Comparative statics: shifts in demand and supply, and equilibrium that keeps moving

Detail

1. Core ideas and definitions

• Equilibrium: the point where the quantity buyers want equals the quantity sellers offer (qᴰ = qˢ).
• Equilibrium price (p*): the price at which this happens.

• Equilibrium quantity (q*): the amount bought and sold at that price.

• Comparative statics: comparing the old equilibrium with the new equilibrium after something shifts a curve.

• It compares two resting points. It does not trace the path or the time taken between them.

• Here the number of firms is fixed. In the free-entry case, entry and exit keep price at minimum average cost, so a demand shift changes mainly quantity.

• Shift of a curve vs movement along a curve:

• A change in the good's own price causes a movement along the curve.

• A change in any other factor (income, tastes, number of buyers, input prices, technology) shifts the whole curve.

• Normal good: a good whose demand rises when income rises, e.g. clothes.

• Marginal cost (MC): the extra cost of producing one more unit. The firm's supply curve is built from its MC. If MC rises, supply shifts left.

2. Effect of a demand shift (supply unchanged)

• What shifts demand only: consumers' income, number of consumers, tastes, and prices of related goods. These do not move the supply curve.
• Rightward shift of demand (e.g. income rises for a normal good like clothes, or more consumers join the market):
• At the old price there is excess demand (buyers want more than sellers offer).
• Buyers compete, so price is bid up.

• Result: P↑ Q↑.

• Leftward shift of demand:

• At the old price there is excess supply (unsold stock).
• Sellers cut prices to clear stock.

• Result: P↓ Q↓.

• Rule: with a demand shift, price and quantity move in the same direction.

3. Effect of a supply shift (demand unchanged)

• What shifts supply only: input prices (wages, raw materials), technology, and the number of firms.
• Leftward shift of supply (e.g. an input price rise raises marginal cost):
• At the old price there is excess demand, so price rises.

• Result: P↑ Q↓.

• Rightward shift of supply (more firms, better technology):

• At the old price there is excess supply, so price falls.

• Result: P↓ Q↑.

• Rule: with a supply shift, price and quantity move in opposite directions.

4. Simultaneous shifts of demand and supply (Class 12, Table 5.1)

Demand	Supply	Quantity	Price
Left	Left	Decreases	Ambiguous
Right	Right	Increases	Ambiguous
Left	Right	Ambiguous	Decreases
Right	Left	Ambiguous	Increases

• Same direction: Q is certain, P is ambiguous.
• Opposite directions: P is certain, Q is ambiguous.
• Ambiguous means the answer depends on which shift is bigger.
• Worked example: both curves shift right. Start with qᴰ = 1,000 − p and qˢ = 700 + 2p, so p* = 100 and q* = 900.
• Case A: demand moves to 1,300 − p and supply to 1,000 + 2p. Then 300 = 3p, so p = 100, q = 1,200. Price is unchanged and quantity rises.
• Case B: demand moves to 1,300 − p but supply only to 850 + 2p. Then 450 = 3p, so p = 150, q = 1,150. Price rises because demand shifted more.
• In both cases Q rose, which is certain. P depended on the size of each shift, which is the ambiguous part.

5. Exercise drills (Class 12)

• Complements are goods used together, like shoes and socks.

• Shoes become dearer → fewer shoes are bought → demand for socks shifts left → socks: P↓ Q↓.

• Substitutes are goods used in place of each other, like tea and coffee.

• Coffee becomes dearer → people switch to tea → demand for tea shifts right → tea: P↑ Q↑.

• Salt market (numerical): qᴰ = 1,000 − p and qˢ = 700 + 2p.

• 1,000 − p = 700 + 2p → 300 = 3p → p* = 100, q* = 900.

• Input-cost rise: supply becomes qˢ = 400 + 2p.

• 1,000 − p = 400 + 2p → 600 = 3p → p* = 200, q* = 800.

• Supply shifted left, so P↑ Q↓, as the rule predicts.

• ₹3 per-unit tax on sellers: a per-unit (specific) tax is a fixed rupee amount charged on each unit sold.

• The seller keeps only the net price (p − 3), so supply becomes qˢ = 700 + 2(p − 3) = 694 + 2p.
• 1,000 − p = 694 + 2p → 306 = 3p → consumer price = 102, producer net price = 99, q = 898.
• Tax incidence means who actually bears the tax. Buyers pay ₹2 more (100 → 102). Sellers get ₹1 less (100 → 99).
• Rule: the side that responds less to price carries more of the tax.
  • Buyers here are less price-responsive: demand changes by 1 unit per ₹1 of price.
  • Sellers are more responsive: supply changes by 2 units per ₹1.
  • So buyers bear ₹2 and sellers bear ₹1, a 2:1 split. Full incidence detail is in the taxation notes.

6. Real-world equilibrium is never static (Class 9, The Price Puzzle)

• The curves keep shifting because of technology, wages, interest rates, wars, pandemics and weather.
• So the market is always moving towards a new equilibrium. It rarely rests at one point.
• COVID-19 masks (2020):
• Demand jumped suddenly (demand shifted far right). Supply could not grow at once, so prices rose sharply.
• New producers entered and supply caught up (supply shifted right), so prices fell.
• After the pandemic, demand fell back and prices returned to pre-pandemic levels.
• Lesson: supply adjusts more slowly than demand in the short run, so price spikes are sharpest early on.

