Econ 12
Unit 2 · Markets & Consumer Behaviour
BC Economics 12

Unit 2: Markets &
Consumer Behaviour

Why do we buy what we buy — and why do markets work the way they do? Explore utility, demand, supply, market equilibrium, and the psychology behind our choices.

Section 1

What is Utility?

In economics, "utility" means satisfaction. It explains why we buy what we buy.

Key Vocabulary — click any word to see its definition
Utility Utils Total Utility Marginal Utility Rational Consumer Budget Constraint
Utility is the satisfaction or enjoyment a person gets from using a good or service. It is personal — the same product can give different satisfaction to different people. Example: A hot coffee gives high utility to someone who loves coffee, but low utility to someone who does not.
Utils are imaginary units that economists use to measure utility. They are not real — they are just a way to compare satisfaction levels. Example: "The first slice of pizza gave me 80 utils."
Total Utility (TU) is the total satisfaction a person gets from consuming all units of a good. It usually goes up as you consume more — but it rises more slowly each time.
Marginal Utility (MU) is the extra satisfaction from consuming one more unit. MU = change in TU ÷ change in quantity. In most decisions, it is the last unit that matters most.
Rational Consumer — economists assume that people try to get the most satisfaction they can from their money. They compare what they gain (satisfaction) with what they pay (price).
Budget Constraint is the limit on what a consumer can buy. It is set by their income and the prices of goods. You cannot spend more money than you have.

When you choose to buy a coffee instead of a juice, you are making an economic decision. You chose the option that gave you more utility — more satisfaction — for your money. Economics studies this kind of decision carefully.

Why utility matters

Utility helps us answer three questions: Why do people buy certain things? How much are they willing to pay? And when do they stop buying?

If you understand utility, you can explain most consumer behaviour.

The rational consumer

Economists assume that consumers are rational. This means they compare the satisfaction they expect to get (utility) with the price they must pay. When the utility is worth the price, they buy. When it is not, they do not.

How do we measure utility?

Utility is personal and cannot be measured exactly. But economists use imaginary units called utils to compare satisfaction levels. The number itself is not important — what matters is whether utility is going up or down as you consume more.

Real-world example: You are very hungry and you eat your first burger. You feel great — high utility. The second burger is still good. By the third burger, you feel too full. Your utility from burgers peaked at the second one and then fell.
💡 Did You Know?

In the 1700s, a philosopher named Jeremy Bentham believed that happiness could be measured like a number. He wanted to calculate the "total happiness" of a decision. This was an early version of the utility idea in economics.

🔑 Key Idea

Utility is about the last unit. Your decision to buy "one more" depends on how much satisfaction that extra unit gives you — not how much you enjoyed the first one.

📝 Knowledge Check
1. What does "utility" mean in economics?
2. What are "utils"?
3. A "rational consumer" in economics is someone who:
Section 2

Marginal Utility & the Law of Diminishing Marginal Utility

Each extra unit gives you a little less satisfaction than the one before.

Key Vocabulary
Law of DMU Diminishing Disutility MU Formula
Law of Diminishing Marginal Utility — as you consume more units of a good, the extra satisfaction (MU) from each unit decreases. This law applies to almost everything we consume.
Diminishing means getting smaller over time. In this context, your satisfaction from each extra unit becomes smaller and smaller, even though it is still positive.
Disutility — when consuming more of something actually makes you feel worse. Total utility begins to fall. Example: eating a 9th slice of pizza when you are already very full.
MU = ΔTU ÷ ΔQ
Marginal Utility = the change in Total Utility divided by the change in quantity. Since we usually look at one extra unit (ΔQ = 1), MU simply tells us how much TU changes after one more unit.

The Law of Diminishing Marginal Utility is one of the most important ideas in this unit. It says that as you consume more of any good, each extra unit gives you less satisfaction than the one before.

MU = ΔTU ÷ ΔQ
Marginal Utility = the change in Total Utility when one more unit is consumed.
Because ΔQ = 1 in most examples: MU = TU after − TU before

Why does this happen?

Your most urgent need is satisfied first. Think about a very hot day. The first glass of cold water is incredibly satisfying. The second is good. By the fifth glass, you probably do not want any more. Your need for water was satisfied after the first few glasses.

When does a consumer stop buying?

A rational consumer keeps buying a good as long as the extra satisfaction (MU) is worth the price. When the MU falls below the price, they stop buying. This is why people do not buy an unlimited amount of things they enjoy — at some point, one more unit is just not worth the money.

Example: You pay for a monthly movie streaming service. The first film you watch feels very worthwhile. By your 15th film of the month, you are only half-watching it. The satisfaction from "one more film" has become very low, even though the price (your monthly subscription) has not changed.
🌏 Real World

The Law of DMU explains why all-you-can-eat buffets make money. Restaurants know that after the first few plates, customers' satisfaction from extra food drops so much that they do not eat very much more than a normal meal.

🔑 Two Curves

Total Utility goes up, but more and more slowly. It may eventually fall.

Marginal Utility always falls — it starts high and gets lower with each unit.

