Thermal Energy End-of-Unit Exam

What is the TOTAL kinetic energy of the molecules in this system (Sample A + Sample B) at the moment shown in the diagram? Please input ONLY numbers.

(a)  

Answer explanation

The total kinetic energy of the molecules in the system is 0 because the diagram likely indicates that the molecules are at rest or not in motion, resulting in no kinetic energy.

What will the AVERAGE kinetic energy of the molecules in this system be when it reaches equilibrium? Please input ONLY numbers.

(a)  

Answer explanation

At equilibrium, the average kinetic energy of molecules is determined by temperature. If the system is at absolute zero (0 Kelvin), the kinetic energy is 0. Therefore, the correct answer is 0.

True or False: For things at the same temperature, the thing with more molecules has more total kinetic energy (thermal energy) than the thing with less molecules.

• True

• False

Answer explanation

True. At the same temperature, the average kinetic energy per molecule is the same, but more molecules mean more total kinetic energy. Therefore, the object with more molecules has more thermal energy.

Thermal energy is the

• the TOTAL kinetic energy of all the molecules that make up a sample

• measure when something becomes different over time

• a measure when there is a balanced state in which a system is stable

• measure of the AVERAGE kinetic energy of the molecules of a thing.

Answer explanation

Thermal energy refers to the TOTAL kinetic energy of all the molecules in a sample, which encompasses the energy due to their motion. This distinguishes it from average kinetic energy, which only considers the mean.

When hot water shoots up from underground, making the ground warmer, what happens to the molecules in the ground?

• Heat molecules combine with the molecules in the ground.

• The energy of the molecules in the ground decreases. ⬇️

• The energy of the molecules in the ground increases. ⬆️

• The cold energy of the molecules in the ground decreases.

Answer explanation

When hot water rises, it transfers heat to the ground, increasing the energy of the molecules in the ground. This results in higher molecular motion, making the ground warmer.

On a space shuttle covered with two layers of carbon, the outer layer and the inner layer are the same size and have the same number of molecules. Part 1: Before the layers touch, how does the temperature of the outer layer compare to the temperature of the inner layer? Part 2: What happens after the layers have been touching for a while?

• Part 1: the OUTER layer is HOTTER than the INNER layer. Part 2: When they touch, the outer layer shares its heat with the cooler inner layer until they both have the SAME temperature.

• Part 1: the OUTER layer is COOLER than the INNER layer. Part 2: Once the layers are touching, the inner layer will transfer kinetic energy to the molecules in the cooler outer layer until both layers reach the SAME temperature, which will be in between their starting temperatures.

• Part 1: the layers are the SAME temperatures. Part 2: Once the layers are touching, kinetic energy and cold energy will transfer between the molecules in the two layers until both layers reach the same temperature, which will be in between their starting temperatures.

Answer explanation

The outer layer is hotter due to exposure to space conditions. When they touch, heat transfers from the outer to the cooler inner layer until both reach the same temperature, equalizing their thermal energy.

Lin cooked a pot of soup and put half of it in the freezer. Why is the soup in the pot hot while the soup in the freezer is cold? What's different about their molecules?

• The molecules of the soup in the pot are moving and the molecules of the soup in the freezer are NOT moving.

• The molecules of the soup in the pot are moving FASTER than the molecules of the soup in the freezer.

• Nothing because all of the soup is made of the same type of molecules.

• The molecules of the soup in the freezer are SMALLER than the molecules of the soup in the pot.

Answer explanation

The soup in the pot is hot because its molecules are moving FASTER due to higher temperature, while the soup in the freezer is cold because its molecules are moving slower. This difference in molecular motion explains the temperature difference.

Joseph puts hot toast on a plate. The plate's energy increases ⬆️. What happens to the energy of the toast when he does this?

• The toast's energy stays the same.

• The toast's energy decreases ⬇️

• The toast's energy increases ⬆️

• It is impossible to know without data on the number of molecules in each object.

Answer explanation

When Joseph puts hot toast on the plate, the plate absorbs energy, causing the toast's energy to decrease. Heat transfers from the toast to the plate, resulting in a loss of energy for the toast.

Sophia is cooking rice in a pot. When she leaves the pot on the stove, the rice gets hotter. What happens to the rice's molecules when it gets hotter?

• The kinetic energy of the molecules in the rice increases. ⬆️

• The potential energy of the molecules in the rice decreases. ⬇️

• The kinetic energy of the molecules in the rice decreases. ⬇️

• More heat molecules combine with the molecules in the rice.

Answer explanation

As the rice gets hotter, the kinetic energy of its molecules increases, causing them to move faster. This is why the correct answer is that the kinetic energy of the molecules in the rice increases.