Is ALL transfer of energy by electromagnetic radiation classified as "heat"?

Question

In my uni notes my lecturer wrote that energy is either transferred as HEAT or WORK. (he said "all other modes of energy transfer other than heat are called work". Heat can obviously be transferred by convection, conduction or radiation. In many places it says that heat radiation is 'infrared radiation'. This makes sense as infrared is the type of radiation that makes us feel the warmest (although other electromagnetic radiations such as light and microwaves can also make us feel warm... but perhaps this is just due to a bit of infrared radiation being mixed in with them?) So, what I want to know is, are all the forms of electromagnetic radiation (such as x-rays, light, radio waves and microwaves) in addition to infrared classified as 'heat transfer of energy'?

I have 2 hypotheses...

1. Perhaps my lecturer got it wrong... energy can be transmitted/transferred by heat, work OR radiation

OR

2. All types/wavelengths of electromagnetic radiation ARE in fact heat, but our bodies sense the infrared as heat/warmth more so than the other types, as it hits the outside of our bodies where there are heat sensors. For example, if we stand near lots of microwaves, we may not initially sense the thermal energy as 'heat' as it is heating up water inside our bodies rather than the heat sensors in the skin. However, our body would gradually heat up and we would start to feel the thermal energy we had acquired as heat. And another example, we can get sunburnt from UV rays even without feeling too hot (ie the burn can be out of proportion to the feeling of heat). This may be because the heat and therefore thermal energy we get from UV rays also doesn't interact with heat sensors in the skin as well as infrared can. So it is more of a 'human sense' issue on how well we can 'sense' heat rather than whether heat is actually transferred or not.

Are either of my hypotheses correct? Or is there another explanation? Thank you!

Answer 1

"Heat" has eveveryday definitions that overlap with temperature and energy. But in one particular strict physics sense, heat is a process, not an object. Heat transfer can be accomplished by any form of radiation (but you'd need a very hot object for some of them).

In addition, you can use the exact same kind of radiation to do work. So you can't look at the wavelength and say "that's IR radiation, therefore it is heat". You could use a device in some cases that will allow work to be extracted from the radiation.

energy is either transferred as HEAT or WORK.

This is a description that we can (at least in physics assignments) often lump energy transfer into two different classes. We can harness the energy transfer to do some sort of work, or we can let it dissipate on its own. The energy is moved from one place to another in both cases, but we call one work and the other heat.

If we build a fire in the middle of the woods, almost all of the combustion energy goes into raising the temperature of the immediate surroundings, which then heat other things and themselves cool off. After several hours everything has cooled to the same temperature as the surroundings because of heat transfer.

We could instead put that fire under a boiler, make it into some steam, and have it drive a piston. Now instead of all the energy leaving as heat, some of the energy has been harnessed to do work.

We can even imagine a thermophotovoltaic system that can absorb IR radiation and turn it into electrical energy. Now you can have some IR radiation that is heat and other IR radiation that is doing work.

Perhaps my lecturer got it wrong... energy can be transmitted/transferred by heat, work OR radiation

Radiation isn't a separate category from those two. Radiation can be a heat transfer mechanism and radiation can do work.

Answer 2

So, what I want to know is, are all the forms of electromagnetic radiation (such as x-rays, light, radio waves and microwaves) in addition to infrared classified as 'heat transfer of energy'?

No. There is a subtle difference between the transfer of energy and the transport of energy. All forms of "waves" transport energy from one location to another. On the other hand, the transfer of energy from a wave to a substance can occur by means of heat or work.

Heat is energy transfer due solely to temperature difference. So if the temperature of the electromagnetic radiating source is greater than the temperature of the substance interacting with the radiation, energy is transferred in the form of heat. This is the case for infrared radiating sources.

Work is energy transfer due to forces times translational displacement or torque times angular displacement.

This is technically how microwave radiation cooks food. Microwave radiation doesn't "heat" food. From the wave view, it cooks primarily due to the interaction between the alternating electric field and the electric dipole of the water molecule. The field exerts a rapidly alternating torque on the dipole giving the dipole rotational kinetic energy. That rotational kinetic energy is then randomized by collisions into increasing the translational kinetic energy of the water molecules, and thus increasing the temperature of the molecules (increasing the temperature of the food). So technically, microwave ovens cook food by electromagnetic work.

The manner in which electromagnetic radiation interacts with matter depends on the frequency of the wave (or energy of the photon, from the particle view). For an overview of this interaction, I recommend you visit this site:

http://hyperphysics.phy-astr.gsu.edu/hbase/mod3.html

Hope this helps.