Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Microwave oven

  1. May 29, 2009 #1
    just wondering if i put oil in a microwave oven will the oil get heated and why?

    i know polar substances will be heated but im not sure if the same thing will happen to non-polar substance like oil
     
  2. jcsd
  3. May 29, 2009 #2
    The microwave oven send out waves of energy that heats water, sugar and fat molecules,
    if any substance contain one of these elements, then it will be heated.

    If you put a piece of metal into the microwave oven then, the metal will reflect the microwaves which will ruin your microwave oven. The microwave oven also sets up electric currents within the metal.

    I have heard stories of the microwave ovens blowing up when oil or metallic objects are put in them, however, I have not seen any hard proof that will support this.
     
    Last edited: May 29, 2009
  4. May 30, 2009 #3
    Before I answer let me tell you that I am a microwave scientist and have worked microwaves and microwave ovens for nearly 50 years - I also teach this stuff.
    1. microwaves heating oil: while a much poorer microwave absorber than water, oils still do absorb microwave energy and heat, especially if the quantity of oil is large - say 4 ounces or more. Also, oils have a specific heat capacity of about 0.5, which is half that of water (1.0) and that means that for a given amount of microwave energy absorbed oil will heat twice as much as water. But be extraordinarily careful heating oil inside a microwave oven because oils can easily reach temperatures of over 400 F! This can cause serious burns. So, it is best not to heat them inside a microwave oven.

    2. metals in the microwave oven - they will not destroy the oven or cause it to blow up. I routinely heat my coffee with a spoon in the cup. I also did the definitive early research on this in the late 70's and early 80's. But it is possible for metals to arc (spark) under certain conditions. This can be dangerous especially with things like metal twist ties and steel wool. Also, things like the metal trim (silver or gold) around the rims of fine china is dangerous in that the dish or cup can easily beak or shatter - but this due to the trim not being perfectly continuous like a wire that would carry current, Instead the trim has microscopic gaps and that can cause micro-arcs and temperatures exceeding 1000 F locally.
     
  5. May 30, 2009 #4

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    Welcome to PF!

    Hi microwaveguru! Welcome to PF! :smile:
    I did a forum search and noticed you've corrected some errors in old threads!

    :biggrin: You're hot stuff! :biggrin:
     
  6. May 30, 2009 #5
    Thank you tiny tim - I'll do my best not to disappoint.
     
  7. May 30, 2009 #6

    RonL

    User Avatar
    Gold Member

    Would metal, completely submerged, in water still arc or would it help concentrate energy into the heating of the water?:smile:
     
  8. May 30, 2009 #7
    The "why" part of the original poster's question is still pending. It's about how to reconcile -- on one occasion the schools teach that water molecules are polar and oil molecules nonpolar -- on another occasion they say that microwave ovens work on the principles of making polar molecules spin around to align themselves. So why does the microwave oven heat oil?
     
  9. May 31, 2009 #8
    This is a fascinating question and not easy to answer because microwave ovens are extremely complex and not as well understood as you might think. Anyway, here are some thoughts.

    First, you have to understand that microwaves are a form of electromagnetic energy, just like visible light, TV, radio, X-rays, etc. Each of these is distinguished by its frequency and associated wavelength. For the microwave oven the nominal (average –it’s really a range) frequency is 2450 Megahertz (MHz) or million of cycles/second, or 2.45 Gigahertz (GHz) billions of cycles/second; and this has an associated “free space” wavelength (effectively air or vacuum) of 12.2.cm. The wavelength decreases as it passes through an absorber such as water where the wavelength at room temperature is a bit over one cm. It’s called an electromagnetic wave because it has an electrical component and a magnetic component – both have to be there – one can’t exist without the other. It is the electrical component that heats things like water, food, oils, etc. The magnetic component heats magnetic materials such as iron (ferrous) containing materials.

    The answer to your question is very complex because there are so many factors (I originally wrote an answer early this morning but, because I am new to this site, it got lost in never-never land. Since then I’ve thought of more things). The thing that stands out in my mind is the relative volume of water & metal. Let’s say you have a liter of water in a glass beaker and you place a small magnet on the center bottom. It is unlikely the magnet would get any microwave energy because the water would have absorbed all of it since the penetration depth of microwaves into water at room temperature (the distance at which 63% of the energy is absorbed and causes heating) is a little more than a centimeter. I’m neglecting microwave energy coming through the center bottom of the beaker – very complex! So, all the microwaves heat the water….. But now, let’s assume the magnet’s volume is large enough to come within a centimeter of the glass side wall – first microwaves would be absorbed by the water, but there is likely to be sufficient microwave energy left to heat the magnet. It sounds good, but there are lots of complications. For example, if the magnet is that large, then there isn’t much water so the magnet will be heating a small volume of water and that represents a particular case of the problem. I suggest you consider how you would experiment to learn the answer. A warning – I’ve been thinking about how to run the experiment and it is very difficult. I’d be happy to try t help you. Good luck! By the way, it couldn't possibly arc - the electric field energy would be far too small because of the water.
     
