This hot afternoon, I went to the freezer to take some water. I opened the door of the refrigerator, took out a bottle of chilled water and shut the door. I quickly poured out enough cold water into a glass cup and then tried to replace the bottle back in the fridge but the fridge door refused to budge. It was shut tight like as tho locked with a key and no matter how hard i pulled it was useless. Many of us have had such exp with our refrigerators at home and we all soon learnt that we have to wait for some time (say 30-40seconds) before the door opens with ease again. Now the question is, what is the principle behind this process? A good physicist here should be able to give a convincing explanation.

When the pressure between the inside of a container and its outside are pretty much same, you feel no resistance opening the lid or door to that container, as the case may be. When there is differential pressure between the inside and outside, then there is resistance. The direction of differential pressure will determine where resistance is coming from. Because the fridge is much colder than the outside, it is at a lower temperature. Remember the relationship between temperature and pressure? It's a direct relationship. If the temperature inside a container is really low, the gas particles in that container dont move rapidly, which means they are sluggish. That translates to lower pressure. Think about a cooking gas container; its gas particles are bouncing all about the container looking for an escape, which is why when you create an outlet, they immediately rush out. These gas particles are at a higher pressure. Anyways, I digress.

Back to the explanation, the lower temperature inside the fridge means the gas (air) particles inside the fridge have created a lower pressure situation inside it, lower than the atmospheric pressure (which is the pressure at room temperature). Because there is now differential pressure, when you try to pull the fridge open, there is resistance. But why is the resistance against opening the fridge? Because matter in the classical world generally tends to move towards lower pressure. This is the same reason your gas particles in the cooking gas (which are at higher pressure) are seriously looking for a way to escape. So, your fridge door continues to resist your pull because the door is gravitating towards the lower pressure in the fridge.
The small seal areas on the door lip make this pressure translate into greater force and that's why you often need to pull hard to open the door.

MathsChic:
When the pressure between the inside of a container and its outside are pretty much same, you feel no resistance opening the lid or door to that container, as the case may be. When there is differential pressure between the inside and outside, then there is resistance. The direction of differential pressure will determine where resistance is coming from. Because the fridge is much colder than the outside, it is at a lower temperature. Remember the relationship between temperature and pressure? It's a direct relationship. If the temperature inside a container is really low, the gas particles in that container dont move rapidly, which means they are sluggish. That translates to lower pressure. Think about a cooking gas container; its gas particles are bouncing all about the container looking for an escape, which is why when you create an outlet, they immediately rush out. These gas particles are at a higher pressure. Anyways, I digress.

Back to the explanation, the lower temperature inside the fridge means the gas (air) particles inside the fridge have created a lower pressure situation inside it, lower than the atmospheric pressure (which is the pressure at room temperature). Because there is now differential pressure, when you try to pull the fridge open, there is resistance. But why is the resistance against opening the fridge? Because matter in the classical world generally tends to move towards lower pressure. This is the same reason your gas particles in the cooking gas (which are at higher pressure) are seriously looking for a way to escape. So, your fridge door continues to resist your pull because the door is gravitating towards the lower pressure in the fridge.
The small seal areas on the door lip make this pressure translate into greater force and that's why you often need to pull hard to open the door.

Wow... Bravo!!!. I'm stoked. I couldn't have done any better. You must be an Engineer, you're most definitely one of the rare ones.

Synopsis from mathschic's analysis: immediately you open your fridge door, a chunk of warmer air(different from the Cold air from fridge) floods in. When you close it, that air contracts while it's mixing with the cold air. It makes a suction(absorption) for about 60-120secs

Guy na lite Ooo dey cos am... if d frdge is cold, u wil xperience dat..
It mins u r getin constant lite 4 ur area

MathsChic:
When the pressure between the inside of a container and its outside are pretty much same, you feel no resistance opening the lid or door to that container, as the case may be. When there is differential pressure between the inside and outside, then there is resistance. The direction of differential pressure will determine where resistance is coming from. Because the fridge is much colder than the outside, it is at a lower temperature. Remember the relationship between temperature and pressure? It's a direct relationship. If the temperature inside a container is really low, the gas particles in that container dont move rapidly, which means they are sluggish. That translates to lower pressure. Think about a cooking gas container; its gas particles are bouncing all about the container looking for an escape, which is why when you create an outlet, they immediately rush out. These gas particles are at a higher pressure. Anyways, I digress.

