Market Equilibrium Class 11: Price Determination under Perfect Competition and Applications

What Market Equilibrium is about

Weightage: This chapter is part of Unit 7, Forms of Market and Price Determination under perfect competition with simple applications, which carries 8 of the 80 theory marks in the school annual exam.Source: CBSE Economics (030) syllabus 2026-27, Class XI course structure, Unit 7 Forms of Market and Price Determination under perfect competition with simple applications

Key concepts

Equilibrium, excess demand and excess supply

NCERT section 5.1

Market equilibrium occurs where market demand = market supply (qᴰ = qˢ). The price at which this happens is the equilibrium price (p*) and the quantity bought and sold is the equilibrium quantity (q*).

• Excess demand (qᴰ > qˢ) at a price below p*: buyers cannot get all they want; competition among buyers raises the price.
• Excess supply (qˢ > qᴰ) at a price above p*: sellers cannot sell all they want; competition among sellers lowers the price.

The price keeps adjusting until excess demand and excess supply disappear: this is the invisible hand of the market.

Wheat market with qᴰ = 200 − 4p and qˢ = 20 + 2p:

Price (₹)	Quantity demanded	Quantity supplied	Market situation
20	120	60	Excess demand 60
25	100	70	Excess demand 30
30	80	80	Equilibrium
35	60	90	Excess supply 30
40	40	100	Excess supply 60

Setting 200 − 4p = 20 + 2p gives p* = ₹30 and q* = 80.

Worked example

In the wheat market above, what is the market situation at ₹25, and what will happen to the price?

Try it first, then show the solutionSolution

1. qᴰ = 200 − 100 = 100; qˢ = 20 + 50 = 70
2. Excess demand = 30
3. Buyers compete for scarce wheat, so the price rises towards ₹30

Answer: Excess demand of 30; the price rises to ₹30.

Solving equilibrium with linear demand and supply

NCERT section 5.1.1

With qᴰ = a − bp and qˢ = c + dp:

At equilibrium a − bp = c + dp, so

$p^{\ast }=\frac{a-c}{b+d}$, and $q^{\ast }=a-b{p}^{\ast }$ (check: $=c+d{p}^{\ast }$).

Always substitute back into both equations: if they give different quantities, there is an arithmetic slip.

For wheat: p* = (200 − 20) ÷ (4 + 2) = 30; q* = 200 − 4 × 30 = 80; check: 20 + 2 × 30 = 80.

Worked example

Find equilibrium if qᴰ = 90 − 3p and qˢ = 10 + 5p.

Try it first, then show the solutionSolution

1. 90 − 3p = 10 + 5p ⇒ 8p = 80 ⇒ p* = 10
2. q* = 90 − 30 = 60; check 10 + 50 = 60

Answer: p* = ₹10, q* = 60.

Effect of shifts in demand (supply unchanged)

NCERT section 5.1.1

With an upward-sloping supply curve and a fixed number of firms:

• Increase in demand (rightward shift: higher income for a normal good, rise in the price of a substitute, favourable tastes): at the old price there is excess demand, so the price rises. Equilibrium price rises and quantity rises.
• Decrease in demand (leftward shift): excess supply at the old price; price falls and quantity falls.

Example: if wheat demand rises to qᴰ = 260 − 4p (supply unchanged), the new equilibrium is p* = (260 − 20) ÷ 6 = ₹40 and q* = 100: both higher than before (₹30, 80).

The size of the price change depends on how steep the supply curve is: with inelastic (steep) supply, a demand shift mainly raises price.

Worked example

What happens to the equilibrium price and quantity of tea if the price of coffee (a substitute) rises?

Try it first, then show the solutionSolution

1. Coffee dearer: demand for tea increases (rightward shift)
2. Excess demand at the old tea price pushes the price up
3. Supply is unchanged, so quantity rises along the supply curve

Answer: Equilibrium price and quantity of tea both rise.

