Supply
What is Supply?
Supply refers to the quantity of a good or service that producers are willing and able to sell at various prices during a specific time period. It is not just about what they can make, but what they are willing to bring to the market.
Building on the concept of demand (which looks at buyers), supply looks at the seller's side of the market. The fundamental relationship is that, ceteris paribus (all other factors remaining constant), as the price of a good rises, producers are willing to supply more of it. This creates an upward-sloping supply curve.
Extension and Contraction of Supply
It is crucial to distinguish between movements along the curve and shifts of the curve. These specific terms describe changes in quantity supplied due solely to price changes.
| Cause |
|---|
| Caused by an increase in the price of the good. |
| Caused by a decrease in the price of the good. |
Individual vs. Market Supply
Individual supply is the quantity supplied by a single firm at various prices. Market supply is the total quantity supplied by all firms in the industry at various prices.
To find market supply, we perform horizontal summation. This means adding up the quantities supplied by each individual firm at every specific price point. We sum along the horizontal axis (quantity), not the vertical axis (price).
| Market Supply (A + B) |
|---|
| 15 units |
| 30 units |
| 45 units |
Notice that at 20, Firm A supplies 10 and Firm B supplies 20. The market supply is not30 (price) or the average; it is the sum of quantities: 10 + 20 = 30 units.
Movements Along the Supply Curve
A movement along the supply curve occurs only when the price of the good itself changes. This is called a change in 'quantity supplied'.
Why does this happen? Higher prices increase potential profit margins, incentivizing firms to produce more (extension). Lower prices reduce profit margins, causing firms to cut back production (contraction).
⚠︎ Confusing Movements with Shifts
The Error: Students often think that if production costs rise, the supply curve shifts left. While true for cost changes, students frequently confuse this with a movement along the curve when the price of the good changes.
The Correction:
- If the price of the good changes → Move along the existing supply curve.
- If a non-price determinant (like cost, technology, or tax) changes → Shift the entire supply curve.
Shifts in Supply (Non-Price Determinants)
A shift in the supply curve means that at every price, producers are willing to supply a different quantity. This is caused by factors other than the current price of the good.
| Reasoning |
|---|
| Higher costs reduce profit margins at the same price, so firms produce less. |
| Better technology lowers production costs and increases efficiency. |
| Taxes act like an extra cost. Subsidies act like a negative cost (income). |
| More producers mean more total output at every price. |
| Directly affects the ability to produce the good. |
Describing Shifts Correctly
When to use: When asked to explain a change in supply due to non-price factors (e.g., 'Explain two reasons why the supply of tea may increase').
Why examiners accept this: Examiners look for the mechanism. Don't just say 'supply increases.' You must link the factor to the cost or incentive.
Correct Phrasing Example: 'A fall in the price of fertiliser reduces the cost of production, increasing profit margins and incentivizing farmers to supply more tea at every price level, shifting the supply curve to the right.'
Past Paper Style Questions
Q:
Identify what brings about a movement upwards along a supply curve for rice. [1]
A:
An increase in the price of rice.
Q:
Explain two causes of an increase in supply. [4]
A:
- Lower costs of production (e.g., cheaper raw materials or wages) which increases profit margins.
- Improvement in technology which increases productivity and lowers unit costs.
(Note: Each point needs the cause + the explanation of why it shifts supply).
Q:
Draw a demand and supply diagram showing the effect of lower energy costs on the processed food market. [4]
A:
- Axes labelled Price (vertical) and Quantity (horizontal).
- Original Demand (D) and Supply (S1) curves drawn correctly.
- New Supply curve (S2) shifted to the right of S1.
- Equilibrium points marked showing a fall in Price and an increase in Quantity.
Q:
What would NOT cause a shift in the supply curve? [1]
A:
A change in the price of the good itself (this causes a movement along, not a shift).