📝 Knowledge Check
1. The Law of Diminishing Marginal Utility states that as you consume more of a good:
2. Using the formula MU = ΔTU ÷ ΔQ: if Total Utility rises from 100 to 120 after one extra unit, what is the Marginal Utility?
3. A consumer experiences "disutility" when:
Interactive Activity

🍕 The Pizza Lab

Move the slider to "eat" slices of pizza. Watch Total Utility and Marginal Utility change in real time.

Marginal Utility of Pizza Slices
Move the slider left and right to choose how many slices you eat. The table and charts update automatically.
Slices eaten: 0 slices
Total Utility 0 utils
Marginal Utility — utils / slice
Status —
Total Utility — rises, then falls
Marginal Utility — always falling
Slices (Q) Total Utility (TU) Marginal Utility (MU) Assessment
📝 Knowledge Check
1. Looking at the pizza data, at which slice does Total Utility reach its highest point?
2. When Marginal Utility becomes negative (e.g., at slice 7), what happens to Total Utility?
3. The pizza data shows the Law of Diminishing Marginal Utility because:
Section 4

Consumer Choice

How do consumers decide where to spend their money?

Key Vocabulary
Consumer Choice Value for Money Consumer Surplus
Consumer Choice — the decision a consumer makes about which goods to buy, how many to buy, and in what combination. Rational consumers try to get the most satisfaction from their available money.
Value for Money — in economics, this means getting the most utility (satisfaction) per dollar spent. Consumers naturally prefer options that give them more satisfaction for the same price.
Consumer Surplus — the difference between what a consumer was willing to pay and what they actually paid. It represents the extra benefit the consumer received. Example: You were willing to pay $20 for a book. It was on sale for $12. Your consumer surplus = $8.

When you have a limited amount of money, you must choose how to spend it. A rational consumer tries to get the most satisfaction (utility) possible from their budget.

The basic idea

Consumers compare the satisfaction they get from different goods. If one item gives more satisfaction per dollar, they will prefer to buy that item first. As they buy more of it, the marginal utility falls (remember the Law of DMU). Eventually, the satisfaction per dollar from different goods becomes similar, and the consumer feels happy with their choices.

Example: You have $5. Pizza slices cost $2 and drinks cost $1. You are very thirsty and a little hungry. Your first drink gives you much more satisfaction per dollar than a pizza slice. So you buy a drink first. After the drink, the next best choice might be a pizza slice. You keep buying in the order that gives you the most satisfaction per dollar.

Consumer Surplus

Sometimes, the price you pay for something is lower than the maximum amount you would have been willing to pay. This gap is called consumer surplus. It is a real benefit that you receive as a buyer.

Example: You would pay up to $50 for a concert ticket. You buy one for $35. Your consumer surplus is $15. You feel like you got a good deal.
🛒 Shopping Behaviour

When you choose a larger pack of something because it costs less per unit, you are already thinking about value for money — which is exactly how economists model consumer decision-making.

🔑 Key Idea

A rational consumer always asks: "Which option gives me the most satisfaction for my money right now?" As they buy more, diminishing MU changes the answer.

🚀 Going Further The Formal Rule: Consumer Equilibrium Not required for the unit test ▶

Economists have a precise rule for when a consumer has made the best possible choices with their budget. It is called consumer equilibrium. At this point, the consumer cannot improve their total satisfaction by shifting money from one good to another.

The rule uses the idea of MU per dollar — how much satisfaction you get for each dollar you spend on a good. You calculate it by dividing the MU of a good by its price.

MU_A / P_A = MU_B / P_B
When the MU per dollar is equal for all goods you buy, you have maximised your total utility. If one ratio is higher than another, you should buy more of that good and less of the other.

This is called the Equimarginal Principle. It is a central idea in university-level microeconomics. Understanding the basic concept — that consumers compare satisfaction per dollar — is what matters at this stage.

📝 Knowledge Check
1. A consumer has $10. Snacks give MU per dollar = 8; drinks give MU per dollar = 4. To maximise utility, where should they spend their next dollar?
2. You were willing to pay $30 for a book but you find it on sale for $20. What is your consumer surplus?
3. Why do rational consumers typically buy a mix of goods rather than spending all their money on just one item?
Section 5

Demand & the Law of Demand

Why do buyers buy more when prices fall?

Demand Law of Demand Quantity Demanded Demand Curve Demand Schedule Non-Price Determinants
Demand — the entire relationship between price and the quantities buyers are willing and able to purchase at each price, all else equal.
Law of Demand — as price rises, quantity demanded falls; as price falls, quantity demanded rises. Price and quantity demanded move in opposite directions.
Quantity Demanded — the specific amount buyers will purchase at one particular price. A change in price moves you along the curve; it does NOT shift the curve.
Demand Curve — a downward-sloping line (or curve) on a price–quantity graph showing how quantity demanded changes at every price.
Demand Schedule — a table listing the quantities buyers would purchase at a series of different prices.
Non-Price Determinants — factors other than price that shift the entire demand curve: income, tastes, prices of related goods, number of buyers, and expectations.

What Is Demand?

Demand is not just "wanting" something — it means being willing and able to pay for it. A student might want the newest iPhone, but if they cannot afford it, that is not demand in the economic sense.

