exits because it cannot cover its average cost. So min AC = Rs 20.

Step 1 — Equilibrium price. Free entry-exit pins price at min AC:

p₀ = Rs 20

Step 2 — Equilibrium quantity. At p₀ = 20 the market quantity is whatever the demand curve says:

q₀ = 200 − 20 = 180 kg

Step 3 — Output per firm. At p₀ = 20 each firm supplies:

q0f = 10 + 20 = 30 kg

Step 4 — Equilibrium number of firms.

n₀ = q₀ / q0f = 180 / 30 = 6 firms

So the long-run equilibrium of this wheat market is: price Rs 20 per kg, quantity 180 kg, and 6 firms each producing 30 kg.

5.2.4 What Happens When Demand Shifts Under Free Entry-Exit?

This is the most striking implication of the long-run model. Because price is pinned at min AC by the entry-exit mechanism, a demand shift can never move the equilibrium price. Suppose demand rises from DD₀ to DD₁:

  • At the original price p₀, the new demand is greater than supply — there is excess demand.
  • Price tends to rise, opening a window of supernormal profit.
  • New firms enter, market supply expands, and the price is pushed back down to p₀ = min AC.
  • The market settles at a higher quantity q₁ > q₀, supplied by a larger number of firms n₁ > n₀, but at the same price p₀.

Conversely, a leftward shift in demand to DD₂ reduces q and n while leaving p₀ unchanged — some existing firms exit until losses are eliminated.

Quantity → Price → p₀ = min AC DD₂ DD₀ DD₁ E₂ q₂ E q₀ F q₁
Figure 5.4 — Under free entry-exit, demand shifts move quantity and the number of firms but leave the equilibrium price unchanged at p₀ = min AC. Compare with fixed-firms case: there, demand shifts moved both p* and q*.
⚙️ Fixed Firms vs. Free Entry-Exit — the contrast
  • Fixed firms: a demand shift moves both p* and q*. Larger price effect, smaller quantity effect.
  • Free entry-exit: a demand shift moves only q* (and the number of firms). Zero price effect, larger quantity effect.

Competency-Based Questions — Equilibrium & Free Entry-Exit

Scenario: A perfectly competitive market for organic basmati rice in Punjab has identical farms. The market demand and supply schedules are qD = 500 − 4p and qS = 200 + p, where p is in rupees per kg and q is in tonnes. Each individual farm has a U-shaped AC curve whose minimum lies at Rs 60 per kg, with output qf = 25 tonnes at that minimum.
Q1. With the number of farms fixed, calculate the short-run equilibrium price and quantity. Verify by computing excess demand at p = Rs 50 and excess supply at p = Rs 70.
L3 Apply
Answer: Set qD = qS: 500 − 4p* = 200 + p* ⇒ 5p* = 300 ⇒ p* = Rs 60. Substitute: q* = 200 + 60 = 260 tonnes (also 500 − 240 = 260 ✓). At p = 50: qD = 300, qS = 250, ED = 50 tonnes (positive, price rises). At p = 70: qD = 220, qS = 270, ES = 50 tonnes (positive, price falls). The Walrasian Invisible Hand drives the price back to Rs 60.
Q2. The short-run equilibrium price (Rs 60) coincides with the minimum AC of each farm. What is the equilibrium number of farms in the long run, and explain why entry-exit will not be triggered.
L4 Analyse
Answer: Equilibrium number of firms n₀ = q₀ / q0f = 260 / 25 = 10.4 farms. (In a worked NCERT problem we round to 10 — interpret the leftover as residual capacity at the marginal farm.) Because p* = min AC already, every existing farm earns exactly normal profit. No farm has the incentive to leave (no losses) and no outsider has the incentive to enter (no supernormal profit). So entry-exit is not triggered. This short-run equilibrium also happens to be the long-run equilibrium.
Q3. A consultant claims, "If consumer income rises and demand shifts rightward, both price and quantity will increase." Evaluate this claim under (i) fixed firms and (ii) free entry-exit, and identify the implicit assumption it is making.
L5 Evaluate
Answer: Under fixed firms the claim is correct: a rightward demand shift creates excess demand at the old price; price rises and so does quantity supplied. Under free entry-exit the claim is wrong: at the original price (= min AC), any temporary excess demand attracts new firms into the market, expanding supply and pushing the price back down to min AC. Quantity rises but price is unchanged. The consultant has implicitly assumed a fixed number of firms. Whether their conclusion is correct depends entirely on whether the time horizon is short or long.
Q4 (HOT). Construct an example where the algebraic equilibrium price exists but the equilibrium is "unstable" — i.e. a small price disturbance does not bring the market back to p*. What feature of the curves would have to be unusual?
L6 Create
Answer: Stability of the standard textbook equilibrium relies on (i) DD sloping down and (ii) SS sloping up. Reverse one of these and the corrective force flips sign. Example: suppose SS has a "backward-bending" segment (as the labour-supply curve sometimes does at high wages) and the intersection happens on this backward segment. A small increase in price now raises ED rather than ES — and the price spirals away from the original p*, not back to it. So the unusual feature is a positively sloped DD or a negatively sloped SS at the intersection. NCERT's standard model rules these out by assumption, which is why every textbook equilibrium is stable.
Assertion–Reason Questions — Equilibrium & Free Entry-Exit

Options for all items: (A) Both A and R are true and R is the correct explanation of A. (B) Both A and R are true but R is not the correct explanation of A. (C) A is true, R is false. (D) A is false, R is true.

