Price determination

Edexcel International A-Level Economics· 1.3.4 (2018 spec)· 60 min read

1. Market Equilibrium and Shifts★★☆☆☆⏱ 10 min

📘 Definition

Equilibrium Price and Quantity

The price and quantity where quantity demanded equals quantity supplied, with no inherent pressure for price to change in a free market.

At prices above equilibrium, excess supply (surplus) occurs: producers cut prices to clear unsold stock, driving price back to equilibrium. At prices below equilibrium, excess demand (shortage) occurs: consumers bid up prices to access scarce goods, pushing price back to equilibrium. Shifts in demand or supply create a new equilibrium point.

📐 Worked Example

The market for coffee has demand function $Q_{d} = 100 - 2 P$ and supply function $Q_{s} = 10 + 3 P$ . Calculate equilibrium price and quantity, and explain what happens if the market price is set at £12.

1
Set $Q_{d} = Q_{s}$ to find equilibrium price: $100 - 2 P = 10 + 3 P$
$90 = 5P \implies P = 18$
2
Substitute $P = 18$ back into $Q_{d}$ to find equilibrium quantity:
$Q = 100 - 2(18) = 64 \text{ units}$
3
At $P = £12$ : $Q_{d} = 100 - 2 (12) = 76$ , $Q_{s} = 10 + 3 (12) = 46$ . Excess demand of 30 units exists, so price will rise to £18 to clear the market.

Exam tip:

Always label excess supply/demand areas on your diagrams, and add a 1-sentence explanation of how market forces eliminate the disequilibrium for full KAA marks.

2. Consumer and Producer Surplus★★★☆☆⏱ 12 min

📘 Definition

Consumer Surplus (CS) and Producer Surplus (PS)

CS = difference between what consumers are willing to pay for a good and what they actually pay, shown as the triangle above equilibrium price and below the demand curve. PS = difference between the price producers receive and their minimum acceptable price, shown as the triangle below equilibrium price and above the supply curve.

Shifts in demand or supply change the size of CS and PS. For example, an outward shift in supply (lower costs of production) lowers equilibrium price, increasing CS as consumers pay less for more units, while PS may rise or fall depending on the size of the price fall and quantity increase.

📐 Worked Example

A stationary market has equilibrium price £2, equilibrium quantity 100 units. The maximum willingness to pay for the first unit of notebooks is £10, and the minimum acceptable price for the first unit is £0.50. Calculate total CS and PS at equilibrium.

1
CS is the area of the triangle: base = quantity = 100, height = max willingness to pay minus equilibrium price = £10 - £2 = £8
$CS = 0.5 \times 100 \times 8 = £400$
2
PS is the area of the triangle: base = 100, height = equilibrium price minus minimum acceptable price = £2 - £0.50 = £1.50
$PS = 0.5 \times 100 \times 1.5 = £75$
3
Total welfare at equilibrium is CS + PS = £475.

3. Functions of the Price Mechanism★★★☆☆⏱ 10 min

📘 Definition

Price Mechanism

The system by which market forces of supply and demand allocate scarce resources in a free market, performing three core functions across local, national and global markets.

Signalling function: Price changes send information to producers and consumers about changing market conditions, e.g. rising oil prices signal to producers to drill more, and to consumers to use less oil.
Incentive function: Higher prices incentivise producers to increase supply to earn more revenue, while lower prices incentivise consumers to increase consumption.
Rationing function: Prices ration scarce resources to consumers willing and able to pay the highest price, e.g. concert tickets are rationed to those who can afford the market clearing price.

📐 Worked Example

Explain how the price mechanism responds to a global shortage of lithium used in electric vehicle batteries.

1
Signalling: Rising lithium prices signal to lithium miners that demand exceeds supply, so they should expand production. They also signal to electric vehicle manufacturers to explore alternative battery materials.
2
Incentive: Higher lithium prices give miners a direct incentive to invest in new mining projects, as they can earn higher profits from increased output.
3
Rationing: Higher lithium prices raise the cost of electric vehicles, rationing limited lithium supplies to consumers willing to pay higher prices for EVs until supply expands to meet demand.

Exam tip:

When asked about the price mechanism in context, always link each function explicitly to the example given, and specify if the market is local, national or global to pick up level 3 KAA marks.

4. Indirect Taxes and Subsidies: Impact and Incidence★★★★☆⏱ 15 min

📘 Definition

Tax and Subsidy Incidence

The share of an indirect tax paid by consumers vs producers, or the share of a subsidy received by consumers vs producers, determined by the relative price elasticity of demand (PED) and price elasticity of supply (PES).

A specific indirect tax shifts the supply curve upwards parallel by the value of the tax, raising equilibrium price and lowering equilibrium quantity. A subsidy shifts the supply curve downwards parallel by the value of the subsidy, lowering equilibrium price and raising equilibrium quantity. The tax revenue rectangle is split between consumer incidence (price rise x new quantity) and producer incidence (remaining tax per unit x new quantity).

