A change in quantity supplied is caused only by a change in the good’s own price, with all other factors held constant (ceteris paribus).

Quick Scoop: Core Idea

When the price of a product changes, producers move along the existing supply curve, and the amount they are willing to sell changes. That movement is what economists call a change in quantity supplied.

  • Higher price → producers supply more units (expansion in quantity supplied).
  • Lower price → producers supply fewer units (contraction in quantity supplied).

Other factors (like technology, input costs, taxes) do not cause a change in quantity supplied; they change supply itself (shift of the curve), which is a different concept.

Mini Story: The Coffee Seller

Imagine a street vendor selling cups of coffee:

  • At a price of 2, they’re happy to sell 50 cups.
  • If the price rises to 3, selling each cup brings more profit, so they’re now willing to sell 80 cups.

Nothing else changed: same stall, same machine, same workers. Only the price per cup changed, and the number of cups they’re willing to offer rose. That jump from 50 to 80 cups is a change in quantity supplied , a movement up along the same supply curve.

If, instead, a new coffee machine made brewing cheaper, they’d be able to supply more at every price – that would be a change in supply , not just quantity supplied.

Key Points in Bullet Form

  • “Quantity supplied” = how many units producers are willing and able to sell at a specific price.
  • A change in quantity supplied happens when the good’s own price changes , other conditions unchanged.
  • Graphically, this is a movement along the supply curve , not a shift of the curve.
  • Other things (technology, input costs, number of firms, taxes) change supply , not quantity supplied, and shift the entire curve.

In one line:
What causes a change in quantity supplied?
A change in the product’s own price, holding all other factors constant.

TL;DR:
Only a change in the good’s price causes a change in quantity supplied (a move along the supply curve); all other factors cause a shift in supply, not quantity supplied.

Information gathered from public forums or data available on the internet and portrayed here.