Effect of shifts in supply (demand unchanged)

NCERT section 5.1.1

• Increase in supply (rightward shift: better technology, fall in input prices, subsidy, more firms in the short-run setting): excess supply at the old price, so the price falls. Equilibrium price falls and quantity rises.
• Decrease in supply (leftward shift: rise in input prices, a tax, crop failure): excess demand at the old price; price rises and quantity falls.

Example: if wheat supply rises to qˢ = 50 + 2p (demand unchanged), p* = (200 − 50) ÷ 6 = ₹25 and q* = 100: price lower, quantity higher.

Worked example

Floods destroy part of the onion crop. What happens to the price and quantity of onions?

Try it first, then show the solutionSolution

1. Supply decreases (leftward shift)
2. At the old price there is excess demand
3. Price rises; quantity bought and sold falls

Answer: Price rises and quantity falls.

Simultaneous shifts in demand and supply

NCERT section 5.1.1

When both curves shift, the effect on one variable is certain and on the other it depends on the relative size of the shifts.

Shift	Price	Quantity
Demand ↑, supply ↑	Rises, falls or unchanged (depends on which shift is bigger)	Rises
Demand ↓, supply ↓	Depends	Falls
Demand ↑, supply ↓	Rises	Depends
Demand ↓, supply ↑	Falls	Depends

Example: if wheat demand becomes 260 − 4p and supply becomes 80 + 2p (both increase), p* = (260 − 80) ÷ 6 = ₹30 (unchanged) and q* = 140 (higher): here the two shifts are equal in size at the old price (60 units each), so price is unchanged.

Worked example

Demand for smartphones rises sharply while better technology also increases their supply slightly. What happens?

Try it first, then show the solutionSolution

1. Both increase: quantity certainly rises
2. Demand increase is larger than supply increase, so price rises

Answer: Quantity rises; price rises (because the demand shift is bigger).

Price ceiling

NCERT section 5.2.1

A price ceiling is a maximum price fixed by the government below the equilibrium price, so that essential goods (food grains, kerosene, medicines) remain affordable to the poor.

At the ceiling price, quantity demanded exceeds quantity supplied: excess demand (shortage).

Wheat example: equilibrium ₹30; ceiling ₹25: qᴰ = 100, qˢ = 70, shortage = 30.

Consequences:

• Goods must be rationed (ration cards, fair price shops, queues), so each household gets a limited quantity.
• Long queues and waiting.
• Some buyers willing to pay more lead to a black market, where goods are sold illegally above the ceiling.
• Sellers may cut quality or supply less.

A ceiling above equilibrium has no effect.

Worked example

If the government fixes the wheat price at ₹25 (equilibrium ₹30), find the shortage and name two consequences.

Try it first, then show the solutionSolution

1. qᴰ = 100, qˢ = 70; shortage = 30
2. Rationing through ration cards and fair price shops
3. Possible black market where wheat is sold above ₹25

Answer: Shortage of 30 units; rationing and black marketing.

Price floor

NCERT section 5.2.2

A price floor is a minimum price fixed by the government above the equilibrium price, so that producers get a fair return. Examples: minimum support price (MSP) for crops; minimum wage for workers.

At the floor price, quantity supplied exceeds quantity demanded: excess supply (surplus).

Wheat example: floor ₹35: qˢ = 90, qᴰ = 60, surplus = 30.

Consequences:

• To keep the floor effective, the government must buy the surplus (procurement), store it (buffer stocks) or export it. Here it buys 30 units at ₹35 = ₹1,050.
• Storage costs and wastage; consumers pay a higher price.
• For a minimum wage set above the market wage, labour supply exceeds demand: some unemployment.

A floor below equilibrium has no effect.

Worked example

Why must the government usually buy grain when it announces an MSP above the market price?

Try it first, then show the solutionSolution

1. At the MSP, farmers want to sell more than buyers want to buy (surplus)
2. Without purchase, farmers could not sell all their grain and the price would fall below the MSP

Answer: To absorb the surplus so that the floor price actually holds.

