Unit 4.2 Thermal Energy Review with Answers
1. Which direction does Thermal Energy ALWAYS transfer?
Thermal/heat energy always moves from high temperature to low temperature; hot to cold; warm to
cool.
2. When does Thermal Energy stop transferring?
Thermal energy stops when all temperature is equal, equilibrium. For example coffee in a mug is 100⁰F
and the air in the room is 75⁰F. The heat from the coffee goes into the air until it’s the same as the air in
the room, 75⁰F.
3. What does it mean to reach Equilibrium?
As heat energy is transferred from one object to another, it’ll keep moving until both objects become and
stay at the same temperature.
4. If “coldness” does not really exist, how can an object become colder?
“Coldness” is the decreasing of thermal energy
5. Explain how boiling water using a hot plate is an example of both Conduction and Convection.
It’s conduction at the point of the pot touching the hot plate because heat from the hot plate goes into the
pot by touch/contact. Then the pot transfers the heat into the water by convection. Water at the bottom
gets hot and starts moving because the molecules are energized and spread out to touch other molecules.
As the more energized/hot molecules spread out to the top, less energized/cooler molecules will sink to
the bottom where they’ll get heated creating a convection cycle/current.
6. Explain why someone would want to use a wooden spoon to stir a very hot liquid and not a metallic
spoon.
Wood is a great insulator or a poor conductor. A metal spoon is a great conductor, allowing energy to
move through it easily, making it a poor insulator.
7. Define conduction in your own words.
Heat/thermal energy moves from one thing to another by touch/contact.
8. Define convection in your own words.
Heat/thermal energy moves through fluids like gas and liquids. Convection only works with the heat
source being at the bottom.
9. Define radiation in your own words.
Heat/thermal energy moves through space and the air as electromagnetic waves.
The image below describes the 3 terms well.
10. Define a convection current (convection cycle) in your own words.
In order for convection to work, the heat source needs to be at the bottom of the fluids, gas or liquids.
The heat source energizes fluid molecules making them move faster, bumping into other fluid
molecules. As they continue this pattern of moving faster because of having a lot of thermal energy, they
begin bumping nearby fluid molecules that have less thermal energy. The fluid molecules that have a lot
of thermal energy become less dense and go up, making room for fluid molecules that have less energy
and are more dense to sink. These fluid molecules with less thermal energy and more density get thermal
energy from the fire and keep this cycle going.
11. Explain how fire can be an example of both Radiation and Convection.
Fire is energy that moves as electromagnetic waves, radiation and as it moves out it transfers energy to
nearby fluid molecules, such as liquid or gas. Those energized fluid molecules move faster, bumping
into other fluid molecules. As they continue this pattern of moving faster and bumping fluid molecules,
they become less dense and go up, making room for fluid molecules that have less energy and are more
to sink. These fluid molecules with less thermal energy and more density get thermal energy from the
fire and keep this cycle going. See below.
12. Explain or Draw an example of a situation that would involve all three methods of thermal energy
transfer. Label or explain where the conduction, convection, and radiation is taking place.