If you put an empty mug in a microwave oven and press start for just one second, several things could happen:
No noticeable effect: One second may not be sufficient time for any noticeable changes to occur. The mug may remain cool, and the microwave's energy may not have had enough time to generate any significant heat.
Superheating: In some cases, when a container with pure water is heated in a microwave, it can become superheated. Superheating happens when the liquid's temperature exceeds its boiling point without actually boiling. If the mug contained water, there is a small risk that it could become superheated during that brief period. When disturbed or when an object (like a spoon) is placed into the superheated water, it can rapidly boil and potentially cause a dangerous eruption of hot water.
Mug damage: Depending on the material of the mug, intense and uneven heating can occur within just one second, potentially causing the mug to crack, shatter, or become damaged due to the thermal stress.
It's important to note that putting empty containers in the microwave is generally not recommended. Microwaves are designed to heat food and liquid, and heating an empty container can lead to uneven heating, damage to the container, or even damage to the microwave itself. It is best to follow the manufacturer's instructions and use microwave-safe containers with appropriate contents when operating a microwave oven.