Back to the explanation, the lower temperature inside the fridge means the gas (air) particles inside the fridge have created a lower pressure situation inside it, lower than the atmospheric pressure (which is the pressure at room temperature). Because there is now differential pressure, when you try to pull the fridge open, there is resistance. But why is the resistance against opening the fridge? Because matter in the classical world generally tends to move towards lower pressure. This is the same reason your gas particles in the cooking gas (which are at higher pressure) are seriously looking for a way to escape. So, your fridge door continues to resist your pull because the door is gravitating towards the lower pressure in the fridge.
The small seal areas on the door lip make this pressure translate into greater force and that's why you often need to pull hard to open the door.

Good one mathschic,please are you a mathematician or a physicist?

Danfuster:


Wow... Bravo!!!. I'm stoked. I couldn't have done any better. You must be an Engineer, you're most definitely one of the rare ones.

ladygudhead:

Good one mathschic,please are you a mathematician or a physicist?

Lol. Thanks.
I studied engineering. However, I'm not practicing as one. I'm squarely into finance these days.

MathsChic:


Lol. Thanks.
I studied engineering. However, I'm not practicing as one. I'm squarely into finance these days.

OK,nice.

[quote author=MathsChic post=43721726]When the pressure between the inside of a container and its outside are pretty much same, you feel no resistance opening the lid or door to that container, as the case may be. When there is differential pressure between the inside and outside, then there is resistance. Because the fridge is much colder than the outside, it is at a lower temperature. Remember the relationship between temperature and pressure? It's a direct relationship. If the temperature inside a container is really low, the gas particles in that container dont move rapidly, which means they are sluggish. That translates to lower pressure.


nice one MathsChic. The cooling of the warm air dat goes into d fridge results in drop in pressure as d air contracts. Really love d analysis. Where i'm nt perfectly clear is on d process dat occurs whil we wait for d door to open wit ease again. I believe the pressure in d fridge would hav to increase again for d ease of openin. Hw does dis pressure increas happen?

odijeks

Actually this is a very good point you have made. Immediately after shutting the fridge, it's more difficult to open, because of the rapid cooling of the air that just entered. After a while, it's easier because, as you rightly noted, the air expands. What causes the increase? Most fridges have a small outlet, some kind of pressure-relief mechanism, that lets out air and allows for the expansion. If this were not there, pressure will just build up infinitely inside the fridge. Don't you think? But some fridges are not just air-tight and overtime, air will seep in through the door and pressure will equalize.

ismoh90:
Guy na lite Ooo dey cos am... if d frdge is cold, u wil xperience dat..
It mins u r getin constant lite 4 ur area

No be say light constant like that o, the fridge is kinda superb. E sabi maintain cool temp after small light

MathsChic:

Actually this is a very good point you have made. Immediately after shutting the fridge, it's more difficult to open, because of the rapid cooling of the air that just entered. After a while, it's easier because, as you rightly noted, the air expands. What causes the increase? Most fridges have a small outlet, some kind of pressure-relief mechanism, that lets out air and allows for the expansion. If this were not there, pressure will just build up infinitely inside the fridge. Don't you think? But some fridges are not just air-tight and overtime, air will seep in through the door and pressure will equalize.

Thanks., you've done justice to this one

You good!

MathsChic:


Lol. Thanks.
I studied engineering. However, I'm not practicing as one. I'm squarely into finance these days.

The quoted, one of my greatest fears!

That 1st poster is disillusioned....btw it is sealed with a tube like seal to keep the gas in,use ur brain boy

Abudu2000:
That 1st poster is disillusioned....btw it is sealed with a tube like seal to keep the gas in,use ur brain boy

MathsChic analysis is scientific, to reasonable extent, though. But my opinion is that the rubber-like sealant around the tip of the fridge body and its door is the cause of what the poster experienced.

