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Everyone in this book takes the price as given — consumers in Chapter 2, firms in Chapter 4. So who sets it? The answer is nobody. It emerges.
Chapter 2 — the market demand curve: how much all consumers together will buy at each price, each taking price as given.
Chapter 4 — the market supply curve: how much all firms together will sell at each price, each taking price as given (built on the cost curves of Chapter 3).
If every consumer and every firm treats the price as something handed to them, where does it come from?
Nobody chooses it. It is the one price at which the plans of all of them happen to fit together — and this chapter finds it.
What equilibrium means, and what excess demand and excess supply are.
How p* and q* are found, and how the market corrects itself when it is out of equilibrium.
What happens when demand shifts, when supply shifts, and when both shift together.
The same tools, applied to wages instead of goods.
Why, in the long run, price is driven all the way down to minimum average cost.
Price ceilings and price floors — what happens when government fixes a price.
Almost every diagram below uses the same wheat market — qD = 200 − p
and qS = 120 + p. Equilibrium, shortage, surplus, price ceiling and price floor
are all read off one set of numbers you can check yourself.
An equilibrium is a situation where the plans of all consumers and firms in the market match and the market clears. The aggregate quantity all firms wish to sell equals the quantity all consumers wish to buy.
The price p* at which equilibrium is reached.
The quantity q* bought and sold at that price.
Excess demand exists when, at a price, market demand exceeds
market supply.
Excess supply exists when, at a price, market supply is greater
than market demand.
Equilibrium in a perfectly competitive market can be defined as a zero excess demand – zero excess supply situation. Whenever supply and demand are unequal, the market is not in equilibrium and there will be a tendency for the price to change.
Wheat: qD = 200 − p and qS = 120 + p
(firms supply nothing below ₹10).
qD(p*) = qS(p*)
200 − p* = 120 + p*
2p* = 80
p* = ₹40
q* = 200 − 40 = 160 kg
or q* = 120 + 40 = 160 kg ✓
Always substitute into both equations — if they disagree, you have made an arithmetic slip.
ED(p) = qD − qS = (200 − p) − (120 + p) = 80 − 2p
ES(p) = qS − qD = (120 + p) − (200 − p) = 2p − 80
ED > 0 whenever 80 − 2p > 0, i.e. whenever p < 40.
ES > 0 whenever 2p − 80 > 0, i.e. whenever p > 40.
So every price below p* has excess demand and every price above it has excess supply. The equilibrium price is the unique price where both are zero.
Demand 180, supply 140 → excess demand of 40.
Some consumers are unable to obtain the good at all, or get too little. They are willing to pay more than ₹20, and bid the price up.
As price rises: quantity demanded falls (movement along DD) and quantity supplied rises (movement along SS). The gap closes.
The market moves up to ₹40.
Supply 180, demand 140 → excess supply of 40.
Some firms are unable to sell the quantity they want, so they lower their price.
As price falls: quantity demanded rises and quantity supplied falls. The gap closes.
The market moves down to ₹40.
Since Adam Smith (1723–1790) it has been maintained that in a perfectly competitive market an "Invisible Hand" changes price whenever there is an imbalance: it raises price when there is excess demand and lowers it when there is excess supply. We assume throughout that this process actually reaches equilibrium.
In Chapter 1 you met this as "prices send signals that coordinate millions of strangers". This slide is that claim with the mechanism written out.
A student writes: "At a price of ₹20 there is excess demand of 40 kg. This means the market is short of 40 kg of wheat, so the government should arrange to produce 40 kg more. Then the shortage will disappear at a price of ₹20."
(a) Is 40 kg the amount by which output must rise for the market to clear at ₹20?
(b) What actually happens to the shortage as price rises to ₹40, and how much does each
side move?
(c) What does this tell you about the difference between a quantity adjustment and a
price adjustment?
