[Physics] Why do two heaters with the same power produce different heat

electricitythermodynamics

I have two portable electric heaters and their power is exactly the same: 2000W. One is using ceramic technology, and the other resistor technology. The one with ceramic technology produce a lot more heat.

Considering they have the same power, where has the energy not converted into heat (by the less efficient heater) gone?

I didn't expect there would be so much difference, as heat is usually one of the unwanted byproducts of machines intended for different purposes, like motion. And I know from the thermodynamics laws that energy is not destroyed or created. So where has the missing heat gone? And how can two heaters with the same power produce different heat outputs?

Best Answer

A ceramic heater works by convective heat transfer, i.e., the energy consumed directly goes towards increasing the kinetic energy of air molecules, which are then blown into the surrounding space. The warmth felt due to such a heater is due to molecules in that warm air colliding with your skin, causing random motion of molecules in your skin due to the collisions.

In contrast, an infrared heater with a tungsten filament or other resistive element that grows red hot works by emitting electromagnetic radiation in the infrared range. The energy consumed by an infrared heater mainly goes into producing that electromagnetic radiation. When that electromagnetic radiation interacts with the molecules of your skin or other surface, it causes random motion of the molecules in your skin (heat) by causing the molecules to transition to a higher-energy vibrational state.

Which one will feel warmer sooner depends on the circumstances. An infrared heater can be more efficient by transferring heat energy directly to your skin, which is where you want it to be, instead of wasting energy by warming air molecules that might not even ever wind up subsequently hitting your skin, for example if you're in a drafty garage. On the other hand, the electromagnetic radiation emitted by an infrared heater travels in a straight line, so you might not be directly heated by the electromagnetic radiation if you're in a part of the room where there isn't an uninterrupted straight line between the heating element and your skin.

If you're measuring heater effectiveness by measuring how long it takes to get the air in a room up to a certain temperature, a convective heater is likely to win, because heating air is what a convective heater does directly. An infrared heater heats whatever the radiation hits, which means you could for example be wasting energy by heating a wall or something directly, instead of nearly all of the energy going into heating air.