The Law of Demand

The law of demand states that when price rises, quantity demanded falls; when price falls, quantity demanded rises — all else being equal. This happens for two reasons:

  • Substitution effect — when bubble tea gets more expensive, students switch to cheaper drinks like water or coffee.
  • Income effect — a higher price means your yuan buys less, so you cut back on purchases.

Movement vs. Shift

There are two very different things that can happen to demand — and students often confuse them:

  • Movement along the curve — only a price change causes this. If HEYTEA raises its price from ¥28 to ¥35, you move up the same demand curve and buy fewer cups. The curve itself does not move.
  • Shift of the curve — a non-price factor changes, and the whole curve moves left or right. If student incomes rise, they buy more at every price — the whole demand curve shifts right.

HEYTEA Demand Schedule

HEYTEA (喜茶) is one of China's most popular premium tea brands. The table below shows how many cups per day a typical HEYTEA location might sell in Guangzhou at different prices.

Price per CupCups Sold per DayWhat Happens
¥12520Very affordable — high demand
¥20380Mainstream price — steady demand
¥28260Current typical price — moderate demand
¥35160Expensive — many switch to substitutes
¥4580Premium price — only loyal fans remain
🧋 Real World: China

China's bubble tea market is worth over ¥300 billion and has more than 500,000 shops. Brands like HEYTEA, Nayuki, and Mixue compete fiercely on price — a perfect example of demand in action.

💡 Key Idea

The demand curve slopes downward because of two forces working together:

  • Substitution effect — higher price → switch to alternatives
  • Income effect — higher price → feel poorer → buy less
📋 5 Shifters of Demand
  1. Income — more income = more demand (for normal goods)
  2. Tastes & preferences — trending = higher demand
  3. Prices of related goods — cheaper substitute = lower demand
  4. Number of buyers — more buyers = higher demand
  5. Expectations — expect price rise = buy now = higher demand today
📊
Interactive: Demand Curve Shifts
Click any scenario to see a fresh demand curve — click again to close.
⬆ Demand Increases (Shifts Right)
⬇ Demand Decreases (Shifts Left)
✏️
Knowledge Check — Demand
1. The price of HEYTEA bubble tea rises from ¥28 to ¥38. According to the law of demand, what happens?
Quantity demanded increases and the demand curve shifts right
Quantity demanded decreases — we move up along the same demand curve
Quantity demanded stays the same because people love HEYTEA
The demand curve shifts left because the price rose
2. A Guangzhou-based influencer goes viral promoting a new fruit tea brand. What happens to the demand for that tea?
The price falls, causing a movement down the demand curve
Tastes improve, shifting the demand curve to the right
Supply increases, shifting the demand curve to the right
Nothing changes — social media does not affect economics
3. Which of the following would shift the demand curve for HEYTEA to the LEFT (decrease demand)?
The price of HEYTEA increases from ¥28 to ¥38
Student incomes rise after part-time jobs pay more
A health study finds excessive sugar is harmful and people change their tastes
HEYTEA opens 50 new shops in Guangzhou, increasing the number of buyers
Section 6

Supply & the Law of Supply

Why do sellers produce more when prices rise?

Supply Law of Supply Quantity Supplied Supply Curve Non-Price Determinants (Supply) Input Costs
Supply — the entire relationship between price and the quantities producers are willing and able to sell at each price, all else equal.
Law of Supply — as price rises, quantity supplied rises; as price falls, quantity supplied falls. Price and quantity supplied move in the same direction.
Quantity Supplied — the specific amount producers will sell at one particular price. A price change moves you along the supply curve; it does NOT shift it.
Supply Curve — an upward-sloping line on a price–quantity graph showing how quantity supplied rises as price rises.
Non-Price Determinants (Supply) — factors other than price that shift the entire supply curve: input costs, technology, number of sellers, government policies, and expectations.
Input Costs — the cost of resources used in production (labour, raw materials, energy). When input costs rise, supply decreases; when they fall, supply increases.

What Is Supply?

Supply is the producer's side of the market. It describes how much of a good or service sellers are willing and able to offer at various prices. Just like demand, supply is a relationship — not a single number.

The Law of Supply

The law of supply states that when price rises, quantity supplied rises; when price falls, quantity supplied falls. This happens because:

  • Higher profit motive — a higher selling price makes it more profitable to produce more units.
  • New firms enter — high prices attract new competitors who add to total market supply.
  • Opportunity cost — producers shift resources toward the good that pays more.

Movement vs. Shift

Just like demand, there are two things that can happen to supply:

  • Movement along the curve — only a price change causes this. If EV prices rise, BYD produces more — you move up the existing supply curve.
  • Shift of the curve — a non-price factor changes. If battery costs fall due to new technology, BYD can produce more at every price — the whole supply curve shifts right.

BYD Electric Vehicle Supply Schedule

BYD (比亚迪) is China's largest EV manufacturer. The table below shows how many EVs per month BYD might supply at different price levels.