(a) No — and this is the trap. If output were forced up to 180 kg while the price stayed at ₹20, the market would "clear" only in the sense that everyone who wants wheat at ₹20 gets it. But firms do not want to supply 180 kg at ₹20 — their supply curve says they will offer only 140. Producing 180 would require compelling or subsidising them; it is not an equilibrium, because the plans of firms are being overridden rather than matched.
(b) The gap closes from both sides at once — this is the key observation.
| Price | qD | qS | Excess demand |
|---|---|---|---|
| ₹20 | 180 | 140 | 40 |
| ₹30 | 170 | 150 | 20 |
| ₹40 | 160 | 160 | 0 |
Quantity supplied rises by 20 (140 → 160) and quantity demanded falls by 20 (180 → 160). Neither side moves by 40. The 40 kg gap is eliminated by a 20 kg rise in supply plus a 20 kg fall in demand.
The student assumed only the supply side responds. But a price change moves both curves' quantities — that is exactly what "movement along the curve" means, and it happens on both curves simultaneously.
(c) A quantity adjustment tries to force one side to meet the other at an unchanged price. A price adjustment makes both sides move toward each other voluntarily. That is why the market mechanism needs no one to compute the 40, and why it settles at 160 rather than 180.
Looking ahead: a government that does hold the price at ₹20 — a price ceiling — gets exactly the student's problem: a permanent shortage of 40 kg that has to be rationed somehow. That is Part F of this chapter.
Demand shifts right → at p₀ there is now excess demand → price is bid up → new equilibrium at higher price and higher quantity.
Demand shifts left → at p₀ there is now excess supply → price is bid down → new equilibrium at lower price and lower quantity.
Price and quantity always move together.
Supply shifts left (e.g. input prices rise, a unit tax) → price ↑, quantity ↓
Supply shifts right (e.g. better technology, more firms) → price ↓, quantity ↑
Demand shift → same direction. Supply shift → opposite directions.
Did the price of this good itself change? If yes → movement along the curve. If no → the curve shifts. Nothing that shifts a curve is ever the good's own price.
Clothes are a normal good, so demand rises at each price → DD shifts right. Income does not affect firms' costs, so SS is unchanged.
Result: price of clothes rises and quantity rises — panel (a) of the demand-shift diagram.
| Shift in demand | Shift in supply | Quantity | Price |
|---|---|---|---|
| Leftward | Leftward | Decreases | May rise, fall or stay unchanged |
| Rightward | Rightward | Increases | May rise, fall or stay unchanged |
| Leftward | Rightward | May rise, fall or stay unchanged | Decreases |
| Rightward | Leftward | May rise, fall or stay unchanged | Increases |
Same direction (both right, or both left) → quantity is unambiguous, price depends on the relative magnitudes of the shifts.
Opposite directions → price is unambiguous, quantity depends on the magnitudes.
Take both curves shifting right. A rightward demand shift pushes price up; a rightward supply shift pushes price down. The two pressures oppose, so the net effect depends on which shift is larger.
But both shifts push quantity up. They reinforce, so quantity certainly rises.
Newspapers report: "Onion prices have risen sharply this year, and less onion was sold than last year."
(a) Using shifts alone, deduce what must have happened in the onion market. Is the answer
unique?
(b) Suppose instead prices rose and quantity sold also rose. What would you conclude?
(c) A student says: "Price rose, so demand must have risen." Why is that reasoning
unreliable?
(a) Supply must have shifted left. Price ↑ with quantity ↓ means the two moved in opposite directions — the signature of a supply shift.
Is it unique? No. Two other combinations in Table 5.1 can also produce price ↑ with quantity ↓:
Supply left + demand right (row 4) — price certainly rises; quantity falls if the supply contraction outweighs the demand expansion.
Supply left + demand left (row 1) — quantity certainly falls; price rises if supply's upward pull outweighs demand's downward pull.
So the only thing we can assert with certainty is that supply shifted left. Demand may have shifted right, shifted left, or not moved at all — the observation cannot distinguish these.