  10. May 31, 2009 #9
    Water molecules are dipolar – two hydrogen atoms and an oxygen atom. The heating of pure water is essentially due to the dipolar rotation phenomenon as you’ve probably been taught (actually more complex than that but OK for now). But add a pinch of salt to the water and a new mechanism dominates: ionic conduction – the acceleration of ions such as sodium, chloride, hydronium and hydroxyl ions, by the alternating polarity of the electric field, and that causes these ions to collide with non-ionized molecules like water and the collision impact translates to heat. So, there are many different heating mechanisms.

    Every molecule has intrinsic electrical properties called its dielectric properties. These are: the relative dielectric constant, the dissipation factor and the dielectric loss factor. The mathematics and physics is hairy, so let’s just say these govern how well a molecule or material “supports” (absorbs) an electric field and converts that to heat. Water heats well so its dielectric properties values are relatively high. A reason they are high is that the water molecule is small and mobile – it is able to rotate in an attempt to keep up with the flip-flopping electric field component of the microwave and that results in heat.

    Oils on the other hand are esters of long chain fatty acids, which are much less mobile and have difficulty rotating with the field. So there is very little energy absorption (or “loss”) and not much heating. Oil’s dielectric loss properties are about 2 orders of magnitude less (1.5%) than that of water. But understand: anything will heat in a microwave oven if it is in there long enough. So, if you have a fairly large beaker of oil – say a liter, it will absorb the microwaves and convert them to heat. Also, oil has a specific heat capacity of about 0.5, which is half that of water (1.0) – which mean that, for a given amount of microwaves, oils will heat twice as much as water. But, heating oil in a microwave oven can be dangerous – you can reach temperatures over 400 F in a confined space. You can get a very nasty burn form this. Please don’t do it.
     
  11. Jun 1, 2009 #10
    microwaveguru, thank you for your helpfulness!
     
  12. Jun 1, 2009 #11
    mikelepore: it's a pleasure!
     
  13. Jun 1, 2009 #12

    RonL

    User Avatar
    Gold Member

    Thanks for your answer,

    I was thinking of a solid ring of metal, like copper, and am not quit sure why you mentioned a magnet.
    I can't seem to find my way out of never never land, (much to the dismay to most on this and other forums:smile:) reading your reply, about 10 things popped into my mind.

    I'm always too close to the edge on this forum, so I won't go into any detail, but a couple of thoughts involved a coil of tubing, with the first part (two or three rings) being cermic and the rest copper, submereged in water and capable of rotation. principles of induction heating along with the principles of action, in a bubble jet printer.

    Needless to say the most important thoughts of the day will suffer at the expense of trivial nonsense.:mad: (but I do love never never land):!!)

    Again Thanks for your time.

    RonL
     
  14. Jun 1, 2009 #13

    QuantumPion

    User Avatar
    Science Advisor
    Gold Member

    While we're on the subject of microwaves, and since we have a resident expert, I have a question for you. How do those grey-colored materials that are packed with some microwave foods work? What exactly do they do to help heat something such as a pizza?
     
  15. Jun 1, 2009 #14
    Hi mikelpore - thought you might enjoy this factoid about microwave heating.

    There is a relationship between the microwave oven and what is being heated – one influences the other. This is a weird phenomenon. The temperature of water affects how much and how fast it heats. Cold water, say 10 degrees C (50 F) will heat faster than water at 20 C (68 F). Try it – first run the oven with about a liter of tap water for 10 minutes – then carefully remove it from the oven – be careful – it will be very hot. Let the oven cool down, with the door open, for about 20 minutes – wipe it dry and check to see that the glass turntable is at room temperature – if it is still too warm wait another 10 minutes. Then put ½ liter of 10 C water in a one liter beaker & measure the temperature before microwaving; and then heat for 2 minutes and measure the temperature again – stir like crazy until you get a stable temperature. Remove the beaker of water and discard. Cool the oven down for 5 minutes, then repeat the test with the 20 C water. Be sure you use the exactly same amount of water each time. You should see that the cold water heated more in the same amount of time.

