It sounds like you've got a really interesting science project. But let me say first that I would recommend that you should NOT do it! Or if you do do it, do it with a microwave that no one will ever want to use again, because you will probably destroy the microwave if you run it for very long without any water inside. Also, if you do do it, make sure there’s an adult around in case something catches on fire.
Microwaves work by shooting tiny waves called microwaves through food. Microwaves are a lot like light waves, except that they’re a lot bigger. When the microwaves go through the food, they make the water molecules vibrate. (This is because water molecules happen to have positive and negative ends and respond to the electric fields generated by the waves. The explanation for this is a bit tricky, but that’s the general idea.) Molecules other than water won’t be affected by this much.
If you put a cup of water (or anything with water in it) into the microwave, nearly all of the microwaves are absorbed by the water. If you run the microwave without anything in it, the microwaves aren’t absorbed by anything except for the microwave itself. There are certain parts in the microwave that can be damaged by absorbing microwaves. This is why you should not run a microwave empty or without anything with water inside in it. (Remember that food has water in it.)
Microwaves can go straight through most (but not all) kinds of paper, plastic or glass with no effect, so running the microwave with just paper, plastic, or glass inside should in most cases be about the same as if there were nothing inside at all. (This is why many microwaveable foods are in containers made of these things.)
But microwaves can not go through metal (including gold and silver). When they hit the metal, they are reflected, just like light off a mirror. If there is only metal inside the microwave, this is even worse than running it with nothing inside, because the microwaves will be reflected right back into the parts of the microwave that will be damaged. Also, if the metal has sharp points or bends or odd things about its shape, you can get sparks or arcs of electricity between two points on the metal. This can cause the microwave to catch on fire. (This why you should never put metal twist-ties in the microwave. The sharp points of the metal create sparks that cause the paper of the twist-tie to burn.)
In theory, if you were to put the metal piece inside of some water, it should cause the water to heat up more quickly, because the metal will reflect stray microwaves back into the water. But you should not put two pieces of metal within 1/2 inch of each other or the edge of the microwave. The microwave has some metal in its walls to reflect microwaves back into the food, and you can get electric arcs between the sides of the microwave and metal pieces inside. But I would not recommend trying anything with metal in the microwave (even if it /should/ be safe), because you run the risk of something unexpected happening.
Different liquids should heat up fine. The bigger the proportion of water there is in them, the faster they’ll heat up. You could probably make a very good experiment just out of looking at different liquids (and/or foods with water in them). You could also consider how well the microwave heats liquids in different shapes/sizes of containers...and possibly find out what the best type of container is to use in the microwave. Another thing you could do is to get some relatively solid food and find out which parts of it heat up first by measuring the temperature at different places inside after heating it for different amounts of time.
But you really shouldn’t do any sort of experiment where you run the microwave without any water inside, especially not with metal inside.
(published on 10/22/2007)