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IB DP Economics Study Notes

2.3.1 Equilibrium Price & Quantity

Understanding the dynamics of market equilibrium is crucial for grasping the fundamental principles of microeconomics. The equilibrium price and quantity offer a snapshot of where the market stands in terms of supply and demand.


At the heart of market operations is the concept of equilibrium. This is the point where the forces of supply and demand balance out, ensuring stability in prices and quantities.

  • Equilibrium Price: This is the price at which the quantity of a product that consumers desire matches the quantity that producers are willing to offer. It's the price where demand and supply curves intersect on a graph.
  • Equilibrium Quantity: At the equilibrium price, the amount of the product that consumers want to buy equals the amount that producers want to sell. This quantity is referred to as the equilibrium quantity.

How it's Determined

The equilibrium price and quantity aren't arbitrary figures but are determined by the interplay of demand and supply in the market. Here's a step-by-step breakdown:

Plotting the Curves

1. Demand Curve: This downward-sloping curve showcases the relationship between the price of a product and the quantity that consumers are prepared to purchase. At higher prices, consumers tend to buy less, while at lower prices, they buy more. Understanding the individual vs market demand is essential for grasping how these curves are constructed.

2. Supply Curve: This upward-sloping curve illustrates the relationship between the price of a product and the quantity that producers are prepared to supply. At higher prices, it becomes more profitable for producers to supply more of the product, while at lower prices, they supply less. The law of supply and the non-price determinants of supply play a critical role in shaping this curve.

Finding the Equilibrium

1. Intersection Point: By plotting the demand and supply curves on the same graph, the point where they intersect represents the market equilibrium. The price corresponding to this point is the equilibrium price, and the quantity is the equilibrium quantity.

Graph of market equilibrium

A graph illustrating market equilibrium (equilibrium price and equilibrium quantity) at the point of intersection of demand curve and supply curve.

Image courtesy of economicshelp

If the demand and supply curves don't intersect, it indicates a state of disequilibrium, leading to either a surplus or a shortage in the market. This imbalance prompts price adjustments until a new equilibrium is established.

Market Forces and Their Role

The invisible hand of the market, driven by the forces of demand and supply, ensures that prices and quantities adjust in response to changes in market conditions.

The Twin Laws of the Market

1. Law of Demand: This law states that, all else being equal, an increase in the price of a product will lead to a decrease in the quantity demanded. Conversely, a decrease in price will lead to an increase in quantity demanded. This inverse relationship stems from consumers' rational behaviour of trying to maximise their utility or satisfaction.

2. Law of Supply: According to this law, a rise in the price of a product, with everything else held constant, will lead to an increase in the quantity supplied. Similarly, a fall in price will lead to a decrease in quantity supplied. This direct relationship arises from producers' aim to maximise profits.

Adjusting to Disequilibrium

Markets rarely remain static. Changes in external conditions can lead to disequilibrium, prompting adjustments:

  • Excess Supply (Surplus): When the current market price is higher than the equilibrium price, there's a surplus. Producers, eager to offload excess stock, will typically reduce prices. As prices decrease, the quantity demanded by consumers rises, and the quantity producers are willing to supply falls. This continues until the market returns to equilibrium.
Graph of excess supply (market surplus)

A graph illustrating excess supply (market surplus) and how it is converted to market equilibrium.

Image courtesy of economicshelp

  • Excess Demand (Shortage): A situation where the current market price is lower than the equilibrium price results in a shortage. Consumers, faced with limited stock, are often willing to pay a higher price. Recognising this, producers raise prices. As prices ascend, the quantity consumers demand reduces, and the quantity producers supply increases, moving the market back to equilibrium.
Graph of excess demand (market shortage)

A graph illustrating excess demand (market shortage) and how it is converted to market equilibrium.

Image courtesy of economicshelp

Unknown block type "table", specify a component for it in the `components.types` option

A summary table of the main features of market shortage and market surplus.

External Factors and Their Impact

Various external factors can disrupt market equilibrium:

  • Shift in Demand: A multitude of factors, from changing consumer tastes to fluctuations in income or the prices of complementary and substitute goods, can cause the demand curve to shift, leading to a new equilibrium. Understanding how externalities and welfare loss influence demand is critical.
  • Shift in Supply: External events, be they technological breakthroughs, variations in production costs, or unforeseen natural disasters, can shift the supply curve. This shift results in a new equilibrium point. The role of government in correcting market failures often involves interventions that can shift supply curves.

