What is a supply and demand model?
It is the simplest picture of a market. A downward demand line says buyers want less at higher prices; an upward supply line says sellers offer more at higher prices. Their crossing is the equilibrium, the one price at which the market clears.
This simulator draws both lines on a price-and-quantity chart and lets you slide them, so you can see how the equilibrium responds to a change in the market.
Why look at it?
Shifts happen constantly: incomes change, tastes change, costs change. The model predicts the direction the price and quantity should move when a curve shifts, without tracking individual shoppers.
Two ideas matter most: moving one line changes both price and quantity, and the gaps between the price line and each curve measure the value buyers and sellers gain from trading.
1. The axes frame the market
Quantity runs along the horizontal axis and price up the vertical one. Every pair of numbers on the chart is one possible deal between buyers and sellers.
2. Demand and supply are drawn
The red demand line falls from top-left to bottom-right, and the blue supply line climbs from bottom-left to top-right. Their slopes set how strongly buyers and sellers respond to price.
3. A slider slides a whole line
The demand slider lifts or lowers the demand line; the supply slider slides the supply line. Each one is a shift of the whole curve, not a move along it.
4. The crossing point moves
Wherever the two lines meet is the new equilibrium. Dashed guides carry that point down to the equilibrium quantity Qe and across to the price Pe on the axes.
5. The surplus triangles redraw
The red triangle above the price line is consumer surplus and the blue triangle below it is producer surplus. Both are rebuilt around the new equilibrium, and their combined area is the total surplus readout.
What does the equilibrium point show?
It is where the demand and supply curves cross. At that price the quantity buyers want equals the quantity sellers offer, so the market clears.
What is a shift of the demand curve?
It is a change in how much people want at every price, such as a rise in incomes. The whole curve moves; it is not the same as moving along the curve.
What is a shift of the supply curve?
It is a change in how much sellers offer at every price, such as a fall in production costs. The curve slides left or right as the slider changes.
What is consumer surplus?
It is the area under the demand curve and above the price line: the extra value buyers get because they pay less than they were willing to. Shown as the red triangle.
What is producer surplus?
It is the area above the supply curve and below the price line: the extra sellers receive above the least they would accept. Shown as the blue triangle.
Why is total surplus used as a measure?
Adding the two triangles gives total surplus, a simple gauge of how much value the trade creates at the current market price.