The Student Room Group

Physics teaser to keep you amused!

You have two identical refrigerators in a room.
One, fridge A, is empty and the other, fridge B, is full.
The room is at a constant temperature and both fridges have reached a steady internal temperature.

Which fridge, A or B, costs the most to run?

Edit to add that both fridges are kept shut.
(edited 12 years ago)

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Is it the full one? :biggrin: I reckon since the full one will have a larger internal surface area the heat loss over time will be greater and therefore will burn a bigger hole in one's pocket. :^_^:
Reply 2
I would say that fridge A costs more to run.

Does it have something to do with the contents of fridge B having a higher heat capacity and greater mass than the air in fridge A? If so, then is it that the contents of fridge B would take longer to be heated up by the outside warmth than the air in fridge A?

So, once a stable temperature has been reached, fridge B would be easier (and cheaper) to keep at that temperature?
(edited 12 years ago)
Reply 3
A related teaser is as follows:

You have a thermally sealed room (no heat exchange with the outside) with a perfectly efficient fridge inside. When the fridge is on, does the temperature in the room increase, stay the same, or decrease?
Reply 4
The one you have to buy food to fill it with.
Reply 5
Fridge A costs more to run even if they're kept shut.

The fridge which is more full will experience a smaller increase in temperature (same heat input divided into larger mass). This leads to the temperature of the full fridge staying above the trip level for longer. When enough heat is added to the fridge, the compressor for the full fridge will be on for longer to achieve the same temperature change (more mass, more energy to remove for the same temperature change). So fridge B is more efficient after it's contents are cold because there is longer runtime and less start/stop losses.
Reply 6
Has no one considered the 3rd option? :wink:
The same? Since they both have reached a steady internal temperature before we count them one being full and one being closed doesn't really change their running costs unless they are opened and closed (if they were open and closed a full refrigerator may present an obstacle to convection currents, which i think would help reduce heat flowing into the refrigerator so an empty one would cost more if they were opened and closed the same amount)
(edited 12 years ago)
Reply 8
Fridge B would cost more because people would be opening and closing it to get the food out.
Reply 9
Ah I see. Was I supposed to assume there is 100% efficient transfer of heat out of the fridge?

Thats hard to believe because heat energy will only go from high to low,(I forget which fundamental law that is) ie a fridge can never remove the heat from the air without an input of energy to increase the temperature in the first place.
Reply 10
Original post by emi_sarb
Fridge B would cost more because people would be opening and closing it to get the food out.


Just to clear up any confusion - the door is kept shut.
It's one of those fridges that only exists in physics problems, where it can be full of food but nobody bothers to get any out; or empty and no one puts anything in. :wink:
Reply 11
Original post by Stonebridge
You have two identical refrigerators in a room.
One, fridge A, is empty and the other, fridge B, is full.
The room is at a constant temperature and both fridges have reached a steady internal temperature.

Which fridge, A or B, costs the most to run?

Edit to add that both fridges are kept shut.


The empty one. The full one has a greater mass of material at lower temperature, hence needs more energy to warm up. Hence needs less energy to keep cool, and costs less to run.
Reply 12
Original post by Fallen
A related teaser is as follows:

You have a thermally sealed room (no heat exchange with the outside) with a perfectly efficient fridge inside. When the fridge is on, does the temperature in the room increase, stay the same, or decrease?


The thermal energy in the room remains constant, but electrical energy is coming into the room via a cable (I assume). This energy is used to cool down the fridge..so the room cools down? But the thermal energy must remain the same? I'm torn, I'll go with stays the same.
Reply 13
Original post by LeeC
The thermal energy in the room remains constant, but electrical energy is coming into the room via a cable (I assume). This energy is used to cool down the fridge..so the room cools down? But the thermal energy must remain the same? I'm torn, I'll go with stays the same.

So very close.
It is indeed the electrical energy coming in which sways the balance, but in fact it adds to the rooms total energy and heats it.

I may be very much mistaken, but I believe it is the Second Law of Thermodynamics which prevents a fridge from working without an overall increase in heat, as otherwise you would get a decrease in entropy (if the fridge started spontaneously cooling itself). May be wrong, though, but certainly the final answer is correct.
Reply 14
Original post by Stonebridge
You have two identical refrigerators in a room.
One, fridge A, is empty and the other, fridge B, is full.
The room is at a constant temperature and both fridges have reached a steady internal temperature.

Which fridge, A or B, costs the most to run?

Edit to add that both fridges are kept shut.


They're both identical therefore they have the same internal temperature. I agree with Stewie here and say that they cost the same based on my understanding that the amount of contents in the fridge does not affect the temperature inside it.

Although putting something hot inside the fridge may increase the temperature a little, but our internal temperature is steady therefore both fridge's cost the same to run at this period. I think.....
Reply 15
the fridge motors job is to maintain a steady temperature in the fridge.

Assuming that the contents of the fridge have reached thermal equilibrium with the inside of the fridge then there is no net flow of heat from them to their surroundings.

The answer is therefore that the power required, once thermal equilibrium is reached, is the same.

Unless the contents are lager in which case the door will be opened quite regularly
Original post by Stonebridge
You have two identical refrigerators in a room.
One, fridge A, is empty and the other, fridge B, is full.
The room is at a constant temperature and both fridges have reached a steady internal temperature.

Which fridge, A or B, costs the most to run?

Edit to add that both fridges are kept shut.

yo man! i just randomly entered this forum so that i can message you (coz its not allowed to PM you)
so where i can post my a2 phy questions?
haha you closed that forum RIP!!
Reply 17
It's 2 years since I did these "teaser" threads.
If you have any questions just post them on the forum here.
Press the "post new thread" button



and post.
Original post by Stonebridge
It's 2 years since I did these "teaser" threads.


So what was the answer?
Reply 19
Is it another fridge at an another dimension as particles can exist in two places at once.

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