In summary, the market, through the continuous interplay of supply and demand, tends to gravitate towards equilibrium. This self-adjusting mechanism ensures that resources are allocated optimally, harmonising consumer preferences with producers' capabilities.


External shocks, such as natural disasters, can have profound effects on market equilibrium. Natural disasters can disrupt the supply chain, reducing the availability of goods, leading to a leftward shift in the supply curve. With demand remaining constant, this results in a higher equilibrium price and a lower equilibrium quantity. For instance, if a major flood affects a rice-producing region, the supply of rice will decrease. If the demand for rice remains unchanged, the price of rice will increase, and the quantity available in the market will decrease, reflecting the new equilibrium.

Price floors and price ceilings are government-imposed limits on how low or how high prices can be charged for goods and services. A price floor, set above the equilibrium price, results in a surplus since the quantity supplied exceeds the quantity demanded at that price. An example is minimum wage laws, where the wage rate might be set above the equilibrium, leading to potential unemployment. A price ceiling, set below the equilibrium price, leads to a shortage as the quantity demanded exceeds the quantity supplied. Rent control in housing markets is a classic example, where a cap on rent can lead to a shortage of rental units available.

There are several reasons a market might not reach equilibrium. Government interventions, such as price controls, taxes, or subsidies, can prevent prices from adjusting to their equilibrium levels. Imperfect information can also hinder the market from reaching equilibrium; if consumers or producers lack full knowledge about market conditions, they might not respond optimally to price signals. Sticky prices, where prices are slow to change despite shifts in demand or supply, can also prevent equilibrium. This is often seen in labour markets where wages don't adjust quickly. Lastly, externalities, where third parties bear costs or receive benefits from a transaction they aren't a part of, can distort market outcomes and prevent equilibrium.

If both demand and supply increase simultaneously, the effect on the equilibrium price and quantity can be a bit complex. The increase in demand, holding supply constant, would generally lead to a higher equilibrium price and quantity. Conversely, an increase in supply, with demand held constant, would typically result in a lower price but higher quantity. When both curves shift rightward simultaneously, the equilibrium quantity will definitely increase. However, the change in equilibrium price is ambiguous and depends on the magnitude of the shifts. If the increase in demand is greater than the increase in supply, the price will rise. If the increase in supply is greater than the increase in demand, the price will fall.

Consumer surplus and producer surplus are concepts that highlight the benefits consumers and producers derive from market transactions at the equilibrium price. Consumer surplus is the difference between what consumers are willing to pay for a good or service and what they actually pay. It's the area between the demand curve and the price level up to the equilibrium quantity. On the other hand, producer surplus is the difference between the price producers are willing to sell a good or service for and the price they actually receive. It's the area between the supply curve and the price level up to the equilibrium quantity. Both surpluses maximise at equilibrium, ensuring the most efficient allocation of resources in a market.

Practice Questions

Explain the concept of equilibrium price and quantity in a competitive market. How are they determined?

Equilibrium price and quantity in a competitive market refer to the point where the quantity demanded by consumers matches the quantity supplied by producers. This state of balance ensures stability in prices and quantities. They are determined by the intersection of the demand and supply curves on a graph. Specifically, the demand curve, which slopes downwards, represents the quantities consumers are willing to buy at different prices. Conversely, the supply curve, sloping upwards, indicates the quantities producers are willing to offer at various prices. The point where these two curves meet is the equilibrium point, with the corresponding price being the equilibrium price and the quantity being the equilibrium quantity.

Describe the role of market forces in adjusting to a state of disequilibrium. Provide an example of how the market adjusts to a surplus.

Market forces, specifically the laws of demand and supply, play a pivotal role in adjusting to disequilibrium. When there's a discrepancy between quantity demanded and supplied, prices adjust to restore balance. For instance, in the case of a surplus, where the market price is above the equilibrium price, there's more quantity supplied than demanded. Recognising unsold stock, producers typically reduce prices. As prices fall, consumers' quantity demanded increases, while the quantity producers are willing to supply decreases. This adjustment continues until the market returns to equilibrium. An example would be unsold seasonal clothing; as the season ends, retailers reduce prices to clear stock, increasing demand until the surplus is eliminated.

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Written by: Dave
Cambridge University - BA Hons Economics

Dave is a Cambridge Economics graduate with over 8 years of tutoring expertise in Economics & Business Studies. He crafts resources for A-Level, IB, & GCSE and excels at enhancing students' understanding & confidence in these subjects.

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