Supply and Price Determination

The supply curve shows the quantity firms are willing and able to sell at each price, over a given period. Like demand, it separates cleanly into two kinds of change. A rise or fall in the good's OWN price causes a movement along a fixed curve — a higher price makes previously-unprofitable units worth producing, so more gets supplied without anything about the underlying conditions of production changing. Everything else that affects supply shifts the entire curve to a new position, because it changes how much firms are willing to supply at EVERY price, not just the current one.

The spec names five shift factors. Costs of production rising (more expensive raw materials, higher wages) shifts supply LEFT — the same output now costs more to make, so less is worth producing at any given price. New technology cuts the cost of producing each unit, shifting supply RIGHT. Indirect taxes raise the effective cost of supplying each unit, shifting supply LEFT — but the two types of indirect tax raise that cost differently, and the difference is mechanical, not decorative: a specific tax adds a fixed £ amount that doesn't care what the pre-tax price is, so it adds the same constant £ amount at every output — a parallel shift. An ad valorem tax adds a fixed PERCENTAGE of the pre-tax price instead, so the £ amount it adds is bigger wherever the pre-tax price is already bigger — and since price rises as you move up a normal supply curve, that means the wedge itself widens the further up the curve you go, a pivot rather than a shift, exactly as the diagram below plots with real numbers. Government subsidies are the exact mirror image of a tax — a payment per unit that lowers the effective cost, shifting supply RIGHT. Natural disasters can destroy productive capacity outright, shifting supply sharply LEFT, sometimes with no gradual warning at all.

Price elasticity of supply (PES) measures how responsive quantity supplied is to a change in price: PES = %ΔQs ÷ %ΔP. Because the supply curve slopes upward — a higher price always calls forth more supply, never less — %ΔQs and %ΔP always move in the SAME direction, so PES comes out positive by construction. That's a genuine, derivable contrast with price elasticity of demand, which needs a negative-sign convention specifically because a downward-sloping demand curve moves %ΔQd and %ΔP in OPPOSITE directions. Supply's upward slope removes the need for that correction entirely — there's nothing to correct for. Worked example: price rises from £8 to £10 (a 25% rise) and quantity supplied rises from 120 to 144 units (a 20% rise); PES = 20 ÷ 25 = 0.8 — inelastic, since it's below 1, and correctly positive with no sign attached.

The five-point scale: PES = 0 is perfectly inelastic (a vertical supply curve — no amount of price change calls forth any more output at all); 0 < PES < 1 is inelastic; PES = 1 is unit elastic; PES > 1 is elastic; PES → ∞ is perfectly elastic (a horizontal supply curve — any amount can be supplied at one fixed price, none below it).

The spec names five determinants. Time period is the dominant one, developed in the mechanism below. Availability of stock and perishability: a firm sitting on unsold stock of a non-perishable good can release extra supply almost immediately when price rises, without producing a single new unit — a genuinely different situation from a fresh, perishable good that has to be grown or made before it can be sold at all. Mobility of factors of production: how easily labour and capital already used elsewhere can be redirected INTO this industry — specialist semiconductor fabrication capacity, confirmed in an examiner report as leaving manufacturers 'struggling to respond to the increase in demand' during the 2021-22 chip shortage, is a real example of a market where factor immobility keeps PES low even when the financial incentive to expand is enormous. Legal constraints — licensing, planning permission, patents — can cap how fast supply expands regardless of price. Capacity: how much spare productive capacity currently sits unused; a firm already running its factories flat out has close to zero PES until new capacity is actually built, however high the price rises.

Nuclear power is the clearest confirmed example of how long "the short run" can genuinely last: reactor construction takes 7.5 years on average, and one Argentinian reactor took 33 years — no price signal, however large, buys its way past a physical construction timeline like that.

A stylised illustration of the same mechanism in numbers: the same 20% price rise might call forth only a 5% rise in quantity supplied in the short run (PES = 0.25, sharply inelastic) but a 35% rise once the long run has played out (PES = 1.75, elastic) — the price signal is identical both times; only the time available to respond to it differs.

Equilibrium is the single price where quantity demanded exactly equals quantity supplied — the only price at which neither buyers nor sellers have any reason to push it further. A shift of demand or supply moves that point in a predictable direction: a rightward demand shift raises both equilibrium price and quantity; a leftward demand shift lowers both; a rightward supply shift lowers price and raises quantity; a leftward supply shift raises price and lowers quantity. Where both curves shift at once, the direction of the price change and the direction of the quantity change can be pinned down independently, and one of the two directions is sometimes genuinely indeterminate without knowing the RELATIVE size of the two shifts — a distinction worth stating explicitly rather than guessing at a combined outcome the diagram alone can't actually show.

Consumer surplus is the difference between the maximum price a consumer was willing to pay for each unit and the price they actually had to pay, summed across every unit bought — not the price itself, and not the total amount spent. Examiners have confirmed this specific gap in wording costs the mark, independently, three separate times. The identical discipline applies to producer surplus: it is the difference between the price a producer actually received and the minimum price they would have been willing to accept, summed across every unit sold — not simply 'profit,' and not the revenue itself.

