Bobby Sharf, age 11, asked:

Why do I get shocked sometimes when touch something?

Prof. Foletta answers:

Bobby, I hope you don't mean that you get emotionally disturbed after touching something; if so please talk to your parents about it. If you mean that you are disgusted when you touch something gross, that's normal. So there's nothing to worry about. However, if you mean that you get a shock of pain from touching a metal object, that is something I can explain to you.

The shock you feel sometimes when first touching a metal object is due to a spark or small bolt of lightning from static electricity, as you may know. But what is the spark? The spark is caused by a large difference in electrical charge between you and the metal object. An electrical charge is due to small atomic particles called electrons (negative) and protons (positive). All matter is made up of atoms with these particles, normally in equal numbers. Since everything has about the same electrical charge, a spark is not produced when you touch something. If the air is dry (like in winter) and if you walk rapidly or slide across certain materials (like a cat, wool, nylon, polyester, etc.) a large quantity of electrons can collect on you. These electrons build up on you if there is little moisure in the air to drain them off to other objects. Electrons rub off some material very easily because electrons are very much lighter than protons, and are on the outside of the atom.

The pain you feel with a shock is due to electronic charge quickly equalizing with some other object. The spark jumps the air gap just before you touch the metal object and is very small in size or area. The small area of the spark contact point on your body magnifies its effect as pain. You can prove this by doing what ever you did to get a shock but touch the metal object in only a certain way. First, do the charge build-up process, the rubbing or walking, holding a key or spoon in your hand. Then before touching the metal object, touch it with the key or spoon first. Then touch the metal object directly with your hand. Unless you built up a very, very large charge you may only hear the spark, but not feel it when touching the object. Then after the charge is equalized and you touch the object directly there is no electricity to flow and no pain. The spark struck the key or spoon surface and the charge was equalized. The same amount of electricity is flowing when you hold the key or spoon but the flow is spread over the surface of your hand and is below your feeling of pain.

You can learn more about static electricity and properties of materials in physics courses and general physics books.

Kay Gates, asked:

How does a microwave oven make food hot and not get the oven hot?

Prof. Foletta answers:

Kay, a microwave oven works by heating the food itself and not the air around the food as with a kitchen convection oven. Heat is the motion of molecules in substances. Normally heat is transferred between objects by convection or the direct contact of the molecules. The microwave oven generates large amounts of microwave electromagnetic radiation inside the oven cavity. That microwave radiation would go out into space like a microwave radar system, except the oven cavity is designed to keep the radiation inside. In fact, the microwave generator in a home oven is powerful enough (600 Watts) that it could be used in a system to bounce a signal off the moon and receive it back on Earth. Microwave energy more than few microwatt is harmful to living beings. So it is important to keep a microwave oven in good working order and never stay close to it while it is operating.

Microwave energy heats a substance if its molecules have a nonsymmetrical structure or effective charge dipole spacing and if the molecules are free to move. Water, the most common molecule in food, will move rapidly in response to microwave energy and heat up the other food molecules that do not respond. Some substances (like air, glass, plastics and some oils) have a molecular structure that does not respond to the microwaves. Metals that are good electricity conductors and thick enough reflect the microwaves, as a mirror reflects light, without heating up. A metal chamber is used to keep the microwave radiation contained within the oven.

You can learn more about microwaves and electromagnetic radiation effects in advanced electromagnetics books.