Price per EVUnits Supplied / MonthWhat Happens
¥80,0008,000Low price — barely covers costs, low supply
¥110,00018,000Moderate price — reasonable profit margin
¥140,00032,000Good price — full production capacity used
¥170,00046,000High price — overtime shifts, new factories
¥200,00055,000Premium price — maximum production effort
🚗 Real World: China EVs

China produces over 60% of the world's electric vehicles. BYD alone sold more than 3 million EVs in 2023 — more than any other manufacturer in the world. Government subsidies and battery technology improvements have massively shifted supply rightward.

💡 Key Idea

The supply curve slopes upward because higher prices make production more profitable — so sellers supply more.

Remember: supply and demand curve in opposite directions. Demand slopes down ↘; Supply slopes up ↗

📋 5 Shifters of Supply
  1. Input costs — cheaper inputs = more supply
  2. Technology — better tech = more efficient = more supply
  3. Number of sellers — more firms = more supply
  4. Government policy — subsidies = more supply; taxes = less supply
  5. Expectations — expect price drop = sell now = more supply today
📊
Interactive: Supply Curve Shifts
Click any scenario to see a fresh supply curve — click again to close.
⬆ Supply Increases (Shifts Right)
⬇ Supply Decreases (Shifts Left)
✏️
Knowledge Check — Supply
1. The price of BYD electric vehicles rises from ¥140,000 to ¥170,000. According to the law of supply, what happens?
Quantity supplied decreases and the supply curve shifts left
Quantity supplied increases — we move up along the same supply curve
Quantity supplied stays the same regardless of price
The supply curve shifts right because price rose
2. A breakthrough in battery technology cuts BYD's production costs by 30%. What happens to the supply of BYD EVs?
The price of EVs falls, causing movement down the supply curve
Lower input costs shift the supply curve to the right — more EVs supplied at every price
Lower costs reduce supply because profits fall
Technology does not affect supply — only price does
3. The Chinese government imposes a new ¥5,000 tax on every EV manufactured. What happens to the supply curve for EVs?
Supply curve shifts right — higher tax motivates more production
We move down along the existing supply curve because buyers buy fewer EVs
Supply curve shifts left — higher input costs make production less profitable at every price
The demand curve shifts left because the tax makes EVs more expensive
Section 7

Market Equilibrium

Where demand and supply meet to set price and quantity

Equilibrium Equilibrium Price Equilibrium Quantity Shortage Surplus Market Clearing
Equilibrium — the point where the quantity demanded equals the quantity supplied. There is no pressure for the price to change.
Equilibrium Price — the price at which Qd = Qs. Also called the market-clearing price. Every unit offered for sale finds a buyer.
Equilibrium Quantity — the quantity bought and sold at the equilibrium price.
Shortage — when quantity demanded exceeds quantity supplied (Qd > Qs). Occurs when price is below equilibrium. Competition among buyers pushes price up.
Surplus — when quantity supplied exceeds quantity demanded (Qs > Qd). Occurs when price is above equilibrium. Sellers cut prices to clear unsold inventory.
Market Clearing — when the market reaches equilibrium and all goods offered are sold. No shortage or surplus remains.

What Is Market Equilibrium?

Now that you understand both demand and supply separately, we can put them together. Equilibrium is the point where buyers and sellers agree — where the quantity buyers want to purchase exactly equals the quantity sellers want to sell.

At equilibrium, there is no shortage (everyone who wants the good at this price can get it) and no surplus (sellers can sell everything they produce). The market "clears."

How the Market Finds Equilibrium

Markets are self-correcting. They automatically adjust toward equilibrium:

  • If price is too low → shortage → buyers compete → price is bid up → we move toward equilibrium
  • If price is too high → surplus → sellers compete → price is cut → we move toward equilibrium

Changes in Equilibrium

When demand or supply shifts, equilibrium changes. The interactive diagram below lets you explore all four cases:

  • Demand increases (D shifts right) → equilibrium price rises, equilibrium quantity rises
  • Demand decreases (D shifts left) → equilibrium price falls, equilibrium quantity falls
  • Supply increases (S shifts right) → equilibrium price falls, equilibrium quantity rises
  • Supply decreases (S shifts left) → equilibrium price rises, equilibrium quantity falls

HEYTEA Market Example

PriceQd (buyers)Qs (sellers)Result
¥1252080Shortage of 440 cups
¥20380160Shortage of 220 cups
¥28260260✅ Equilibrium!
¥35160360Surplus of 200 cups
¥4580480Surplus of 400 cups
🏪 Real World: Meituan

Meituan (美团), China's largest food delivery platform, processes data from millions of orders daily. When a restaurant's price is too high (surplus of unsold food), the app automatically suggests discounts — a real-world market seeking equilibrium.

