Supply: ISC Class 12 Economics Notes on the Law of Supply and Elasticity of Supply

What Supply is about

Weightage: Supply is part of the unit Microeconomic Theory: Demand, Elasticity of Demand, Supply, which carries 16 of the 80 theory marks, shared with 2 other chapters. Questions on it can appear in any section of the paper.Source: CISCE ISC Economics syllabus, examination year 2027, distribution of marks

Key concepts

Supply versus stock (actual versus intended supply)

Stock is the total quantity of a good available with the seller at a point of time: what he could offer (intended or potential supply).

Supply is the part of the stock that the seller is actually willing to offer for sale at a particular price during a particular period (actual supply).

Basis	Stock	Supply
Meaning	Total quantity available	Quantity actually offered at a price
Time	A point of time (stock concept)	A period of time (flow concept)
Price link	Independent of current price	Depends on price
Size	Supply can never exceed stock (for storable goods)	Part of stock

For perishable goods (fish, flowers), supply tends to equal stock because they cannot be held back.

Worked example

Suppose a wheat trader has 800 quintals in his warehouse and sells 300 quintals this week at ₹2,400 per quintal, expecting a higher price next month. Identify stock and supply and give one difference between them. [2 marks]

Try it first, then show the solutionSolution

1. Stock = 800 quintals (total available); supply = 300 quintals (actually offered at ₹2,400 this week).
2. Stock is measured at a point of time; supply over a period of time, at a given price.

Answer: Stock 800 quintals; supply 300 quintals.

Determinants of supply and the supply function

Sx = f(Px, Pr, Pf, T, G, E, N)

• Px: own price: a higher price makes production more profitable.
• Pr: prices of related goods: if wheat becomes more profitable, a farmer shifts land from rice, reducing rice supply.
• Pf: prices of factors (wages, raw materials, power): higher costs reduce supply.
• T: technology: better technology lowers cost and raises supply.
• G: government policy: taxes reduce supply; subsidies increase it.
• E: expectations of future prices: an expected rise may reduce present supply.
• N: number of firms in the market (for market supply).
• Goal of the firm: profit maximisation versus sales maximisation.
• Natural factors for agricultural goods (monsoon).

Worked example

Suppose the government gives a subsidy on solar panels to manufacturers. Which determinant changes and how does the supply curve respond? [2 marks]

Try it first, then show the solutionSolution

1. Government policy (G): the subsidy lowers the cost of production per unit.
2. Producers offer more at every price: the supply curve shifts to the right.

Answer: Increase in supply: rightward shift of the supply curve.

Law of supply; schedule, curve and market supply

Law of supply: other things remaining the same, the quantity supplied of a good rises when its price rises and falls when its price falls.

Reasons: higher profit at higher prices; higher prices cover rising marginal cost; new firms enter when prices rise; firms release stocks when prices are high.

The supply schedule lists quantities offered at different prices; the supply curve slopes upwards from left to right. The market supply curve is the horizontal summation of individual supply curves.

Number of firms: an increase in the number of firms shifts the market supply curve to the right, but the supply curve of an individual firm may shift to the left: more firms compete for the same inputs, factor prices rise and each firm's cost increases.

Worked example

Suppose only two firms supply a product. Derive the market supply schedule and state what the market supply curve shows. [2 marks]

Price (₹)	Firm A (units)	Firm B (units)	Market supply
10	0	5	5
20	10	15	25
30	20	25	45
40	30	35	65

Try it first, then show the solutionSolution

1. Add quantities at each price (horizontal summation): 5, 25, 45, 65.
2. Quantity supplied rises with price: the market supply curve slopes upwards.

Answer: Market supply 5, 25, 45 and 65 units at ₹10, ₹20, ₹30, ₹40.

Movement along and shift of the supply curve

Movement (change in quantity supplied) is caused only by a change in the good's own price:

• Extension: price rises → move up along the curve.
• Contraction: price falls → move down along the curve.

Shift (change in supply) is caused by other determinants:

• Increase in supply (rightward shift): fall in input prices, better technology, subsidy, fall in price of other goods produced with the same resources, more firms.
• Decrease in supply (leftward shift): rise in input prices, a tax, natural calamity, rise in prices of other goods the firm can produce, fewer firms.

A rightward shift means more is offered at the same price (or the same quantity at a lower price).