Plausible cause: unseasonal rain destroying the crop — a leftward supply shift.
(b) Price ↑ with quantity ↑ means the two moved in the same direction — the signature of a demand shift, rightward. Again a small opposing supply shift cannot be ruled out, but demand must have expanded and dominated.
(c) Because a price rise alone is consistent with two completely different causes.
Demand shifts right → price ↑, quantity ↑
Supply shifts left → price ↑, quantity ↓
Both raise price. Only the quantity tells you which happened.
The general method: the price alone is never enough. You need both the price change and the quantity change to identify which curve moved:
Same direction → demand shifted. Opposite directions → supply shifted.
This is one of the most useful things in the whole chapter, and it is exactly why economists always want the quantity data alongside the price data.
In the market for goods: households demand, firms supply.
In the market for labour: households supply and firms demand — the roles are reversed.
Note also that "labour" means hours of work, not the number of labourers.
A firm hires up to the point where the extra cost of the last unit of labour equals the extra benefit from it:
For a perfectly competitive firm MR = price, so MRPL equals the value of marginal product, VMPL.
If the wage rises, MPL must rise to keep the equality. By the law of diminishing marginal product (Chapter 3), a higher MPL means less labour employed. Hence a downward-sloping demand curve.
A household's supply decision is essentially a choice between income and leisure. People enjoy leisure and find work irksome, but they value the income work brings.
A higher wage raises the opportunity cost of leisure — every hour off now costs more forgone income. So the individual wants less leisure and works longer.
Pushes labour supply up.
A higher wage also makes the individual richer, so she wants to spend more on leisure activities — and therefore work less.
Pushes labour supply down.
At low wages the first effect dominates → more labour supplied.
At high wages the second dominates → less labour supplied.
So an individual's supply curve bends backward.
But the market supply curve of labour is still upward sloping: although some individuals work less at higher wages, many more individuals are attracted into the market, and that outweighs the reduction.
Effect 1 is a substitution effect; Effect 2 is an income effect. Here they work in opposite directions — the same substitution-versus-income-effect logic that also explains the Giffen good, though the two cases are not identical: a Giffen good is a demand-side case needing a strongly inferior good with a large budget share, whereas this is a wage–leisure trade-off.
So far the number of firms was fixed. Now we allow free entry and exit — one of the four defining features of perfect competition from Chapter 4 — and assume all firms are identical.
With free entry and exit, in equilibrium no firm earns super-normal profit or incurs a loss. The equilibrium price will be equal to the minimum average cost of the firms.
Firms earn super-normal profit → this attracts new firms → market supply shifts right → price falls → super-normal profit is wiped out.
Entry stops when profit is back to normal.
Firms earn less than normal profit → some firms exit → market supply shifts left → price rises → profits return to the normal level.
Exit stops when profit is back to normal.
Min AC was the firm's break-even point — the price at which it earns exactly normal profit. Free entry and exit forces the whole market to that price.
qD = 200 − p
Single firm: qfS = 10 + p for p ≥ 20
(the firm produces nothing below ₹20, so min AC = ₹20)
p₀ = min AC = ₹20q₀ = 200 − 20 = 180 kgqf = 10 + 20 = 30 kgn₀ = q₀ / qf = 180/30 = 6With a fixed number of firms, demand helped determine the price. With free entry and exit, cost alone determines the price, and demand determines only the quantity and the number of firms.
At p₀ there is now excess demand → price tends to rise → this creates the possibility of super-normal profit → new firms enter → their extra supply wipes out the profit and pushes price back to p₀.
The final result: a higher quantity supplied at the same price, by a larger number of firms.
Compared with a fixed number of firms, a demand shift under free entry and exit has:
a larger effect on quantity and no effect at all on price.
With a fixed number of firms the rising supply curve resists the expansion, so part of the demand increase is absorbed by a higher price. With free entry, new firms supply whatever is needed at p₀, so the whole adjustment falls on quantity.