    Why does it happen? Because all molecules vibrate at temperatures above absolute zero (minus 273 C) and the frequency of vibration increase as the temperature increases. Water’s vibrational frequency at room temperature is almost 10 times greater than the frequency of the microwaves in the oven – so the energy capture by the water is fairly inefficient. Heat the water and its frequency of vibration increase while that of the microwaves stays the same – so it is more inefficient and so it heats less.

    But here are many materials that heat faster as they get hotter. Polypropylene is a good example – it is a large molecule with a very low vibrational frequency – it is solid at room temperature, But when it melts (at about 240 ) its vibrational frequency increases enough that it starts to capture microwaves and heat – that means it heats more efficiently as its temperature increases, and, like an avalanche – the warmer it becomes the faster it eats causing a phenomenon known as “thermal runaway” or “runaway heating” – very difficult to control and can be dangerous – don’t try it – it really needs a proper laboratory setting and lots of controls.

    Microwave heating can be fascinating!
     
  16. Jun 1, 2009 #15
    I chose a magnet because it is magnetic and easily understood regarding the interaction with the magnetic field. It would be the same with any material with magnetic properties.

    Your ceramic/copper,etc. is very complex and I wouldn't know where t begin. It may be worth experimenting. But such experiments are very difficult - even accurately measuring the temperature of water in the oven is extremely difficult - I'm presenting a paper on this in July. Among things to consider are:
    - how much water?
    - the temperature of the water.
    - size of your samples.
    - where located in the water
    - how to compensate for the reduction of water volume due to the presence of your sample.
    - even if it doesn't heat due to microwave interaction, the sample can act as a heat sink and steal heat from the water.
    And that's just the beginning of the list and I haven't even started tlking about the oven.
    But, I do wish you good luck and admiration if you can put together a careful experiment!
     
  17. Jun 1, 2009 #16
    The grey-colored materials are called “microwave susceptors”. They are specially manufactured microwave interactive packaging materials that get very hot – 375 F – very quickly and provide a hot surface for the pizza to crisp. A susceptor is made by coating a thin film of plastic (PET: polyethylene teraphthalate) with vacuum deposited aluminum (that looks like minute island of aluminum under an electron microscope). That film is then bonded (glued) to paper or paperboard – depending on whether you want a flexible or rigid susceptor – your pizza tray is rigid. That then gets packaged with the pizza and it’s placed under the pizza when microwaving it.

    There is a lot of complicated physics here but to make it as simple as I can: You may remember this heating equation:
    P = I2R , where
    P = Power in watts – that tells us how hot it gets
    I = current flow (amperes)
    R = resistance to current flow (ohms).

    Aluminum foil will let a current flow though it with no resistance to the flow – so P is zero – no heating

    Paper has lots of resistance, but it won’t allow a current to flow though it, so again P = zero.

    In a susceptor those tiny aluminum islands allow a current to flow but there are gaps between the islands, which represent a resistance, and if the current can jump the gap you get Power – watts – heat. It’s sophisticated though – the island size and the gap size is critical. If he gap is too large = too much resistance so no power; too small and there is not enough resistance = not enough power to heat.
     
  18. Jun 1, 2009 #17

    QuantumPion

    User Avatar
    Science Advisor
    Gold Member

    Fascinating. I didn't know that the material itself was getting so hot, since when you are done cooking they don't feel particularly hot to the touch. I thought they just reflected microwaves back into the pizza or something :smile:
     
  19. Jun 1, 2009 #18
    But wax will.o:)
     
  20. Jun 1, 2009 #19
    Actually they get very hot - 375 degrees - and then they stop heating, so it probably lost a lot of its heat to the pizza. I know they get hot because I touched one that didn't ave any pizza on it and burned my hand - on television!
     
  21. Jun 1, 2009 #20
    No -it shouldn't - wax is interesting (I gave a paper in Japan last year that involved heating wax). Wax is almost microwave transparent when it is solid and can stay like that for minutes of microwaving. Then, once the wax melts it becomes very microwave absorptive and heats like mad - it exhibits "runaway heating" - see my response to another question. Now, what happens when the wax isn't absorbing microwaves and why doesn't it blow up the oven? Microwave ovens are constructed to protect the magnetron from lots of reflected power when it is empty or has something like solid wax in it - in this case, the glass turntable is made to absorb the microwaves (technically it is called a "matching load"). Also, the steel that makes up the walls of the oven are slightly magnetic, so the walls will also absorb some power. The trick in microwave oven design is to get most of the energy to the food when you need it. Who needs hot walls?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Microwave oven
  1. Microwave oven (Replies: 6)

Loading...