Determination of Equilibrium Market Prices (AQA A-Level Economics): Revision Notes

Determination of Equilibrium Market Prices

Introduction to market equilibrium

In a competitive market, the interaction between demand and supply determines the price at which goods and services are bought and sold. This section explores how the market reaches a balance point called equilibrium, where the plans of consumers and producers align perfectly.

To understand this concept, we'll use the tomato market as our main example. In this market, numerous consumers wish to buy tomatoes at various prices, whilst many farmers and firms wish to sell tomatoes. The question is: at what price do these opposing forces find balance?

Understanding equilibrium and disequilibrium

Before examining how prices are determined, we need to understand two fundamental concepts that describe market conditions.

Equilibrium refers to a point where market forces are balanced, creating a state of rest. When a market is in equilibrium, there is no tendency for change because the opposing forces of supply and demand are equal. Think of it like a set of perfectly balanced scales - neither side tips up or down.

Disequilibrium, by contrast, describes a situation where these market forces are not in balance. When a market is in disequilibrium, there are pressures pushing for change, just like unbalanced scales that tip to one side.

The interaction of demand and supply

Let's examine how market demand and supply work together to determine price and quantity in a competitive market.

The market demand curve shows the total quantity of a good that all consumers in the market plan to purchase at different price levels during a particular time period. This curve slopes downward from left to right, reflecting the inverse relationship between price and quantity demanded. When prices are high, fewer consumers are willing or able to buy tomatoes; when prices fall, more consumers enter the market or existing buyers purchase more.

The market supply curve shows the total quantity of a good that all producers in the market wish to supply at different price levels during the same time period. This curve slopes upward, demonstrating the positive relationship between price and quantity supplied. Higher prices make production more profitable, encouraging firms to supply greater quantities.

Equilibrium price (P*) is determined at the exact point where these two curves intersect. At this price, the quantity demanded by consumers equals the quantity supplied by producers. The market 'clears' - meaning all the goods that firms wish to sell find buyers, and all consumers who want to purchase at this price can do so.

Equilibrium quantity (Q*) is the amount of the good that changes hands at the equilibrium price. This represents the actual volume of transactions that take place in the market.

Market disequilibrium: excess supply and excess demand

Most of the time, markets are not at their equilibrium price. When prices differ from equilibrium, either excess supply or excess demand occurs, triggering automatic market adjustments.

Excess supply

Excess supply (also called a surplus) occurs when the market price sits above the equilibrium level. At this higher price, firms wish to sell more than consumers wish to buy.

For example, imagine tomato producers set their price at P₁, which is above the equilibrium price P*. At this price:

• Producers would like to supply quantity Q₂
• Consumers only wish to purchase quantity Q₁
• The difference (Q₂ - Q₁) represents excess supply or unsold stock

Producers cannot sell all the tomatoes they wish to at price P₁. Some tomatoes remain unsold, creating unwanted inventory. To eliminate this surplus, producers reduce their prices. As the price falls, two things happen simultaneously: consumers buy more (moving along the demand curve) and some producers reduce their output (moving along the supply curve). This adjustment continues until the price reaches P*, where excess supply disappears.

Excess demand

Excess demand (also called a shortage) occurs when the market price is below equilibrium. At this lower price, consumers wish to buy more than firms wish to supply.

For instance, if the price of tomatoes is P₂ (below equilibrium):

• Consumers would like to buy quantity Q₂
• Producers only wish to supply quantity Q₁
• The difference (Q₂ - Q₁) represents excess demand or unfulfilled demand

Not all consumers can purchase the tomatoes they want at price P₂. This shortage creates upward pressure on price. Some disappointed buyers may offer to pay more, whilst producers realise they can charge higher prices. As the price rises, quantity demanded decreases and quantity supplied increases until equilibrium is restored at P*.

