# Market Equilibrium and Price Changes

> Economics · CIE IGCSE 0455
> Source: https://www.owlsprep.com/study/cie-0455-u2-market-equilibrium-and-price-changes/

This guide teaches you to identify and calculate market equilibrium, analyse how demand and supply shifts change prices and quantities traded, and answer structured exam questions for CIE IGCSE Economics 0455.

**Prerequisites:** [Understanding demand curves and demand shift factors](https://www.owlsprep.com/study/cie-0455-u2-demand/); [Understanding supply curves and supply shift factors](https://www.owlsprep.com/study/cie-0455-u2-supply/)

## Learning objectives

- Define market equilibrium and calculate equilibrium price/quantity from schedules and diagrams
- Analyse how shifts in demand or supply change equilibrium price and quantity traded
- Explain excess supply and excess demand, and how markets adjust back to equilibrium
- Apply equilibrium analysis to structured exam questions for CIE IGCSE Economics 0455

## Defining and Calculating Market Equilibrium

**Market Equilibrium** — The stable market state where quantity demanded equals quantity supplied, with no inherent pressure for price to change.

*Notation:* $D = S$ at $P_e, Q_e$

*Example:* If 100 coffees are demanded at \$2 and 100 coffees are supplied at \$2, \$2 is the equilibrium price and 100 is equilibrium quantity.

At equilibrium, the market clears: there is no unsold leftover stock (excess supply) and no unmet consumer demand (excess demand). You can calculate equilibrium values from a demand/supply schedule, or from a diagram where the demand and supply curves intersect.

**Worked example:** Use the schedule below to find the equilibrium price and quantity of printed t-shirts: 

| Price (\$) | Quantity Demanded | Quantity Supplied |
|---|---|---|
| 5 | 200 | 50 |
| 10 | 150 | 100 |
| 15 | 100 | 100 |
| 20 | 50 | 150 |

1. Find the row where quantity demanded matches quantity supplied.
2. At a price of \$15, both quantity demanded and quantity supplied equal 100 units.
3. State the final values: equilibrium price = \$15, equilibrium quantity = 100 t-shirts.

> **Exam tip:** When drawing equilibrium diagrams, add dotted lines from the intersection point to both axes, and label the equilibrium point $E$ to earn full marks for diagram work.

## Excess Supply and Excess Demand (Disequilibrium)

**Disequilibrium** — An unstable market state where quantity demanded does not equal quantity supplied, creating pressure for price to adjust until equilibrium is restored.

When price is set *above* equilibrium, quantity supplied exceeds quantity demanded, creating excess supply (surplus). Producers will cut prices to clear unsold stock, increasing quantity demanded and reducing quantity supplied until equilibrium is restored. When price is set *below* equilibrium, quantity demanded exceeds quantity supplied, creating excess demand (shortage). Consumers bid up prices to access limited stock, reducing quantity demanded and increasing quantity supplied until equilibrium is restored.

**Worked example:** Using the t-shirt schedule from the previous example, identify the type of disequilibrium at a price of \$20, and explain how the market will adjust back to equilibrium.

1. Extract values at \$20: quantity demanded = 50, quantity supplied = 150.
2. Quantity supplied > quantity demanded, so there is excess supply of 100 t-shirts.
3. Producers reduce prices to sell unsold stock. As price falls, quantity demanded rises and quantity supplied falls, until price reaches \$15 where quantity demanded = quantity supplied = 100.

> **Exam tip:** Always calculate the size of the surplus or shortage (difference between quantity supplied and demanded) when asked, as this earns an extra mark in structured questions.

## Effects of Demand Shifts on Equilibrium

Any non-price demand shift factor (e.g. changes in income, consumer tastes, price of related goods, population) will shift the entire demand curve, changing the equilibrium point. A rightward (increase) in demand raises both equilibrium price and quantity traded. A leftward (decrease) in demand reduces both equilibrium price and quantity traded.

**Worked example:** The market for bicycles is initially in equilibrium. Explain how a successful government campaign promoting cycling for exercise will affect equilibrium price and quantity of bicycles.

