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I wanted to know what would happen if a hairdryer is turned on in an airtight box.

user300778
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2 Answers2

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Energy is delivered into the box via the power cord.

The hair dryer converts the electrical energy into thermal energy, raising the temperature of things inside the box. Since there's no way for this energy to be removed via the power cord, it all has to be transferred to the cooler exterior through the sides of the box.

If the box is well insulated, it could allow the interior to reach very high temperatures. For most real devices, this will continue until a temperature sensor in the dryer trips, shutting off the device.

You've used the "vacuum" tag, but I'm not sure what you might be wondering about that. A hair dryer is just a fan and a heater. There is slightly lower pressure on the intake side of the fan, but it doesn't approach a vacuum.

Xkcd's what if? has covered this topic as well. Hair Dryer

BowlOfRed
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  • "there's no way for this energy to be removed via the power cord, it all has to be radiated" - Well, not completely → thermal conduction. – Gerold Broser Oct 23 '14 at 21:21
  • Yes, my language there was imprecise. – BowlOfRed Oct 23 '14 at 21:24
  • "it could allow the interior to reach very high temperatures" - Pleased define "very high temperatures". Once a temperature is reached where heat generation and heat loss by radiation plus conduction are equal, the temperature inside will not rise any more. – Gerold Broser Oct 23 '14 at 21:26
  • As you say, it's an equilibrium between the power delivered and the insulation provided. The better the insulation, the higher the temperature inside in order to drive the heat loss to equilibrium. We just need the power delivered and the thermal conductivity of the insulation. – BowlOfRed Oct 23 '14 at 21:29
  • I'm terribly sorry having to say this but you're imprecise again. It's not power but energy. Hence, the equilibrium is between energy supplied and energy lost. Power and insulation matter just for when (how fast) this state is reached. – Gerold Broser Oct 23 '14 at 21:36
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    The box will lose heat over time, so the energy delivered per time (power) determines the equilibrium temperature. A 40W light bulb inside will reach equilibrium at a lower temperature than a 1500W hair dryer. When reached, the heat flux through the sides will equal the electrical power draw. – BowlOfRed Oct 23 '14 at 21:41
  • You're right, I was more thinking statically (or differentially if you want ;-), excluding time. My main point was regarding your equilibrium between power (a physical quantity) and insulation (matter). – Gerold Broser Oct 23 '14 at 21:50
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The heating part of the hair dryer has a particular temperature at which it works .After sometime the air hair dryer sucks and the air it blows will come at same temperature for the case of insulated box.So you cannot reach "very high temperatures",Moreover any real hair dryer cannot work for more than a certain time.So if you do this experiment you will never reach the thermal equilibrium and probably you will damage your hair dryer

Sahil Chadha
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