# Demand, supply and market equilibrium

> Edexcel International GCSE Economics · 4EC1 (2017 spec)
> Source: https://www.owlsprep.com/study/edexcel-igcse-economics-s1-demand-supply-and-market-equilibrium/

This guide covers core Edexcel IGCSE Economics 4EC1 Section 1.1.3 content, including demand/supply definitions, curve shifts, equilibrium analysis, and excess demand/supply calculation for exam-style questions.

**Prerequisites:** [Introduction to the market system (Edexcel IGCSE Economics S1_T01)](https://www.owlsprep.com/study/edexcel-igcse-economics-s1-introduction-to-the-market-system/)

## Learning objectives

- Define demand and supply, and distinguish between movements along and shifts of their curves
- Identify and explain all non-price shift factors for demand and supply curves
- Calculate and illustrate excess demand and excess supply from diagrams or data schedules
- Analyse how shifts in demand and supply affect equilibrium price and quantity using labelled diagrams
- Explain how free market forces eliminate shortages and surpluses to restore equilibrium

## Demand: Movements and Shifts

**Demand** — The quantity of a good or service that consumers are willing and able to buy at a range of prices over a given period of time.

*Notation:* $D$, $Q_d$

*Example:* At a price of \$2 per loaf, the demand for bread in a town is 8,000 loaves per week.

The demand curve slopes downward from left to right, reflecting the inverse relationship between price and quantity demanded (ceteris paribus: all other factors held constant). Only a change in the price of the good itself causes a movement along the demand curve: a contraction (price rise, fall in $Q_d$) or extension (price fall, rise in $Q_d$) of demand.

> **Demand Shift Factors (Non-Price)**
>
> All factors below change demand at every price level, shifting the entire curve right (increase) or left (decrease):

- Advertising: successful campaigns increase demand (right shift)
- Income: rising income increases demand for normal goods (right shift)
- Fashion and tastes: growing popularity of a good increases demand (right shift)
- Price of substitutes: if a substitute good rises in price, demand for the original good rises (right shift)
- Price of complements: if a complementary good rises in price, demand for the original good falls (left shift)
- Demographic changes: population growth or changing age structures can increase demand for specific goods (right shift)

**Worked example:** Explain the effect of a successful global advertising campaign for plant-based milk on the demand curve for plant-based milk.

1. 1. Advertising is a non-price shift factor for demand.
2. 2. A successful campaign will increase the quantity of plant-based milk consumers are willing to buy at every price level.
3. 3. This causes a rightward shift of the entire demand curve from D1 to D2, with no movement along the existing curve.

> **Exam tip:** When drawing demand shifts, label the original curve D1 and new curve D2 with a clear arrow showing the direction of shift to gain full diagram marks.

*Calculator:* allowed

## Supply: Movements and Shifts

**Supply** — The quantity of a good or service that producers are willing and able to sell at a range of prices over a given period of time.

*Notation:* $S$, $Q_s$

*Example:* At a price of \$2 per loaf, bakers are willing to supply 8,000 loaves of bread per week.

The supply curve slopes upward from left to right, reflecting the positive relationship between price and quantity supplied: higher prices mean producers can earn more profit, so they increase output. Only a change in the price of the good itself causes a movement along the supply curve: a contraction (price fall, fall in $Q_s$) or extension (price rise, rise in $Q_s$) of supply.

> **Supply Shift Factors (Non-Price)**
>
> All factors below change supply at every price level, shifting the entire curve right (increase) or left (decrease):

- Costs of production: rising raw material or labour costs reduce supply (left shift)
- Changes in technology: more efficient production processes increase supply (right shift)
- Indirect taxes: taxes on goods raise production costs, reducing supply (left shift)
- Subsidies: government payments to producers lower costs, increasing supply (right shift)
- Natural factors: bad weather or natural disasters reduce supply of agricultural goods (left shift)

**Worked example:** Explain the effect of a government subsidy given to solar panel manufacturers on the supply curve for solar panels.

1. 1. Subsidies are a non-price shift factor for supply.
2. 2. The subsidy reduces production costs for manufacturers, so they are willing to supply more solar panels at every price level.
3. 3. This causes a rightward shift of the entire supply curve from S1 to S2.

> **Exam tip:** Always explicitly link the shift factor given in an exam question to the direction of the curve shift to earn full analysis marks.

*Calculator:* allowed

## Market Equilibrium

**Market Equilibrium** — The point where quantity demanded equals quantity supplied, so there is no inherent pressure for price to change.

*Notation:* $P_e$, $Q_e$

*Example:* If consumers demand 8,000 loaves of bread at \$2, and bakers supply 8,000 loaves at \$2, the market is at equilibrium with $P_e = \$2$ and $Q_e = 8,000$.

Equilibrium is found at the intersection of the demand and supply curves. When drawing equilibrium diagrams, always label the vertical axis (Price, P) and horizontal axis (Quantity, Q), plot the downward-sloping D and upward-sloping S curves, and mark the intersection point with $P_e$ on the y-axis and $Q_e$ on the x-axis. Any shift in demand or supply will create a new equilibrium point.

**Worked example:** Using a diagram, analyse the effect of a rise in consumer income on the equilibrium price and quantity of fresh fruit (a normal good).

