Why can you put a can of frozen concentrate juice in the microwave? The metal doesn’t spark or burn.
The microwaves in a microwave oven consist of electric and magnetic fields. Since electric fields push on electric charges, microwaves cause electric currents to flow through any metal objects they encounter. These movements of current don’t necessarily cause any problems in a microwave oven. In fact, metal objects only cause trouble in the microwave oven when they are so thin or narrow that they can’t tolerate the electric currents that flow through them or when they have such sharp ends that electric charges leap off them as sparks. A thin object like a twist-tie can’t tolerate the currents and becomes very hot. Its sharp ends also allow charges to leap out into the air as sparks. But the thick, rounded end of a juice concentrate can easily tolerates the currents sent through it by the microwaves and doesn’t have the sharp ends needed to send charges into the air as sparks. It doesn’t present any problem for the microwave oven.