Under free entry and exit, the equilibrium price equals minimum average cost regardless of how strong demand is.
A student objects: "That cannot be right. If everyone suddenly wants far more of a good, surely producers can charge more for it. The law of demand says higher demand means a higher price."
(a) Is the price permanently unchanged, or unchanged only eventually?
(b) Where exactly does the student's reasoning break down?
(c) Under what condition would the long-run price actually rise?
(a) Unchanged only eventually — the student is right about the short run.
Immediately: demand rises, the existing firms cannot expand instantly, so there is excess demand and the price does rise. Firms earn super-normal profit.
Then: that profit is a signal. New firms enter, market supply shifts right, and the price is pushed back down — coming to rest again at min AC.
So the price rise is real but temporary. The diagram shows only the before and after, not the journey between them.
(b) The student has confused a movement along a curve with a long-run equilibrium.
"Higher demand means higher price" is a statement about a market with a given supply curve. But free entry means the supply side is not fixed — the number of firms is itself a variable. Once supply can expand without limit at min AC, the long-run market supply curve is effectively horizontal at min AC, and a horizontal supply curve pins the price no matter where demand sits.
Note this does not contradict the law of demand at all. Consumers still buy less when price is higher — that is why the demand curve slopes down in every diagram here. What has changed is the supply side.
(c) The long-run price would rise if min AC itself rose.
Since price is pinned to minimum average cost, the only way to move it permanently is to move costs. That happens if:
input prices rise as the industry expands (more firms bidding for the same land, labour or raw material), or technology worsens, or a unit tax is imposed — each raises min AC and therefore the long-run price.
The deep point: in the long run under perfect competition, price is governed by cost, not by demand. Demand determines how much is produced and how many firms produce it — but not what it sells for.
A price ceiling is a government-imposed upper limit on the price of a good or service.
Generally imposed on necessities — wheat, rice, kerosene, sugar — and set below the market-determined price, because at the market price some section of the population cannot afford the good.
At ₹20 consumers demand 180 kg but firms supply only 140 kg — an excess demand of 40 kg.
So although the government's intention was to help consumers, it could end up creating a shortage.
Price can no longer ration the good, so something else must. Typically rationing: ration coupons cap how much any individual may buy, and the good is sold through ration shops (fair price shops).
Each consumer has to stand in long queues to buy the good from ration shops. The price is lower, but the good now costs time as well as money.
Not all consumers are satisfied by the quantity they get from the fair price shop. Some are willing to pay a higher price for more — and that may result in the creation of a black market.
A price is a rationing device. When a good is scarce, something must decide who gets it. Fix the price below equilibrium and you do not abolish that job — you hand it to queues, coupons and illegal trade instead.
Scarcity does not disappear because a price is capped. It reappears in non-price form — exactly the conclusion reached in the Chapter 1 challenge on free healthcare.
A price floor is a government-imposed lower limit on the price that may be charged for a particular good or service.
Agricultural price support — a lower limit on the purchase price of some agricultural goods, normally set above the market-determined price.
Minimum wage legislation — ensures the wage rate does not fall below a particular level, again set above the equilibrium wage.
At ₹60 firms wish to supply 180 kg but consumers demand only 140 kg — an excess supply of 40 kg.
In the case of agricultural support, to prevent the price falling because of this excess supply, the government must buy the surplus at the predetermined price.
| Basis | Price ceiling | Price floor |
|---|---|---|
| What it is | Government-imposed maximum price | Government-imposed minimum price |
| Set relative to p* | Below equilibrium | Above equilibrium |
| Result | Excess demand — a shortage | Excess supply — a surplus |
| Intended to help | Consumers | Producers / workers |
| Typical examples | Wheat, rice, kerosene, sugar; rent control | Agricultural price support; minimum wage |
| Government's follow-up | Rationing through fair price shops | Buying up the surplus |
| Side effects | Queues, black markets | Unsold stocks, storage costs |
A ceiling set above p*, or a floor set below p*, has no effect at all — the market simply settles at p* as usual, because the constraint never bites.
