2008 hsc q12 (1 Viewer)

[Nooblet94]

12 The debate as to whether cathode rays are charged particles or electromagnetic wavescontinued for many years.
Which observation of cathode rays resolved this debate?
(A) Cathode rays can turn a paddle wheel.
(B) An electric field can deflect cathode rays.
(C) Cathode rays can penetrate thin metal foil.
(D) Fluorescent screens glow when struck by cathode rays.

Notes from the marking centre say the answer is B, but as far as I'm concerned, this so called "debate" hasn't been resolved at all and my physics teacher agrees. He also said that if he was doing the exam and was forced to choose one, he'd choose A (which is what I'd choose as well). So my question is firstly, did they end up giving marks for both A and B? If not, why is B correct?

 

[deswa1]

What do you mean this debate isn't settled? Cathode rays are electrons aren't they?

 

[Sanical]

Cathode rays are electrons.
Although electrons can exhibit wave like properties (double slit experiment)

 

[Sanical]

Clarify further, in the cathode ray, the electron acts as a particle. But it can act as a wave as mentioned above, but it's not a wave in a CRT. The electron is definitely not an "electromagnetic wave" (as the question asks) but it does have characteristics of a "wave". So yeah, it ended the debate about it being an electromagnetic wave (which is true).

 

[Nooblet94]

Clarify further, in the cathode ray, the electron acts as a particle. But it can act as a wave as mentioned above, but it's not a wave in a CRT. The electron is definitely not an "electromagnetic wave" (as the question asks) but it does have characteristics of a "wave". So yeah, it ended the debate about it being an electromagnetic wave (which is true).

Ahh true, I must admit I skimmed over the "electromagnetic" part, and just read it as wave. Even so, option A shows that cathode rays can impart momentum, which at the time was thought to be a solely particle like characteristic.

EDIT: Fuckery, just realised it says "charged" particle, which isn't shown by the paddlewheel experiment.

 

[unLimitieDx]

Wasnt Thomson's experiment that proved beyond doubt that cathode rays were negatively charged particles. charge to mass ratio.

 

[someth1ng]

(A) Cathode rays can turn a paddle wheel.
(B) An electric field can deflect cathode rays.
(C) Cathode rays can penetrate thin metal foil.
(D) Fluorescent screens glow when struck by cathode rays.

Okay, looking at this question directly, you can say that C and D are incorrect since C demonstrated that it was EMR until there was a change of model of the atom and fluoescent screens can glow after they absorb light. This leaves A and B - I would pick B over A because the problem was solved "in 1897 when J.J. Thompson measured the mass of cathode rays" and this showed that it was some sort of particle but was much lighter than the lightest atom (Hydrogen). The significance of Thompson in this respect is that he utilised electromagnetic deflection corresponding with option B.

The most important thing about this question is this part "Which observation of cathode rays resolved this debate?" - option A only supported the idea that it was a particle but was not concrete enough to extinguish all doubt and so again, option B prevails.

 

[Sanical]

Ahh true, I must admit I skimmed over the "electromagnetic" part, and just read it as wave. Even so, option A shows that cathode rays can impart momentum, which at the time was thought to be a solely particle like characteristic.

EDIT: Fuckery, just realised it says "charged" particle, which isn't shown by the paddlewheel experiment.

haha you read the particle and the wave part wrong .

 

[Sidekickk]

The question says what RESOLVED this debate. The paddle wheel experiment was definitely one of the main experiments in favour of the particle nature but it wasnt until they proved the electric field deflection that it was accepted.

 

[John Rips]

12 The debate as to whether cathode rays are charged particles or electromagnetic wavescontinued for many years.
Which observation of cathode rays resolved this debate?
(A) Cathode rays can turn a paddle wheel.
(B) An electric field can deflect cathode rays.
(C) Cathode rays can penetrate thin metal foil.
(D) Fluorescent screens glow when struck by cathode rays.

This is what i teach my students:
Heinrich Hertz showed that heat waves (an emr) could also turn a paddle wheel if the heat radiation was aimed correctly at the paddles, so answers A, C and D can all be performed both with particles or emr. Most of the European scientists pulled out of the research argument for emr when Crookes showed cathode ray beams could be deflected by magnetic fields in the same direction as negatively-charged particles. But the final proof was if same CR beams could be deflected by electric fields also as if they were negative particles. It took some years after this to develop "vacuum pumps" which allowed the high potential differences between closely aligned capacitor plates necessary to deflect CR with electric fields. So the electric fields were the final nail to show they were negative particles.

Thompson's work was inportant for two reasons. Firstly a constant q/m value could only be obtained if there was an m value, that is cathode rays had mass. Secondly and in many ways more important, by having the same q/m value no matter which gas was previously in the tube or which metal in the electode led Thompson to suggest that these "negative corpuscles" came from the metal atoms of the electrode. Thompson's idea that cathode ray particles (now called electrons) were components of the atom blew away the accepted norm of the time (Dalton's theory) that atoms were like billiard balls, the tiniest particles unable to be further broken down. I hope that clears it ip for you.