Thermal Energy

Why is thermal energy important to understand? Knowing how the temperatures of objects are affected by factors like the temperature of the surrounding air or other objects that are touching allows us to make good decisions. For example, this will let us know whether it's better to open a window or turn on the air conditioner to cool a room.

Or how we can thaw food quickly to make dinner for our families. Does frozen food thaw faster in water or in the air?

Or which type of mug (ceramic, glass, or plastic) should you buy to keep your drink hot for longer?

(Click the forward arrow to go to the next page.)

But what is thermal energy? An object's thermal energy is defined as the amount of energy of the object due to the energy of motion — or kinetic energy — of all the atoms and/or molecules that make up the object.

So....

When the atoms and/or molecules that make up an object move around faster, the object has more thermal energy.
When the atoms and/or molecules that make up an object move around slower, the object has less thermal energy.

The simulation below represents a microscopic view of a substance; the dots are its atoms. This substance is shown at two different times and with two different amounts of thermal energy. At Time 1, the substance is at room temperature. At Time 2, the substance has been cooled to a very low temperature. This is why the atoms are moving slower at Time 2 than Time 1.

The thermal energy of the substance is greater at Time 1 than Time 2.

But the temperature at Time 1 is also greater than at Time 2. So, what the heck is the difference between temperature and thermal energy??

Thermal energy vs. Temperature. Thermal energy and temperature are closely related concepts. Both have to do with the kinetic energy of the atoms and/or molecules that make up objects or substances. But thermal energy and temperature are not the same thing.

Here's the difference:

The temperature of an object (or non-solid substance, like water in a glass) is defined as the average kinetic energy of all the particles (atoms and/or molecules) that make up the object (or substance).
The thermal energy of an object is the total amount of kinetic energy of the particles (atoms and/or molecules) that make up the object (or substance).
- So, because thermal energy is the SUM of the kinetic energy of all the particles that make up the object, the thermal energy of an object depends on the SIZE of the object.
- BUT, because the temperature of an object is the AVERAGE kinetic energy of all the particles that make up the object, the temperature of an object does NOT depend on the size of the object.

Example. Let's say that there are two objects (A and B) made of the same material and at the same temperature. This means that the particles of Object A (above, left) are moving around with the same average speed as particles of Object B (above, right). But Object A has more mass, so there are more particles that make up Object A than Object B.

Fill-in-the-blank:

Thermal Energy goes from hotter to cooler objects. Thermal energy is transferred between two objects at different temperatures that are touching each other. When two objects that are at different temperatures touch each other, the cooler object will become warmer and the warmer object will cool.

This is represented in the picture below, where the higher-temperature object (red) is on the left and the lower-temperature object (blue) is on the right. Thermal energy will be transferred from the hotter to cooler object. "Heat" is the amount of thermal energy that is transferred in this process.

Why does thermal energy transfer from warmer to cooler objects? This happens because the particles (atoms and/or molecules) that make up the warmer object are moving around faster (on average) than the particles that make up the cooler object. When the faster-moving particles bump into the slower-moving particles, the faster-moving particles tend to slow down and the slower-moving particles tend to speed up.

This idea is represented below, with two particles (P1 and P2):

Because of this transfer of motion between the particles of objects, the warmer object will cool down and the cooler object will warm up. Eventually, the two objects will become the same temperature. This final temperature will be somewhere between the starting temperatures of the objects.

Simulation of thermal energy transfer. In the simulation below, two objects that are touching each other are initially at different temperatures (represented by different colors*). The object on the left is initially at a higher temperature than the object on the right. Thermal energy is transferred from the higher- to the lower-temperature object until they reach the same final temperature (represented by purple) and will remain at that final temperature. (This simulation repeats.)

*In the simulation above, the color red just represents higher temperature, where particles (atoms and/or molecules) are moving around with more speed and kinetic energy. The color blue just represents a lower temperature, where particles are moving around with less speed/kinetic energy. So just think of red as faster-moving particles and blue as slower-moving particles (and purple as particles moving in between these two speeds).

Q1: In the simulation above, if the initial temperature of the object on the left was even higher than shown above, which would be true of the final temperature of the object on the right?

a) It would also be higher than it was above.

b) It would be lower than it was above.

c) It would be the same as it was above.

d) It's impossible to know.

That's right! If the object on the left had a higher initial temperature, its particles would be moving around faster. When the faster-moving particles of the object on the left collide with slower-moving particles of the object on the right, the particles of the object on the right will gain more speed. As a result, the object on the right will reach a higher temperature.

Actually, if the object on the left had a higher initial temperature, its particles would be moving around faster. When the faster-moving particles of the object on the left collide with slower-moving particles of the object on the right, the particles of the object on the right will gain more speed. As a result, the object on the right will reach a higher temperature. (a) is the correct answer.

