Microwave Pizza

1. Oct 23, 2004

Math Is Hard

Staff Emeritus
I have been cooking these mini microwave pizzas lately and they come with a special round cardboard disk that you set the pizza on when you place it in the microwave. The top part of the cardboard disk has a silver-colored coating. I place this silver-side face up (per the directions), and put the pizza on top of it. The pizza almost completely covers the disk.
I have noticed that after a couple of minutes the pizza crust gets heated very well but the cheese and toppings are still cold. The only way to heat the topping is to take out the disk and put the pizza on a paper towel and then nuke it some more. Why is this? I know it's a silly question but I was wondering how the reflective material under the pizza is directing the microwaves. Thanks.

2. Oct 24, 2004

Pieter Kuiper

The silvery cardboard thing is _absorbing_ the microwaves, so it gets hot. It is heating the pizza like a pan pizza is heated. This prevents the pizza crust from getting soggy.

3. Oct 25, 2004

Math Is Hard

Staff Emeritus
Thanks.
I thought the silvery stuff would reflect microwaves instead of absorb them. Why aren't microwaves reflected the way visible light is reflected off shiny surfaces?

4. Oct 25, 2004

Pieter Kuiper

Because of the difference in frequency. Microwaves are a million times slower than visible light. The electrons in metals move with the electric field of the microwaves, as in a resistor. The field accelerates the electrons, they lose energy to the lattice, and the resistor gets warm.

But with visible light the electrons are wiggled back and forth many times before an interaction in which they lose energy to the lattice normally takes place, and this wiggling in its turn sets up the reflected wave.

5. Oct 25, 2004

Math Is Hard

Staff Emeritus
That is so cool! I never knew that. Thanks, Pieter!

6. Oct 25, 2004

Chronos

Metals get hot when microwaved, as Peter noted. I know this firsthand. My granny used to dry silverware in her microwave [she was in her 80's and a bit addled]. She set a number of dish towels on fire, but never blew up the microwave. I never did figure that out.

7. Oct 25, 2004

Math Is Hard

Staff Emeritus
:rofl: :rofl: :rofl:

I guess that silvery coating is specially formulated so it gets hot but not too hot. Microwaving something metal is something I always secretly wanted to do just to see what would happen. That, and licking a frozen flagpole in the dead of winter. :tongue2:

8. Oct 25, 2004

Gokul43201

Staff Emeritus
It's not such a terrible thing to put a metal object into the microwave for a little bit. It'll just get hot fast,'cause the fields are accelerating the elctrons, as Pieter explained. Microwaves are more robust than most people imagine...as chronos and his granny will testify.

I've played with putting different weird things into my microwave :

A large, rusty (and the rusty bit is essential) metal hammer sat in my microwave for about a minute. Nothing happened.

Air-blown soaps, like Ivory, are fun to put into a microwave. They fill the microwave with soap foam.

CDs are fun too...I've put countless AOL CDs in the microwave.

Some freshly cut fruits (lots of electrons in an electrolytic solution) are real good. A freshly gut grape (if cut just right) will ionize the air in the microwave and create a beatuful plasma ball in the microwave. Getting this to work repeatedly is a little hazardous to your microwave oven.

9. Oct 25, 2004

Math Is Hard

Staff Emeritus
Wow! Gokul, you have given me lots of ideas for experiments. In the past, I have limited myself to the nuking of little green plastic army men. There's just a whole new world of things to play with! The grape thing sounds reallllly cool. I am getting a new microwave oven soon, so the old one can be abused a little.

What happens to the AOL CDs? Anything cool?

10. Oct 25, 2004

Gokul43201

Staff Emeritus
Cool ?!! :surprised: It's better than watching 4th of July fireworks !!

