The Cellar - Need science refresher

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Need science refresher

So I was in Sears at the super clearance table and picked up a few USB-connect items. I was able to resist the disco ball because the last box was opened, but I picked up a mini-shredder and mini-refrigerator. They were never worth $40, but for $4 I wanted to try them.

The mini-shredder is fairly straightforward, but the mini-fridge has me puzzled. It a small plastic fridge about 8x4x3 inches. Inside is an led and a cutout for a can bottom. The cutout places the can on a solid aluminum or stainless steel heat sink that has been honeycombed. A small cooling fan at the bottom circulates room temperature air through the chambers. There is no coolant; it is all air cooled. How much cooler can this make a can of soda or beer than the ambient room temperature?

Maybe its for cooling down a hot cup of ramen noodles? FIIK.

I would expect that the heat generated by the fan's motor and, to a lesser extent, the friction from the air it moves would tend to heat the can up slightly faster. That said, keeping it in the small, enclosed space would tend to keep it cooler longer.

So I'd expect it to work better if you don't plug it in.

But give it a try!

It probably has a Peltier Junction, or other thermoelectric device, in the metal plate in the bottom.

EDIT:

Quote:

This effect bears the name of Jean-Charles Peltier (a French physicist) who discovered in 1834, the calorific effect of an electrical current at the junction of two different metals. When a current I is made to flow through the circuit, heat is evolved at the upper junction (at T2), and absorbed at the lower junction (at T1). The Peltier heat absorbed by the lower junction per unit time, http://upload.wikimedia.org/math/5/b...44cf95f637.png is equal to
http://upload.wikimedia.org/math/c/f...d86202d2e5.png where Π is the Peltier coefficient ΠAB of the entire thermocouple, and ΠA and ΠB are the coefficients of each material. P-type silicon typically has a positive Peltier coefficient (though not above ~550 K), and n-type silicon is typically negative, as the names suggest.
The Peltier coefficients represent how much heat current is carried per unit charge through a given material. Since charge current must be continuous across a junction, the associated heat flow will develop a discontinuity if ΠA and ΠB are different. This causes a non-zero divergence at the junction and so heat must accumulate or deplete there, depending on the sign of the current. Another way to understand how this effect could cool a junction is to note that when electrons flow from a region of high density to a region of low density, they expand (as with an ideal gas) and cool.
The conductors are attempting to return to the electron equilibrium that existed before the current was applied by absorbing energy at one connector and releasing it at the other. The individual couples can be connected in series to enhance the effect.
An interesting consequence of this effect is that the direction of heat transfer is controlled by the polarity of the current; reversing the polarity will change the direction of transfer and thus the sign of the heat absorbed/evolved.
A Peltier cooler/heater or thermoelectric heat pump is a solid-state active heat pump which transfers heat from one side of the device to the other. Peltier cooling is also called thermo-electric cooling (TEC).

Quote:

Originally Posted by Bullitt (Post 540450)

Maybe its for cooling down a hot cup of ramen noodles? FIIK.

Now in a way that's what I'm thinking. The fan is probably a 20mm computer fan, and the heat sink is a lot like a CPU heat sink. I can understand it cooling something warmer than room temperature down as low as room temperature. But when I plug it in, it does appear that the circle becomes cool.

When I expect to have the computer on for a few hours, I will get a room temperature can of soda and experiment.

Quote:

Originally Posted by HungLikeJesus (Post 540459)

It probably has a Peltier Junction, or other thermoelectric device, in the metal plate in the bottom.

All I see is the fan and the heat sink.

If there's a Peltier Junction embedded in the plate, the heat sink and fan would probably increase its efficiency.

I know I'd heard of them before, but Peltiers didn't occur to me.

Peltier coolers consist of the peltier element and a heatsink/fan combination to cool the TEC, which sounds like what you're describing. I guess you'll have to do the test to find out.

I stand corrected. What I took for a solid block is actually two pieces with some kind of electrical device sandwiched between.

The great thing about spending only $4 on something is that I'm much less squeamish about disassembling it.

I had to look up Peltier device. I guess this must be one since there is no heat pump.

Quote:

Originally Posted by HungLikeJesus (Post 540471)

Peltier coolers consist of the peltier element and a heatsink/fan combination to cool the TEC, which sounds like what you're describing. I guess you'll have to take it apart to find out.

Fixed it for you.

I have an Igloo around here that works that way. Reverse the plug and it heats. I used it in motels when working away from home.

Someone gave me what looks like a drink coaster with a cord ending in a USB plug. It's supposed to keep cans cool. Never tried it, :blush:

I have had several 12V coolers during my trucking days, and they keep things 40F cooler than ambient. They work on the same principle.

Really....science can work out miracles. nice small invention. i like it.http://photosnag.com/img/3147/n09x0302vnsn/clear.gif

It's really nice that someone else has done the leg work and field tested it. Unfortunately, the review states that it would take an inordinate amount of time to cool a warm can.

But you can get two cans from the fridge, and put one in the gadget. Two cans worth of ice-cold calories for one can's worth of exercise!