Worked example

Suppose (i) the price of mangoes rises during summer, and (ii) a cyclone destroys mango orchards. State the effect on the supply curve of mangoes in each case. [2 marks]

Try it first, then show the solutionSolution

1. (i) Own price change: extension, upward movement along the same supply curve.
2. (ii) Natural factor reduces output at every price: leftward shift (decrease in supply).

Answer: (i) Movement (extension); (ii) leftward shift.

Exceptions to the law of supply

• Backward-bending supply curve of labour: at low wages, a higher wage makes a worker (or his family) offer more hours of work. Beyond a certain income level, a further rise in the wage makes him prefer leisure: he offers fewer hours. The supply curve of labour bends backwards above that wage.
• Perishable goods: must be sold quickly even at low prices.
• Rare goods (antiques, old paintings): supply cannot be increased whatever the price.
• Agricultural goods: supply depends on weather and is fixed in the short run.
• Expectation of a further price rise: sellers may hold back stock as prices rise.
• Urgent need for cash: a seller may sell more at a lower price.

The backward-bending curve must be drawn with wage on the Y-axis and hours of labour on the X-axis: upward-sloping at first, then bending back to the left.

Worked example

Suppose a tailor earning ₹8,000 a month works extra hours as rates rise, but once his earnings reach ₹30,000 he cuts his working hours to spend time with his family, even though rates rise further. Why does this not conform to the law of supply? Describe the supply curve. [2 marks]

Try it first, then show the solutionSolution

1. Up to a point, higher wages raise hours supplied (law holds); beyond it, the income effect makes him prefer leisure, so hours fall as the wage rises.
2. Supply curve of labour: upward-sloping, then bending backwards to the left above the critical wage.

Answer: Backward-bending supply curve of labour: an exception to the law of supply.

Elasticity of supply: degrees, percentage and geometric methods

Elasticity of supply (Es) = percentage change in quantity supplied ÷ percentage change in price = (ΔQ ÷ Q) ÷ (ΔP ÷ P). It is positive.

Degrees:

Degree	Es	Supply curve
Perfectly elastic	∞	Horizontal
Relatively elastic	> 1	Straight line meeting the price (Y) axis, i.e. positive price intercept
Unitary	= 1	Straight line through the origin
Relatively inelastic	< 1	Straight line meeting the quantity (X) axis
Perfectly inelastic	0	Vertical

Geometric method: extend a straight-line supply curve to an axis. If it passes through the origin, Es = 1 at every point; if it cuts the price axis (Y-intercept above zero), Es > 1; if it cuts the quantity axis, Es < 1.

Factors: time period (supply more elastic in the long run), nature of the good (perishable inelastic), cost conditions, availability of spare capacity and inputs.

Worked example

Suppose the price of chocolates rises from ₹10 to ₹12 during a festival week and quantity supplied rises from 200 to 260 boxes. Calculate Es and describe the supply curve. [2 marks]

Try it first, then show the solutionSolution

1. %ΔQ = 60 ÷ 200 × 100 = 30%; %ΔP = 2 ÷ 10 × 100 = 20%.
2. Es = 30 ÷ 20 = 1.5 (> 1).

Answer: Es = 1.5: relatively elastic; a straight-line supply curve that meets the price axis above the origin.

Supply in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Topics in this chapter

• Elasticity of Supply: Degrees, Percentage and Geometric Methods

Supply: key facts

Supply function

Sx = f(Px, Pr, Pf, T, G, E, N)

Elasticity of supply

Es = %ΔQs ÷ %ΔP = (ΔQ ÷ ΔP) × (P ÷ Q)

Geometric rule

Straight line through origin: Es = 1; meets price axis: Es > 1; meets quantity axis: Es < 1

Market supply

Sum of individual quantities supplied at each price

How the board asks Supply

The question patterns to expect, and the method that works for each.

Tricks to remember Supply

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Es by intercept: "Y-axis Yes (> 1), X-axis eXtra small (< 1), Origin One"

  A straight-line supply curve cutting the price axis has Es > 1; cutting the quantity axis, Es < 1; through the origin, Es = 1.

  Use it when: Geometric method.

• Supply shifters: "TIGERN"

  Technology, Input prices, Government policy, Expectations, Related goods, Number of firms.

  Use it when: Determinants of supply.

Supply: board-level solved questions

Ordered from board level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section C style)

Read the situation, then answer every part. In the board paper this is a 8-mark question split into 2 + 2 + 2 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Supply

The slips examiners see most often, and what to write instead.

