MC help (1 Viewer)

[VJ30]

The debate as to whether cathode rays are charged particles or electromagnetic waves
continued 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.


i picked A since that proves that they have momentum. But in the book produced by science teachers, it says both EM waves and charged particle beams will produce effects described in A, C, and D...how can em waves rotate a paddle wheel???

Thanks!

 

[nerdasdasd]

The answer is b. Waves cannot move a paddle wheel, but particles have mass and momentum.

 

[someth1ng]

i picked A since that proves that they have momentum. But in the book produced by science teachers, it says both EM waves and charged particle beams will produce effects described in A, C, and D...how can em waves rotate a paddle wheel???

Light DOES have momentum, therefore, theoretically, it is possible for it to push to a paddle wheel. Obviously, light can pass through solid objects (in this case, metal foil) and light CAN cause fluorescence.

However, the last one, B, light CANNOT be deflected by electric/magnetic fields.

 

[VJ30]

The answer is b. Waves cannot move a paddle wheel, but particles have mass and momentum.

i dont get what you are trying to say..furthermore, i also read somewhere that the deflection was so small that they couldnt observe it and concluded that cathode rays do not get affected by electric field..Hence my choice for A

 

[GoldyOrNugget]

The answer is B because it was known for a fact that EMR cannot be deflected by electricity. This was the nail in the coffin for the idea that cathode rays were waves. It's a "history of physics" question more than anything else.

 

[Demise]

i dont get what you are trying to say..furthermore, i also read somewhere that the deflection was so small that they couldnt observe it and concluded that cathode rays do not get affected by electric field..Hence my choice for A

I don't know where you read that, but Thompson was able to deflect cathode rays with both magnetic and electric fields. Light does have mass (not stationary mass though) as photons contain energy, so theoretically it can push a paddle wheel, although you cannot deflect it. Therefore it's B.

 

[kman16]

i dont get what you are trying to say..furthermore, i also read somewhere that the deflection was so small that they couldnt observe it and concluded that cathode rays do not get affected by electric field..Hence my choice for A

This is the reason as to why its "B".
There was a big debate between English and German scientists as to whether the cathode rays were particles or EMR. These were the observations:
EMR: - Rays penetrated through thin metal foils with no damage
- Rays travelled in straight lines
- Rays didn't SEEM to be deflected by an electric field

Particles: - Rays deflected by magnetic field
- They left at right angles to the surface of the cathode
- They could turn a paddle wheel

So, the last point on EMR was assumed to be proven by Gustav Hertz who did a bad experiment (possibly due to his German bias). He showed that there was no deflection of cathode rays in an electric field. But this turned out to be an invalid conclusion.

J.J Thomson then PUT THE ARGUMENT BEYOND DOUBT when he showed that cathode rays DO deflect in an electric field. However the discharge tube needed to be PARTIALLY EVACUATED to see this observation. When they were evacuated, the cathode rays deflected to the positive plate. This meant that cathode rays are negatively charged particles

The problem with Gustav Hertz experiment was the fact that the cathode ray tube wasn't evacuated which alters the path of the cathode rays

 

[someth1ng]

J.J Thomson then PUT THE ARGUMENT BEYOND DOUBT when he showed that cathode rays DO deflect in an electric field. However the discharge tube needed to be PARTIALLY EVACUATED to see this observation. When they were evacuated, the cathode rays deflected to the positive plate. This meant that cathode rays are negatively charged particles

The problem with Gustav Hertz experiment was the fact that the cathode ray tube wasn't evacuated which alters the path of the cathode rays

The problem with Hertz' experiment was that it wasn't evacuated enough. Also, you need it to be almost fully evacuated for it to deflect in an electric field.

 

[cheepy5]

B is the most correct. There problem solved.