[AMPS] Cold beer

Thu, 19 Jun 1997 10:24:15 +0000

> From:          yo3ctk <yo3ctk@alltrom.ro>
> To:            "'amps@contesting.com'" <amps@contesting.com>
> Subject:       RE: [AMPS] failure
> Date:          Wed, 18 Jun 97 16:39:17 +0000

> 	Then Tom said:
> 
> 	The main heating of tube elements (other than the filament) is
> by 
> 	kinetic energy from electrons. Since the electrons are leaving
> the 
> 	cathode, they are removing energy with their departure.

Hi Mike,

As the electrons leave the cathode, they REMOVE thermal energy from 
the cathode. The amount of energy removed is very small, but it is 
removed.

Electrothermal devices like themocouples and the semiconductors 
used to "pump heat", operate on this principle.

As the electrons are moved from one area to another they carry "heat" 
energy with them.

>> 	Isn't it simple? E times I gives the dissipation, where E is the
>> 	potential difference between the electron source and the target 
>> 	(ie the cathode and anode) and I is the target current (ie anode
>> 	current).
>> 
> >	That's why we calculate the anode or grid dissipation as E*I 
> >	integrated over time, and we don't have to worry about it
>> heating 
>> 	the cathode!

> 	Sorry Tom, I think that your theory is wrong.
> 
> 	Kinetic energy of electrons reaching the anode (or screen grid,
> for that matter) comes from the electric field generated by the anode
> voltage, and NOT from the thermal energy generated by the filament. 

That's what I said! 

There is a tiny contribution due to thermal energy contributing to 
the kinetic energy of the electrons, but it is small and we can 
normally ignore it (unless you own an Igloo cooler that uses a 
semiconductor "heat pump", then the beer we bet with Peter will get 
warmer instead of staying cold).

The bulk of the heating is due to the voltage difference between the 
source and target, and the number of electrons moved.

>You
> are implying that the cathode is cooling due to electron emission and
> this is not correct.

It is correct, but the amount of cooling by this effect is small in 
a vacuum tube. In a properly designed semiconductor the effect is 
very large!

>By the way which type of beer do you prefer?

Cold beer, especially if it is cooled by the effects of electrons 
moving out of one area and into another. It might be unimportant in 
the tube, but it is important in my Igloo dc operated cooler! ;-)

73, Tom W8JI 

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