Is a hotter heat source always necessary for an object to be heated?

Heat always moves from a high-temperature object to a low-temperature object. Therefore, a low-temperature object cannot become hotter than the relative high-temperature object from which it is receiving heat, assuming the high-temperature object's temperature remains constant. What about cases where electrical energy is converted into thermal energy or when an object is heated by solar energy? Additionally, in the case of frictional heat, mechanical energy is converted into thermal energy. In these situations, what serves as the relative high-temperature heat source? Is a relative high-temperature heat source always necessary for an object to gain heat, regardless of the circumstances? Can heat not transfer directly?

• "What about cases ... when an object is heated by solar energy?" At least in this case, there is an obvious high-temperature object (namely, the Sun) from which the heat flows.
– d_b
Commented Aug 4 at 9:01
• A heat pump or a refrigerator transfers thermal energy from a cooler object to a warmer object, using work (typically from electrical energy) to do so. The electrical energy does not need high temperatures to be generated: think about wind or hydroelectric or tidal power. Commented Aug 4 at 17:53
• @Henry Wind and rain/hydroelectric power are ultimately powered by the sun as well (or more specifically, the temperature differential between the sun and outer space). Tidal power is a bit more complicated (primordial gravitational potential energy). Commented Aug 4 at 22:49
• " what serves as the relative high-temperature heat source?" Water flows downhill, right? (I'm oversimplifying by ignoring surface tension. Some of the best craft plumbers make a living investigating water leaks that move upward, assisted by wind.) So what happens at the source of a river? Rain. And what is rain? Water vapour condensing to liquid water. Isn't this like kinetic energy transforming to thermal energy? Remember: the laws of thermodynamics tell us about heat flow, but not how other forms of energy may be transformed to heat. Commented Aug 5 at 8:31
• How would you ever start a fire if this were the case? Besides, your frictional energy example is surely points to the fact that the answer is No. Commented Aug 6 at 10:27

3 Answers

In thermodynamics the terms "heat" and "heating" have a very definite meaning. "Heating" is the mechanism by which energy is transferred solely due to a temperature difference. The other mechanism is work, transferring energy by means of force times displacement. Thus in thermodynamics when we say something is "heated" we mean exposing something to something else at higher temperature.

In every day language, however, the term "heated" is commonly (and technically incorrectly) used to describe any situation in which the temperature of something is raised, even when such situations do not involve a temperature difference.

An example is vigorously rubbing your hands together to make them feel "warmer" on a cold day. The temperature on your skin increases not due to heat (exposure to something with a temperature higher than your skin temperature), but due to work, in this case friction work. This is commonly called "friction heating", but in reality is due to work not heat. On the other hand, putting your hands near a fire does raise the skin temperature due to heating, in this case, by radiant heating.

Another example is so called "electrical heating", which is raising the temperature of an object (e.g., an electrical resistor) due to passing electrical current through it. This is technically not "heating" since it does not involve exposing the object to something at higher temperature. It is due to electrical work (the work done at the microscopic level to force electrons through the object). The proper use of the term electrical “heating” is in reference to the subsequent transfer of energy from the resistor to its surroundings due a temperature difference with its surroundings.

So in the context of thermodynamics, it is important to distinguish between transferring energy to something by means of heat, which is "heating" (requiring a temperature difference) and transferring energy to something by means of work (requiring force times displacement).

Hope this helps.

• Often, and especially when a flame is small, hands are heated by convection of the hot air raising from the fire, not only radiation Commented Aug 6 at 14:58
• @AlessioSangalli sure I’d the hand is close to the fire. But the farther the more it is radiant heat Commented Aug 6 at 15:00

Consider an induction cook top. An alternating magnetic field induces a current in the bottom of the pan, which heats it. Heat flows from the bottom of the pan into the food, and also back into the cook top. So yes, the heat always flows from hot to less hot.

• An example where something is true is not a proof or rationalization that it is always true. Commented Aug 5 at 12:56
• In a heat pump, heat flows from cold to hot (which requires work). Commented Aug 5 at 14:08
• @Beska Congratulations on winning the Banal Observation of the Week award. Commented Aug 5 at 18:56
• My point was that you weren't answering the question. An "Is it always the case...?" type question can't be answered with "here's a case where it happens." Commented Aug 5 at 19:21

You can't make something hotter than the source using a passive system (such as a system of lenses). You can set something on fire using sunlight and a lens, but no system of lenses, but no system of lenses could allow you to do the same with moonlight, as the maximum brightness you could achieve would be that of the surface of the moon. You can make something hotter than the source using an active system, such as one which generates electricity and uses that for heating, but it is important to note that such a generator will need a temperature differential to work with, a cold 'rest of the world' into which waste heat is transferred (and there will necessarily be some waste heat)