1. The campaign changes consumer tastes in favour of cycling, increasing demand for bicycles at every price.
2. The demand curve shifts right from $D_1$ to $D_2$.
3. At the original equilibrium price, there is now excess demand for bicycles.
4. Price rises until a new equilibrium is reached at a higher price ($P_2$) and higher quantity traded ($Q_2$).

> **tip**
>
> Always state the *cause* of the demand shift first, before describing the effect on equilibrium, to hit all mark scheme points.

## Effects of Supply Shifts on Equilibrium

Any non-price supply shift factor (e.g. changes in production costs, technology, number of producers, government taxes/subsidies) will shift the entire supply curve, changing the equilibrium point. A rightward (increase) in supply reduces equilibrium price and raises equilibrium quantity traded. A leftward (decrease) in supply raises equilibrium price and reduces equilibrium quantity traded.

**Worked example:** The market for wheat is initially in equilibrium. A global drought reduces wheat harvests across all major producing countries. Explain the effect on equilibrium price and quantity of wheat.

1. The drought reduces the amount of wheat that can be produced at every price, decreasing supply of wheat.
2. The supply curve shifts left from $S_1$ to $S_2$.
3. At the original equilibrium price, there is now excess demand for wheat.
4. Price rises until a new equilibrium is reached at a higher price ($P_2$) and lower quantity traded ($Q_2$).

> **Exam tip:** When drawing shift diagrams, label original curves $D_1/S_1$ and new curves $D_2/S_2$ clearly, plus original equilibrium $E_1$ and new equilibrium $E_2$ to avoid losing marks.

## Common pitfalls

- **Wrong:** Confusing movements along curves with curve shifts when explaining price changes
  - Why it fails: Price changes cause movements along existing D/S curves, only non-price factors cause shifts. Citing price as a shift cause loses marks.
  - Correct: First state the non-price factor causing the D/S shift, then explain the resulting price change as the effect of that shift.
- **Wrong:** Mixing up equilibrium changes when supply shifts left
  - Why it fails: Many students incorrectly assume a leftward supply shift leads to lower price, but reduced supply makes goods scarcer so price rises.
  - Correct: Leftward supply shifts = higher price, lower quantity; rightward supply shifts = lower price, higher quantity.
- **Wrong:** Skipping the disequilibrium adjustment step in explanations
  - Why it fails: Mark schemes explicitly award marks for explaining the excess demand/supply stage between old and new equilibrium.
  - Correct: After stating the curve shift, explain the excess demand/supply at the original price, then describe how price adjusts to the new equilibrium.
- **Wrong:** Labelling diagram axes incorrectly
  - Why it fails: Swapping axes or using non-standard labels (e.g. 'Cost' instead of 'Price') loses all marks for diagram work.
  - Correct: Always label the y-axis $P$ (Price) and x-axis $Q$ (Quantity) before drawing any curves.
- **Wrong:** Assuming both demand and supply shift unless explicitly stated
  - Why it fails: Nearly all IGCSE questions ask you to analyse the effect of a single shift (only D or only S) unless told otherwise.
  - Correct: Only analyse shifts in both curves if the question explicitly states factors affecting both demand and supply are changing.

## Cheatsheet

| Market Change | Curve Shift Direction | Effect on $P_e$ | Effect on $Q_e$ |
| --- | --- | --- | --- |
| Increase in Demand | Demand shifts right | Rises | Rises |
| Decrease in Demand | Demand shifts left | Falls | Falls |
| Increase in Supply | Supply shifts right | Falls | Rises |
| Decrease in Supply | Supply shifts left | Rises | Falls |
| Price above equilibrium | No curve shift | Falls to $P_e$ | Adjusts to $Q_e$ |
| Price below equilibrium | No curve shift | Rises to $P_e$ | Adjusts to $Q_e$ |

## What's next

Now that you have mastered market equilibrium analysis, you can apply this knowledge to understand government interventions in markets, such as price controls, taxes and subsidies, which are core topics in the rest of the Allocation of Resources unit. Practice drawing equilibrium shift diagrams for different real-world scenarios, as these are frequently tested in Paper 2 structured questions worth 4-6 marks each. Make sure you can explain every step of the adjustment process clearly, as this is where most students lose marks in exam answers. Next, move on to learn about government market interventions and their effects.

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