Supply: Meaning, Determinants, Law & Curve Movements vs. Shifts
Understand producer behavior in market supply. Learn the crucial distinction between supply and actual sales, analyze supply functions and determinants, derive market supply via horizontal summation, and master movements along the curve (expansion/contraction) vs. shifts (increase/decrease).
1. Meaning of Supply & Supply vs. Actual Sales
Supply of a commodity by a firm or seller is defined as the quantity of a commodity that a firm or seller is willing to offer for sale at a given price during a given period of time.
Supply vs. Actual Sale (Textbook Example)
Supply refers to willingness to sell, whereas actual sale depends on market transactions.
Example: A farmer is willing to sell 50 quintals of wheat at ₹ 15 per kg (Supply = 50 quintals). However, buyers purchase only 30 quintals at this price (Actual Sale = 30 quintals). Supply is 50 quintals, but actual sale is 30 quintals.
The specific quantity of the commodity the firm is willing to offer for sale.
The specific price per unit at which the firm offers that quantity.
The specific time frame (per day, week, month, year) during which supply is offered.
2. Determinants of Supply & The Supply Function
The willingness of a seller to offer goods depends on profitability. Key determinants include:
Direct relationship. Higher price increases profit margins, inducing producers to supply more. Lower price reduces supply.
If the price of pulses rises relative to wheat, a farmer diverts land to pulses, increasing pulse supply and decreasing wheat supply at unchanged wheat prices.
Rise in wages or raw material costs (e.g. cotton for cloth) increases per-unit cost, reducing profit margin and decreasing supply at the same price.
Improved technology reduces per-unit cost of production, raising profit margins and expanding supply at unchanged prices.
Lower excise duty reduces production cost, increasing supply. Higher excise duty (e.g. on cigarettes/liquor) increases cost and reduces supply.
If goal is profit maximization, supply increases only at higher prices. If goal is sales maximization (capturing market share), supply increases even at lower prices.
Supply Function Formula:
Textbook Example: qₛ = -15 + 3P. The positive coefficient (+3) indicates that for every ₹ 1 increase in price, quantity supplied increases by 3 units. Supply is zero when P ≤ ₹ 5.
3. Law of Supply & Market Supply (Horizontal Summation)
Law of Supply: Other factors determining supply remaining constant (ceteris paribus), there is a direct relationship between the price of a commodity and its quantity supplied.
Textbook Schedule: Market Supply of Sugar (₹ per kg)
| Price per kg (₹) | Firm A Supply (kg) | Firm B Supply (kg) | Firm C Supply (kg) | Market Supply (A + B + C) (kg) |
|---|---|---|---|---|
| 25 | 100 | 200 | 0 | 300 |
| 30 | 200 | 300 | 100 | 600 |
| 35 | 300 | 400 | 200 | 900 |
| 40 | 400 | 500 | 300 | 1200 |
| 45 | 500 | 600 | 400 | 1500 |
4. Movement Along Supply Curve vs. Shift in Supply Curve
Movement Along Supply Curve (Change in Qₛ)
Occurs strictly due to a change in the price of the commodity, while non-price factors remain constant.
- Expansion of Supply: Price rises → Qₛ increases → Upward movement along same curve (A to B).
- Contraction of Supply: Price falls → Qₛ decreases → Downward movement along same curve (A to C).
Shift of Supply Curve (Change in Supply)
Occurs due to changes in non-price factors (technology, input costs, taxes, related goods) at the same price.
- Increase in Supply: Better technology / lower input cost → Rightward shift of supply curve (S to S₁).
- Decrease in Supply: Higher excise tax / higher input cost → Leftward shift of supply curve (S to S₂).