📐 Worked Example

The government imposes a £2 per unit specific tax on soft drinks. PED for soft drinks is -0.4, PES is 1.6. Calculate the share of the tax paid by consumers and producers, and explain why the burden falls more on one group.

1
The consumer share of tax burden is calculated as $\frac{P E S}{∣ P E D ∣ + P E S}$
$Consumer\ share = \frac{1.6}{0.4 + 1.6} = 0.8 = 80%$
2
Producer share of tax burden is $\frac{∣ P E D ∣}{∣ P E D ∣ + P E S} = \frac{0.4}{2} = 0.2 = 20$
3
Consumers pay 80% of the £2 tax = £1.60 per unit, producers pay 20% = £0.40 per unit. The burden falls more on consumers because demand is relatively price inelastic compared to supply, so consumers are less responsive to price rises.

Exam tip:

Remember the rule: the more inelastic side of the market bears the greater burden of a tax, or receives the greater benefit of a subsidy. Always state the elasticity values in your answer to justify the incidence split.

5. Common Pitfalls

Wrong move:

Drawing an ad valorem tax as a parallel shift of the supply curve

Why:

Ad valorem tax is a percentage of price, so the tax per unit increases as price rises, leading to a pivoted not parallel shift

Correct move:

Draw specific taxes as parallel upward shifts, ad valorem taxes as upward pivots of the supply curve, and label the shift magnitude clearly

Wrong move:

Calculating consumer surplus as the area below the demand curve and above the x-axis

Why:

Consumer surplus is the difference between willingness to pay and actual price paid, so it is bounded above by the demand curve and below by the equilibrium price, not the x-axis

Correct move:

Always calculate CS as the area of the triangle between the demand curve, equilibrium price line, and y-axis

Wrong move:

Attributing tax incidence solely to the party legally required to pay the tax

Why:

The legal incidence of a tax is independent of the economic incidence, which is determined solely by relative PED and PES

Correct move:

Ignore who the law says pays the tax; calculate incidence using the elasticity ratio or the split of the tax revenue rectangle on your diagram

Wrong move:

Forgetting to link diagrams to written analysis in extended answers

Why:

Diagrams are only awarded marks if they are explicitly used to support your chain of reasoning, not just drawn in isolation

Correct move:

After drawing a diagram, reference it in your text e.g. "As shown in Diagram 1, the leftward shift in supply raises equilibrium price, reducing consumer surplus by area $P_{1} P_{2} A B$ "

Wrong move:

Including externality (MSC/MSB) diagrams when asked about tax or subsidy impacts on price

Why:

MSC/MSB diagrams are part of the market failure topic, not price determination, and will not be awarded marks when a standard S&D diagram is required

Correct move:

Use only standard supply and demand curves for all price determination questions, including tax and subsidy incidence questions

6. Quick Reference Cheatsheet

Concept	Key Rule	Diagram Requirement
Equilibrium	$Q_{d} = Q_{s}$ , excess demand/supply eliminated by price changes	Label P, Q axes, D/S, $E_{0}$ (original equilibrium), $E_{1}$ (new equilibrium, shift arrow)
Consumer/Producer Surplus	CS = willingness to pay minus actual price; PS = actual price minus minimum cost	Shade CS: above P, below D; Shade PS: below P, above S; label areas
Price Mechanism Functions	Signalling (information), Incentive (profit motive), Rationing (scarcity allocation)	Link each function explicitly to the case study context in answers
Indirect Tax Incidence	More inelastic side of market bears larger tax burden	S shifts up by tax value, shade tax revenue rectangle, split into consumer/producer incidence
Subsidy Incidence	More inelastic side of market receives larger subsidy benefit	S shifts down by subsidy value, shade total subsidy cost rectangle

7. Frequently Asked

Do I need to label all curves on S&D diagrams for marks?

Yes: label original D/S, shifted D1/S1, axes P/Q, equilibrium points, and shaded surplus/incidence areas to get full draw marks. Unlabelled axes lose 2 marks automatically.

How do I distinguish between specific and ad valorem tax shifts?

Specific tax causes a parallel upward shift of the supply curve, while ad valorem tax causes a pivot upwards, as the tax is a percentage of price so increases as price rises.

Do I need to include MSC/MSB diagrams for tax/subsidy price questions?

No: MSC/MSB diagrams are part of the market failure topic, not price determination, and will not be awarded marks when a standard S&D diagram is required.

Going deeper

What's Next

Now that you have mastered price determination, you are ready to move to the next Unit 1 topics covering market failure and government intervention, where you will explore why governments sometimes intervene in free markets to correct inefficient outcomes, and how to evaluate the effectiveness of different policy tools. Price determination is the foundational building block for all microeconomic analysis in Edexcel IAL Economics, and you will apply these concepts repeatedly to topics like labour markets, monopoly, and international trade across both IAS and IA2 units. Make sure you practice drawing all required diagrams from memory, and work through past paper questions on price determination to build your confidence with extended response answers and evaluation points.

→Market Failure (Edexcel IAL Unit 1)