AP® Microeconomics · Unit 2 · Topic 2.1
Shifting a Curve Versus Moving Along It
A change in a good's own price moves you along a fixed demand or supply curve (a change in quantity demanded or supplied); a change in anything else shifts the whole curve (a change in demand or supply itself).
Demand and supply curves plot quantity against price, holding every other influence constant. The good's own price lives on that graph's vertical axis, so when it changes, you slide along the curve that's already drawn. Nothing about buyers' willingness to pay or sellers' willingness to produce has changed — you're just reading a different point on the same relationship. Economists call this a change in quantity demanded or quantity supplied, and they guard the distinction closely because mixing it up makes you think the wrong thing caused the change.
A shift is different. It happens when something off the axes changes: income, tastes, the price of a related good, input costs, technology, expectations, or the number of buyers or sellers. These factors redraw the whole curve, so that at every price, people now want to buy or sell a different amount. That's a genuine change in demand or supply, not just a move along it.
The two can happen together. If coffee becomes cheaper and a health study also boosts demand, the demand curve shifts right at the same moment the price change would otherwise move you along it — the new equilibrium reflects both forces, and untangling which one did what is exactly the skill the rest of this unit builds on, especially when taxes or price controls push price and quantity around at once.
Key terms
- Quantity demanded/supplied
- The specific amount bought or sold at one particular price, read as a point on a fixed curve.
- Demand/supply
- The entire relationship between price and quantity, shown as the whole curve.
- Movement along the curve
- A change in the good's own price that slides buyers or sellers to a different point on the same curve.
- Shift of the curve
- A change in some factor other than the good's own price that redraws the entire curve left or right.
Watch it explained
- How it worksHow does a change in the good's own price move you along a curve instead of shifting it?
- Common mistakeWhy isn't a drop in price a decrease in demand?
- Worked exampleShow me which one moves when coffee gets cheaper, and which when a study says coffee is healthy.
- Going furtherWhat happens on the graph when a curve shifts and the price changes at the same moment?
- Common mistakeWhy isn't a rise in quantity supplied the same thing as an increase in supply?
Check yourself
End of topic.