Demand and Supply

Demand and Supply is the engine room of Microeconomics. Nearly every question about prices, shortages, taxes, or market behaviour eventually traces back to this one framework: how much do buyers want, how much do sellers offer, and where do the two sides agree to trade? Once you can read a demand-supply diagram fluently, concepts like elasticity, taxation, price controls, and market failure all become easier to reason through.

Learning Objectives

By the end of this section, you will be able to:

• Define demand and supply and state the law of demand and the law of supply
• Distinguish between a "change in quantity demanded/supplied" (movement along a curve) and a "change in demand/supply" (shift of a curve)
• Identify the determinants that shift the demand curve and the supply curve, and predict the direction of each shift
• Calculate and interpret price elasticity of demand and supply, and connect elasticity to total revenue
• Determine market equilibrium price and quantity algebraically and graphically, and analyze what happens when a shock hits either curve
• Apply the demand-supply framework to real markets (agriculture, fuel, technology) to explain price changes and predict outcomes

Quick Answer

Demand is the quantity of a good buyers are willing and able to purchase at each price, and supply is the quantity sellers are willing and able to offer at each price. The law of demand says quantity demanded falls as price rises (all else equal); the law of supply says quantity supplied rises as price rises. Market equilibrium occurs where the demand and supply curves intersect — the price at which quantity demanded exactly equals quantity supplied, leaving no shortage or surplus. Elasticity measures how strongly quantity responds to price changes, which determines who bears a tax, how a firm should price, and how volatile a market's prices will be. Together, these tools explain why prices rise, fall, and settle where they do in real markets.

Topics at a Glance

Topic	What You Will Learn	Key Question
Demand	The law of demand, the demand curve, and its determinants	Why does quantity demanded fall as price rises?
Supply	The law of supply, the supply curve, and its determinants	Why are producers willing to sell more at higher prices?
Elasticity	How responsive quantity is to price and other changes	Who really bears the burden of a tax or price rise?
Equilibrium	Where demand meets supply, and what happens after a shock	What happens to price and quantity when a curve shifts?

Movements vs. Shifts: The Idea Students Mix Up Most

This single distinction accounts for a huge share of exam mistakes, so it deserves its own walkthrough before you dive into the individual topics.

• A movement along the curve happens only when the good's own price changes. Nothing else moves — the whole curve stays put, and you slide up or down it. Example: petrol prices rise from ₹100 to ₹110 per litre, so commuters cut their weekly fuel purchase from 20 litres to 18 litres. This is a fall in quantity demanded, not a fall in demand.
• A shift of the curve happens when something other than the good's own price changes — income, tastes, the price of related goods, expectations, input costs, technology, or the number of buyers/sellers. Example: a fuel-efficient electric scooter enters the market. Even at the same petrol price, people now buy less petrol. The entire demand curve for petrol shifts left.

Worked Numeric Example: Finding Equilibrium

Suppose the demand and supply for a good are given by:

• Demand: Qd = 100 − 2P
• Supply: Qs = 20 + 3P

At equilibrium, Qd = Qs:

100 − 2P = 20 + 3P 80 = 5P P = 16, Q = 68

Check: Qd = 100 − 2(16) = 68 ✓ Qs = 20 + 3(16) = 68 ✓

Now suppose incomes rise and demand shifts to Qd = 130 − 2P (a normal good). Setting the new Qd equal to the unchanged Qs:

130 − 2P = 20 + 3P 110 = 5P P = 22, Q = 86

Both equilibrium price and quantity rise — exactly what you'd expect when demand shifts right along an unchanged upward-sloping supply curve.

Real-World Application: Onion Prices in India

India's onion price spikes are a textbook demand-supply case. A poor monsoon shrinks the onion harvest — supply shifts left (Qs = f(P) falls at every price because there is physically less to sell). With demand for onions largely unchanged (onions are a near-necessity with few substitutes, so demand is inelastic), the leftward supply shift causes a sharp price rise with only a small drop in quantity consumed. This is also why onion prices are so volatile compared to, say, rice: inelastic demand means even a modest supply shock produces a large price swing.

