# What mechanism at the microscopic level determines whether a system heats up or not?

When placed in an ordinary or a microwave oven, a beaker of water heats up except during boiling (i.e., a phase change involving latent heat). Now, suppose a system absorbs energy in such a way that the electrons are excited to higher energy levels. Will this necessarily heat up the system? In other words, is heating caused by the electrons or atoms in an object being excited to higher energy levels?

## 3 Answers

The microwave photon energy level gives rotational kinetic energy to water molecules due to interaction of the electromagnetic field with the water dipole molecule. The rotational kinetic energy is subsequently randomized to increase the average translational kinetic energy of the molecules, increasing the temperature of the water molecules. The microwave energy levels are well below those needed to excite electrons to higher levels.

A regular oven cooks with infrared electromagnetic energy. The photon energy level corresponds to molecular vibration. These energy levels are likewise too low for electron excitation.

Hope this helps

• A toaster oven certainly uses radiation, but I'd argue that a bake-a-cake oven also uses conduction and convection. – psitae Dec 22 '18 at 20:17
• @psitae What's your point? Whether its conduction, convection, radiation (microwave or infrared) none of these cook by exciting electrons. – Bob D Dec 22 '18 at 21:26
• Why mention photons in this context? – Pieter Dec 23 '18 at 19:11

On the case of a microwave oven the water molecules in the food are made to vibrate. Water molecules are tiny electric dipoles and these are made to vibrate more strongly. The molecules stay in the electronic ground state.

• What happens in ordinary ovens/burners? For heating to take place, a necessary condition is that the system must absorb energy. For this to happen, there must be a coupling between the environment that supplies energy and the system that absorbs it. For the microwave example, it is the electromagnetic microwave that couples to the dipole moment of water molecules. – mithusengupta123 Dec 22 '18 at 17:14
• The microwave photon energy level causes rotation not vibration of water dipole molecules. The vibration energy is higher than the microwave oven photon energy. For a good explanation of the interaction of radiation with matter, check out the Hyperphysics web site – Bob D Dec 22 '18 at 17:24
• @BobD "Photon" does not explain anything here. The electric field polarizes the dipole orientation which relaxes when the field is switched off again. The field changes direction $2.45\cdot10^9$ times per second, so that becomes noticeable as heat. – Pieter Dec 23 '18 at 19:17
• @Pieter With all due respect, photon energy explains why microwave and infrared radiation do not excite electrons resulting in cooking, which I thought the OP was wondering. It also explains why the electric field in the kilohertz range does not cause molecule rotation. – Bob D Dec 24 '18 at 14:43

The heat contained in a system can be thought of as the total kinetic energy of a system. my2cts' answer is correct, but I'll add that if you excite an electron to a higher energy level, that will indeed cause heating in many cases (see for example FRET).

The friction between the water molecules and the rest of the system is what causes things to heat up in a microwave. In a conventional oven, it's a direct transfer of kinetic energy from hot air.

Homework question: would something heat up in a microwave if it didn't contain any water?

• Heat is energy transfer from one substance to another solely due to a temperature difference between them. Systems do not "contain" heat. You cannot think of heat as the total kinetic energy of a system. The proper term for the total kinetic energy (plus potential energy) is internal energy. – Bob D Dec 22 '18 at 21:31
• @BobD that's correct. I'll edit my answer. – psitae Dec 22 '18 at 21:34