The pull force the poster exerted while trying to open the fridge door wasn't enough to against the friction existing between the two surfaces.

phetty:


MathsChic analysis is scientific, to reasonable extent, though. But my opinion is that the rubber-like sealant around the tip of the fridge body and its door is the cause of what the poster experienced.

The pull force the poster exerted while trying to open the fridge door wasn't enough to against the friction existing between the two surfaces.

Did you say friction? That, of course, is not correct.

MathsChic:

The small seal areas on the door lip make this pressure translate into greater force and that's why you often need to pull hard to open the door.

phetty:


MathsChic analysis is scientific, to reasonable extent, though. But my opinion is that the rubber-like sealant around the tip of the fridge body and its door is the cause of what the poster experienced.

The pull force the poster exerted while trying to open the fridge door wasn't enough to against the friction existing between the two surfaces.

the seal is related like MathsChic rightly pointed out but she explains that it serves in preventing exchange of pressure between the fridge compartment and the atmosphere, which adds up. i don't think it can be related to friction cos that would mean there is no friction when the fridge finally opens easily, which really doesn't make sense.

MathsChic:
When the pressure between the inside of a container and its outside are pretty much same, you feel no resistance opening the lid or door to that container, as the case may be. When there is differential pressure between the inside and outside, then there is resistance. The direction of differential pressure will determine where resistance is coming from. Because the fridge is much colder than the outside, it is at a lower temperature. Remember the relationship between temperature and pressure? It's a direct relationship. If the temperature inside a container is really low, the gas particles in that container dont move rapidly, which means they are sluggish. That translates to lower pressure. Think about a cooking gas container; its gas particles are bouncing all about the container looking for an escape, which is why when you create an outlet, they immediately rush out. These gas particles are at a higher pressure. Anyways, I digress.

Back to the explanation, the lower temperature inside the fridge means the gas (air) particles inside the fridge have created a lower pressure situation inside it, lower than the atmospheric pressure (which is the pressure at room temperature). Because there is now differential pressure, when you try to pull the fridge open, there is resistance. But why is the resistance against opening the fridge? Because matter in the classical world generally tends to move towards lower pressure. This is the same reason your gas particles in the cooking gas (which are at higher pressure) are seriously looking for a way to escape. So, your fridge door continues to resist your pull because the door is gravitating towards the lower pressure in the fridge.
The small seal areas on the door lip make this pressure translate into greater force and that's why you often need to pull hard to open the door.

Great explanation! Your explanation doesn't explain why the door opens some secs afterwards as stated in the problem. Because as long as the low temp regime exists inside the refrigerator, I expect atmospheric pressure to prevail over the door due to the low pressure inside the refrigerator.

Thanks for the help guys

MathsChic:
When the pressure between the inside of a container and its outside are pretty much same, you feel no resistance opening the lid or door to that container, as the case may be. When there is differential pressure between the inside and outside, then there is resistance. The direction of differential pressure will determine where resistance is coming from. Because the fridge is much colder than the outside, it is at a lower temperature. Remember the relationship between temperature and pressure? It's a direct relationship. If the temperature inside a container is really low, the gas particles in that container dont move rapidly, which means they are sluggish. That translates to lower pressure. Think about a cooking gas container; its gas particles are bouncing all about the container looking for an escape, which is why when you create an outlet, they immediately rush out. These gas particles are at a higher pressure. Anyways, I digress.

Back to the explanation, the lower temperature inside the fridge means the gas (air) particles inside the fridge have created a lower pressure situation inside it, lower than the atmospheric pressure (which is the pressure at room temperature). Because there is now differential pressure, when you try to pull the fridge open, there is resistance. But why is the resistance against opening the fridge? Because matter in the classical world generally tends to move towards lower pressure. This is the same reason your gas particles in the cooking gas (which are at higher pressure) are seriously looking for a way to escape. So, your fridge door continues to resist your pull because the door is gravitating towards the lower pressure in the fridge.
The small seal areas on the door lip make this pressure translate into greater force and that's why you often need to pull hard to open the door.

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