Q2: The object on the right side of the simulation starts at a higher temperature than it is above (but it is still at a lower starting temperature than the object on the left). Which of the following would be true?

a) It would take longer for the objects to reach the same temperature than it did above.

b) It would take less time for the objects to reach the same temperature than it did above.

c) It would not affect how long it takes for the objects to reach the same temperature.

d) I don't know.

That's right! If the object on the right started at a higher temperature, then it would take less time for the objects to reach the same temperature. (b) is the correct answer.

Well, actually, if the object on the right started at a higher temperature, then it would take less time for the objects to reach the same temperature. (b) is the correct answer.

Speed of Thermal Energy Transfer. How quickly thermal energy is transferred from the hotter to colder object depends on the temperature difference between the objects.

The greater the temperature difference is between the objects, the faster thermal energy is transferred to the colder object.
As the difference in temperatures between the two objects gets smaller, the amount of thermal energy transferred in a given time decreases.

This relationship is shown in the graph below (for two objects of the same material and mass). The height of each line (the red and blue line) shows the total amount of thermal energy transferred to the colder object at that time. The red line shows this for two objects that initially had a temperature difference of 40°C. The blue line shows this for two objects that initially had a temperature difference of 80°C.

As you can see, the amount of thermal energy transferred to the colder object is always higher when the two objects had a greater initial temperature difference (the blue line). Also, the speed of thermal energy transfer, which is the steepness (or "slope") of each line at any point in time, is always higher for the blue line.

Let's say that the cooler object needed to gain a certain amount of thermal energy (for example, to reach a certain temperature or to start melting). This is shown in the graph below, where the dotted line represents this amount of energy. The cooler object would gain this amount of thermal energy faster when there was a bigger initial temperature difference between the objects. You can see that the time to gain this energy is less for the 80°C initial difference (blue line) than for the 40°C initial difference (red line).

In general, how fast thermal energy is transferred from the hotter to colder object depends on the temperature difference between these objects.

It also depends on other things like the mass of the objects and how much thermal energy it takes for the object's temperature to change (called the "heat capacity"). The heat capacity of an object depends on what the object is made of. Some materials (like metals) need less thermal energy to increase in temperature compared to other materials (like water). This is because the thermal energy transferred to the cooler object may be converted to other types of kinetic energy (such as rotational and vibrational energies of molecules) that are not related to temperature.

Thermal energy transfer between solids and gases. Thermal energy will be transferred between two solid objects at different temperatures. But thermal energy will also transfer between a solid and a gas (from whichever is warmer to whichever is cooler). For example:

In the picture below, a balloon is sitting in the sun. The picture shows the cross-section of the balloon and the air inside the balloon. The outside surface of the balloon is warmed by the air and by the sunlight hitting the surface of the balloon. Some of the sunlight will be converted into thermal energy in the balloon. Thermal energy will transfer from the (warmer) outside surface of the balloon to the (cooler) inside surface of the balloon. Thermal energy will continue to be transferred from the (now warmer) inside material of the balloon to the (cooler) air inside the balloon. The arrows in the picture show the direction of transfer of thermal energy.

How exactly is thermal energy transferred from the balloon material to the air inside the balloon? The particles that make up the material of the balloon will transfer their kinetic energy (through millions or billions of collisions) to gas molecules that make up the air inside the balloon. The molecules of air in the balloon will move around faster as a result; that is, they will gain kinetic energy.

You can watch a fun video demonstration of gas in a balloon cooling here.

Thermal energy can also be transferred between a solid and a liquid. For example:

Thermal energy due to the motion of the water molecules will transfer from the (warmer) water to the (colder) ice cube in the water. The arrows in the picture show the direction of thermal energy transfer: from the water to the ice. The water molecules surrounding the ice cube will collide with the molecules that make up the ice cube and transfer their energy of motion to the molecules of the ice cube. Of course, as you know, this leads to the ice cube melting in the water. It also causes the water to cool, since the liquid water molecules lose kinetic energy to the molecules of the ice cube.

Cup of hot water with ice cube and energy arrows pointing to the ice cube.

Q4: You are learning how to cook and offered to make dinner for your family. But you forgot to thaw the two identical jars of spaghetti sauce you left in the freezer! So, you fill Pot A with 1 liter of water at 50°C and an identical pot, Pot B, with 1 liter of water at 90°C. You put a jar of the sauce into each pot. Which jar will thaw first?

a) The jar in the 50°C water (Pot A).

b) The jar in the 90°C water (Pot B).

c) Neither. They would thaw at about the same time.

d) It's impossible to know.

That's correct! The spaghetti sauce in the hotter water (Pot B) will thaw first. This is because thermal energy is transferred to the spaghetti sauce faster when the water is warmer. So, the sauce will gain the thermal energy needed to thaw it faster.

Well, the spaghetti sauce in the hotter water (Pot B) will thaw first. This is because thermal energy is transferred to the spaghetti sauce faster when the water is warmer. So, the sauce will gain the thermal energy needed to thaw it faster.

Thermal Energy

Table of Contents: Thermal Energy