Price elasticity of supply

Detail

1. What price elasticity of supply means

• Price elasticity of supply (eS) shows how much the quantity sellers offer changes when the price changes.
• Formula:
• eS = %ΔQs / %ΔP
• eS = (ΔQ/Q) × (P/ΔP), which is the same as (ΔQ/ΔP) × (P/Q)

• Here ΔQ is the change in quantity supplied, ΔP is the change in price, and P, Q are the starting price and quantity. NCERT uses the starting values as the base.

• How to read the number:

• The measure is a ratio of two percentage changes (%Δy/%Δx).

• If it is greater than 1, supply is elastic. If it is less than 1, supply is inelastic. If it is exactly 1, supply is unit elastic [2].

• Sign:

• The law of supply says higher price → more quantity supplied.
• So price and quantity move in the same direction, and eS is positive.
• Price elasticity of demand is usually negative. This sign difference is a common MCQ trap.

2. NCERT cricket-ball example

• Price: rises from ₹10 to ₹30.

• %ΔP = (30 − 10)/10 × 100 = 200%

• Output: rises from 200 to 1,000 balls.

• %ΔQ = (1000 − 200)/200 × 100 = 400%

• eS = 400/200 = 2, so supply is elastic.

• Meaning: every 1% rise in price brings a 2% rise in output.

3. Properties of eS

Shape of supply curve	eS	Meaning
Vertical	0 (perfectly inelastic)	Quantity does not change at any price. Example: fish already brought to the market that morning.
Horizontal (beyond NCERT)	∞ (perfectly elastic)	Firms supply any amount at one price. Even a small price fall makes supply drop to zero.
Upward-sloping	> 0	Higher price brings more output.

• Unit-free: eS is a ratio of two percentages, so the units cancel out.
• Quantity in kg or quintals, and price in rupees or paise, give the same eS.
• Price elasticity of demand is unit-free for the same reason.
• The slope (ΔP/ΔQ) is different. It changes when you change units, so slope and elasticity are not the same thing.

4. Geometric measure of elasticity (straight-line supply curve)

• Formula: at any point S on a straight-line supply curve, eS = Mq₀ / Oq₀
• q₀ is the quantity at point S.
• M is where the line meets the quantity axis, after extending it if needed.

• O is the origin.

• Why it works:

• The slope of the line is ΔP/ΔQ = Sq₀/Mq₀. So ΔQ/ΔP = Mq₀/Sq₀.
• At point S, P = Sq₀ and Q = Oq₀.
• So eS = (Mq₀/Sq₀) × (Sq₀/Oq₀) = Mq₀/Oq₀.

Straight-line supply curve	Position of M	eS	Worked example
Cuts the price axis (panel a)	In the negative quantity range, so Mq₀ > Oq₀	> 1 at every point	Qs = −10 + 2P. At P = 10, Q = 10 and M = −10, so Mq₀ = 20 and Oq₀ = 10. eS = 2. Check: 2 × 10/10 = 2 ✓
Passes through the origin (panel b)	M = O, so Mq₀ = Oq₀	= 1, whatever the slope	Qs = 3P or Qs = 0.5P: eS = 1 at every point
Cuts the quantity axis (panel c)	In the positive quantity range, so Mq₀ < Oq₀	< 1 at every point	Qs = 10 + P. At P = 10, Q = 20 and M = 10, so Mq₀ = 10 and Oq₀ = 20. eS = 0.5. Check: 1 × 10/20 = 0.5 ✓

• Trap: steepness does not decide eS here.
• A very steep line and a very flat line through the origin both have eS = 1.

• What decides eS is where the line meets the axes, not its slope.

• eS changes along the line:

• In panel (a), eS stays above 1 but falls towards 1 as output grows.
• In panel (c), eS stays below 1 but rises towards 1 as output grows.

5. NCERT exercise patterns (solved)

• Exercise 25: find eS from total revenue (TR)
• Quantity = TR ÷ price.
• At ₹10, TR = ₹50, so Q = 5. At ₹15, TR = ₹150, so Q = 10.
• %ΔQ = (10 − 5)/5 = 100%. %ΔP = (15 − 10)/10 = 50%.

• eS = 100/50 = 2.

• Exercise 26: find the original quantity

• Price ₹5 → ₹20, so %ΔP = 300%.
• With eS = 0.5, %ΔQ = 0.5 × 300 = 150%.

• ΔQ = 15 = 1.5 × Q₁, so Q₁ = 10 and Q₂ = 25.

• Exercise 27: find the new quantity

• Price ₹10 → ₹30, so %ΔP = 200%.
• With eS = 1.25, %ΔQ = 1.25 × 200 = 250%.

• ΔQ = 2.5 × 4 = 10, so the new quantity is 4 + 10 = 14.

• Exam method: first write %ΔP, then %ΔQ = eS × %ΔP, then ΔQ = %ΔQ × Q₁.