Key Terms

Term	Definition	Related Concept
Demand	Quantity of a good buyers are willing and able to purchase at each price, over a given period	Law of Demand, Demand Curve
Supply	Quantity of a good sellers are willing and able to offer at each price, over a given period	Law of Supply, Supply Curve
Law of Demand	As price rises, quantity demanded falls, ceteris paribus	Demand Curve
Law of Supply	As price rises, quantity supplied rises, ceteris paribus	Supply Curve
Movement along a curve	Change in quantity demanded/supplied caused only by a change in the good's own price	Change in Demand/Supply
Shift of a curve	Change in demand/supply caused by a non-price determinant	Determinants of Demand/Supply
Equilibrium	The price-quantity pair where Qd = Qs; no shortage or surplus	Market Clearing Price
Shortage	Qd > Qs at a given price; typically below equilibrium price	Excess Demand
Surplus	Qs > Qd at a given price; typically above equilibrium price	Excess Supply
Price Elasticity of Demand (PED)	% change in quantity demanded ÷ % change in price	Elastic, Inelastic
Price Elasticity of Supply (PES)	% change in quantity supplied ÷ % change in price	Elastic, Inelastic
Normal Good	A good whose demand rises as income rises	Income Elasticity
Inferior Good	A good whose demand falls as income rises	Income Elasticity

Common Mistakes

Misconception 1: "A change in price causes a shift in the demand curve." Why it's wrong: This confuses cause and effect. A change in the good's own price is what you read along an existing curve, not something that moves the curve itself. Correct understanding: A price change produces a movement along the curve — a change in quantity demanded or supplied. Only non-price factors (income, tastes, related goods' prices, expectations, technology, number of buyers/sellers) shift the curve itself.

Misconception 2: "Demand and supply always shift in opposite directions when 'the market changes.'" Why it's wrong: Students often assume every event affects both curves symmetrically and in opposite directions, but many shocks affect only one side of the market. Correct understanding: A monsoon failure shifts supply left without directly shifting demand. A festival season shifts demand right without directly shifting supply. Always ask separately: "does this change buyers' willingness/ability to buy?" and "does this change sellers' willingness/ability to sell?"

Misconception 3: "High elasticity means a good is unimportant, and low elasticity means it's important." Why it's wrong: Elasticity measures responsiveness to price, not importance or necessity in an absolute sense. Correct understanding: Inelastic demand often reflects necessity (insulin, staple food) or lack of substitutes, not unimportance. Elastic demand often reflects the presence of substitutes (a specific brand of soft drink), not triviality. Elasticity tells you how quantity reacts to price, which is a separate question from how much people value the good.

Comparison and Connections

Concept	Demand-Side Version	Supply-Side Version	Key Distinction
Own-price effect	Movement along demand curve	Movement along supply curve	Only the good's own price is changing
Non-price shift	Shift of demand curve (income, tastes, related goods, expectations, buyers)	Shift of supply curve (input costs, technology, taxes/subsidies, expectations, sellers)	A determinant other than own price is changing
Elasticity	Price Elasticity of Demand (PED)	Price Elasticity of Supply (PES)	Both measure % change in quantity ÷ % change in price, but demand elasticity is (by convention) reported as a positive absolute value despite the negative relationship
Market imbalance	Shortage (Qd > Qs) triggers price rises	Surplus (Qs > Qd) triggers price falls	Both point the market back toward equilibrium
Time horizon effect	Demand tends to become more elastic over time (substitutes found)	Supply tends to become more elastic over time (capacity can adjust)	Short-run and long-run curves look different for both

Practice Questions

Recall

1. State the law of demand and the law of supply in one sentence each. Answer guidance: Law of demand — quantity demanded falls as price rises, ceteris paribus. Law of supply — quantity supplied rises as price rises, ceteris paribus.
2. List four determinants (other than price) that can shift the demand curve. Answer guidance: Any four of — income, tastes/preferences, price of substitutes/complements, expectations of future price, number of buyers, advertising/fashion.

Understanding

3. Explain why a movement along the demand curve is different from a shift of the demand curve. Answer guidance: A movement is caused solely by a change in the good's own price and is read along the existing, unchanged curve. A shift is caused by a change in any other determinant and moves the entire curve to a new position, changing quantity demanded at every price level.
4. Why does a shortage at a given price push the market price upward? Answer guidance: At a price below equilibrium, Qd exceeds Qs. Frustrated buyers bid the price up (or sellers realize they can charge more), which raises Qs (movement along supply) and lowers Qd (movement along demand) until they meet at equilibrium.

Application

5. A severe drought destroys half of a country's wheat crop. Using a demand-supply diagram description, explain what happens to the equilibrium price and quantity of wheat, and to farmers' total revenue if demand for wheat is inelastic. Answer guidance: Supply shifts left (Qs falls at every price). With unchanged demand, equilibrium price rises and equilibrium quantity falls. If demand is inelastic, the percentage price rise exceeds the percentage quantity fall, so farmers' total revenue (P × Q) actually increases despite selling less wheat.
6. Smartphone manufacturing costs fall due to cheaper semiconductor chips. Explain the effect on the equilibrium price and quantity of smartphones. Answer guidance: Lower input costs shift the supply curve right (more supplied at every price). With demand unchanged, equilibrium price falls and equilibrium quantity rises.