So a price control only does something when it prevents the market from reaching equilibrium — which is precisely why it creates a shortage or a surplus.
Rents in a city are too high for ordinary people, so the government imposes rent control — a ceiling well below the market rent.
(a) Show the immediate effect on the market for rented apartments.
(b) The policy aims to help those seeking apartments. Identify precisely who gains and
who loses.
(c) What happens over a longer period, once landlords can adjust the quantity of
housing offered? Why is the long-run effect worse than the short-run one?
(a) The controlled rent lies below equilibrium, so quantity demanded exceeds quantity supplied: an excess demand for apartments — a housing shortage. More families want flats at the controlled rent than there are flats being offered.
(b) The outcome splits the intended beneficiaries in two.
Gainers: tenants who already have an apartment, or who manage to get one. They pay a genuinely lower rent. This is a real benefit and should not be dismissed.
Losers: the people still looking. Because there are now fewer flats available than before, many who would have found housing at the market rent now find none at all. They are the very people the policy was meant to help.
Also losers: landlords, whose rental income falls.
Since price can no longer ration, allocation falls to queues, waiting lists, personal connections — and, as with any ceiling, black market payments such as unofficial "deposits" demanded on the side.
(c) In the long run the shortage gets substantially worse, because supply is far more elastic over a longer period.
Short run: the stock of buildings is essentially fixed — supply is nearly vertical, so the quantity offered falls only a little.
Long run: landlords can convert flats to other uses, sell them, or simply let them deteriorate, and developers stop building new rental housing because the return no longer justifies it. The supply curve becomes much flatter, so the same controlled rent now produces a much larger shortage.
The general lesson: the harm from a binding price control grows over time, because both sides get more responsive to price the longer they have to adjust. This is the elasticity point from Chapter 2 — demand and supply are more elastic in the long run — applied to policy.
Keep positive and normative separate. Everything above is positive analysis of what rent control does. Whether it is a good policy also depends on how you weigh a gain to sitting tenants against a loss to those shut out — and that is a value judgement.
22. qD = 700 − p, qS = 500 + 3p for
p ≥ 15 (and 0 below). Why is market supply zero below ₹15? Find p* and q*.
23. Same demand curve, but now with free entry and exit and identical firms,
each with qfS = 8 + 3p for p ≥ 20.
(a) What is the significance of p = 20? (b) Find the equilibrium price. (c) Find the
equilibrium quantity and number of firms.
22. Supply is zero below ₹15 because ₹15 is the minimum average variable cost of the firms. Below it, price does not cover even the avoidable costs of production, so every firm shuts down (Chapter 4, Condition 3).
700 − p = 500 + 3p → 200 = 4p → p* = ₹50
q* = 700 − 50 = 650 · check: 500 + 3(50) = 650 ✓
23. (a) ₹20 is the minimum average cost of a firm. Below it a firm cannot cover its full costs and would exit in the long run — so it is the firm's break-even/shut-down point.
(b) With free entry and exit, the equilibrium price is driven to minimum average cost: p₀ = ₹20. Any higher and super-normal profit attracts entry until price falls back; any lower and firms exit until it rises back.
(c) q₀ = 700 − 20 = 680
Each firm: qf = 8 + 3(20) = 68
Number of firms: n₀ = 680 / 68 = 10
qD = 1000 − p and qS = 700 + 2p
(a) Find the equilibrium price and quantity.
(b) An input price rise changes supply to qS = 400 + 2p.
How do p* and q* change? Does this conform to your expectation?
(c) Instead, the government imposes a tax of ₹3 per unit of salt sold. How does
this affect equilibrium?