💡 Key Idea

At equilibrium: Qd = Qs

  • Price below equilibrium → shortage → price rises
  • Price above equilibrium → surplus → price falls
  • Markets automatically self-correct toward P*
📋 Equilibrium Rules
  1. D↑ → P* rises, Q* rises
  2. D↓ → P* falls, Q* falls
  3. S↑ → P* falls, Q* rises
  4. S↓ → P* rises, Q* falls
📊
Interactive: Market Equilibrium Shifts
Watch how demand and supply shifts change the equilibrium price and quantity.
Demand Shifts
✏️
Knowledge Check — Equilibrium
1. At a HEYTEA price of ¥12, quantity demanded is 520 cups but quantity supplied is only 80 cups. What is happening in this market?
There is a surplus of 440 cups — sellers have too much unsold inventory
The market is at equilibrium because everyone is happy
There is a shortage of 440 cups — buyers want more than sellers supply at this price
Demand is less than supply, so price will fall further
2. A health study causes a large fall in demand for bubble tea. Supply stays the same. What happens to equilibrium price and quantity?
Equilibrium price rises; equilibrium quantity rises
Equilibrium price falls; equilibrium quantity falls
Equilibrium price rises; equilibrium quantity falls
Neither price nor quantity changes because supply did not change
3. New battery technology dramatically lowers BYD's production costs. Demand for EVs stays the same. What is the new equilibrium?
Lower cost shifts supply right → equilibrium price falls, equilibrium quantity rises
Lower cost shifts supply left → equilibrium price rises, equilibrium quantity falls
Lower cost shifts demand right → equilibrium price rises, equilibrium quantity rises
Nothing changes at equilibrium — only price changes affect equilibrium
Section 8

Price Controls

What happens when governments set prices above or below equilibrium?

Price Control Price Ceiling Price Floor Binding vs. Non-Binding Deadweight Loss Black Market
Price Control — a government-imposed limit on the price of a good. Can be a maximum (ceiling) or a minimum (floor).
Price Ceiling — a maximum price set by law. If set below equilibrium, it creates a shortage. Example: rent control, maximum food prices.
Price Floor — a minimum price set by law. If set above equilibrium, it creates a surplus. Example: minimum wage, agricultural price supports.
Binding vs. Non-Binding — a price control is binding if it changes market outcomes. A ceiling below P* is binding (creates shortage). A floor above P* is binding (creates surplus).
Deadweight Loss — the lost value of trades that would have occurred at equilibrium but do not happen because of the price control. A measure of market inefficiency.
Black Market — illegal trade that develops when binding price controls prevent the market from clearing. Prices on black markets typically exceed the ceiling.

Why Do Governments Impose Price Controls?

Governments sometimes intervene in markets for social reasons. They may want to keep essential goods affordable (using price ceilings) or protect producers' incomes (using price floors). While well-intentioned, these policies almost always create unintended consequences.

Price Ceiling — Maximum Price

A price ceiling sets a maximum price below equilibrium. The goal is to make goods affordable. The result?

  • Quantity demanded > quantity supplied → shortage
  • Long queues, rationing, reduced quality, or black markets may emerge
  • Example: If the government caps HEYTEA at ¥15 when equilibrium is ¥28, far more people want tea than HEYTEA can supply

Price Floor — Minimum Price

A price floor sets a minimum price above equilibrium. The goal is to protect producers or workers. The result?

  • Quantity supplied > quantity demanded → surplus
  • Unsold goods pile up; government may have to buy the surplus
  • Example: A minimum wage above the equilibrium wage → surplus of labour = unemployment

China Case Study: Rice Price Floors

The Chinese government maintains price floors for rice and wheat to protect farmers. When market prices fall, the government buys grain at the floor price, maintaining farmer incomes — but also building large government grain stockpiles (surplus). This is a deliberate trade-off to ensure rural incomes and food security.

🏠 Real World: Rent Control

Many Chinese cities have discussed rent controls to make housing affordable. Economic research consistently shows rent control creates housing shortages and reduces housing quality over time — landlords have less incentive to maintain properties when rents are capped.

💡 Key Idea

Price controls only have effects if they are binding:

  • Ceiling below P* → binding → shortage
  • Ceiling above P* → non-binding → no effect
  • Floor above P* → binding → surplus
  • Floor below P* → non-binding → no effect
⚠️ Unintended Consequences
  • Black markets develop
  • Quality declines
  • Shortages or surpluses persist
  • Resources misallocated
  • Deadweight loss created
📊
Interactive: Price Ceiling vs. Price Floor
See how government price controls create shortages or surpluses.
Price Ceiling (Maximum)
Price Floor (Minimum)
← Select a price control
✏️
Knowledge Check — Price Controls
1. The equilibrium price for bubble tea in Guangzhou is ¥28. The government sets a price ceiling at ¥18. What is the result?
A surplus — at ¥18 sellers produce more than buyers want
No effect — the ceiling is above the equilibrium price
A shortage — at ¥18 buyers want more than sellers are willing to supply
The equilibrium adjusts automatically so there is no shortage
2. A minimum wage is set at ¥25 per hour. If the equilibrium wage in the market is ¥20, what is the economic effect?
The minimum wage is non-binding — it has no effect since it is above equilibrium
The minimum wage is binding — it creates a surplus of labour (more workers seek jobs than employers hire)
The minimum wage creates a shortage of workers because employers cannot find enough labour
The minimum wage shifts the demand for labour to the right
3. The government sets a price floor at ¥20 for a good whose equilibrium price is ¥28. What happens?
A binding price floor — creates a large surplus of unsold goods
A non-binding price floor — the market continues at equilibrium with no effect
A binding price floor — creates a shortage as buyers want more at this low price
A price ceiling — since ¥20 is below the equilibrium price
Section 9