• ✗Mistake: Not explaining the backward bend

  CISCE examiners' comments, 2025: "Could not explain that the law applies only up to a certain point after which supply falls"; "Drew a positively sloped supply curve instead of a backward bending one"; "Left the diagram unlabelled or labelled it wrongly".

  ✓Do this: State both phases (more work, then more leisure) and draw the curve bending back, with axes labelled Wage and Hours of labour.

  Usually costs: 1-2 marks.

• ✗Mistake: Wrong intercept for elastic supply

  CISCE examiners' comments, 2025: "Drew a supply curve passing through the origin instead of one with positive Y-intercept"; "A few could not recognise the degree of elasticity of supply".

  ✓Do this: Es > 1 → cut the price axis; Es = 1 → origin; Es < 1 → cut the quantity axis.

  Usually costs: 1 mark.

• ✗Mistake: Stock and supply without the time dimension

  CISCE examiners' comments, 2024: "Defined them as stock and supply without clear definitions"; "Did not mention the time dimension".

  ✓Do this: Stock: at a point of time; supply: over a period at a price.

  Usually costs: 1 mark.

• ✗Mistake: Off-topic justification

  CISCE examiners' comments, 2024: "Wrote about other factors influencing supply instead of the number of firms"; "Could not justify the leftward shift of the individual supply curve".

  ✓Do this: Answer only the factor asked: more firms → higher input prices → individual supply falls.

  Usually costs: 1 mark.

Practice questions on Supply

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQBoard level

   A vertical supply curve shows elasticity of supply equal to:
   (a) 1
   (b) 0
   (c) ∞
   (d) more than 1

   Hint

   Quantity fixed.

   Answer

   (b) 0

2. Q2 · 1 mark · Very short answerBoard level

   Give one reason why an increase in the price of wheat may reduce the supply of rice.

   Hint

   Related goods.

   Answer

   Farmers shift land and resources to the more profitable wheat, so less rice is offered at every price.

3. Q3 · 2 marks · Short answerBoard level

   Price rises from ₹20 to ₹25 and quantity supplied rises from 500 to 550 units. Calculate Es and state its degree.

   Hint

   Original base.

   Answer

   Es = 10% ÷ 25% = 0.4: relatively inelastic.

4. Q4 · 2 marks · Short answerHarder board level

   Distinguish between extension of supply and increase in supply.

   Hint

   Cause.

   Answer

   Extension: more supplied due to a rise in own price (movement up the curve). Increase: more supplied at the same price due to another factor (rightward shift).

5. Q5 · 1 mark · Very short answerHarder board level

   State the effect of a GST increase on a good on its supply curve.

   Hint

   Tax raises cost.

   Answer

   Supply decreases: the curve shifts to the left.

6. Q6 · 2 marks · Short answerHarder board level

   Describe how to tell the elasticity of a straight-line supply curve that cuts the quantity axis at a positive quantity.

   Hint

   Intercept rule.

   Answer

   If the line cuts the X-axis (quantity), Es < 1 at every point on it.

7. Q7 · 2 marks · Short answerHOTS / competency

   Explain why supply is more elastic in the long run.

   Hint

   Capacity and entry.

   Answer

   In the long run firms can expand plants, new firms can enter and inputs can be reallocated, so quantity can respond more to price.

8. Q8 · 2 marks · Short answerHOTS / competency

   A seller has 500 units in stock and sells 500 units at whatever price. What kind of good might this be, and what is the elasticity of supply?

   Hint

   Cannot be stored.

   Answer

   A perishable good (fresh fish); supply equals stock and is perfectly inelastic in the very short run (Es = 0).

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Law of supply?Show answer

   Other things constant, quantity supplied varies directly with price.

2. 2.Market supply curve is obtained by?Show answer

   Horizontal summation of individual supply curves.

3. 3.Es of a straight line through the origin?Show answer

   One.

4. 4.Main exception to the law of supply?Show answer

   Backward-bending supply curve of labour.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Supply = quantity offered at a price over a period; stock = total available at a point of time.
• ☐Own price → movement; others → shift.
• ☐Tax or input cost up → left shift; subsidy or technology → right shift.
• ☐Es = %ΔQs ÷ %ΔP (positive).
• ☐Intercept rule: Y-axis > 1, origin = 1, X-axis < 1.

Supply: frequently asked questions