The first heating mechanism is caused by the molecules in a polar liquid “hooking on” each other due to the uneven charge distribution of the molecule, so that one end gets an excess of positive charge and the other a negative charge – see the picture to the right which shows water molecule groups. The microwave field (+ and – and downwards directed arrows in the picture) will then exert a turning force on the molecules – they attempt to align with the field and energy is supplied to them. When the field direction changes (this occurs 4,9 billion times per second at 2450 MHz), the molecules return the alignment energy to the field – the system behaves like an electric capacitor connected to an alternating voltage. If the microwave frequency is very high, the molecule groups will however rotate so sluggishly (in relative terms) that they do not at all align with the field – no energy transfer takes place. In a certain frequency interval, the molecule groups will (statistically) still rotate but with some lag, which depends on their mass inertia. All energy will then not be recovered when the field direction is reversed. A part will instead be permanently given off to the molecule groups, which will heat up by a general net increase in their movement. The phenomenon is called dipole relaxation.