How a shift changes each: a rightward shift of supply caused by a genuine fall in production costs (new technology, for example), with demand unchanged, has an unambiguous effect on consumer surplus specifically — price falls and quantity rises, and both of those effects independently widen the region between the (unchanged) demand curve and the new, lower price, so consumer surplus rises, for any downward-sloping demand curve, regardless of exactly how big the shift is. Producer surplus rises too in this case, and for a reason worth deriving rather than assuming: producer surplus isn't measured against the OLD cost curve once costs have genuinely fallen, it's measured against the NEW one — so even though producers receive a lower price than before, the minimum they'd have been willing to accept fell by more than the price did, at the new equilibrium quantity. A cost-reducing supply increase is not, in general, a transfer from one side of the market to the other — both sides can genuinely gain from the same shift at once.

The price mechanism is the process by which price alone — with no central planner — allocates a scarce good and coordinates the independent decisions of buyers and sellers. The spec names three distinct functions it performs simultaneously, and mistaking one for another is a confirmed, repeated MCQ trap. The rationing function: a higher price rations a limited supply toward the buyers who value it most, and induces buyers as a group to cut back consumption when a good becomes scarce — a demand-side response to price. The incentive function: a higher price rewards producers already in the market with a direct financial reason to expand output, since the extra output is now worth more revenue than before. The signalling function: a price change tells resources OUTSIDE the current market — capital, entrepreneurs, workers deciding which industry to join — where they're needed, without anyone issuing an instruction.

These three functions operate identically whether a market is local (a single town's fresh produce stall), national (the UK's electricity market), or global (crude oil, semiconductors) — what changes across that scale is how fast and how completely the price signal reaches every affected party, not whether the same three functions are happening. A local shortage rations, incentivises and signals within days; the global semiconductor shortage — where manufacturers were confirmed, in an Oct 2022 examiner report, to be 'struggling to respond to the increase in demand' — took years to fully work through, precisely because building new fabrication capacity is a long-run response, not a short-run one, the same time-based logic that governs PES above.

An indirect tax is a tax on spending, collected from the seller at the point of sale — contrasted with a direct tax on income or profit. The spec names two structurally different types. A specific tax is a fixed £ amount per unit, regardless of price — a specific tax of £2 per litre of fuel is £2 whether fuel costs £1 or £1.50 a litre. An ad valorem tax is a fixed PERCENTAGE of price — the £ amount collected rises automatically as price rises. Both raise the effective cost of supplying each unit, so both shift supply up/left — but, as the diagram above shows, not the same shape: specific shifts in parallel, ad valorem pivots.

A subsidy is the exact mirror image: a payment PER unit to producers, lowering the effective cost of supplying each unit and shifting supply down/right — more is supplied at every price, equilibrium price falls, equilibrium quantity rises. The spec asks for the impact on consumers, producers, AND government for both a tax and a subsidy: consumers face a higher price (tax) or a lower price (subsidy); producers receive a lower price (tax) or a higher price (subsidy) — note this is the price actually RECEIVED once the wedge is added, not the price on the new shifted curve itself; government collects tax revenue (a receipt) or pays a subsidy cost (an expenditure), in both cases equal to the £ amount per unit multiplied by the NEW, post-change equilibrium quantity, not the original one.

Incidence is the specific question of who, of the total amount raised or spent, actually bears it — how much comes out of the consumer's pocket, and how much out of the producer's margin. That is not the same question as whether a tax exists or how large it is: a £5 tax could split 90/10 in either direction depending entirely on the market it's imposed on, which is exactly what the derivation below shows rather than asserts.

A specific tax doesn't just redistribute £8 from consumers and £4 from producers to the government (the burden rectangles already computed) — it also destroys some surplus outright. Consumer surplus falls from £25 to £16 (a £9 loss — £1 more than the £8 consumer-burden rectangle) and producer surplus falls from £12.50 to £8 (a £4.50 loss — £0.50 more than the £4 producer-burden rectangle). The extra £1.50 lost by the two sides combined, which the government never collects (£13.50 lost vs only £12 raised), is the deadweight loss from the 1-unit fall in quantity traded (Q0=5 to Q1=4) — trades that would have benefited both a buyer and a seller but no longer happen.

A subsidy is not simply this in reverse, though: consumer surplus rises from £25 to £36 (+£11) and producer surplus rises from £12.50 to £18 (+£5.5) — a combined £16.50 gain that is actually LESS than the £18 the government pays out, not more. The missing £1.50 is the subsidy's own deadweight loss, from the opposite cause to a tax's: the subsidy pushes quantity traded up to Q2=6, past the point (Q0=5) where the value to buyers still exceeds the true cost of production, so the extra unit produced is a net waste of resources even though both CS and PS individually rise.

Two further evaluation points apply to that same deadweight-loss finding, both explicit indicative-content bullets in a genuine WEC11 mark scheme on this exact spec point [Jun21 MS, Q12(e)]: government cannot simply read off the £1.50 waste figure in advance, because sizing a subsidy correctly is itself a real measurement problem — "it is hard to measure the level of subsidy that should be paid," so a subsidy set too high wastes more than this worked example shows, and one set too low under-corrects whatever justified the intervention in the first place. And a subsidy paid over time creates its own separate risk beyond the static deadweight-loss calculation: an industry that comes to rely on it can make withdrawal politically and economically costly — a genuine "dependency culture," where "the growers may become dependent on the subsidy and its withdrawal would cause significant problems," not a one-off cost but an ongoing constraint on ever reversing the policy.