Like if a put a toast with jam on top in the microwave and warm it up altogether, jam feels a lot hotter than bread. Why?
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Microwaves, specifically, tend to heat things unevenly. That’s because microwaves work by causing the water within the food to vibrate, and so are dependent on having (liquid) water available in order to heat foods. In foods where available water is very uneven (say, the water-based gel of your jam + the relatively dry bread), parts of the food with high water content will heat up first and faster.
Also, not part of the explanation, but why the hell are you putting toast in a microwave?
There are two thing going on in this question:
First, microwaves have a frequency that is close to the natural harmonic frequency of water so they are really good at heating things that have lots of water in them but bad at heating dry foods. The ELI5 version is to consider pushing someone on a swing. The swing has a natural frequency that it “wants” to swing at. If you time your pushes right and push at the same frequency, the swing will go higher. But if you just push randomly or at the wrong frequency, the swing won’t work very well: sometimes you will be pushing against the swing and slowing it down.
Second, all substances have a property called “specific heat capacity” which determines how its temperature changes when you add or remove energy. You have to add a lot of energy to water to raise its temperature but lead only needs a little energy. If you put a kilogram of lead and a kilogram of water in the same oven, the lead would get hot faster.
Going back to the swing analogy, if the person on the swing is heavier, you will have to add more energy by pushing longer or harder to get them up to the same height as a lighter person. Also, the swing could rotate or go side to side so some of the energy of your push might go to movement that does not increase the height of the swing. In a real substance, the configuration of its molecules determines how it can vibrate, rotate, or wiggle in ways that don’t contribute to temperature.
Because microwaves don’t apply heat. They add energy to water molecules present. Its energy is then presented as heat.
Of course different objects have different water contents which cause this uneven heating.
1. Energy transfer through different materials is different. This the same reason that touching any metal object on a hot day feels hotter than touching a branch right next to it.
We have an equation “Q = mcΔT” which states that: “c” the specific heat capacity of an object affects the the amount of energy is transferred from the change in temperature. Higher the specific heat capacity, the faster heat is transferred.
2. Microwaves don’t “heat” objects in the commonly thought of way. Microwaves emit electromagnetic waves in the “microwave” category (around 10^8hz and a wavelength of around 1cm). This wave shakes the water particles in the object and transfers energy to the water. This shaking of particles is what we know as “heat” and thus the water, and therefore the surrounding food is heated. Foods that transfer energy faster or foods with more water may be hotter than others that are not.
3. Solids tend to transfer energy (heat) faster (especially in small quantities). This is due to the fact that in fluids, they tend to create a “convection current” which transfers heat instead of conduction (liquids tend to do both). So the solid bread transfers heat faster than a viscous fluid like jam that can’t create a convection current.
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