Shift in demand or supply curve occurs when a non-price determinant changes; movement along the curve occurs when only price changes.
## Core concept
Movement along the curve happens when the quantity demanded or supplied changes *solely* due to a change in the commodity's own price. The demand or supply curve itself remains stationary.
Shift of the curve occurs when factors *other than* the commodity's own price change, causing the entire curve to relocate. A new demand or supply schedule emerges at each price level.
This distinction is critical because it affects market equilibrium differently and requires different analytical approaches.
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## Movement along the demand curve
- Change triggered: Own price of the commodity only
- Direction: Inverse relationship (price ↑ → quantity demanded ↓; price ↓ → quantity demanded ↑)
- Graphical representation: Movement *along* the same curve (upward or downward)
- Example: Apple price rises from ₹50 to ₹80 per kg → quantity demanded falls from 100 units to 60 units (movement along curve)
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## Shift in the demand curve
- Change triggered: Determinants other than own price — income, tastes, preferences, prices of related goods, expectations, population, etc.
- Direction: Rightward shift (increase in demand) or leftward shift (decrease in demand)
- Graphical representation: Entire curve relocates; at *same* price, quantity demanded changes
- Example: A rise in consumer income (normal good) → at ₹50/kg, quantity demanded rises from 100 to 150 units → entire curve shifts right
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## Movement along the supply curve
- Change triggered: Own price of the commodity only
- Direction: Direct relationship (price ↑ → quantity supplied ↑; price ↓ → quantity supplied ↓)
- Graphical representation: Movement *along* the same curve (upward or downward)
- Example: Producer receives higher price (₹100 → ₹150) → willing to supply more units
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## Shift in the supply curve
- Change triggered: Determinants other than own price — input costs, technology, number of sellers, natural disasters, government subsidies/taxes, future price expectations, etc.
- Direction: Rightward shift (increase in supply) or leftward shift (decrease in supply)
- Graphical representation: Entire curve relocates; at *same* price, quantity supplied changes
- Example: Introduction of better farming technology → at ₹50/kg, suppliers willing to sell 200 units instead of 150 → curve shifts right
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## Common exam applications
- Distinguishing scenarios: "When income rises for a normal good, does demand shift or move along the curve?" (Answer: shift right)
- Effect on equilibrium: Movement changes quantity and price; shift changes both, but direction depends on curve direction and magnitude
- Policy analysis: Subsidy causes supply shift (not movement); price ceiling causes movement along demand curve
- Law of Demand: Operates *along* the curve; other determinants cause *shifts*
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## Common mistakes
- Confusing "more quantity demanded" with "demand increase" — demand increase = rightward shift; more quantity = movement down the curve
- Assuming all changes in quantity demanded = demand shift (only non-price changes cause shifts)
- Forgetting that at equilibrium, if one curve shifts, both price AND quantity change, not just one
- Misinterpreting "demand falls" — could mean movement (price rose) or shift (determinant changed)
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## Worked example
Onion market: Initial equilibrium at ₹40/kg, 500 units demanded and supplied.
Scenario 1 (Movement): Price increases to ₹60/kg → quantity demanded falls to 300 units, quantity supplied rises to 700 units. *No shift; movement along both curves.*
Scenario 2 (Shift): Heavy rain damages crop → at ₹40/kg, suppliers now only willing to supply 300 units (previously 500). *Supply curve shifts left; price will rise and equilibrium quantity falls.*