Assertion (A): If the prevailing market price is below the equilibrium price, the price will tend to rise.
Reason (R): At a price below p*, market demand exceeds market supply, so dissatisfied buyers offer higher prices.
Correct option: (A). Both statements are true and R is the precise explanation of A. Excess demand at p < p* generates the upward Walrasian price pressure, restoring equilibrium.
Assertion (A): Under free entry and exit of identical firms, the long-run equilibrium price equals the minimum of the firm's average cost curve.
Reason (R): At any price greater than min AC firms earn supernormal profit, attracting entry; at any price below min AC firms incur losses, prompting exit.
Correct option: (A). Both true, and R explains A. Entry pushes price down whenever p > min AC; exit pushes price up whenever p < min AC. The only price compatible with no entry and no exit is p = min AC.
Assertion (A): Under free entry-exit, a rightward shift of the demand curve has a larger effect on equilibrium quantity than under a fixed number of firms.
Reason (R): Under free entry-exit the equilibrium price does not change, so the entire excess demand is absorbed through quantity expansion via new entry rather than partly through price increase.
Correct option: (A). Both statements are true and R correctly explains A. Under fixed firms a demand rise is split into a price rise (which damps the quantity response by reducing qD) and a quantity rise. Under free entry-exit there is no price rise, so the entire demand expansion translates into a quantity expansion.

5.2.5 Quick Recap of Part 1

Five Take-aways

  • Equilibrium is the price p* at which qD(p*) = qS(p*); the corresponding traded quantity is q*.
  • Excess demand (p < p*) drives price up; excess supply (p > p*) drives price down. This is the Walrasian "Invisible Hand" at work.
  • The textbook wheat example with qD = 200 − p and qS = 120 + p gives p* = Rs 40, q* = 160 kg. Excess demand function: ED(p) = 80 − 2p.
  • With a fixed number of firms, both p* and q* are jointly determined by DD and SS at their intersection.
  • With free entry and exit, the long-run price is pinned at p = min AC regardless of demand. Demand only fixes q* and the equilibrium number of firms n₀ = q₀ / q0f.

Continue to Part 2 — applications of demand-supply analysis: price ceilings, price floors (MSP), demand and supply shifts, simultaneous shifts, and a brief look at monopoly equilibrium.

Frequently Asked Questions — Market Equilibrium: Excess Demand, Excess Supply & Free Entry-Exit

What is market equilibrium in Class 12 Microeconomics?

Market equilibrium is the price at which the quantity demanded by buyers equals the quantity supplied by sellers, with no tendency for price to change. At this price both buyers and sellers are simultaneously satisfied — every buyer who wants to buy at the price can do so, and every seller who wants to sell at the price can do so. The equilibrium price and equilibrium quantity are read off the intersection of demand and supply curves.

What is excess demand and what is excess supply?

Excess demand exists when the price is below equilibrium — the quantity buyers want to buy exceeds the quantity sellers want to sell. Excess supply exists when the price is above equilibrium — the quantity sellers want to sell exceeds the quantity buyers want to buy. Excess demand pushes price up; excess supply pushes price down. Together they ensure the market converges to equilibrium.

How is equilibrium price determined under perfect competition?

Equilibrium price under perfect competition is determined by the intersection of the market demand curve and the market supply curve. At this price, quantity demanded equals quantity supplied. NCERT Class 12 shows the determination both algebraically (solving the demand and supply equations simultaneously) and graphically (intersection of the two curves).

What is the long-run equilibrium under free entry and exit?

In the long run under perfect competition with free entry and exit, the equilibrium price equals the minimum of the representative firm's average total cost. If price exceeds minimum ATC, supernormal profits attract entrants who push the supply curve right and the price down. If price is below minimum ATC, firms make losses and exit, pushing the supply curve left and the price up. Equilibrium is reached at price = minimum ATC, where every firm earns normal profit only.

Why does the long-run equilibrium price equal minimum ATC?

Free entry erodes supernormal profits — as new firms enter, market supply increases and price falls. Free exit eliminates losses — as firms leave, supply falls and price rises. The two forces stop only when economic profit is zero, which occurs at price equal to the minimum point of the average total cost curve. At this point every firm earns just enough to cover its opportunity cost — a normal profit.

What is the algebra of market equilibrium with simple linear demand and supply?

Suppose demand is qd = a − b·p and supply is qs = c + d·p, where a, b, c, d > 0. Equilibrium requires qd = qs, giving a − b·p = c + d·p. Solving for p: p* = (a − c) / (b + d). Substituting back gives the equilibrium quantity q* = (a·d + b·c) / (b + d). NCERT Class 12 uses such linear schedules in worked examples and exercises.

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