Consumer & Producer Surplus

Measuring who gains from market exchange

Willingness to Pay Consumer Surplus Producer Surplus Total Surplus Market Efficiency Deadweight Loss
Willingness to Pay — the maximum price a buyer is willing to pay for a unit. Represented by the height of the demand curve at that quantity.
Consumer Surplus (CS) — the benefit buyers receive beyond what they actually pay. CS = Willingness to Pay − Actual Price. Shown as the area below the demand curve and above the price line.
Producer Surplus (PS) — the benefit sellers receive above their minimum acceptable price (cost). PS = Actual Price − Minimum Acceptable Price. Shown as the area above the supply curve and below the price line.
Total Surplus — CS + PS. The total gain from all trades in the market. Maximised at equilibrium.
Market Efficiency — a market is efficient when total surplus is maximised — all mutually beneficial trades occur and no deadweight loss exists. Equilibrium achieves this.
Deadweight Loss — the reduction in total surplus from price controls, taxes, or other market distortions that prevent some mutually beneficial trades from occurring.

What Is Consumer Surplus?

Imagine you would pay up to ¥50 for a cup of HEYTEA, but the market price is only ¥28. You pay ¥28 and receive ¥22 worth of extra benefit. That ¥22 is your consumer surplus — the difference between what you were willing to pay and what you actually paid.

On a graph, consumer surplus is the triangular area below the demand curve and above the market price. The demand curve represents willingness to pay — the further above the price, the more surplus that buyer gets.

What Is Producer Surplus?

A HEYTEA seller can profitably make a cup for ¥18, but sells it for ¥28. The extra ¥10 is producer surplus — the revenue received above the minimum they need to supply.

On a graph, producer surplus is the triangular area above the supply curve and below the market price. The supply curve represents the minimum price sellers need — the further below the market price, the more surplus that seller gets.

Total Surplus and Efficiency

Total surplus = Consumer Surplus + Producer Surplus. At equilibrium, total surplus is maximised — every buyer who values the good more than the cost gets to buy it, and every seller who can produce it for less than the value gets to sell it.

Price controls reduce total surplus by preventing some of these mutually beneficial trades — creating deadweight loss.

Surplus Summary

ConceptDefinitionGraph Location
Consumer SurplusWTP − Price PaidBelow D, above P*
Producer SurplusPrice Received − CostAbove S, below P*
Total SurplusCS + PSEntire shaded area at E*
Deadweight LossLost trades from distortionTriangle away from E*
📱 Real World: Pinduoduo

Pinduoduo's group-buying model creates huge consumer surplus — buyers pay well below what they'd be willing to pay for goods. In 2023, Pinduoduo reported that Chinese consumers saved over ¥600 billion compared to standard retail prices. That's consumer surplus at scale.

💡 Key Idea

At equilibrium, total surplus is maximised. Any policy that moves price away from P* (a ceiling, floor, or tax) shrinks the surplus pie and creates deadweight loss.

📐 On the Graph
  • CS = triangle above P*, below D curve
  • PS = triangle below P*, above S curve
  • Total surplus = CS + PS = both triangles
  • DWL = triangle of lost trades
📊
Interactive: Visualise Surplus
See consumer surplus, producer surplus, and how price controls create deadweight loss.
Show Surplus Areas
Show Deadweight Loss
← Select a view
✏️
Knowledge Check — Surplus
1. You are willing to pay ¥50 for a HEYTEA drink but the market price is ¥28. What is your consumer surplus?
¥50 — your full willingness to pay
¥28 — the price you paid
¥22 — the difference between what you were willing to pay and what you actually paid
¥0 — consumers never get a surplus because they pay the market price
2. On a supply-and-demand graph, where is producer surplus shown?
The area below the demand curve and above the market price
The area above the supply curve and below the market price
The area above the demand curve and below the supply curve
The area to the right of the equilibrium quantity
3. A binding price ceiling is set below equilibrium. What happens to total surplus (CS + PS) compared to the free-market equilibrium?
Total surplus increases because consumers benefit from lower prices
Total surplus stays the same — it just transfers from producers to consumers
Total surplus decreases — some trades no longer occur, creating deadweight loss
Producer surplus increases because lower prices attract more buyers
Section 10

Price Elasticity of Demand

How responsive are buyers to price changes?