The market adjustment mechanism

The process described above is called the market adjustment mechanism. It's helpful to think about markets having two 'sides':

• The short side of the market consists of the economic agents (buyers or sellers) who can always fulfil their plans
• The long side of the market contains those who cannot fulfil their plans at the current price

When price is above equilibrium (P₁), households and consumers are on the short side - they can buy exactly the quantity of tomatoes they wish (Q₁). Tomato producers are on the long side - they want to sell Q₂ but can only sell Q₁. The difference represents unsold stock.

When price is below equilibrium (P₂), the situation reverses. Producers are on the short side and can sell all they produce (Q₁), whilst consumers are on the long side, unable to buy as much as they want. The difference represents unmet demand.

Summary of disequilibrium conditions

Market disequilibrium occurs when:

• Planned demand is less than planned supply, causing the price to fall, or
• Planned demand is greater than planned supply, causing the price to rise

Market equilibrium occurs when:

• Planned demand equals planned supply, and the price remains stable

How supply shifts disturb market equilibrium

Once a market reaches equilibrium, it remains there until an external event causes either the supply curve or the demand curve to shift position. Let's first examine what happens when supply changes.

Consider the tomato market where equilibrium initially exists at point X, with price P₁ and quantity Q₁. The market remains stable at this point until something changes the conditions of supply.

Suppose a bumper harvest occurs due to favourable weather conditions. This causes the supply curve to shift rightward from S₁ to S₂, indicating that producers can and will supply more tomatoes at every price level.

However, at the original equilibrium price P₁, the market is no longer in equilibrium. Looking at the diagram:

• At price P₁, producers now wish to supply quantity Q₃ (point V on the new supply curve S₂)
• Consumers still only wish to purchase quantity Q₁ (point X on the demand curve)
• The difference between Q₃ and Q₁ represents excess supply

To eliminate this surplus stock, tomato producers must reduce their prices. The market price falls from P₁ towards P₂. As the price decreases:

• Consumers move along their demand curve, purchasing more tomatoes
• Producers move along their new supply curve S₂, reducing the quantity they supply

The market settles at a new equilibrium (point Z) where the new supply curve S₂ intersects the unchanged demand curve D. At this point:

• The new equilibrium price is P₂ (lower than before)
• The new equilibrium quantity is Q₂ (higher than before)
• Planned supply once again equals planned demand

How demand shifts disturb market equilibrium

Just as supply shifts create disequilibrium, changes in demand also disturb market equilibrium and trigger price adjustments.

Let's examine what happens in the tomato market when consumer demand increases. Tomatoes are usually considered a normal good - a product for which demand increases as consumer incomes rise. When household incomes grow, people tend to buy more tomatoes (and other goods).

Initially, the market is in equilibrium where demand curve D₁ intersects the supply curve S at price P₁ and quantity Q₁. Consumers and producers can both fulfil their plans at this point.

Now suppose consumer incomes increase significantly. This shifts the market demand curve rightward from D₁ to D₂, showing that at every price level, consumers now wish to purchase more tomatoes than before.

At the original price P₁, equilibrium no longer exists:

• Consumers now wish to buy a much larger quantity (shown at point K on demand curve D₂)
• Producers are only willing to supply quantity Q₁ (point H on the supply curve)
• The difference between these quantities (distance H to K) represents excess demand

Consumers cannot buy all the tomatoes they want at price P₁. Competition among buyers drives the price upward. As the price increases to P₂:

• The quantity of tomatoes purchased and sold rises from Q₁ to Q₂
• There is movement along the supply curve from point H to point J, representing an extension of supply as producers respond to higher prices
• The excess demand is eliminated

The market establishes a new equilibrium at point J where:

• The new equilibrium price is P₂ (higher than before)
• The new equilibrium quantity is Q₂ (greater than before)
• The market once again clears, with planned demand equalling planned supply

Calculating equilibrium price: a worked example

We can determine equilibrium price and quantity not only through diagrams but also by using supply and demand schedules - tables showing the quantities demanded and supplied at different prices.

Exam tips

Understanding equilibrium and disequilibrium is fundamental to economics. These concepts help explain why markets move from one state to another and form the foundation for more advanced topics you'll encounter later.

Remember!