# Supply

> Edexcel International A-Level Economics · Edexcel IAL Econ 2018
> Source: https://www.owlsprep.com/study/edexcel-ial-economics-u1-supply/

This guide covers core supply concepts for Edexcel IAL Economics Unit 1, including supply curve movements vs shifts, non-price shift factors, PES calculations, determinants, and required exam diagrams. No demand-equilibrium content is included per scope constraints.

**Prerequisites:** [Edexcel IAL Econ U1 basic microeconomic principles](https://www.owlsprep.com/study/edexcel-ial-economics-u1-intro-to-microeconomics/); [Basic percentage change calculation skills](https://www.owlsprep.com/study/edexcel-ial-maths-basic-percentages/)

## Learning objectives

- Define supply and distinguish between movements along the supply curve and shifts of the curve
- Identify and explain non-price factors that shift the supply curve
- Define Price Elasticity of Supply (PES), calculate it and interpret its values
- Analyse PES determinants, including short-run vs long-run differences
- Draw and label all required supply and PES diagrams for exam questions

## What is Supply? Movements vs Shifts

**Supply** — The quantity of a good or service that producers are willing and able to sell at a given price, over a specific time period, ceteris paribus.

*Example:* A coffee roaster is willing to sell 500 bags of coffee per week at a price of \$10 per bag.

The supply curve is upward sloping because higher prices create an incentive for producers to increase output, and higher-cost producers will enter the market at higher price points. Changes to the market cause two distinct types of change to supply:

- **Movement along the curve**: Caused *only* by a change in the own price of the good, leading to an expansion (price rise) or contraction (price fall) of quantity supplied.
- **Shift of the curve**: Caused by non-price factors, leading to an increase (right shift) or decrease (left shift) in supply at every price level.

**Exam command terms**

Common command words for supply questions:

- **Define** — 2-mark question, include ceteris paribus for full marks *(Define supply (2 marks))*

- **Explain** — 4-mark question, require a 2-stage reasoning chain *(Explain the effect of a price rise on quantity supplied (4 marks))*

- **Draw** — 4-mark question, label all axes and curves clearly

**Worked example:** Explain whether a 10% increase in the price of wheat causes a movement along or shift of the wheat supply curve.

1. Step 1: Identify the cause of the change: a rise in the own price of wheat, not a non-price supply factor.
2. Step 2: Conclude this causes a movement along the existing supply curve, specifically an expansion of quantity supplied, as higher prices incentivize wheat farmers to sell more.

> **tip**
>
> Always label supply curves S (original) and S1 (shifted) with clear arrows showing shift direction in exams to earn full diagram marks.

## Non-Price Factors Shifting the Supply Curve

Five core non-price factors cause shifts of the supply curve, all linked to changes in production costs or producer capacity:

- **Costs of production**: Higher input costs (wages, raw materials) reduce supply (left shift); lower costs increase supply (right shift).
- **New technology**: More efficient production reduces costs, increasing supply (right shift).
- **Indirect taxes**: Specific (per-unit) and ad valorem (percentage) taxes raise production costs, reducing supply (left shift).
- **Subsidies**: Government payments to producers reduce costs, increasing supply (right shift).
- **Supply shocks**: Natural disasters, wars or harvest failures destroy production capacity, reducing supply (left shift).

**Worked example:** Draw the effect of a government subsidy given to electric vehicle (EV) manufacturers on the supply curve for EVs.

1. Step 1: Label the y-axis Price (P) and x-axis Quantity of EVs (Qs).
2. Step 2: Draw the original upward-sloping supply curve, labelled S.
3. Step 3: A subsidy reduces per-unit production costs for EV manufacturers, so producers are willing to supply more EVs at every price point.
4. Step 4: Draw the new supply curve to the right of S, labelled S1, with an arrow showing the rightward shift.

> **warning**
>
> Do not discuss tax/subsidy incidence or welfare impacts in this topic: these are out of scope and only covered in later equilibrium and government intervention units. Only link taxes/subsidies to cost changes and supply shifts here.

> **Exam tip:** When explaining supply shifts, always explicitly link the factor to producer costs or incentives for full KAA marks, e.g. 'higher raw material costs increase per-unit production costs, so producers supply less at every price, shifting supply left'.

## Price Elasticity of Supply (PES): Definition & Calculation

**Price Elasticity of Supply (PES)** — A measure of the responsiveness of quantity supplied of a good to a change in its own price.

*Notation:* PES = \frac{\% \Delta Q_s}{\% \Delta P}

PES values are always positive because of the upward-sloping supply curve: price and quantity supplied move in the same direction. The table below outlines the interpretation of PES values and their corresponding curve shapes:

| PES Value | Classification | Curve Shape |
| --- | --- | --- |
| 0 | Perfectly inelastic | Vertical line |
| 0 < PES < 1 | Inelastic | Steep upward slope |
| 1 | Unitary elastic | Straight line passing through origin |
| PES > 1 | Elastic | Shallow upward slope |
| ∞ | Perfectly elastic | Horizontal line |

**Worked example:** Calculate PES if the price of coffee rises from \$2 to \$2.40, and quantity supplied rises from 1000 units to 1400 units. Interpret your result.