Market Equilibrium in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Topics in this chapter

• Equilibrium Price and Shifts in Demand and Supply
• Price Ceiling and Price Floor

Market Equilibrium: key facts

Equilibrium condition

q^D = q^S

Market clears; no excess demand or supply.

Equilibrium price (linear)

q^D = a - bp,\ q^S = c + dp \Rightarrow p^* = \frac{a - c}{b + d}

Substitute back into both to find q*.

Excess demand

ED = q^D - q^S > 0 \text{ (price below equilibrium)}

Price tends to rise.

Excess supply

ES = q^S - q^D > 0 \text{ (price above equilibrium)}

Price tends to fall.

Shortage under a ceiling

Shortage = q^D(p_c) - q^S(p_c)

p_c below p*.

Surplus under a floor

Surplus = q^S(p_f) - q^D(p_f)

p_f above p*; government purchase cost = surplus × p_f.

How the school exam asks Market Equilibrium

The question patterns to expect, and the method that works for each.

Tricks to remember Market Equilibrium

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Single shifts

  "Demand moves price and quantity together; supply moves them apart."

  Use it when: Effects of one shift.

• Ceiling and floor

  Ceiling below → Shortage; Floor above → Surplus ("CS below, FS above").

  Use it when: Controlled prices.

• Linear equilibrium

  "Subtract intercepts, add slopes": p* = (a − c) ÷ (b + d).

  Use it when: Equation questions.

Market Equilibrium: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section E style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 4-mark question split into 1 + 1 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Market Equilibrium

The slips examiners see most often, and what to write instead.

• ✗Mistake: Placing a price ceiling above equilibrium in the diagram

  A "ceiling" sounds like it is high.

  ✓Do this: An effective ceiling is below equilibrium; an effective floor is above it.

  Usually costs: 1-2 marks.

• ✗Mistake: Not checking q* in both equations

  Students stop after finding p*.

  ✓Do this: Substitute p* into both demand and supply; they must give the same q.

  Usually costs: 1 mark.

• ✗Mistake: Treating a change in price as a shift

  In equilibrium questions, price changes are results, not causes.

  ✓Do this: Only non-price factors shift curves; price then adjusts and quantity moves along the curves.

  Usually costs: 1 mark.

• ✗Mistake: Saying both price and quantity are certain when both curves shift

  Applying the single-shift rule twice.

  ✓Do this: With two shifts, one variable is certain and the other depends on the relative size of the shifts.

  Usually costs: 1-2 marks.

NCERT exercise guide: Market Equilibrium

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

Practice questions on Market Equilibrium

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQExam level

   An increase in demand with supply unchanged leads to:
   (a) higher price, lower quantity
   (b) higher price, higher quantity
   (c) lower price, higher quantity
   (d) lower price, lower quantity

   Hint

   Move along supply.

   Answer

   (b)

2. Q2 · 1 mark · MCQExam level

   Minimum support price for wheat is an example of a:
   (a) price ceiling
   (b) price floor
   (c) equilibrium price
   (d) black market price

   Hint

   Minimum.

   Answer

   (b) price floor

3. Q3 · 1 mark · Very short answerExam level

   What is meant by equilibrium price?

   Hint

   Demand = supply.

   Answer

   The price at which market demand equals market supply.

4. Q4 · 3 marks · Short answerExam level

   Explain how the market moves to equilibrium when there is excess demand.

   Hint

   Buyers compete.

   Answer

   At a price below equilibrium, demand exceeds supply. Buyers compete for the limited goods and bid the price up. As price rises, quantity demanded falls and quantity supplied rises, until they are equal at the equilibrium price.

5. Q5 · 3 marks · Short answerHarder exam level

   Find equilibrium: qᴰ = 150 − 5p; qˢ = 30 + 7p.

   Hint

   12p = 120.

   Answer

   p* = 10; q* = 100 (check 30 + 70 = 100).

