inconsistent behaviour of cathode rays (1 Viewer)

[mrpotatoed]

So the inconsistent behaviour of cathode rays was due to Hertz concluding that it was a wave as his experiments showed that they were not deflected by electric fields, which occurred because his electric field was not strong enough so that there was no observable change in direction (right?). This confusion went for a while as other scientists (such as crooks) did observe a deflection.

If cathode rays cause a paddle wheel to spin, why is there any confusion, clearly they MUST be particles due to p=mv? The paddle can only be moving if there is momentum from the rays being transferred to the wheel and the ray can only have momentum in the first place if it has mass. So my question is, why was there any debate as the observations from the paddle wheel experiment seem highly conclusive.

It eventually took JJ Thomson finding the q/m ratio to prove it was a particle, right? Why was the paddle wheel experiment not enough on its own?

 

[InteGrand]

I think the real explanation for the spinning was actually something called the radiometric effect. See this paragraph: https://en.wikipedia.org/wiki/Crookes_tube#Paddlewheel

(However, 'real' explanations are not always required or even accepted for HSC purposes. Also, see Q11 of 2005 HSC Physics: http://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2005exams/pdf_doc/physics_05.pdf

The answer by the Board of Studies is B: http://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2005exams/pdf_doc/physics_er_05.pdf)
So it's safest for the HSC to just say what reputed HSC textbooks say.

 

[InteGrand]

the ray can only have momentum in the first place if it has mass.

Incidentally, light has momentum, though not mass.

 

[mrpotatoed]

Your board of studies links aren't working for me ._. Even when I tried manually typing the url, guess is something wrong on my half.

Also, they didn't know it was due to heating until a while after the experiment was first carried out though did they? (the Wikipedia article said JJ Thomason found out in 1900's). What caused them, when the experiment first occurred, to not just assume that it was the cathode rays transferring momentum and ending the argument right then and there?

Also with the dual nature of light, my understanding is that for classical physics we treat it as a wave, but for quantum physics, as a particle? (that's what my teacher told me, but he was probably dumbing it down haha) Quantum physics didn't exist then so they would assume light was a wave? Sorry haha, just started to write notes for each dot point and im at first one where you need to explain the apparent inconsistent behaviour of cathode rays... and trying to figure out why things went as they did.

 

[InteGrand]

Your board of studies links aren't working for me ._. Even when I tried manually typing the url, guess is something wrong on my half.

They didn't know it was due to heating until a while after the experiment was first carried out though did they?

Also with the dual nature of light, my understanding is that for classical physics we treat it as a wave, but for quantum physics, as a particle? Quantum physics didn't exist then so they would assume light was a wave? Sorry haha, just started to write notes for each dot point and im at first one where you need to explain the apparent inconsistent behaviour of cathode rays... and trying to figure out why things went as they did.

Sorry, I've updated the links; they should work now.

And I think it's true that they didn't know it was due to heating until a while after the experiment was first carried out.

And light was thought of as a wave before quantum theory was developed (i.e. in classical physics). So yes, we'd think of it as a wave. It's because of things like the double slit experiment that we had to readjust this view. And in quantum physics, light's thought of as both a particle, and a wave (mind-bending!).

 

[mrpotatoed]

So regarding my initial statement (I edited it like 50 times so you probably saw an older version, trying to wrap my head around my thoughts lol), What caused them, when the paddle wheel experiment first occurred, to not just assume that it was the cathode rays transferring momentum and ending the argument right then and there?

Am I overthinking this whole scenario, and instead of wasting time on this I should just memorise the timeline of events that the BoS follows instead of trying to flaws in it

 

[InteGrand]

I'm not exactly sure, but probably due to the other experiments providing evidence against them having mass.

 

[InteGrand]

So regarding my initial statement (I edited it like 50 times so you probably saw an older version, trying to wrap my head around my thoughts lol), What caused them, when the paddle wheel experiment first occurred, to not just assume that it was the cathode rays transferring momentum and ending the argument right then and there?