Elasticity Price Elasticity of Demand Elastic Demand Inelastic Demand Unit Elastic Perfectly Elastic Perfectly Inelastic
Elasticity — a measure of how much one variable responds to a change in another. In economics, it usually refers to how sensitive quantity demanded or supplied is to a change in price.
Price Elasticity of Demand (PED) — measures the percentage change in quantity demanded caused by a 1% change in price. Formula: PED = (% ΔQd) ÷ (% ΔP). Always negative (law of demand), but we use the absolute value |PED|.
Elastic Demand — |PED| > 1. Buyers respond strongly to price changes. A 1% price rise causes more than 1% fall in quantity demanded. Demand curve is relatively flat.
Inelastic Demand — |PED| < 1. Buyers respond weakly to price changes. A 1% price rise causes less than 1% fall in quantity demanded. Demand curve is relatively steep.
Unit Elastic — |PED| = 1. The percentage change in quantity demanded exactly equals the percentage change in price. Demand curve is at a specific intermediate slope.
Perfectly Elastic — |PED| = ∞. The demand curve is horizontal. Consumers buy any quantity at exactly one price; any price increase drives quantity to zero. Example: a single wheat farmer selling into a world market.
Perfectly Inelastic — |PED| = 0. The demand curve is vertical. Quantity demanded does not change at all when price changes. Example (approximate): life-saving medication with no substitute.

What Is Price Elasticity of Demand?

Not all markets respond to price changes in the same way. When HEYTEA raises its bubble tea price by ¥5, some customers switch to cheaper alternatives — quantity demanded falls noticeably. But when the city water authority raises rates, most households pay up and use nearly the same amount. Economists measure this responsiveness using Price Elasticity of Demand (PED).

The Formula

PED = % change in Quantity Demanded ÷ % change in Price
Always use absolute value |PED| for interpretation

Example: The price of HEYTEA rises 20% (from ¥25 to ¥30). Quantity demanded falls 30% (from 1,000 to 700 cups per day).

PED = 30% ÷ 20% = 1.5 → |PED| > 1 → Elastic demand

Interpreting |PED|

  • |PED| = 0 → Perfectly inelastic (vertical curve, no response)
  • |PED| < 1 → Inelastic (steep curve, weak response)
  • |PED| = 1 → Unit elastic (proportional response)
  • |PED| > 1 → Elastic (flat curve, strong response)
  • |PED| = ∞ → Perfectly elastic (horizontal curve)
🧋 China Example: Bubble Tea vs. Rice

Bubble tea (HEYTEA, Nayuki) has elastic demand in Guangzhou — it's a luxury, has many substitutes, and takes a notable share of a student's spending. Rice, on the other hand, is a staple necessity with almost no substitutes — its demand is highly inelastic. That's why China's rice price controls (price floors) work: farmers know demand won't fall much if rice prices shift slightly.

💡 Reading the Graph

The slope of a demand curve shows elasticity visually:

  • Steep curve → inelastic (small ΔQ for given ΔP)
  • Flat curve → elastic (large ΔQ for given ΔP)
  • Vertical → perfectly inelastic
  • Horizontal → perfectly elastic

⚠️ Slope and elasticity are related but NOT the same — elasticity also depends on where on the curve you are.

📊
Interactive: Elasticity & the Demand Curve
See how the same price increase causes different quantity responses depending on elasticity. All curves start at the same point.
Choose elasticity type:
← Select an elasticity type
✏️
Knowledge Check — Price Elasticity
1. The price of milk rises from ¥8 to ¥10 per litre. Quantity demanded falls from 100 to 95 litres per week. What does this suggest about demand for milk?
Elastic — buyers responded strongly to the price change
Inelastic — buyers responded very little to the price change
Unit elastic — the percentage changes in price and quantity were equal
Perfectly elastic — buyers stopped buying milk entirely
2. Which formula correctly calculates the Price Elasticity of Demand?
PED = % change in Price ÷ % change in Quantity Demanded
PED = change in Quantity ÷ change in Price (using raw numbers)
PED = % change in Quantity Demanded ÷ % change in Price
PED = Total Revenue ÷ Quantity
3. A perfectly elastic demand curve looks like which of the following?
A vertical straight line — consumers buy the same amount regardless of price
A downward-sloping diagonal line at exactly 45°
A horizontal straight line — consumers buy any quantity at one price, nothing above it
A curved line bowing inward toward the origin
Section 11

Determinants of PED & Total Revenue Test

What makes demand elastic or inelastic — and what does that mean for revenue?

Normal Good Inferior Good Determinants of PED Substitutes Necessity vs Luxury Proportion of Income Time Horizon Total Revenue Total Revenue Test
Normal Good — a good for which demand increases when consumer income rises, and falls when income falls. Most goods are normal goods: as people earn more, they buy more restaurant meals, branded clothing, and electronics.
Inferior Good — a good for which demand decreases when consumer income rises. When people get richer, they switch away from inferior goods to higher-quality alternatives. Examples: instant noodles, bus rides, low-cost cooking oil. As incomes rise in China, demand for instant noodles falls as consumers switch to restaurant meals.
Determinants of PED — the factors that determine whether demand for a good is elastic or inelastic. The main determinants are: availability of substitutes, necessity vs. luxury status, proportion of income spent, and time horizon.
Substitutes — goods that can replace each other. The more (and better) substitutes a good has, the more elastic its demand — if its price rises, consumers easily switch. HEYTEA has many substitute drinks; tap water has none.
Necessity vs Luxury — necessities (rice, water, medication) tend to have inelastic demand because consumers cannot easily give them up. Luxuries (designer clothing, entertainment) tend to be elastic because consumers can cut them without major harm.
Proportion of Income — goods that take a large share of income tend to be more elastic (buyers are sensitive to price because the impact is large). Goods that take a tiny share (e.g., salt) tend to be inelastic.
Time Horizon — over time, demand becomes more elastic. Consumers find more substitutes, change habits, or invest in alternatives. Short-run: gasoline demand is inelastic. Long-run: people buy EVs and demand becomes more elastic.
Total Revenue (TR) — the total income a seller receives: TR = Price × Quantity. Total revenue is what businesses care about — a price change affects TR depending on whether demand is elastic or inelastic.
Total Revenue Test — a method using TR to determine demand elasticity. If price rises and TR falls → elastic demand. If price rises and TR rises → inelastic demand. If price rises and TR is unchanged → unit elastic demand.

