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Heat transfer

Learn how energy moves from warmer to cooler objects, like your hand to a glass of water. Dive into the difference between heat and temperature, and explore how heat transfer shapes our everyday experiences. Created by Jasmine Rana.

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  • blobby green style avatar for user Varun Jhavar
    heat is defined as the energy that is caused by the random motion of molecules..............then why does heat flow from a hot object to a cold one, and why not randomly from any object to any object?
    (7 votes)
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    • hopper cool style avatar for user Steven Yan
      So I think you may mixed up your definitions there. Absolute temperature (T) is defined as being proportional to the average translational kinetic energy of the particles (its random motion). That is to say, the hotter something is, the faster the molecules are vibrating, stretching, bending, and moving.

      Heat (Q), however, is the transfer of energy due to a difference in temperature. So an object cannot "have" heat. It is one of two ways you can transfer energy between the system and the surroundings. The other is work (W), which is mechanical transfer of energy, and that relationship is described by the First Law of Thermodynamics: delta(U) = Q - W(by the system), where U is internal energy.

      When we are talking about heat, we are looking at it more macroscopically rather than the individual motions of the molecules. But this is akin to thinking about why ions travel down their concentration gradient as opposed to some up and some down its gradient. It is driven by entropy and the Second Law, which states that heat flows from higher to lower temperatures.
      (18 votes)
  • blobby green style avatar for user TV2932
    At she mentions the heat transferral of your e to some of the water's. Is this convection,conduction or radiation?Thanks
    (4 votes)
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  • purple pi teal style avatar for user Sophia
    At what exactly is the (q) that physical scientists use.
    (3 votes)
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  • marcimus pink style avatar for user Ci Qian W
    does steady state means thermal equilibrium? what are the differences and similarities? pls explain the concept of steady state. I wonder why the temperature gradient stay constant at steady state, isn't it the object will finally have same temperature at all points and there is no temperature gradient?
    (2 votes)
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    • blobby green style avatar for user Rebecca Barrett
      Steady state is a more general concept that describes a process that is unchanging over time. It can describe a number of things. I have often heard it used to describe a chemical reaction where the forward and the reverse reaction are equal.

      The difference between steady state and thermal equilibrium is that a steady state can include heat being generated. If you held the water long enough eventually it would be the same temperature as your skin. You would not be at thermal equilibrium with the water. Thermal equilibrium can only occur when no new heat is being generated. In order to be at thermal equilibrium you would have to be a corpse holding the water in your cold lifeless hand.
      (3 votes)
  • blobby green style avatar for user Alex Chirica
    At first, the video says that you (at 37 Celsius) are holding a cup of water (at 20 Celsius). Wouldn't heat transfer average out those temperatures? I thought that after time, both you and the glass are going to have 28.5 Celsius in temperature. (The average of 37 and 20) However, 28.5 Celsius is extremely low for a human's body temperature. I am very confused.
    (1 vote)
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    • female robot grace style avatar for user tyersome
      There are two factors you are overlooking.

      First factor:
      By this logic, if you put an drop of ice water (at 0°C) into a glass of room temperature (say 20°C) water, the final temperature should be about 10°C.

      Does that seem reasonable?

      How about if you put that same drop of ice water into a swimming pool?

      Your prediction would still be that the final temperature is 10°C!?

      Does this help you see something you've been overlooking?

      Second factor:

      What happens when you start to feel cold?

      What does that process do?

      In general, mammals maintain a relatively constant body temperature — this is an example of homeostasis. So unless you are dead, extremely sick, or suffering from hypothermia your body will automatically expend energy to maintain your temperature.

      Does this help?
      (4 votes)
  • blobby green style avatar for user Jijo Mathew
    What are the applications of Thermal Radiation?
    (1 vote)
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  • blobby green style avatar for user devin.nair
    Wait, what was the system and what was the surrounding. I am confused. Was the surrounding the air around the thing she was holding or the surrounding of the cup?
    (1 vote)
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  • blobby green style avatar for user Marvin Kelly
    Which will cool quicker, a kettle at 100 degrees or kettle at 70 degrees?
    (1 vote)
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  • blobby green style avatar for user Majed Al
    Hey There,
    I have a question hope to get an answer for it.
    (Q) Whats the relationship between geometry and heat transfer..?
    thank you in advance
    (1 vote)
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  • blobby green style avatar for user woodishree2
    When the cup becomes colder then definitely the heat is transferred from the cup to the body.......
    (1 vote)
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Video transcript

Imagine that you are holding in your hands a glass of water. We're going to say that the glass of water is at room temperature, so that's about 20 degrees Celsius, and you are at body temperature, which is about 37 degrees Celsius. Now intuitively, you know that after a while, your hand starts to feel kind of cold, right? So what's going on here? In the physical sciences, we say that what's going on here is called a heat transfer. And the idea is that because we have two different things at different temperatures, specifically because our water's at a lower temperature, energy in the form of heat is going to travel from our body to the water, which means that our body is losing energy, and the cup of water here is gaining that energy. So we are becoming cold, and the cup of water is warming up. Now when I was first learning about heat transfer, I didn't quite understand the difference between heat and temperature, so I just want to go ahead and briefly discuss what the difference is. Heat is the amount of energy that's transferred due to a change in temperature. So I'm going to go ahead and write that out here. Remember that whenever you see a gradient, whether it's a gradient of pressure, or concentration, or in this case, a gradient of temperature, it always means that there is some type of potential energy that's stored up. And since the system wants to achieve its lowest energy possible, if there is no opposing force, the gradient wants to disappear, so this is the basis behind heat transfer. Of course, what sets up this gradient is temperature, and temperature is an absolute quantity. Specifically, it's defined as the average kinetic energy of molecules in whatever we are measuring the temperature of. So to summarize, the point I really want to underscore here is that while temperature is an absolute measure of energy, there is really no such thing as an absolute heat content. Heat should always be described as heat transfer, because it is measuring the amount of heat that is either lost or gained. It's also important to emphasize that heat transfer occurs between a system and its surroundings, which I'm going to abbreviate as surr. So to understand this, let's go ahead and revisit our example above. Now, what's really important to determine here is the direction of heat transfer. Now as I talked about before, we can't really attribute an absolute value of heat content to either the system or the surroundings, but what we can say is, what direction does this heat transfer occur in? What's getting hotter, essentially, and what's getting colder? So the way that physical scientists kind of denote heat transfer is with the lowercase letter q. And so in the case of our system, which is our hand which is becoming colder, the way that this would be denoted is with a minus q, because heat energy is being lost from the system. On the other hand, what you would find after a while if you kept holding this glass of water and then set it down on the counter and measure the temperature is that the temperature of the water would have increased. And essentially, what is going on here is that the energy lost from the system is being absorbed by the surrounding. And so in this case, because heat energy is being gained, this would be denoted as plus q. To state this phenomenon more generally, it's fair to say that the heat that is either lost or gained by the system is equal and opposite in magnitude to the heat that's either lost or absorbed by the surroundings.