Am I overthinking this whole scenario, and instead of wasting time on this I should just memorise the timeline of events that the BoS follows instead of trying to flaws in it

Memorising the timeline of events would be sufficient for getting marks in the exam. If you're curious about the history, you could research it further, but it probably wouldn't matter much for marks haha.

 

[mrpotatoed]

That's the thing though isn't it, The only experiment that went against cathode rays being a charged particle was Hertz' experiment where it wasn't deflected by his electric fields, but that only showed they weren't charged.

Oh well, I'll have to interrogate my teacher when I see him next lol.

 

[Fizzy_Cyst]

There were a few other things the HSC wants you to say:

Cathode Rays passed through thin gold foil - how could a 'particle' pass through an interlinked arrangement of gold atoms if atoms are the smallest thing known in the universe (as they were back then), therefore must not be a particle.
Moving charges create a magnetic field -- the magnetic field associated with these cathode Rays could not be measured (simply because there was not a sensitive enough magnetic field probe)
The reason why the electric field did not cause deflection was not because the field was too weak, rather the tube was not properly evacuated, which caused the residual molecules to ionise when struck by cathode Rays and effectively cancelled out the charge on the plates

 

[mrpotatoed]

There were a few other things the HSC wants you to say:

Cathode Rays passed through thin gold foil - how could a 'particle' pass through an interlinked arrangement of gold atoms if atoms are the smallest thing known in the universe (as they were back then), therefore must not be a particle.
Moving charges create a magnetic field -- the magnetic field associated with these cathode Rays could not be measured (simply because there was not a sensitive enough magnetic field probe)
The reason why the electric field did not cause deflection was not because the field was too weak, rather the tube was not properly evacuated, which caused the residual molecules to ionise when struck by cathode Rays and effectively cancelled out the charge on the plates

I've never seen that in any textbook I've read, weird. Was it crookes who did that experiment? That then would make sense about the whole disagreement.

 

[keepLooking]

Gold foil is Ruthorford's experiment.

 

[Crisium]

Gold foil is Ruthorford's experiment.

Yeah and it the experiment that led to the discovery of the nucleus; but it can be related to this

 

[keepLooking]

As Crisium has mentioned, the gold foil experiment will probably be in your Q2Q section of your textbook.

 

[Fizzy_Cyst]

It's not Rutherfords experiment! He used alpha particles.

Hertz passed Cathode Rays through thin gold foil in 1892

 

[Crisium]

It's not Rutherfords experiment! He used alpha particles.

Hertz passed Cathode Rays through thin gold foil in 1892

Oops sorry

I haven't actually gone through From Ideas to Implementation and I've only heard about a "gold foil" experiment ever being related to Rutherford

 

[keepLooking]

It's not Rutherfords experiment! He used alpha particles.

Hertz passed Cathode Rays through thin gold foil in 1892

Sorry! I shouldn't have said anything if I wasn't sure.

Fizzy is right.

 

[Fizzy_Cyst]

All G ppls, that's what this forum is for!

Don't know why that piece of info isn't in textbooks though. It's pretty useful!

 

[mrpotatoed]

Asked my teacher about it yesterday.... he said he agreed with what I was saying (Crookes had a much stronger case than Hertz) but that it was mainly due to the slow speed with which information travelled in those days that caused the apparent inconsistent behaviour.

 

[Fizzy_Cyst]

So my question is, why was there any debate as the observations from the paddle wheel experiment seem highly conclusive.

Yes, but by the same token, the observations of no deflection in electric field, no measurable magnetic field associated with the cathode rays and ability to penetrate thin gold foil seem highly conclusive that cathode rays are waves!

Not so much the slow speed which information travelled, but rather the fact that two groups of scientists each had 'conclusive' evidence (based on the theories and scientific instruments of the day) that their theory was correct!