What Makes Demand Elastic or Inelastic?

Four key factors determine the price elasticity of demand for any good. Use the mnemonic SNIP:

  • S — Substitutes: More and better substitutes → more elastic. HEYTEA competes with Nayuki, Coco, homemade tea. Rice has almost no substitute.
  • N — Necessity vs. Luxury: Necessities are inelastic; luxuries are elastic. Insulin is a necessity; a cinema ticket is a luxury.
  • I — Income proportion: A larger share of income → more elastic. Designer sneakers (¥2,000+) are elastic; chewing gum is inelastic.
  • P — Period of time: Longer time → more elastic. In the short run, gasoline is inelastic. Over 5 years, people switch to EVs.

Normal Goods vs. Inferior Goods

Income affects demand differently depending on the type of good. This is related to the I (Income proportion) factor in SNIP — but it goes further:

✅ NORMAL GOODS

Income ↑ → Demand ↑
Income ↓ → Demand ↓
Examples: restaurant meals, branded clothing, smartphones, cars

📉 INFERIOR GOODS

Income ↑ → Demand ↓
Income ↓ → Demand ↑
Examples: instant noodles, bus rides, low-cost cooking oil, generic brands

China example: As incomes rose in Chinese cities, demand for instant noodles (Nissin, Master Kong) fell — consumers switched to restaurant meals and food delivery apps. Instant noodles are an inferior good.

The Total Revenue Test

Total Revenue = Price × Quantity. When price changes, what happens to TR depends entirely on elasticity:

Demand Type Price rises → Price falls →
Elastic (|PED|>1) TR falls ▼ TR rises ▲
Inelastic (|PED|<1) TR rises ▲ TR falls ▼
Unit Elastic (|PED|=1) TR unchanged → TR unchanged →

Why? Elastic demand: a price increase drives away so many buyers that the quantity drop outweighs the higher price. Inelastic demand: the price increase more than compensates for the small quantity drop.

🏭 Real World: China Tobacco

The Chinese government taxes cigarettes heavily. Tobacco demand is highly inelastic (few substitutes, habit-forming, small income proportion for heavy smokers). This means a price increase raises total revenue for the government — the quantity fall is small relative to the price rise. This is the economic logic behind sin taxes.

💡 TR Test in Practice

A business can use the TR test to make pricing decisions:

  • Elastic demand → lower prices to raise TR
  • Inelastic demand → raise prices to raise TR
  • Unit elastic → price change has no effect on TR

This is why airlines charge different prices to different travellers — business travellers (inelastic) pay far more than leisure travellers (elastic).

📊
Interactive: Total Revenue Test
Price increases from P₁=¥10 to P₂=¥20. See how total revenue (P×Q) changes with elasticity. ■ Green = TR₁ before (P₁=¥10 × Q₁); ■ Red = TR₂ after (P₂=¥20 × Q₂). The demand curve colour shows the elasticity type.
Choose demand type:
← Select demand type to see TR change
✏️
Knowledge Check — Determinants & Total Revenue
1. HEYTEA raises its bubble tea price from ¥25 to ¥30. Total revenue falls from ¥50,000 to ¥42,000 per week. What does this tell you about demand for HEYTEA?
Demand is inelastic — the price rise caused only a small fall in quantity
Demand is elastic — the fall in quantity more than offset the higher price, so TR fell
Demand is unit elastic because total revenue changed by less than 10%
The law of demand does not apply to premium teas
2. Which of the following goods most likely has INELASTIC demand, and why?
Designer handbags — expensive luxury with many substitutes
A specific brand of instant noodles — many competing brands at the same price
Insulin for diabetics — a medical necessity with no close substitute
A cinema ticket — optional entertainment with many alternatives
3. When demand is unit elastic and price rises by 10%, what happens to total revenue?
Total revenue rises by 10%
Total revenue falls by 10%
Total revenue is unchanged — the price rise is exactly offset by the quantity fall
Total revenue doubles
📚
Sources & Further Reading
References for Unit 2 content
  • BC Ministry of Education — Economics 12 Curriculum
  • Mankiw, N.G. — Principles of Economics (South-Western, 2021)
  • Case, Fair & Oster — Principles of Economics (Pearson, 2020)
  • National Bureau of Statistics of China — Consumer Price Index Reports
  • Meituan & iResearch — China Food Delivery Market Reports
  • Kahneman, D. — Thinking, Fast and Slow (Farrar, Straus and Giroux, 2011)