(a) 1000 − p = 700 + 2p → 300 = 3p → p* = ₹100,
q* = 1000 − 100 = 900 ✓ (check: 700 + 200 = 900)
(b) 1000 − p = 400 + 2p → 600 = 3p → p* = ₹200,
q* = 1000 − 200 = 800
Price rises (100 → 200) and quantity falls (900 → 800). This does conform to expectation: a rise in input prices raises marginal cost, shifting supply leftward, and a leftward supply shift always moves price and quantity in opposite directions.
(c) With a unit tax of ₹3 the seller keeps only (p − 3) per unit, so
supply becomes:
qS = 700 + 2(p − 3) = 694 + 2p
1000 − p = 694 + 2p → 306 = 3p → p* = ₹102,
q* = 1000 − 102 = 898
Who actually bears the ₹3 tax? The buyer's price rose from ₹100 to ₹102, so
buyers bear ₹2. Sellers now receive 102 − 3 = ₹99 instead of ₹100, so
they bear ₹1.
The tax is shared, and the split is decided by the relative slopes of the two curves — not by who legally pays it.
9. How are p* and q* affected when consumers' income (a) increases? (b) decreases?
10. How does an increase in the price of shoes affect the market for socks?
11. How does a change in the price of coffee affect the equilibrium price of tea?
12. How do p* and q* change when the price of an input rises?
13. If the price of a substitute Y of good X increases, what happens in the market for X?
9. For a normal good: (a) income up → demand shifts right → p* and q* both rise. (b) income down → demand shifts left → both fall. (For an inferior good the directions reverse.)
10. Shoes and socks are complements. A rise in the price of shoes reduces the demand for socks → demand for socks shifts left → the price of socks falls and fewer pairs are bought and sold.
11. Tea and coffee are substitutes. If the price of coffee rises, consumers switch to tea → demand for tea shifts right → the equilibrium price of tea rises and the quantity rises. (If coffee's price falls, both fall.)
12. A rise in an input price raises marginal cost → supply shifts left → price rises and quantity falls.
13. Y is a substitute for X, so a rise in the price of Y shifts the demand for X rightward → price of X rises and quantity of X rises.
Notice the pattern across all five. Questions 9, 10, 11 and 13 are demand shifts, so p and q always move together. Question 12 is a supply shift, so they move oppositely. Identify which curve moves and the answer follows immediately.
14 & 21. Compare the effect of a shift in the demand curve when the number of
firms is fixed with the case where entry and exit are permitted.
25. If the market rent for apartments is too high and the government imposes rent
control, what impact will it have on the market for apartments?
14 & 21.
| Demand shifts right | Fixed number of firms | Free entry and exit |
|---|---|---|
| Equilibrium price | Rises | Unchanged (stays at min AC) |
| Equilibrium quantity | Rises a little | Rises a lot |
| Number of firms | Unchanged | Increases |
Why: with a fixed number of firms the upward-sloping market supply curve resists the expansion, so part of the increased demand is absorbed by a higher price and only part by higher quantity.
With free entry, any tendency for price to rise creates super-normal profit, which attracts new firms until price is pushed back to min AC. The long-run market supply is effectively horizontal, so the entire adjustment falls on quantity.
A demand shift has a larger effect on price and smaller effect on quantity when firms are fixed; the reverse when entry and exit are free.
25. Rent control is a price ceiling set below the equilibrium rent. It produces excess demand for apartments — a shortage. Tenants who secure a flat pay less, but many seekers find none. Since price can no longer ration, allocation falls to waiting lists, queues and connections, and a black market in unofficial side payments is likely. Over time landlords withdraw property and new rental construction dries up, so the shortage worsens.
Chapter 1 asked three questions: what to produce, how, and for whom. It could not answer them — it had no machinery. Four chapters later, you have built the machinery and the answers have appeared almost as a by-product.
What to produce? → the equilibrium quantity q*, set where demand meets supply.
How to produce? → the least-cost input combination, from the cost function of
Chapter 3.
For whom? → whoever has the purchasing power — determined by price, and by the
wage rate set in the labour market.
And the single idea underneath all of it: scarcity forces choice, and every choice has an opportunity cost.