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Key Idea: In a fluid, regions that have different temperatures have different densities. The action of a gravitational force on regions of different densities cause them to rise or fall, creating currents that contribute to the transfer of energy.

Students should know that:

  1. In a fluid system in which the fluid is the same temperature throughout, the upward forces and downward gravitational force are balanced and natural convection will not occur.
  2. When a region of a fluid is heated, it becomes less dense. When a region of a fluid is cooled, it become denser. The gravitational force on a denser region is greater than the gravitational force on a less dense region of equal volume.
  3. Heating a fluid from below will create a warmer region that is less dense than the cooler, denser region above it. When a warmer region of a fluid is below a cooler region, the upward force and downward gravitational force are unbalanced so that the warmer region rises and the cooler region sinks.
  4. As the warmer region rises, its thermal energy is transferred from one location in the fluid to another.
  5. As a region of fluid moves to new locations, adjacent regions of fluids move in to fill the space left behind.
  6. When the rising fluid encounters a boundary such as the surface of the fluid, it moves horizontally. As regions warmer than the surroundings move horizontally, they transfer energy to the surroundings, and this causes the fluid to cool and become denser. This region then begins to sink when it becomes denser than the region below it. As the region sinks, its gets closer to a heat source causing its temperature to increase and its density to decrease, and the cycle continues.

Boundaries:

  1. Items do not ask students to calculate how much energy is transferred by convection in a particular situation.
  2. Students are not expected to know how much denser a fluid needs to be than the fluid around it in order to fall.
  3. Assessment items do not use the term “buoyant force,” nor will they assess students’ understanding of buoyant forces.
Frequency of selecting a misconception

Misconception
ID Number

Student Misconception

Grades
4–5

Grades
6–8

Grades
9–12

RGM102

Density and gravity don't play a role in why an object rises. An object rises because it is hotter than its surrounds.

N/A

35%

32%

RGM073

During convection, heat molecules form and spread throughout the material.

N/A

33%

30%

EGM026

Thermal energy is produced by atoms and molecules rubbing against one another (Kesidou & Duit, 1993; Wiser, 1986)

N/A

27%

28%

CLM034

When water is moving, its thermal energy moves separately from the water (AAAS Project 2061, n.d.).

N/A

26%

24%

RGM002

Convection currents start at the bottom of the fluid and then the convection region gradually extends toward the top of the fluid (Chiou, NARST 2012).

N/A

27%

20%

RGM001

During convection, thermal energy gradually diffuses from the heat source toward the top of the fluid. The mechanism is the same as conduction (Chiou, NARST 2012).

N/A

18%

18%

CLM029

When a container of water is heated from the bottom, the water will not rise until it starts to boil (AAAS Project 2061, n.d.).

N/A

18%

18%

RGM003

The temperature of all of the fluid is the same when convection currents are cycling (Chiou, NARST 2012). The amount of thermal energy the fluid has while convection occurs within the fluid is the same throughout the fluid.

N/A

10%

12%

Frequency of selecting a misconception was calculated by dividing the total number of times a misconception was chosen by the number of times it could have been chosen, averaged over the number of students answering the questions within this particular idea.