Analysis

7. Two goods, A and B, both experience a 10% price rise. Quantity demanded for A falls by 2%; for B it falls by 15%. Compare the elasticity of demand for A and B, and suggest a reason for the difference. Answer guidance: PED(A) = 2/10 = 0.2 (inelastic); PED(B) = 15/10 = 1.5 (elastic). A is likely a necessity with few substitutes (e.g., a staple food or medicine); B likely has many substitutes or is a discretionary purchase (e.g., a specific brand of clothing).
8. A government imposes a price ceiling below the market equilibrium price for cooking gas cylinders. Analyze the likely consequence using the demand-supply framework, and explain why this differs from a price floor set above equilibrium. Answer guidance: A ceiling below equilibrium creates a persistent shortage (Qd > Qs at the capped price) because sellers are unwilling to supply as much at the lower price while buyers want more — this can lead to rationing, black markets, or queues. A price floor above equilibrium instead creates a surplus (Qs > Qd), as seen with agricultural minimum support prices, requiring the government to purchase or store the excess.

FAQ

Q1: What's the real difference between "demand" and "quantity demanded"? "Demand" refers to the entire relationship between price and quantity — the whole curve/schedule. "Quantity demanded" refers to one specific point on that curve at one specific price. When people say "demand increased," they usually mean the whole curve shifted; when they say "quantity demanded increased," they mean price fell and buyers responded by moving along the same curve.

Q2: Why does the demand curve slope downward and the supply curve slope upward? Demand slopes downward mainly because of the substitution effect (buyers switch to cheaper alternatives as price rises) and the income effect (a price rise reduces real purchasing power). Supply slopes upward because higher prices make production more profitable at the margin, encouraging existing sellers to produce more and new sellers to enter.

Q3: Can both demand and supply shift at the same time? Yes, and this is common in real markets. When both shift, the direction of the price change is often predictable, but the direction of the quantity change (or vice versa) can be ambiguous without knowing the relative size of the shifts — this is a classic "indeterminate" case examiners like to test.

Q4: Is elasticity the same at every point on a straight-line demand curve? No. Even on a straight-line (linear) demand curve, PED changes along its length — it is more elastic near the top (high price, low quantity) and more inelastic near the bottom (low price, high quantity), because elasticity depends on percentage changes, not the slope alone.

Q5: Why do economists say "ceteris paribus" so often when discussing demand and supply? Because the law of demand and the law of supply only hold when everything else is held constant. In the real world, many things change simultaneously (income, tastes, weather, technology), so "ceteris paribus" is the assumption that lets us isolate the pure effect of price on quantity — it's a simplifying tool for building the model, not a claim about how the real world always behaves.

Quick Revision

• Demand: quantity buyers want and can afford at each price; law of demand — price up, quantity demanded down.
• Supply: quantity sellers offer at each price; law of supply — price up, quantity supplied up.
• Movement along a curve = only the good's own price changed. Shift of a curve = a non-price determinant changed.
• Demand shifters: income, tastes, prices of substitutes/complements, expectations, number of buyers.
• Supply shifters: input costs, technology, taxes/subsidies, expectations, number of sellers.
• Equilibrium: the price/quantity where Qd = Qs; solve by setting demand equation equal to supply equation.
• Shortage (Qd > Qs) pushes price up; surplus (Qs > Qd) pushes price down — both move the market toward equilibrium.
• PED = %ΔQd / %ΔP; PES = %ΔQs / %ΔP. Inelastic: |E| < 1; elastic: |E| > 1; unit elastic: |E| = 1.
• Inelastic demand + supply shock → large price swing, small quantity change (e.g., onions, wheat during drought).
• A rightward demand shift with unchanged supply raises both equilibrium price and quantity; a leftward supply shift with unchanged demand raises price but lowers quantity.
• Elasticity is not constant along a straight-line demand curve — it varies by point, not just by slope.
• When both curves shift simultaneously, one of price or quantity may be impossible to determine without knowing relative shift sizes.

Related Topics

Prerequisites: Basic Economics concepts — scarcity, opportunity cost, the price mechanism, ceteris paribus

Related Topics within this section: Demand, Supply, Elasticity, Equilibrium

Next Topics after this section: Market Structures (Perfect Competition, Monopoly, Oligopoly), Consumer and Producer Surplus, Taxation and Price Controls, Market Failure and Externalities