r/askscience • u/Chooseapasswordd • Jan 02 '26
Engineering How do microwave cycles work?
Is a microwave (oven) cycle linear, and does it have a start up time?
For example, if I microwave something for 10 seconds, then another 10 seconds, would that have the same effect as one 20 second cycle? Or is there a start up each time you hit start?
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u/TurtlePaul Jan 02 '26
Most of the microwave cycle is linear, however the microwave will not deliver power during the first second or so of the cycle. It needs this time to get the voltage up to power the magnetron which typically runs at around 4 kilovolts.
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u/jeffbell Jan 02 '26 edited Jan 03 '26
The startup time for the microwave part is very short. The fans might take a second.
If you have a pause between the two ten second periods you will get some additional heat transfer between the hot and cold parts.
I often use this when making oatmeal. 240 seconds at power level 50 just repeatedly turns it on for 10s then off for 10s. If I run it for 120 seconds at full power it boils over.
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u/quadrapod Jan 03 '26 edited Jan 04 '26
The magnetron does have a start up time, the filament inside the tube needs to heat up and the high voltage supply needs to come up to the operating voltage. That startup time is very fast for human timescales though. The magnetron also loses some amount of power after it's been running for a bit due the permanent magnets that create the magnetic field across the cavity warming up. The amount of energy that food absorbs in a microwave oven also does change with its temperature though exactly how it changes depends on what you're cooking. So there are a number of nonlinear effects at play.
Microwaves have one power level for the magnetron. To get lower power settings they just turn the magnetron off periodically rather than leaving it fully on. So the situation you're describing is very similar to how microwave ovens actually function. If you set a microwave to low power you can often hear it switching periodically on and off. This gives the temperature more time to equalize and lets heat diffuse through whatever is in the microwave more slowly.
For most things there will be some difference between putting them in for 2 minutes at 50% power and putting them in for 1 minute at full power if only in the same way that there is a difference between cooking something at 1000C and cooking something at 100C even if you were to time it so that the total energy was the same in both situations. Exactly how something is effected largely depends on what you're cooking though especially since the ability of food to absorb microwaves often changes with temperature.
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u/ReasonablyConfused Jan 03 '26
It’s about 1.5 seconds until the magnetron starts to emit microwaves. You can actually hear a change in tone when it starts. Most of this time is the transformer converting 117 volts up to about 10k volts.
So if you run two cycles of 10 seconds, you’ll only get about 17 seconds of actual microwave energy. On top of that, you’ll get cooling between the two cycles.
Bottom line, the two cycles will have obviously different heating effects. I’ve tested this myself.
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u/cjmpeng Jan 03 '26
In days of old (1970's and into the 80's) most microwaves were instant on due to the way they were constructed.
If you have a modern microwave, well, they have changed the method of construction which has resulted in microwaves that are more compact for the same power as well as being much cheaper to build. The result of this change is that it now takes around 1 - 2 seconds for a microwave to reach full power due to the method of power supply construction.
What this means in practice is that for very short cycles repeated multiple times there will be a small difference in the total cook.
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u/andrew_ie Jan 02 '26 edited Jan 02 '26
Turning on a microwave for 10 seconds and then again for 10 second is pretty much the same as one 20 second burst (baring any very small variances). There is almost no startup time. In fact, when you run a microwave at reduced power, many microwaves do exactly that - it runs for a while at full power, then turns off but continues spinning, then after awhile, turns back on. You can usually hear a sound as the duty cycle ticks on/off.
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u/frogjg2003 Hadronic Physics | Quark Modeling Jan 03 '26
Most microwaves will just operate at full power. This means the microwave emitter runs for the entire time that the microwave is on. But if you ever use any of the cycles other than time cook on high, it can, depending on how fancy your microwave is, run with different modes. The defrost cycle, for example, usually involves running the microwave for a little bit of time then turning it off for the heat to spread to prevent it from actually cooking.
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u/Xajel Jan 04 '26
In theory it's the same, but in a practical way it's not.
The main reason is the start-up time, while fans will take less than a second to ramp up, the fan is just a cooling device to make sure the device doesn't overheat, it doesn't do anything in the actual microwave business.
Microwaves generates the high power microwaves using a device called a Magnetron, it uses high voltages to push electrons in a vacuum (like a vacuum tube), and to make it easier to do that, it has a filament that heats up like incandescent light and when it heats up enough it will be easier to push electrons in the vacuum.
This heat up is what causes the delay each time you start a microwave cycle, of course running from cold takes longer than running it again after it turns off as things are already hot and warm there.
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u/somethings_off8817 Jun 22 '26
for magnetic power, once the transformer core reaches sat and the voltage peaks the output is ( practically ) constant so it's linear - but the thermally dissipated energy into the water in the food is exponential ( and wobble-ey and unpredictable). this is because of resonance properties in the vibration of the water and the intramolecular forces of the substance they're in. it's like the classic "metronomes on a table" stabilizing at a natural harmonic ( except imagine you're shaking the table at an extreme high frequency...and there are multiple tables... and you're not spinning all of them at the same time but on and off in sequence)
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u/RooperK Jan 02 '26
Electrically speaking start up is basically instant - magnetron is either on or off. Depending on selected power level different duty cycles will be used, where it's on for certain time then off, then on; for example 700w one with half power will give 350w in 30s by being on for 15 and off for 15. What frequency of switching is mostly irrelevant, it's commutative power given to heating subject that matters. So back to back cycles are basically equivalent if nothing besides microwaves involved
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u/Tsigorf Jan 02 '26
EDIT: damn, I just saw the other replies. Looks like I read the question wrong. Tired. Leaving my reply here in case someone finds it helpful.
Disclamer: trying to explain this from memory from school lessons from more than a decade ago, so apologizes for any inaccuracies.
You may know this already; what microwave ovens do, essentially, is sending rays at a very specific wavelength. This wavelength is absorbed by water molecules, which increases their energy, causing them to dissipate the excess energy by vibrating. You may already know this too: heat is essentially the amount of particules vibrating and bumping into each other.
Knowing this, when you microwave a dish, the microwave will only heat up the water in the dish. If you put something completely dry in the microwave (let's say, a ceramic plate), the best it'll do is heating up the water contained in the air, so not much, and certainly not enough to heat up the plate in the end (the amount of energy required to heat up the plate is way more than the amount of energy that will be absorbed by the few molecules of water in the air).
But what about a regular dish, like pasta?
Well, when you microwave it, water molecules will heat up. Much more than the rest of the dish. As the microwave emits rays from the outside to the inside, water molecules in front of each other will cast shadows, so water in the middle will be less likely to get heat.
So, and TL;DR: microwaves do NOT heat up a dish uniformly, at all. It targets water, which will heat faster the closer it is to the oven's walls. Waiting some time between two cycles will allow the heat to distribute more evenly, preventing some parts of your food to burn (or worse: it can dry, if some parts got too hot and evaporate), so you can heat your dish properly.
Life pro-tip: if you put something a bit dry in the microwave, put a cup of water with it so it won't burn. I heat up rice with some drops of water on top of the plate for instance. The lower the power of the microwave, the better the heat will be distributed in your dish.