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How do electromagnetic waves transfer energy?

  1. Jan 16, 2016 #1
    I was thinking about a laser, a very strong laser, how does it "burn" things? And what about the microwave oven? What happens in the atomic scale? I know that when something has an increase in temperature the atoms moves quicky because the the temperature is proportional to the kinetic energy. How the waves transfer kinetic energy to the atoms and molecules of those object's

    I think is because the waves transfer energy to the electrons of the atoms but it doesn't make sense because all the atom (then the nucleus) has to acquire energy, not just the electron.
    I think quantum mechanics is involved in a possible explication of the phenomenon so please explain me the process (how the light tranfer energy) step by step if you can use also equations (if they are differential equations show them in the simpler form)

    Thanks
     
  2. jcsd
  3. Jan 16, 2016 #2
    Electromagnetism interacts with electrons in atoms and can raise them to a higher energy state, which can be measured as increase in the atom's temperature.
    Nuclear interactions occur only via the two nuclear forces, (the strong and the weak nuclear force.)
    Some nuclear reactions do EMIT electromagnetic energy in the form of gamma rays though.
     
  4. Jan 17, 2016 #3

    Svein

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    Science Advisor

    The microwave oven is equivalent to a capacitor with a very high frequency field across it. When something with bad dielectric behavior is put inside that capacitor, there will be a dielectric loss → heat.
     
  5. Jan 17, 2016 #4
    I'm no expert, but I believe microwaves transfer energy to water molecules.

    "Quanta" are all about geometry. A quantum of energy is produced in some geometric context. Think of it as a gap between two particles. If the gap is small, the quantum is large. That quantum travels through space until it encounters another gap of the same size, which can then absorb it.

    So a gamma ray laser would be poor at burning Earthly things because it has trouble finding a gap small enough to fit into. Microwaves are much longer wave length and can excite bigger things, like water molecules. The atoms in the water molecules are jiggling around so the gaps are constantly changing in size, so there is a broad band of radiation they can absorb.
     
  6. Jan 17, 2016 #5
    First of all clear the concept that wave is a periodic motion meant to transfer energy from one point to other. Many people simply use the word "wave" being confused what it exactly is. Electromagnetic Wave is the same thing. When an electromagnetic wave is emitted, it means energy transfers from source to target. You could think of EM wave as itself being made of energy. EM waves are made of photons(energy packets) which move in a periodic manner and travel to the target.

    When we say an EM wave is travelling, we mean that energy is travelling.

    As soon as the wave hits the target, the energy of the wave is at first absorbed by the electrons.Electrons themselves are stationary waves. When EM waves approach the electrons, it gets stored in the orbit of the electron,thus more energy is now packed in a small orbit. Thus to make it stable, electrons(or simply wave) move away from nucleas When energy of electrons is increased, they jump to a higher orbit and their distance from the nucleas increases. Thus each and every atom enlarges and that is what thermal expansion is at a large scale. If electrons get enough energy to cause a chemical reaction, a reaction occurs and that is what we see in the form of BURN.

    Energy is absorbed by both nucleas and electrons. When electron acquire energy they change their orbit. When nucleas acquires energy, it vibrates more vigorously than before. Thermal energy or temperature is nothing but just a measure of the average kinetic energy of the nucleas(or simply atoms).
     
  7. Jan 17, 2016 #6

    Nugatory

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    Intuitively, the mechanism is straightforward: when an electromagnetic wave passes through a region of space, that means the electrical field in that region is oscillating back and forth, pointing first in one direction and then the other. Electrons and atomic nuclei are both electrically charged, so they are pushed back and forth by the wave - and that motion ends up as heat. The kinetic energy of the molecules comes from the energy of the electric field, which is weakened as it pushes the charged particles back and forth.

    You are right that this phenomenon is quantum mechanical at heart - we're dealing with subatomic particles and quantum mechanics is an important part of their behavior. However, when we're presented with a real problem ("How powerful of a laser do I need to drill a hole one centimeter wide in a piece of steel plate ten centimeters thick in five seconds?") we don't use quantum mechanics. Instead we work with macroscopic properties of the steel (things like the fraction of the incident laser light it reflects and absorbs, how quickly it heats up when energy is added to it, what temperature it melts at, what temperature it boils at, is there enough oxygen around to burn it to iron oxide as it heats up) to see how much energy we have to dump into it in a given time, then choose our laser accordingly.
     
    Last edited: Jan 17, 2016
  8. Jan 17, 2016 #7

    Nugatory

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    Neither of these statements are correct, although considering how often they are repeated in pop-sci writing one can be forgiven for repeating them as if they are fact.
     
  9. Jan 17, 2016 #8

    Drakkith

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    I don't recommend thinking of an EM wave simply as being "moving energy". It's certainly true that an EM wave transfers energy, but that fact tells you nothing about its properties or behaviors. More info below.

    It's perhaps better to say that when an EM wave is traveling, we mean that a disturbance in the electromagnetic field, in the form of periodic oscillations of the field vectors, is moving from one location to another. Since the field vectors represent the direction and magnitude of the electric and magnetic fields, these oscillating field vectors are able to exert forces on objects and, by virtue of these forces, transfer energy to the objects.

    Don't get too caught up in using energy as an explanation. Energy is a tricky thing. For many, it's simply bookkeeping. A consequence of the symmetry of certain natural laws. It's certainly a useful concept that can make it much easier to build a model or ease calculations, but relying on it to be an explanation of why things happen is sometimes not very useful.

    The EM wave interacts with the whole atom (or molecule) all at once, not just the electrons. The atoms/molecules can absorb and 'store' energy in a variety of ways, including the vibration of the atoms, electronic transitions (electrons changing orbitals), or the breaking and/or making of molecular bonds.

    Note that thermal expansion is not a result of each atom enlarging, but the result of the increasing average distance between atoms or molecules.

    Not just the nucleus or the atom, but the electrons too. For example, a metal has a great many electrons that are free to 'roam about' the whole material. As you increase the temperature of the metal, these electrons are accelerated to larger average velocities and momentums. Temperature is also a measure of the energy stored in the vibrational and rotational states of the atoms and molecules.
     
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