6. Determinants of supply elasticity (beyond NCERT)

• Time period (Marshall's three periods). This is the most important factor.
• Market period: the stock is already fixed. The supply curve is vertical and eS = 0.
• Short run: firms can change variable inputs such as labour and raw material, but not plant size. Supply is more elastic.

• Long run: firms can change plant size, and new firms can enter or old ones leave. Supply is most elastic.

• Spare capacity. Idle machines and workers let output rise quickly, so supply is more elastic.

• Storability and perishability.
• Storable goods such as grain, onion and steel can be held back or released as prices change, so supply is more elastic.

• Perishable goods such as tomato, milk and flowers must be sold quickly, so supply is less elastic.

• Gestation lags. Some output takes years to expand, so short-run eS is near 0.

• Examples: tea and rubber plantations, power plants, mines.

• Factor mobility. When land, labour and machines can easily move from other uses, supply is more elastic.

7. Indian relevance: farm supply, price swings and policy

• Farm supply is inelastic in the short run.
• Farmers decide their sowing a season ahead.

• Once the crop is in the ground, output cannot rise when prices rise.

• Result: shocks show up in price, not quantity.

• Demand rises or weather ruins the crop → supply cannot respond → prices spike.

• This explains the price swings in tomato, onion and potato (TOP).

• Seasonality matters more than overall shortage. For TOP crops, the Union Government says seasonal arrival of these highly perishable crops is a major cause of their price swings, more than any overall gap between demand and supply [3].

• Cobweb cycle (linked to market-equilibrium-price-controls). A cobweb cycle is a repeating boom and bust in farm prices caused by the time lag between sowing and harvest.
• High price this year → farmers sow more → a glut next year → the price crashes.

• Low price → farmers sow less → shortage → the price rises again.

• Elastic supply dampens price shocks.

• When supply can respond, extra demand is met by extra output, so prices rise less.

• This is the case for cold chains, storage and faster capacity creation.

• Policies that make supply more elastic:

• Market Intelligence and Early Warning System (MIEWS): a portal that tracks TOP prices, run through NAFED (National Agricultural Cooperative Marketing Federation of India). It sends low-price alerts to states [3][4].
• Operation Greens: added to PMKSY (Pradhan Mantri Kisan SAMPADA Yojana) in November 2018 with ₹500 crore to stabilise TOP prices [5][6]. It works in two ways:
  • Short term: a 50% subsidy on transport and storage [5][6].
  • Long term: grants of 35–70% of project cost for value-addition projects in production clusters [5][6].
• Expanded Operation Greens (2021-22): the scheme was widened to 22 perishable products, including mango, banana, apple, green peas, garlic, ginger and shrimp [5][6].
• Price stabilisation buffer: the Centre bought 3 lakh tonnes of onion in 2025, with targeted release planned from September 2025 [7]. Releasing stock from a buffer works like extra elastic supply in a lean season.

Prelims Hooks

• eS = %ΔQs / %ΔP = (ΔQ/ΔP) × (P/Q). It is positive for an upward-sloping supply curve.
• Vertical supply curve → eS = 0. Horizontal supply curve → eS = ∞.
• Any straight-line supply curve through the origin has eS = 1, whatever its slope. This is the classic "steepness" trap.
• A straight line that cuts the price axis gives eS > 1. One that cuts the quantity axis gives eS < 1.
• Geometric measure: eS = Mq₀/Oq₀.
• eS is unit-free. The slope of the curve is not.
• Marshall's market period gives perfectly inelastic supply (eS = 0). The long run gives the most elastic supply.
• Perishable and long-gestation goods (tomato, plantation crops) have low short-run eS. Storable goods have higher eS.
• Operation Greens began in 2018-19 under PMKSY for TOP crops and was expanded to 22 perishables in 2021-22 [5][6].
• MIEWS tracks TOP crop prices through NAFED [3][4].

Mains Points

• Food inflation is mostly a supply-side problem.
• Farm supply is inelastic in the short run, so weather and seasonal shocks hit prices hard.

• Raising the repo rate does little to fix this. Cold chains, storage and processing are needed to make supply more elastic.

• Buffers turn fixed supply into flexible supply.

• Stocks such as the 3 lakh tonne onion buffer (2025) [7], together with Operation Greens storage subsidies [5], let the government release supply when prices spike.

• Trade-offs: fiscal cost, storage losses, and possible harm to farmers' price gains.

• The cobweb cycle hurts both farmers and consumers.

• Farmers face a glut and low prices one year, then consumers face shortage and high prices the next.

• Remedies: price forecasts (MIEWS [4]), crop planning advisories, contract farming and futures markets. These help farmers base sowing on expected prices, not last season's prices.

• Low capacity makes supply inelastic outside farming too.

• In power, mining and housing, long gestation keeps supply inelastic, so demand growth turns into price rises.
• This supports faster clearances and infrastructure investment to raise long-run elasticity.