Chapter 3: Demand and Supply

Part 3.4 – Market Equilibrium, Price Mechanism, Shortages and Surpluses

By Dr. Anthony Fok

Introduction

After studying demand and supply separately, we are now ready to combine both concepts.

This is where Economics becomes particularly powerful.

Neither consumers nor producers determine market prices independently.

Instead, prices emerge from the interaction between buyers and sellers.

Economists call this process the price mechanism, and the point where demand equals supply is known as market equilibrium.

Understanding equilibrium is essential because it forms the basis for many later topics, including price controls, indirect taxes, subsidies, market failure and government intervention.

What Is Market Equilibrium?

Market equilibrium is the situation where:

Quantity demanded equals quantity supplied.

At the equilibrium price:

• Consumers are willing to purchase exactly the quantity producers are willing to sell.
• There is neither excess demand nor excess supply.
• There is no natural tendency for the market price to change, assuming other factors remain constant.

This is often referred to as the market-clearing price because all goods supplied can be sold, and consumers can purchase the quantity they desire.

Understanding Equilibrium Using a Demand and Supply Diagram

A standard demand and supply diagram contains:

• A downward-sloping demand curve (D).
• An upward-sloping supply curve (S).

The point where the two curves intersect is the equilibrium point (E).

The corresponding price is called the equilibrium price (Pe).

The corresponding quantity is called the equilibrium quantity (Qe).

Every A-Level Economics student should be able to draw and explain this diagram accurately.

However, drawing the diagram alone is insufficient.

Marks are awarded for explaining the economic reasoning behind it.

Why Does Equilibrium Occur?

Imagine a new café opens in Bishan.

Initially, the owner charges S$8 per cup of coffee.

At this price:

• Customers are willing to buy 300 cups per day.
• The café is willing to sell 300 cups per day.

Demand equals supply.

There is no pressure for the café to increase or decrease prices.

The market has reached equilibrium.

The Price Mechanism

The price mechanism refers to the process by which changes in demand and supply cause prices to adjust until equilibrium is restored.

Prices perform three important functions.

1. Signalling Function

Prices provide information.

A rising price signals that a product has become relatively scarce or demand has increased.

A falling price suggests weaker demand or greater availability.

Businesses use these signals when deciding what to produce.

2. Incentive Function

Higher prices create incentives for producers.

When prices increase:

• firms expand production,
• new firms may enter the industry,
• investment becomes more attractive.

Conversely, falling prices discourage production.

3. Rationing Function

Prices also allocate scarce goods.

When demand exceeds supply, higher prices reduce demand and ensure that limited goods are allocated among consumers willing and able to pay.

Although this may not always be socially equitable, it is one way markets allocate scarce resources.

What Is a Shortage?

A shortage exists when:

Quantity demanded exceeds quantity supplied.

This occurs when the market price is below equilibrium.

Consumers wish to purchase more than producers are willing to supply.

Why Do Shortages Occur?

Suppose the equilibrium price of bottled water is S$2.

If retailers sell it for only S$1:

• consumers buy much more,
• producers supply less.

Demand now exceeds supply.

Some consumers are unable to obtain the product.

The market experiences a shortage.

How Does the Market Eliminate a Shortage?

In a free market, shortages create upward pressure on prices.

As prices rise:

• consumers reduce quantity demanded,
• producers increase quantity supplied.

Eventually, quantity demanded equals quantity supplied again.

The market returns to equilibrium without direct government intervention.

Singapore Example: Face Masks During COVID-19

During the early stages of the COVID-19 pandemic, demand for face masks increased dramatically.

Initially, supply could not respond quickly enough.

The result was a shortage.

Many countries experienced empty supermarket shelves.

Over time:

• manufacturers expanded production,
• governments imported additional supplies,
• new firms entered the market.

Supply increased, helping restore market equilibrium.

This example demonstrates how markets respond to changes in demand over time.

What Is a Surplus?

A surplus exists when:

Quantity supplied exceeds quantity demanded.

This occurs when the market price is above equilibrium.

Producers supply more than consumers are willing to purchase.

Why Do Surpluses Occur?

Suppose the equilibrium price of strawberries is S$6.

If producers attempt to sell them for S$10:

• consumers purchase fewer strawberries,
• farmers produce more because prices are attractive.

Excess supply develops.

Unsold inventories begin to accumulate.

How Does the Market Eliminate a Surplus?

When producers cannot sell all their output, they often reduce prices.

As prices fall:

• consumers purchase more,
• producers reduce production.

Eventually, the surplus disappears and equilibrium is restored.

Changes in Market Equilibrium

Market equilibrium is not permanent.

Whenever demand or supply changes, equilibrium also changes.

Economists therefore analyse how markets adjust following changes in economic conditions.

Increase in Demand

Suppose consumer incomes increase.

Demand for restaurant meals rises.

The demand curve shifts to the right.

At the original price:

• demand now exceeds supply,
• a shortage develops,
• prices rise,
• firms increase production.

A new equilibrium is established with:

• a higher price,
• a higher quantity traded.

Decrease in Demand

If consumer confidence falls during an economic slowdown:

• households reduce discretionary spending,
• demand decreases,
• prices fall,
• firms reduce output.

The new equilibrium features both lower prices and lower quantities.

Increase in Supply

Suppose technological improvements reduce production costs.

Supply shifts to the right.

At the original price:

• firms produce more,
• excess supply develops,
• prices fall,
• consumers purchase more.

The new equilibrium generally involves:

• lower prices,
• higher quantities.

Decrease in Supply

Natural disasters, supply chain disruptions or rising production costs may reduce supply.

The supply curve shifts leftwards.

The new equilibrium typically results in:

• higher prices,
• lower quantities.

Singapore Case Study: COE Market

The Certificate of Entitlement (COE) system provides an excellent illustration of demand and supply.

The number of COEs released each bidding exercise is limited.

When demand for cars increases while COE supply remains relatively fixed:

• bidding intensifies,
• COE premiums rise.

Conversely, weaker demand tends to reduce COE premiums.

Although the COE system has unique institutional features, it still reflects the interaction between demand and constrained supply.

Dr. Anthony Fok’s Exam Tip

Many students memorise outcomes without explaining the adjustment process.

Whenever discussing equilibrium, structure your answer logically:

1. Identify whether demand or supply changes.
2. State the direction of the curve shift.
3. Explain whether a shortage or surplus develops.
4. Describe how prices adjust.
5. Explain how producers and consumers respond.
6. Conclude with the new equilibrium price and quantity.

This step-by-step explanation demonstrates strong analytical reasoning and is rewarded in examinations.

Common Student Mistake

❌ “Prices rise because demand increases.”

This explanation is incomplete.

A stronger answer is:

“An increase in demand creates excess demand (a shortage) at the original equilibrium price. The resulting competition among consumers pushes prices upward until a new equilibrium is established.”

Always explain why prices change, not merely that they change.

Quick Revision Summary

You should now be able to:

• Define market equilibrium.
• Explain the functions of the price mechanism.
• Distinguish between shortages and surpluses.
• Explain how markets return to equilibrium.
• Analyse how changes in demand and supply affect equilibrium price and quantity.
• Apply equilibrium analysis to real-world Singapore examples.

Coming Up in Part 3.5

The final part of this chapter will cover:

• Government intervention in markets
• Price ceilings
• Price floors
• Minimum wages
• Rent controls
• Agricultural price support
• Examination-style questions
• Common Cambridge examination mistakes
• Chapter summary and practice questions