Supply
The law of supply, its determinants, and how to measure the elasticity of supply.
Definition
Supply is the quantity of a good that a producer is willing and able to offer for sale at a given price during a given period of time.
Simple Explanation
Supply is the seller's side of the market — how much of something producers are ready to put up for sale at each possible price.
Detailed Explanation
The Law of Supply states that, other things remaining constant, the quantity supplied of a good rises as its price rises, and falls as its price falls — giving the supply curve its characteristic upward slope. The reasoning is straightforward: a higher price makes production more profitable at the margin, encouraging existing producers to supply more and tempting new producers to enter.
Supply is shaped by the good's own price, the cost of production (cheaper inputs make a given price more profitable, raising supply), the state of technology (an improvement lets the same inputs produce more, raising supply), government policy (taxes discourage supply, subsidies encourage it), and the prices of related goods — a rise in the price of one good can pull resources away from a 'competitively supplied' good produced with the same resources, or raise supply of a 'jointly supplied' good produced together with it (e.g. mutton and wool from sheep).
Like demand, supply exists at the individual level (one firm's supply schedule) and the market level (market supply = horizontal summation of every individual firm's quantity supplied at each price).
Price Elasticity of Supply (Es) measures how responsive quantity supplied is to a price change, calculated the same way as price elasticity of demand but for the supply side: Es = (% change in quantity supplied) / (% change in price). Since the supply curve normally slopes upward, Es is conventionally positive.
A simple geometric shortcut works for any straight-line supply curve: extend the line back until it meets an axis. If the line, extended, passes through the origin, supply is unitary elastic (Es = 1) at every point on it. If extended, it cuts the price (Y) axis, supply is elastic (Es > 1) at every point. If extended, it cuts the quantity (X) axis, supply is inelastic (Es < 1) at every point. This works because the ratio of intercept distances mirrors the percentage-change ratio used in the formula.
Real-life Examples
Rising mandi prices, rising supply
When the market price of tomatoes rises sharply, farmers bring more of their stock to the mandi and even delay other crops to grow more tomatoes next season — supply responding positively to price.
Joint supply
A rise in the price of mutton encourages farmers to keep more sheep, which simultaneously raises the supply of wool — the two goods are jointly supplied from the same source.
Step-by-step Graph
- 1
Draw the axes
Plot Price on the Y-axis and Quantity Supplied on the X-axis.
- 2
Plot the supply schedule
Mark each price–quantity combination from the supply schedule as a point.
- 3
Draw the supply curve
Join the points to get an upward-sloping curve (SS), reflecting the law of supply.
- 4
Apply the elasticity shortcut
Extend the straight-line SS curve to an axis: through the origin means unitary elastic, cutting the price axis means elastic, cutting the quantity axis means inelastic.
Formulas
Price Elasticity of Supply
Es = (% change in Qs) / (% change in Price)
Measures responsiveness of quantity supplied to a price change.
Tables
| Determinant | Effect of a favourable change |
|---|---|
| Cost of production | Lower cost raises supply at every price |
| Technology | Improved technology raises supply |
| Government tax/subsidy | Subsidy raises supply; tax lowers supply |
| Price of related (jointly supplied) goods | Higher price of the jointly supplied good raises supply of this good |
| Extended line meets... | Elasticity |
|---|---|
| The origin | Unitary elastic (Es = 1) |
| The price (Y) axis | Elastic (Es > 1) |
| The quantity (X) axis | Inelastic (Es < 1) |
Common Mistakes
- Forgetting that a change in the good's own price moves you along the supply curve, while every other factor shifts it entirely.
- Mixing up competitive supply (resources pulled away towards a more profitable alternative) with joint supply (two goods produced together from the same source).
- Assuming all straight-line supply curves through the origin have different elasticities at different points — on a straight line through the origin, elasticity is exactly 1 everywhere.
Exam Tips
- State 'other things remaining constant' explicitly when writing the law of supply, exactly as for the law of demand.
- For elasticity-of-supply diagrams, always extend the line with a dashed segment to show clearly which axis it meets.
- List determinants of supply separately from determinants of demand in comparison-style answers — examiners penalise mixing the two up.
Summary
- The law of supply gives a direct (positive) price–quantity relationship, other things constant.
- Cost of production, technology, government policy, and related-goods prices all shift the supply curve.
- Market supply is the horizontal summation of individual firms' supply.
- Elasticity of supply measures the responsiveness of quantity supplied to price.
- A straight-line supply curve's elasticity can be read off by checking which axis its extension meets.
Practice MCQs
1. The law of supply describes a relationship between price and quantity supplied that is:
2. A fall in the cost of production, with price unchanged, causes:
3. Mutton and wool are an example of:
4. If a straight-line supply curve, when extended, passes through the origin, its price elasticity of supply is:
5. Market supply at a given price is obtained by:
6. An improvement in production technology, other things constant, will: