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Post by sherlock2308 on Sun Aug 07, 2016 4:44 pm

Can anyone tell me about thomson's experiment and also in the millikan's experiment how was the oil droplet charged?

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Post by Confusions allowed on Sun Aug 07, 2016 6:04 pm

First, make clear what are you trying to say. Also, the lecture notes are there.
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Post by sherlock2308 on Sun Aug 07, 2016 6:25 pm

cathode rays are defined as the rays produced by the cathode when heated in low pressure. Why is the low pressure factor important? would vacuum do?

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Post by Keep Guessing! on Mon Aug 08, 2016 12:12 am

Sir told that in class already. Low pressure ensures less gas atoms so that the electrons don't encounter too many collisions.

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Post by sherlock2308 on Mon Aug 08, 2016 12:15 am

Vacuum has no gas atoms!!! Won't it be better then?

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Post by Aasim Saifee on Mon Aug 08, 2016 12:23 am

sherlock2308 wrote:Vacuum has no gas atoms!!! Won't it be better then?


it is not possible to create perfect vacuum.

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Post by Keep Guessing! on Mon Aug 08, 2016 12:38 am

And we need a gas to ionize. Also, wikipedia says that another reason for him to decrease the pressure was to get better readings and a more accurate value of the e/m ratio.

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Post by The Confused Guy (O.o)? on Mon Aug 08, 2016 8:50 am

For perfect vacum u can go to space and try the experiment :v (jk) and send me the results if u succeed lol!


Last edited by The Confused Guy (O.o)? on Mon Aug 08, 2016 1:38 pm; edited 1 time in total
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Post by The Confused Guy (O.o)? on Mon Aug 08, 2016 1:41 pm

Keep Guessing! wrote:And we need a gas to ionize. Also, wikipedia says that another reason for him to decrease the pressure was to get better readings and a more accurate value of the e/m ratio.
Infact u dont need gas to ionise to give electrons , the electrodes themselves can release electrons under high electric field, heat or light
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Post by Sreejith A Nair on Mon Aug 08, 2016 9:01 pm

To answer this question we need to know the proportion of cathode ray produced from the cathode and the gas in the tube. This could be verified by a series of experiments in which the pressure of the tube is reduced continuously until we get as close as possible to vacume. All the experiments should be conducted at a constant potential difference. With only the pressure changing. Since we are not in a state to cundect such an experiment logical explanations are satisfactory. And I suppose that the major source is the cathode itself as it is highly unlikely that such a small amount of gas produces enough electrons to produce respectable florasence
. If any one has oppositions please point out.

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Post by Keep Guessing! on Tue Aug 09, 2016 12:16 am

The electrons in the cathode ray are from the cathode plate. But first the gas is ionized by the high electric potential. The ions thus formed get attracted to the cathode, collide with it, and release electrons from the cathode. So we do need a small amount of gas to kind of start the emission. However, this was in the olden times. In modern CRTs, thermionic emission through filament is used.

But, I have a question- when the gas atoms are initially ionized, it happens because of the high electric potential across the electrodes. Why isn't the metal ionized before the gas? We all know that the ionization energy of metals is generally lower than gases.

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