## Key Idea: Thermal energy of an object is associated with the disordered motions of its atoms or molecules and the number and types of atoms or molecules of which the object is made.

*Students are expected to know that:*

- The average speed of it atoms or molecules, the number of atoms or molecules, and the types of atoms or molecules of which the object is made
*all*affect the thermal energy of the object. - The thermal energy of an object is the sum of the motion energy (kinetic energy) of all of the individual atoms and molecules that make up the object.
- Since all matter is made up of atoms and molecules that are in constant random motion, all matter has some thermal energy.
- When the average speed of the atoms and molecules of an object increases, the motion energy of the atoms and molecules increases and, therefore, the thermal energy of the object increases. When the average speed of the atoms and molecules of an object decreases, the motion energy of the atoms and molecules decreases and, therefore, the thermal energy of the object decreases.
- When the thermal energy of an object increases, the motion energy and average speed of the atoms and molecules of the object increases. When the thermal energy of an object decreases, the motion energy and average speed of the atoms and molecules of the object decreases.
- For objects that are made of the same number and type of molecules, the object that is made up of the atoms and molecules with the highest average speed will have the most thermal energy and the object that is made up of the atoms and molecules with the lowest average speed will have the least thermal energy.
- For objects that are made of the same number and type of atoms and molecules, the object with the greatest thermal energy is made up of the atoms and molecules with the highest average speed and the object with the least thermal energy is made up of the atoms and molecules with the lowest average speed.
- With all else equal, the greater the number of atoms and molecules, the greater the thermal energy and the fewer the number of atoms and molecules, the lower the thermal energy.
- With all else equal, the greater the thermal energy, the greater the number of atoms and molecules and the lower the thermal energy, the fewer the number of atoms and molecules.

*Boundaries:*

- Students are not expected to know or use the formulas associated with thermal energy, such as 3/2 kT and m(ΔT)Cp. They are also not expected to calculate the thermal energy by summing the kinetic energy of the individual molecules. The sub-ideas above describe qualitative relationships.
- Students are not expected to know which kinds of atoms/molecules have more thermal energy.
- They are also not expected to know about internal energy and the potential energy that exists between the atoms and molecules of a substance.
- Students are not expected to know that absolute zero is the temperature a substance would have if all atomic and molecular motion were to stop.
- Students are not expected to know the term “kinetic energy.” Assessment items will use the phrase “motion energy (kinetic energy)” to avoid confusing students who are familiar with the phrase “kinetic energy.”

Item ID Number |
Knowledge Being Assessed | Grades 6–8 |
Grades 9–12 |
Select This Item for My Item Bank |
---|---|---|---|---|

50% |
55% | |||

46% |
52% | |||

When the average speed of molecules increases, the thermal energy of an object increases. | 45% |
52% | ||

42% |
51% | |||

44% |
47% | |||

40% |
44% | |||

39% |
44% | |||

The thermal energy of an object depends on the speed and type of molecules that make up the object. | 41% |
41% |

Misconception |
Student Misconception |
Grades 6–8 |
Grades 9–12 |
---|---|---|---|

Cold/frozen objects do not have any thermal energy (AAAS Project 2061, n.d.). | N/A |
N/A | |

N/A |
N/A | ||

N/A |
N/A | ||

N/A |
N/A | ||

N/A |
N/A | ||

N/A |
N/A |

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.