11. Oct 25, 2004

Chronos

Raw egg still in the shell [very impressive]
Fresh banana still in the peel
one minute souffles [very tricky]
Fresh whole shrimp
Caviar
brass wire brush, paper towels, and oven cleaner [do not mw, just have available]

12. Oct 26, 2004

rhia

Great! I need to know how this happens in more detail.Any links,pointers?

thanks

13. Oct 26, 2004

Pieter Kuiper

rhia: I cannot find much on the web. HyperPhysics explains resistivity in metals.
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/ohmmic.html

In Nave's exampel of copper, the relaxation time $$\tau=\frac{m_e\sigma}{ne^2}= \frac{d}{v_F} = 2.5\times 10^{-14}$$ seconds.
This corresponds to frequencies in the near infrared.

The conductivity at high frequencies goes down around the inverse relaxation time and the phase of the response changes. The complex frequency-dependent conductivity can be expressed as
$$\sigma(\omega)=\frac{\sigma_0}{1-i\omega\tau}$$.

Last edited: Oct 26, 2004
14. Oct 29, 2004

Gonzolo

I put a boiled egg with the shell in the microwave once. I remember it was a pretty loud noise.

15. Oct 30, 2004

alexepascual

I put an egg in the microwave too years ago. It exploded as I was taking it out. It made a mess. Threre was egg everywhere, even on the ceiling. I would suggest that if you try this you wear goggles. I didn't do it as an experiment but just out of ignorance. (I am wiser now)

16. Oct 31, 2004

Cosmo16

Gokul- How do you have to cut the grape?

My mom will kill me if she sees this.?

17. Oct 31, 2004

Math Is Hard

Staff Emeritus
:rofl: :rofl: LMAO!!! :rofl: :rofl:

18. Oct 31, 2004

Gokul43201

Staff Emeritus
At this point I disclaim any responsibility for the well-being of your microwave oven.

That said :

This has a low success rate, you may have to go through 3 or 4 grapes before you get one to work, but when it does, it's really worth it.

You'll need a good, sharp knife, so be careful.

1. Cut the (seedless) grape in half, and use one of the 2 halves,
2. Put this half-grape down, resting on its curved surface, with the flat (freshly cut surface) facing up towards you,
3. Cut this again, starting from the flat surface and slicing downwards (to the south pole), but stop just short of cutting it into 2 separate pieces,
4. Now you have 2 quarters, just barely attached to each other. Pull these quarters slightly apart, so they are not completely in contact. Do not separate them completely...you just want to create a small wedge-shaped air gap.

That should do it. Again, be careful !

Best of luck !

19. Oct 31, 2004

Gokul43201

Staff Emeritus
I should also warn you, that this may leave a black burn mark on the inside of your microwave, if you're unlucky ! :grumpy:

PS : This just struck me....have you been trying the other objects out ? How did those work ?

20. Nov 1, 2004

Chandler

Man reading a post like this on any other site would just be boring eh :)

21. Nov 14, 2004

Tachyon son

To add some (serious) freakness to the topic:

What kind of interaccion should we expect?

22. Nov 14, 2004

colinr

The coolest thing i ever saw in a microwave was this. Fill a glass with milk, put an ordinary tungsten filament light bulb so that the metal part is completely immersed in the milk. Put the glass in the microwave and the bulb will light up!!

It looks amazing, but still to this day I cant explain it, does anybody know why this works. (try it if you dont believe me - it will not damage your microwave or the bulb)

23. Nov 14, 2004

JosephRombousky

the light bulb thing sounds cool, but i cant understand how that could possibly ever work either. you need a flow of current, if both anode and cathode are in the electrolylte(milk) their shouldn't be any current flow. Unless the filament itself is being excided, and the milk is just a ground or something.....

24. Nov 14, 2004

Gokul43201

Staff Emeritus
I suspect this may simply be from the ionization of the gas (argon?) in the bulb. The milk only serves as a coolant/protection from sparks - only guessing - as i can't see what other functional purpose it serves.

Try this with a broken (filament) bulb, and if it still glows, you can be sure it's not from a current in the filament.

Actually, I'm going to try this when I get home.

PS : I would not do this for too long...if you really heat up the gas in the bulb, might it not rupture at some point ?

25. Nov 14, 2004

Math Is Hard

Staff Emeritus
OMG! I just tried this, but I suspended the light bulb in water instead of milk. It sure enough worked!!!!! I used a blue glass to see the glow better and it lit right up!