6. Q6 · 4 marks · Long answerHarder exam level

   Explain with a diagram the effect of a rise in the price of inputs on the equilibrium price and quantity of a good.

   Hint

   Supply decreases.

   Answer

   Higher input prices raise costs, so supply decreases (shifts left). At the old price there is excess demand; the price rises and quantity falls to the new equilibrium where the new supply curve meets the unchanged demand curve.

7. Q7 · 3 marks · Short answerHarder exam level

   Demand decreases and supply increases at the same time. What happens to price and quantity?

   Hint

   Price is certain.

   Answer

   Price certainly falls (both changes push it down). Quantity may rise, fall or stay the same, depending on whether the supply increase is larger than, smaller than or equal to the demand decrease.

8. Q8 · 3 marks · Short answerHarder exam level

   What is a price ceiling? Explain two of its consequences.

   Hint

   Shortage.

   Answer

   A maximum price fixed below equilibrium. Consequences: excess demand (shortage), leading to rationing and queues; black markets where goods sell illegally at higher prices (also lower quality).

9. Q9 · 3 marks · Short answerHarder exam level

   For qᴰ = 150 − 5p and qˢ = 30 + 7p, what happens if the government fixes a floor of ₹12?

   Hint

   Above equilibrium (₹10).

   Answer

   qᴰ = 90; qˢ = 114; surplus = 24. The government may have to buy 24 units (cost ₹288) to maintain the floor.

10. Q10 · 4 marks · Long answerHarder exam level

    Explain the effect of a fall in the price of a complementary good on the equilibrium of a good (say, petrol cars when petrol becomes cheaper).

    Hint

    Demand for the complement rises.

    Answer

    Cheaper petrol makes running petrol cars cheaper, so demand for petrol cars increases (rightward shift). With supply unchanged, excess demand at the old price raises price, and quantity sold rises. New equilibrium: higher price and higher quantity.

11. Q11 · 3 marks · Short answerHarder exam level

    Explain the effects of a minimum wage set above the market wage in a labour market.

    Hint

    A price floor on labour.

    Answer

    It is a price floor on labour. At the higher wage, more people want to work (supply rises) but firms hire fewer workers (demand falls): excess supply of labour, i.e. unemployment among those who want jobs at that wage, though employed workers earn more.

12. Q12 · 4 marks · Long answerHOTS / competency

    Suppose a city imposes rent control (a ceiling on house rents) below the market rent. Using the ideas of this chapter, predict three effects on the housing market over time.

    Hint

    Ceiling consequences.

    Answer

    (1) Shortage: more people want to rent at the low rent than landlords offer. (2) Rationing by other means: long waiting lists, preference for known tenants, and side payments (a black market in "deposits" or key money). (3) Lower quality and supply over time: landlords spend less on maintenance and fewer new rental homes are built, worsening the shortage.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.What is market equilibrium?Show answer

   Where market demand equals market supply.

2. 2.What happens at a price below equilibrium?Show answer

   Excess demand; price tends to rise.

3. 3.Effect of an increase in supply (demand constant)?Show answer

   Price falls, quantity rises.

4. 4.Is a price ceiling set above or below equilibrium?Show answer

   Below.

5. 5.Result of a price floor?Show answer

   Excess supply (surplus).

6. 6.Give an example of a price floor in India.Show answer

   Minimum support price for crops (also minimum wages).

7. 7.Formula for p* with linear curves?Show answer

   (a − c) ÷ (b + d).

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you define equilibrium price and quantity?
• ☐Can you explain how excess demand and excess supply are removed?
• ☐Can you solve for equilibrium with linear equations?
• ☐Can you show the effect of an increase and a decrease in demand?
• ☐Can you show the effect of an increase and a decrease in supply?
• ☐Can you explain what is certain when both curves shift?
• ☐Can you compute the shortage under a price ceiling?
• ☐Can you compute the surplus and government cost under a price floor?

Market Equilibrium: frequently asked questions