1. 1. Draw the original demand (D1) and supply (S1) curves, mark initial equilibrium $P_{e1}$ and $Q_{e1}$.
2. 2. Rising income is a demand shift factor: it increases demand for normal goods, shifting D1 right to D2.
3. 3. At the original price $P_{e1}$, there is now excess demand, so price rises until a new equilibrium is reached at $P_{e2}$ (higher than $P_{e1}$) and $Q_{e2}$ (higher than $Q_{e1}$).
4. 4. Conclusion: a rise in income increases both the equilibrium price and quantity of normal goods.

> **Exam tip:** When analysing curve shifts, always state the effect on both equilibrium price AND quantity to avoid losing easy marks.

*Calculator:* allowed

## Excess Demand, Excess Supply and Price Adjustment

**Excess Demand and Excess Supply** — Excess demand (shortage) occurs when $Q_d > Q_s$ at a price below equilibrium. Excess supply (surplus) occurs when $Q_s > Q_d$ at a price above equilibrium.

To calculate excess demand or supply from a schedule or diagram, use the formula:

$$Excess = Q_d - Q_s$$

A positive result indicates excess demand, a negative result indicates excess supply, and a result of 0 means the market is at equilibrium. Free market forces automatically eliminate shortages and surpluses: excess demand pushes prices up to ration limited stock, while excess supply pushes prices down to clear unsold stock, until equilibrium is restored.

**Worked example:** The table below shows demand and supply schedules for loaves of bread: | Price (\$) | Qd (thousands) | Qs (thousands) | | --- | --- | --- | | 1.00 | 12 | 4 | | 1.50 | 10 | 7 | | 2.00 | 8 | 8 | | 2.50 | 6 | 9 | | 3.00 | 4 | 11 | Calculate the excess supply or demand at a price of \$2.50, and explain how market forces will adjust the price back to equilibrium.

1. 1. At price = \$2.50, $Q_d = 6,000$, $Q_s = 9,000$.
2. $$Excess = 6,000 - 9,000 = -3,000$$
3. 2. The negative value means there is excess supply (surplus) of 3,000 loaves of bread.
4. 3. Bakers have unsold loaves, so they reduce prices to encourage more consumers to buy bread. As price falls, $Q_d$ rises and $Q_s$ falls, until equilibrium is reached at \$2.00, where $Q_d = Q_s = 8,000$.

> **Exam tip:** Always show your workings when calculating excess demand or supply, as you may earn method marks even if your final answer is incorrect.

*Calculator:* allowed

## Common pitfalls

- **Wrong:** Saying "demand rises" when describing the effect of a price fall, confusing a movement along the curve with a shift.
  - Why it fails: Only non-price factors change demand (shift the curve); price changes only change quantity demanded (movement along the curve).
  - Correct: Refer to "quantity demanded rises" when describing a price fall effect, and "demand rises" only for rightward curve shifts.
- **Wrong:** Drawing demand curves upward sloping or supply curves downward sloping in diagrams.
  - Why it fails: The law of demand states price and $Q_d$ are inversely related (downward slope), and the law of supply states price and $Q_s$ are positively related (upward slope).
  - Correct: Always label curves and check their slope before adding equilibrium labels or shifts in exam diagrams.
- **Wrong:** Calculating excess supply as $Q_s - Q_d$ and stating a positive value is excess demand.
  - Why it fails: The standard formula is $Excess = Q_d - Q_s$, so a negative result equals excess supply, and a positive result equals excess demand.
  - Correct: Always state whether your result is excess demand or supply after calculation, not just the numeric value.
- **Wrong:** Only stating the effect on price (or only quantity) when analysing a curve shift.
  - Why it fails: Exam questions almost always award separate marks for identifying changes to both equilibrium variables.
  - Correct: After explaining the shift, explicitly write "equilibrium price rises/falls and equilibrium quantity rises/falls" in your answer.
- **Wrong:** Mixing up demand and supply shift factors, e.g. attributing a rise in production costs to a demand shift.
  - Why it fails: Shift factors are specific to either demand or supply, and mixing them up leads to incorrect analysis and lost marks.
  - Correct: Memorise the separate lists of demand and supply shift factors, and always link the factor in the question to the correct curve.

## Cheatsheet

| Concept | Key Detail | Diagram Rule |
| --- | --- | --- |
| Demand | Inverse P-Qd relationship; shift factors: advertising, income, tastes, substitutes, complements, demographics | Downward sloping, label D1/D2 for shifts, arrow for direction |
| Supply | Positive P-Qs relationship; shift factors: production costs, technology, taxes, subsidies, natural factors | Upward sloping, label S1/S2 for shifts, arrow for direction |
| Equilibrium | Qd = Qs, no price pressure | Label intersection: Pe (y-axis), Qe (x-axis) |
| Excess Demand | Qd > Qs, price < Pe; price rises to restore equilibrium | Mark gap between Qd and Qs below Pe, arrow up for price rise |
| Excess Supply | Qs > Qd, price > Pe; price falls to restore equilibrium | Mark gap between Qs and Qd above Pe, arrow down for price fall |

## What's next

Now that you have mastered demand, supply and market equilibrium, you are ready to move on to the next core topic in the market system unit: price elasticity of demand and supply, which explores how responsive quantity demanded and supplied are to changes in price and other variables. You will also apply this equilibrium knowledge to analyse government intervention in markets, including price controls, taxes and subsidies, which are common extended response topics in Edexcel IGCSE Economics Paper 1. Practise drawing shift diagrams for real-world contexts (e.g. energy markets, food price changes) to build confidence for 4-8 mark analysis questions in your exam.

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