Tufts OpenCourseware
Author: Fulcrum Institute Development Team

Sample Explanation of a Graph Curve

The story of the heating soup

The soup and the burner were initially at room temperature (24°C). I put my pot of soup on the burner of my electric stove, switched on the heat to medium high, and stirred continuously for the next 4 minutes.

For the first ½ minute, there was no temperature change. On the graph, the pattern type of temperature change (from Analyzing Temperature Graphs) is "constant temperature," showing that the soup was in thermal equilibrium and that there was no net transfer of energy.

Over the next 3 1/2 minutes, the burner heated up. The pattern type of the curve from 1/2 minute to 4 minutes is "increasing, rate increasing."

The temperature increase shows that the amount of energy absorbed by the soup was greater than the amount of energy emitted by the soup. Energy was transferred to the soup by conduction through the pot's bottom and from there by convection (and some conduction) throughout the soup. Energy was transferred from the soup by conduction through the pot walls, by convection of air around the soup's surface, and energy was emitted by radiation.

In the graph, the rate of temperature change increases, showing that the rate at which energy was absorbed by the soup exceeded that at which energy was emitted by the soup. As the stove got hotter, the rate at which it transferred energy to the soup increased. As the soup got hotter, it also emitted more energy - it emitted more energy by radiation and more energy was transferred through the wall by conduction. However, more energy was transferred into the soup than out of it.

After 4 minutes, the burner was red hot and the soup was already 40°C.