The French agency that regulates radio frequencies, the ANFR, has notified Apple of its decision to ban iPhone 12 sales after tests showed the Specific Absorption Rate...
It’s not per kilogram of the device. It’s a measure of absorption per kilogram of body mass of the person using the device.
A good way to think about it is: throw a small piece of meat into the microwave and let it absorb some microwaves. It gets warm. Now imagine a much larger piece of meat. It can absorb a lot more microwaves before it starts getting warm. The smaller the ‘body mass’, the less radiation it can absorb and dissipate before becoming damaged. So when it comes to mobile device radiation levels, they focus on the watts of radiation absorbed per kilogram of body mass.
and different parts of the body resonate at different frequencies…part of the reason ~2.4 GHz was picked for the common household microwave is water molecules resonate there (and other harmonically related frequencies too… it’s why a lot of the unlicensed ISM stuff is allocated there: crappy atmospheric propagation). also the necessary magnetron and waveguide for that freq is conveniently sized for a kitchen appliance and not too complicated.
EDIT: see proper principle of operation in reply below
It is a common misconception that microwave ovens heat food by operating at a special resonance of water molecules in the food. Instead, microwave ovens heat by causing molecules to spin under the influence of a constantly changing electric field, usually in the microwave frequencies range, and a higher wattage power of the microwave oven results in faster cooking times.
It’s not per kilogram of the device. It’s a measure of absorption per kilogram of body mass of the person using the device.
A good way to think about it is: throw a small piece of meat into the microwave and let it absorb some microwaves. It gets warm. Now imagine a much larger piece of meat. It can absorb a lot more microwaves before it starts getting warm. The smaller the ‘body mass’, the less radiation it can absorb and dissipate before becoming damaged. So when it comes to mobile device radiation levels, they focus on the watts of radiation absorbed per kilogram of body mass.
and different parts of the body resonate at different frequencies…
part of the reason ~2.4 GHz was picked for the common household microwave is water molecules resonate there(and other harmonically related frequencies too… it’s why a lot of the unlicensed ISM stuff is allocated there: crappy atmospheric propagation). also the necessary magnetron and waveguide for that freq is conveniently sized for a kitchen appliance and not too complicated.EDIT: see proper principle of operation in reply below