7. Government action that works through shifts (Indian price policy)

• Onion buffer under the Price Stabilisation Fund (PSF): the PSF is a central fund used to step into the market when prices of essential items rise [6][8].
• Onion buffer size: 1.00 lakh tonnes (2020-21) → 2.50 LT (2022-23) → 7 LT (2023-24) → 4.75 LT (2024-25) [6].
• In 2024, 4.7 lakh tonnes of rabi onion was bought for the buffer through NCCF and NAFED, compared with 3.0 lakh tonnes the year before [7].
• Releasing buffer stock during a shortage shifts market supply to the right, so price falls. Mobile vans sold onion at ₹35/kg (Sept 2024) [7].
• The Department of Consumer Affairs tracks daily prices of 38 commodities from 550 centres. This data decides how much onion to release and where [6].
• Under the PSF, States get an interest-free working capital advance shared 50:50 with the Centre to set up State-level PSFs. 7 States have used it, including Andhra Pradesh, Telangana, West Bengal, Odisha, Tamil Nadu, Assam and Nagaland [8].

8. Dynamic pricing

• Dynamic pricing: prices change often, sometimes within hours, as demand, season and events change. It is a real-time form of comparative statics.
• Goa hotel example (100 rooms):
• ₹1,500 on an off-season weekday (a Monday in July); ₹8,000 on a December Saturday; ₹25,000 on New Year's Eve.
• It may cut prices 40% overnight if a group booking is cancelled, because supply of free rooms suddenly rises.

• Tariffs may change several times a day.

• What drives the tariff: how fast rooms are being booked, rivals' rates, festivals and conferences, weather forecasts, days left before arrival, and past booking trends.

• Airline fares: they rise as the flight date comes closer and seats fill up.
• Ride-hailing surge pricing: the fare goes up when many riders want cabs and few drivers are free.
• Motor Vehicle Aggregator Guidelines, 2020 were issued by MoRTH on 27 November 2020 under the Motor Vehicles (Amendment) Act, 2019 [2].
  • They allowed fares down to 50% below the base fare and capped surge at 1.5× the base fare [2].
  • The driver gets at least 80% of the fare on each ride [2].

• Motor Vehicle Aggregator Guidelines 2025 raise the surge cap to 2× the base fare (NCERT scaffold; verify the current rule).

• Time-of-Day (ToD) electricity tariff: the price of electricity changes with the hour of the day.

• It was brought in by the Electricity (Rights of Consumers) Amendment Rules, 2023, notified on 14 June 2023 [3].
• Solar hours are 8 hours a day, set by the State Electricity Regulatory Commission. Tariff in these hours is at least 20% below the normal tariff [3][4].
• Peak hours: at least 1.20× the normal tariff for commercial and industrial consumers, and at least 1.10× for others [3].
• Roll-out: commercial and industrial consumers with maximum demand above 10 kW from 1 April 2024. Most other consumers (not agricultural ones) from 1 April 2025 [3][4].

• Logic: higher peak prices move some demand to cheaper solar hours. This flattens the peak and makes better use of solar power [5].

• Seasonal and perishable pricing (Class 7):

• Onion prices rise when supply dips, for example in the lean season or after crop damage.
• Woollens are discounted at the end of winter because demand shifts left.
• Vegetables are cheaper late at night because sellers must clear perishable stock. Their supply at that hour does not respond to price.

Prelims Hooks

• Demand shift alone: P and Q move in the same direction. Supply shift alone: P and Q move in opposite directions.
• Both curves shift in the same direction: Q is certain and P is ambiguous. Opposite directions: P is certain and Q is ambiguous.
• A rise in consumer income moves the demand curve only. An input-price rise or a new technology moves the supply curve only.
• Coffee price rises → tea (a substitute) sees P↑ Q↑. Shoe price rises → socks (a complement) see P↓ Q↓.
• Salt: qᴰ = 1,000 − p and qˢ = 700 + 2p give p* = 100 and q* = 900. With a ₹3 per-unit tax, buyers pay ₹102, sellers keep ₹99, and q = 898.
• Tax incidence: the less price-responsive side bears the larger share of a per-unit tax.
• ToD tariff (2023 Rules): solar-hour tariff at least 20% below normal. Peak tariff at least 1.2× for commercial/industrial users and at least 1.1× for others [3].
• Surge cap: 1.5× base fare under the 2020 Aggregator Guidelines [2]; 2× under the 2025 Guidelines (NCERT scaffold).
• The onion buffer is kept under the Price Stabilisation Fund, which is run by the Department of Consumer Affairs. Procurement is done through NAFED/NCCF [6][7].

Mains Points

• Supply-side shocks call for supply-side tools.
• Food inflation in India often comes from supply shifting left (bad monsoon, spoilage, crop damage).
• Buffer releases under the PSF shift supply right and cut price and quantity volatility without shrinking demand [6][7].

• Raising interest rates works on demand. It is a blunt tool against this kind of inflation.

• Dynamic pricing: efficiency vs fairness.

• Surge fares and ToD tariffs help clear the market and move demand to off-peak hours [3][5].
• But they can hurt consumers during emergencies.

• India's answer is regulated flexibility: surge caps for cabs [2] and fixed floors and ceilings for ToD tariffs [3].

• Tax design and incidence.

• A per-unit tax on a good with inelastic demand (demand that barely changes with price), such as salt, fuel or essential medicines, falls mostly on consumers.

• This matters for the equity of GST rates and excise duties.

• Moving equilibrium and policy timing.

• Supply catches up with a demand surge only after a lag, as with masks in 2020.
• Hasty price controls or export bans can discourage the very supply response that would bring prices down on its own.
• Help should be temporary and targeted.