Nuke or Oven

Qwakker

Wot is cheaper to cook my food, 45mins in elec oven or 10mins nuked.

Cheers

t0rt0ise

The microwave is cheaper. But to work it out properly you'd need to the wattage of each appliance.

I always wondered why we don't heat water for domestic use by microwave. There must be a reason but I haven't looked to see what it is yet.

Meatballs

In a kettle or immersion heater you are converting electrical energy to thermal energy.

In a microwave you are converting electrical to radiant (the microwaves) then to heat energy.

More chance for energy loss in the system I would guess.

Microwaves are also more sophisticated than a simple element heater, and would require shielding? Also unsure how far they would penetrate into a large body of water from a small source, or if you'd need a really big microwave?

Pincher

t0rt0ise wrote: »

I always wondered why we don't heat water for domestic use by microwave. There must be a reason but I haven't looked to see what it is yet.

A kettle is 3kW, or 2kW if you are a cheapskate
A domestic microwave is typically 800W, or 0.8 kW.
It is therefore four times quicker by kettle.

Despite this, I actually do use the microwave.
I make the tea with 90 degree C water from a hot water dispenser.
Tea bag, hot water, then I put the mug in the microwave for 15 seconds, which makes it almost boil. The water convection is just right to cook the tea bag and release the flavour. Then milk.

samtheman1k

Is it really quite that simple though? thinking back to my Physics days, a microwave excites water molecules on their resonant frequency. Does that technique that mean you get more than 100% heat out for a unit of electricity in (from a physics point of view)? (In a similar way to heat pumps generate more than 100% heat?)

samtheman1k

Meatballs wrote: »

More chance for energy loss in the system I would guess.

But a microwave is a Faraday cage, all the RF energy must go into the water...there is no where else for it to go!

Premier_2

samtheman1k wrote: »

Is it really quite that simple though? thinking back to my Physics days, a microwave excites water molecules on their resonant frequency. Does that technique that mean you get more than 100% heat out for a unit of electricity in (from a physics point of view)? (In a similar way to heat pumps generate more than 100% heat?)

Well that's the first law of thermodynamics kicked into touch if true

penrhyn

Indeed, a statement on a par with Oolon Colluphid's philosophical blockbusters.

Ben84

samtheman1k wrote: »

Is it really quite that simple though? thinking back to my Physics days, a microwave excites water molecules on their resonant frequency. Does that technique that mean you get more than 100% heat out for a unit of electricity in (from a physics point of view)? (In a similar way to heat pumps generate more than 100% heat?)

No, they're very different to heat pumps. A heat pump does seem strange because you can put in one kW of energy and get out two or three kW for example, but it does this by exploiting a loophole. It doesn't actually turn electrical energy in to heat, instead it moves heat from somewhere else. All the energy it gives you is accounted for by cooling something else by the same amount.

Heat pumps exploit the latent energy change when a gas is compressed and vaporised. When a gas is compressed the molecules have much less freedom of movement and simply can contain less energy, so they emit this excess at heat. Conversely, when a compressed gas de-pressurises it adsorbs heat from the surroundings. You may noticed when using a spray can containing pressurised propellant gas, for example CFCs or butane, that the can becomes cooler as you use it. This is because by removing some of the contents from the can there is more room inside it and some of the propellant still inside it can change from liquid to gas, causing it to absorb energy from the surroundings.

Microwaves do not move heat, instead they emit microwave energy which is absorbed by molecules in the food which respond by vibrating. Heat is a property of molecular vibrations and the faster they vibrate the hotter the matter is. Microwave ovens are very efficient compared to conventional ovens for several reasons. They're fast and have no heat up time, the microwaves are almost instantly absorbed by the food. They also have very few losses as only the food absorbs them, while the metal walls of the microwave simply reflect them until they hit the food you're heating. You're not wasting energy heating the oven itself and all the air inside it. Microwaves also penetrate the food to some extent, heating it from within, so they cook/heat the food to the centre faster too. There's no real physics tricks, they just put the energy straight to where you want it, inside the food.

As for the kettle vs microwave for heating water, I'd go for the kettle. Because the kettle element has almost 100% conversion of electrical energy in to heat and is nearly completely surrounded by water, it too suffers very little losses, and as another posted mentioned, kettles come in higher kW ratings so will be faster. The microwave however is useful for quickly and efficiently heating liquids like milk which you wouldn't want to put in the kettle.

samtheman1k

Of course, the problem with kettles is that you nearly always heat more water than required, whereas in a microwave , you'll only heat what you require.

t0rt0ise

In any case I wasn't meaning kettle versus microwave, I was thinking on a bigger scale.

And modern kettles don't have elements that are surrounded with water as the element is under a plate on the bottom of the kettle.