1. Step 1: Calculate percentage change in price: %ΔP = (2.40 - 2)/2 * 100 = 20%
2. Step 2: Calculate percentage change in quantity supplied: %ΔQs = (1400 - 1000)/1000 * 100 = 40%
3. $$PES = \frac{40}{20} = 2$$
4. Step 4: Interpretation: PES = 2 > 1, so supply of coffee is elastic: quantity supplied rises twice as fast in percentage terms as price.

> **warning**
>
> Always use original (not average/midpoint) values for percentage change calculations in Edexcel IAL Economics exams, unless the question explicitly specifies the midpoint method.

## PES Determinants & Short Run vs Long Run

The elasticity of supply depends on five core determinants, with the time period being the most heavily tested in exams:

- **Time period**: Short run (at least one fixed factor of production) → PES inelastic; Long run (all factors variable, firms can enter/exit market) → PES elastic.
- **Stock levels & perishability**: Non-perishable goods with high stock levels have elastic PES; perishable goods that cannot be stored have inelastic PES.
- **Factor mobility**: Ease of moving labour, capital and raw materials between uses: higher mobility = more elastic PES.
- **Capacity limits**: If firms are operating at full production capacity, PES is inelastic.
- **Legal constraints**: Government output quotas or licensing rules restrict supply, making PES inelastic.

**Worked example:** Explain why the PES of fresh strawberries is more inelastic in the short run than the long run.

1. Step 1: In the short run, strawberry farmers have fixed quantities of land and pre-planted crops for the growing season. Strawberries are perishable and cannot be stored for long periods.
2. Step 2: If the price of strawberries rises, farmers cannot immediately increase output, as new crops take 3-6 months to grow, so quantity supplied responds very little to price changes, making PES inelastic.
3. Step 3: In the long run, farmers can plant more strawberries, expand land used for growing, and new farmers can enter the market, so quantity supplied can respond much more to price changes, making PES more elastic.

> **Exam tip:** When answering PES explanation questions, always explicitly reference the short run vs long run distinction, as this is a high-weight mark point for Edexcel examiners.

## Common pitfalls

- **Wrong:** Confusing a movement along the supply curve with a shift when the price of the good itself changes.
  - Why it fails: Students mix up 'quantity supplied' (movement) and 'supply' (shift) terminology.
  - Correct: Only non-price factors shift the supply curve; own price changes only cause movements along the existing curve.
- **Wrong:** Drawing a unitary PES curve that does not pass through the (0,0) origin point.
  - Why it fails: Unitary PES requires the ratio of Qs to P to be constant at all points, which only holds if the curve starts at the origin.
  - Correct: Always draw unitary PES curves as straight upward-sloping lines passing through the origin.
- **Wrong:** Including tax incidence or welfare impacts when discussing taxes as a supply shift factor.
  - Why it fails: Tax incidence and welfare analysis are out of scope for this sub-topic, and only covered in later equilibrium units.
  - Correct: When discussing taxes/subsidies here, only explain that they change production costs, leading to a left/right supply shift respectively.
- **Wrong:** Using the midpoint (arc elasticity) method for PES calculations unless explicitly instructed.
  - Why it fails: Edexcel IAL Economics expects the simple percentage change method using original values for all elasticity calculations by default.
  - Correct: Use %Δ = (new - original)/original * 100 for percentage change calculations unless the question specifies the midpoint formula.
- **Wrong:** Labelling supply curve axes incorrectly, e.g. x-axis as price and y-axis as quantity.
  - Why it fails: Up to 2 marks are lost for unlabelled or incorrectly labelled axes in diagram questions.
  - Correct: Always label the y-axis Price (P) and x-axis Quantity (Qs) for all supply curve diagrams.

## Cheatsheet

| Concept | Key Detail | Exam Reminder |
| --- | --- | --- |
| Supply definition | Willingness + ability of producers to sell at given price | State ceteris paribus for full definition marks |
| Supply shift factors | Costs, tech, taxes, subsidies, supply shocks | Never list price as a shift factor |
| PES formula | PES = %ΔQs / %ΔP | Use original values for % change by default |
| PES values | <1 inelastic, =1 unitary, >1 elastic, 0 perfectly inelastic, ∞ perfectly elastic | Match values to correct curve shapes for diagram questions |
| PES determinants | Time period, stock/perishability, factor mobility, capacity, legal rules | Prioritize short run vs long run in explanation answers |

## What's next

Now that you have mastered supply and PES, you are ready to move on to combining supply with demand to analyse market equilibrium, the next core topic in Edexcel IAL Economics Unit 1. You will also apply supply shift factors when studying government intervention (taxes, subsidies, price controls) and market failure in later units. Ensure you can draw all required supply diagrams accurately and explain PES differences in real-world contexts, as these are frequently tested in both multiple-choice and extended response questions. Practice PES calculation questions regularly to avoid arithmetic errors in your exam.

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