Why can't we put metal in a microwave oven even though the inside of the microwave is all metal?

Answer 1

The magnetron of the microwave is an extremely powerful transmitter of microwaves at the frequency of 2.45GHz. At this frequency fats and water absorb the energy by means of dielectric heating.

The metal inside of the microwave oven will not cause problems since it is earthed and act as a shield, it absorbs any electric field continuously over its relatively large and smooth area. Therefore no charge builds up, because it is earthed. It is so effective as a shield, that it would never leak as much as 10W/m2 at 5cm distance. Most manufacturers would specify leakage at 5mW/m2 as maximum while the magnetron inside produce about 800W of radiative power. Other experiments show that it can reduce a RF signal of 2.4GHz with about 100 times.

It is usually unearthed metal objects with sharp points thats cause arcing. The object act as an antenna and accumulates an electric charge from the electric field that is perpendicular to the magnetic field of the electromagnetic field transmitted by the magnetron. The electric charge tends to be more concentrated at sharp points without any path to discharge. Once the charge is large enough and the distance between this charge and the grounded shielding is short enough. The breakdown potential of air is bridged which acts in the same way as lightning does. Once the charge accumulate to a potential difference that exceeds 3MV per meter between the object and the earthed shielding, then the air becomes a conductor due to the great amount of electrons stored in the field around the conductive object. Then the charge on the metal object will discharge towards the shielding which is the shortest path to the ground. Once the most of the electric charge is transferred to the earth, the charge becomes significantly less and to little to continue the breakdown condition. The air becomes an insulator again and the spark is gone. This will be the case until the charge is generated again to a voltage of 3MV per meter.

This charge and discharge cycle can happen at fast intervals causing a buzzing or arcing sound. The breakdown potential is not only breaking down the air, but it also has to break down through the painted layer in the inside of the shield. It will be the same as with electronic capacitors that blow as the potential passes the breakdown potential, so would the painted layer of the inside of the microwave oven also burn and be seriously damaged. The temperature in the spark can run up to thousands of degrees centigrade.

Another problem can be electric current induced and received in thin foil that will cause heating or even fuse or melt. If a CD or DVD is placed into a microwave oven then the metal coating at the rear side of the disk will most likely melt away not because of the arcing as such, but because of high current flow through the thin metal layer which can even melt the plastic.

Not all metal objects in a microwave would actually spark. Some objects such as grills that are specially designed for microwave ovens often is designed to be large and smooth surfaces or even earthed. They would actually work in a microwave oven without problems. Whether the object will spark or not, has much to do with the shape, sharp points, clearance, earthed or not etc. Often aluminum containers may work well in a microwave as long as its sides are smooth and do not touch the sides, while a fork will spark like crazy due to the high charge that is concentrated at the tips. This however is not easy to assume for all metal objects whether it will spark or not. In some cases aluminum foil can be used in a microwave to shield of some parts of food that do not need as much heating. The normal foil is thick enough not to heat up from the current. But then again, wrinkled foil can be very unsafe because of its unevenness and its pointy texture. Therefore I would not recommend to experiment with metal objects in a microwave oven. Stick to the manufacturers specifications. It can cause serious damage to your microwave if the experiment would fail.

This is also the reason why we shouldn't use cellular phones at gas stations, although the risk is practically very small that the cell phone will cause nearby metal objects to spark and cause the fuel to ignite, it is theoretically possible in some scenarios to happen. However the microwave oven often is about 300 - 3000 times more powerful than a cell phone transmitter section and therefore the effect of a cell phone has on metal objects are significantly less.

Answer 2

To keep the microwaves inside.