significance of price elasticity of demand and of supply in determining the extent of these changes

Consumer and Producer Surplus

Think of buying your favourite snack. The consumer surplus is the extra happiness you get because you paid less than what you were willing to pay. The producer surplus is the extra profit a seller makes because the market price is higher than the minimum they would accept.

Mathematically, for a single good:

\$CS = \int{P}^{P^*} Qd(P')\,dP'\$

\$PS = \int{P^*}^{P} Qs(P')\,dP'\$

Where \$P^*\$ is the equilibrium price, \$Qd\$ the demand curve, and \$Qs\$ the supply curve.

Price Elasticity of Demand (PED)

Elasticity tells us how much quantity demanded changes when price changes.

\$PED = \frac{\% \Delta Q_d}{\% \Delta P}\$

  • 📈 Elastic (|PED| > 1): Quantity changes a lot. A price drop leads to a big increase in sales, boosting consumer surplus significantly.

  • 📉 Inelastic (|PED| < 1): Quantity changes little. A price drop still increases consumer surplus, but the effect is smaller.

Example: If the price of pizza drops from £5 to £4, and demand is elastic, the number of pizzas sold jumps, giving consumers a lot more surplus. If demand is inelastic, the jump in sales is modest.

Price Elasticity of Supply (PES)

Supply elasticity measures how quickly producers can change output when price changes.

\$PES = \frac{\% \Delta Q_s}{\% \Delta P}\$

  • 📈 Elastic (|PES| > 1): Producers can quickly increase output. A price rise leads to a large increase in supply, raising producer surplus.

  • 📉 Inelastic (|PES| < 1): Producers cannot easily change output. A price rise gives less increase in producer surplus.

Analogy: Think of a bakery that can add more ovens (elastic) versus one that has a fixed number of ovens (inelastic).

How Elasticities Affect Surplus Changes

ScenarioEffect on CSEffect on PS
Price ↓, Demand ElasticLarge increase in CSSmall change in PS
Price ↑, Supply ElasticSmall change in CSLarge increase in PS
Price ↑, Demand InelasticSmall decrease in CSLarge increase in PS

Exam Tips 💡

  1. Always state whether the change is in price, demand or supply.

  2. Use the elasticity sign (elastic vs inelastic) to predict the direction and magnitude of surplus changes.

  3. Remember that a larger area under the demand curve above the price increases CS; a larger area above the supply curve below the price increases PS.

  4. When drawing diagrams, label the axes clearly and shade the surplus areas.

  5. Practice sketching quick diagrams for each scenario; this saves time during exams.