# fridge pressure

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The temperature in my kitchen is about 22 centigrade, in my refrigerator about 4 C. Sometimes I open the refrigerator and take somethng out, then close it and after some seconds I open it again. And then I must apply much more force to open the refrigerator door. It means that opening the door for the first time lowered considerably the pressure in the fridge compartment.

Why is it so?

As I understand, when I open the door for the first time the hotter air from the room enters the fridge compartment and the pressure should rise and not fall.

Thanks

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The temperature in my kitchen is about 22 centigrade, in my refrigerator about 4 C. Sometimes I open the refrigerator and take somethng out, then close it and after some seconds I open it again. And then I must apply much more force to open the refrigerator door. It means that opening the door for the first time lowered considerably the pressure in the fridge compartment.

Why is it so?

As I understand, when I open the door for the first time the hotter air from the room enters the fridge compartment and the pressure should rise and not fall.

Thanks

The hotter air that enters is at the ambient pressure. What happens when it cools down? The pressure drops. Atmospheric pressure is around 10^5 N/m^2 and a refrigerator door around 1 m^2, so even a 0.1% drop in pressure is a 10 N force on the door (which requires 5 N to overcome because of the geometry)

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If the fridge door is not 100% airtight, then as the pressure drops inside the fridge it must slowly suck in more air,making it easier to open fridge if left shut for some time.

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The temperature in my kitchen is about 22 centigrade, in my refrigerator about 4 C. Sometimes I open the refrigerator and take somethng out, then close it and after some seconds I open it again. And then I must apply much more force to open the refrigerator door. It means that opening the door for the first time lowered considerably the pressure in the fridge compartment.

Why is it so?

As I understand, when I open the door for the first time the hotter air from the room enters the fridge compartment and the pressure should rise and not fall.

Thanks

$\frac{P_{1}V_{1}}{T_{1}}=\frac{P_{2}V_{2}}{T_{2}}$

V=constant

$\frac{P_{1}}{T_{1}}=\frac{P_{2}}{T_{2}}$

$\Delta P = P_{2}-P_{1}=P_{1}\frac{T_{2}}{T_{1}}-P_{1}$

$\Delta F=\Delta PA=(P_{1}\frac{T_{2}}{T_{1}}-P_{1})A$

P1=101.3x103Pa, T1=273+22=295K, T2=273+4=277K if A=1m2, and we ignore water contents.

F=6580N is required.---directly open the refrigerator like opening the wine bottle with a cork opener.

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$\frac{P_{1}V_{1}}{T_{1}}=\frac{P_{2}V_{2}}{T_{2}}$

V=constant

$\frac{P_{1}}{T_{1}}=\frac{P_{2}}{T_{2}}$

$\Delta P = P_{2}-P_{1}=P_{1}\frac{T_{2}}{T_{1}}-P_{1}$

$\Delta F=\Delta PA=(P_{1}\frac{T_{2}}{T_{1}}-P_{1})A$

P1=101.3x103Pa, T1=273+22=295K, T2=273+4=277K if A=1m2, and we ignore water contents.

F=6580N is required.---directly open the refrigerator like opening the wine bottle with a cork opener.

The calculation seems correct, but the underlying assumptions are probably a little too simple.

It is less than you calculated, for a number of reasons:

1. A part of the air inside the fridge is already cold, and does not cool when you close the fridge. However it is already at the atmospheric pressure. This reduces the effect a lot.

2. Your fridge door leaks air.

3. It takes some time for the air to cool down completely. After just a few seconds, it will not be completely cold yet.

4. It depends if your fridge is full or empty. If it is full, all those cold products will help to cool the air quickly (because of a larger surface area). An empty fridge will probably suffer less from this phenomena.

My experience is that you need to pull quite hard on it. From that, I would guestimate that the actual force is in the order of 100 N (it is comparable to lifting something of roughly 10 kg).

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• 3 weeks later...

The temperature in my kitchen is about 22 centigrade, in my refrigerator about 4 C. Sometimes I open the refrigerator and take somethng out, then close it and after some seconds I open it again. And then I must apply much more force to open the refrigerator door. It means that opening the door for the first time lowered considerably the pressure in the fridge compartment.

Why is it so?

As I understand, when I open the door for the first time the hotter air from the room enters the fridge compartment and the pressure should rise and not fall.

Thanks

(1)- may be the hot air(which has more volume)may enter the fridge .After you close it, the hot air in the fridge may cool down thus creating a partial vacuum. Atmospheric pressure exerts a pressure on the door . thus you may feel difficult to pull the door .

(2)- also may be when you close the door fast, most of the air in the fridge may expel out(like the functioning of a suction cup used in car windshield), thus creating a mild vacuum in the fridge.

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(2)- also may be when you close the door fast, most of the air in the fridge may expel out(like the functioning of a suction cup used in car windshield), thus creating a mild vacuum in the fridge.

Unlikely. A suction cup deforms to create the vacuum — no space between the material and the surface — but a fridge door does not work that way.

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This reminds me of a fridge situation I've always wondered about. I notice that when I empty a plastic gallon jug of milk, if I put the cap back on and place the jug on the counter, within 30 seconds the pressure in the jug increases to the point where I can hear the expulsion of air if I remove the cap. I guess this is simply the opposite of the low pressure in the fridge. Live and learn!

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This reminds me of a fridge situation I've always wondered about. I notice that when I empty a plastic gallon jug of milk, if I put the cap back on and place the jug on the counter, within 30 seconds the pressure in the jug increases to the point where I can hear the expulsion of air if I remove the cap. I guess this is simply the opposite of the low pressure in the fridge. Live and learn!

You can do the opposite and rinse it out with hot water, drain it and quickly put the cap on. The jug will partially collapse.

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Sealing part makes opening easy. Pressure acting direction and opening direction are different. And the pressure applied on the sealing part is about 100N at the maximun situation, i.e., empty and no leaking condition.

Edited by alpha2cen

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