# Demand, supply and market equilibrium

> Economics · CIE A-Level
> Source: https://www.owlsprep.com/study/cie-9708-u2-demand-supply-and-market-equilibrium/

This module covers core microeconomic concepts of demand, supply, and market equilibrium in free markets. You will learn to plot curves, identify equilibrium, and analyze the impact of shocks on market price and quantity.

**Prerequisites:** [Basic introductory microeconomics concepts](https://www.owlsprep.com/study/cie-9708-u1-introduction-to-economics/)

## Learning objectives

- Distinguish between movements along and shifts of demand/supply curves
- Calculate market equilibrium price and quantity from functions and diagrams
- Analyze how changes in demand/supply affect equilibrium outcomes
- Explain the role of price as a market clearing mechanism

## Demand: Core Concepts and Determinants

Demand refers to the quantity of a good that consumers are willing and able to buy at different prices over a given time period. The law of demand states that ceteris paribus, quantity demanded falls as price rises, giving a downward-sloping demand curve.

**Law of demand** — Ceteris paribus, there is an inverse relationship between own price and quantity demanded.

*Example:* As the price of coffee rises from $4 to $5 per cup, consumers buy 10 cups instead of 15 per week.

A key distinction is between movements along the demand curve and shifts of the entire curve. Movements are only caused by changes in the own price of the good. Shifts are caused by non-price determinants: income, prices of related goods, tastes, expectations, and number of buyers.

**Worked example:** The price of apples falls, ceteris paribus. Show what happens to the demand for apples, and state if this is a shift or movement along the curve.

1. Step 1: The change is to the own price of apples, so this is a movement along the existing demand curve.
2. Step 2: Because price falls, we move down and right along the curve from the original higher price point.
3. Step 3: Quantity demanded increases, the position of the entire demand curve does not change.

## Supply: Core Concepts and Determinants

Supply is the quantity of a good that producers are willing and able to sell at different prices over a given time period. The law of supply states that ceteris paribus, quantity supplied rises as price rises, giving an upward-sloping supply curve.

**Law of supply** — Ceteris paribus, there is a positive relationship between own price and quantity supplied.

*Example:* As the price of wheat rises, farmers are willing to supply more wheat because higher prices increase profit margins.

Like demand, we distinguish between movements along the supply curve (caused by own price changes) and shifts of the entire curve (caused by non-price determinants). Non-price determinants include input costs, technology, number of producers, expectations, and government policies like taxes or subsidies.

**Worked example:** A government gives a $1 per kg subsidy to wheat farmers. What happens to the supply curve for wheat? Is this a shift or movement?

1. Step 1: The subsidy changes production costs, a non-price determinant of supply, so it shifts the entire supply curve.
2. Step 2: Lower costs mean farmers are willing to supply more wheat at every possible price.
3. Step 3: The supply curve shifts rightwards (outwards) from its original position.

## Calculating and Identifying Market Equilibrium

Market equilibrium occurs where the plans of consumers and producers match: the quantity that consumers want to buy equals the quantity producers want to sell. At equilibrium, there is no pressure for price to change.

**Market Clearing Price** — Another term for equilibrium price, as it clears the market of all excess demand and excess supply.

**Worked example:** Given demand function $Q_D = 100 - 2P$ and supply function $Q_S = 20 + 2P$, calculate equilibrium price and quantity.

1. At equilibrium, quantity demanded equals quantity supplied, so set $Q_D = Q_S$:
2. $$100 - 2P = 20 + 2P$$
3. Rearrange to isolate and solve for $P$:
4. $$80 = 4P \\ P = 20$$
5. Substitute $P=20$ back into either function to find equilibrium quantity:
6. $$Q = 100 - 2(20) = 60$$
7. Final equilibrium: Price = 20, Quantity = 60 units.

> **tip**
>
> On a diagram, equilibrium is always found at the intersection point of the demand and supply curves.

## Comparative Statics: Adjustment to New Equilibrium

Comparative statics compares the original equilibrium to the new equilibrium after a shift in demand, supply, or both. When a curve shifts, excess demand or excess supply is created at the original price, which pushes price to adjust to the new equilibrium.

**Worked example:** Coffee and tea are substitutes. If the price of coffee rises, what happens to equilibrium price and quantity of tea?

1. Step 1: Coffee price is a non-price determinant of tea demand, so only the demand curve for tea shifts.
2. Step 2: Higher coffee price leads consumers to switch to tea, so demand for tea increases: demand curve shifts right.
3. Step 3: At the original equilibrium price, quantity demanded now exceeds quantity supplied, creating excess demand.
4. Step 4: Excess demand pushes price up, leading to a new equilibrium with higher price and higher quantity of tea.

**Check your understanding**

Test your understanding:

1. A rise in consumer income increases demand for a normal good. This causes:

   - A movement up along the demand curve
   - A rightward shift of the demand curve
   - A leftward shift of the demand curve
   - A movement down along the demand curve

   *Why:* Correct! Income is a non-price determinant of demand, so it shifts the entire curve. Higher demand is represented as a rightward shift.

## Common pitfalls

- **Wrong:** Confusing shifts of the demand/supply curve with movements along the curve for non-price changes
  - Why it fails: Only changes in the own price of the good cause movements along the curve. All other changes shift the entire curve.
  - Correct: Always ask: is the change to the price of this good, or another variable? If it is another variable, the curve shifts.
- **Wrong:** Drawing an increase in supply as a leftward shift of the supply curve
  - Why it fails: An increase in supply means more quantity supplied at every price, which is graphed as a shift to the right.
  - Correct: Remember the rule: *more = right, less = left* for both demand and supply curves.
- **Wrong:** Calculating equilibrium by setting prices equal instead of quantities
  - Why it fails: Equilibrium is defined as quantity demanded equal to quantity supplied, not equal prices.
  - Correct: Always set $Q_D = Q_S$ when solving for equilibrium from linear demand and supply functions.
- **Wrong:** Shifting both demand and supply when only one determinant changes
  - Why it fails: We assume ceteris paribus: only the curve affected by the given change shifts, unless stated otherwise.
  - Correct: Only shift both curves if the question provides information that changes both a demand and a supply determinant.

## Cheatsheet

| Type of change | Effect on equilibrium price | Effect on equilibrium quantity |
| --- | --- | --- |
| Demand increases (shift right) | Rises | Rises |
| Demand decreases (shift left) | Falls | Falls |
| Supply increases (shift right) | Falls | Rises |
| Supply decreases (shift left) | Rises | Falls |

## What's next

Understanding demand, supply, and market equilibrium is the foundation of almost all microeconomic analysis. Every subsequent topic in the price system builds on this core framework, from analyzing the effects of government intervention to understanding consumer and producer surplus, and market failure. Mastering the distinction between shifts and movements, and correctly predicting changes to equilibrium price and quantity, will make complex topics far easier to approach. This framework is also frequently tested in both multiple choice and essay questions, so it is critical to be confident with the concepts here before moving on to more advanced topics.

- [Price, income and cross elasticities of demand](https://www.owlsprep.com/study/cie-9708-u2-price-income-and-cross-elasticities/)
- [Price elasticity of supply](https://www.owlsprep.com/study/cie-9708-u2-price-elasticity-of-supply/)
- [Consumer Surplus](https://www.owlsprep.com/study/cie-9708-u2-consumer-surplus/)

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