HSC Physics Question Thread

[odonataa]

What causes the striation bands to appear? THANK YOU

[jamonwindeyer]

What causes the striation bands to appear? THANK YOU

Hey odonataa!!

Basically, it is the electrons in the Cathode Ray Tube colliding with air particles still in the tube. It is not a perfect vacuum, so electrons hit the air still in there and this causes the striation patterns. Changing the air pressure in the tube will change the pattern.

Hope this helps!! 

[RuiAce]

Better question would be why do the different gas pressures exhibit those specific results and not something else

Speaking of which can someone enlighten me on this topic

[jamonwindeyer]

Better question would be why do the different gas pressures exhibit those specific results and not something else

Speaking of which can someone enlighten me on this topic

Do you mean like the stripes? I was always funny on this, but my best explanation would be as follows (may be slightly off, apologies if I offend any quantum physicists  ).

Electrons moving from the cathode strike atoms in the tube. If the electron has enough energy, it will be 'absorbed' by the atom and dislodge another, with the excess energy being emitted as a photon of light, similar to the photoelectric effect in reverse I suppose? I had it likened to a relay race of sorts, one electron tags in the next.

The new electron then has to speed up before it strikes another atom and the cycle continues. The time it takes to accelerate to the required speed corresponds to a gap in the stripes. The stripes change distance apart based on how far apart the atoms are on average, which is dependent on the air pressure.

[RuiAce]

Do you mean like the stripes? I was always funny on this, but my best explanation would be as follows (may be slightly off, apologies if I offend any quantum physicists  ).

Electrons moving from the cathode strike atoms in the tube. If the electron has enough energy, it will be 'absorbed' by the atom and dislodge another, with the excess energy being emitted as a photon of light, similar to the photoelectric effect in reverse I suppose? I had it likened to a relay race of sorts, one electron tags in the next.

The new electron then has to speed up before it strikes another atom and the cycle continues. The time it takes to accelerate to the required speed corresponds to a gap in the stripes. The stripes change distance apart based on how far apart the atoms are on average, which is dependent on the air pressure.

I suppose so yea but also why different gas pressures cause different patterns. Striations appear for a certain range and then you have Faraday's Dark space and etc., but then if you go further down Crooke's dark space dominates the entire thing and you get this green light and nothing else.

If you go a bit more up then you just get these horizontal purple lines appearing rather than those stripes so to speak! So how does pressure affect that?

[jamonwindeyer]

I suppose so yea but also why different gas pressures cause different patterns. Striations appear for a certain range and then you have Faraday's Dark space and etc., but then if you go further down Crooke's dark space dominates the entire thing and you get this green light and nothing else.

If you go a bit more up then you just get these horizontal purple lines appearing rather than those stripes so to speak! So how does pressure affect that?

Hmm, I suppose it would just be different manifestations of the same idea! I know that Crooke's Dark Space dominates once the air pressure becomes low enough that the electrons stop colliding with particles along the way. Most of them get through and hit the end of the tube, causing the green glow you describe.

I think I remember Faraday's dark space to be the region between the Cathode and Anode (after the negative glow), where the electrons are travelling further and further before they hit an atom (on average) with enough speed to cause fluorescence. Decrease the pressure, increase this gap.

The colours are indicative of the gas inside the tube.

And that's about all I've got, which explains a few of those phenomenon, but not all of them 

[Happy Physics Land]

I suppose so yea but also why different gas pressures cause different patterns. Striations appear for a certain range and then you have Faraday's Dark space and etc., but then if you go further down Crooke's dark space dominates the entire thing and you get this green light and nothing else.

If you go a bit more up then you just get these horizontal purple lines appearing rather than those stripes so to speak! So how does pressure affect that?

I think Rui you are talking about violet streamers appearing in the cathode ray tube. This indicates a high pressure (20mm of Hg) inside the discharge tube. This high pressure indicates a high number of gas particles inside the tube. The reason why you see violet streamers is because collision is occurring everywhere inside the tube. We talking about how when energetic collision between cathode ray and air molecules occur, fluorescence occurs. So when collision occurs everywhere, there will be fluorescence occurring everywhere, forming violet streamers (or purple lines as you call it). These might not be "lines", but our eyes perceive it to be purple lines because collisions are happening at spots very close to one another inside the tube.

[Happy Physics Land]

Do you mean like the stripes? I was always funny on this, but my best explanation would be as follows (may be slightly off, apologies if I offend any quantum physicists  ).

Electrons moving from the cathode strike atoms in the tube. If the electron has enough energy, it will be 'absorbed' by the atom and dislodge another, with the excess energy being emitted as a photon of light, similar to the photoelectric effect in reverse I suppose? I had it likened to a relay race of sorts, one electron tags in the next.

The new electron then has to speed up before it strikes another atom and the cycle continues. The time it takes to accelerate to the required speed corresponds to a gap in the stripes. The stripes change distance apart based on how far apart the atoms are on average, which is dependent on the air pressure.

I think Jamon's second paragraph really effectively summarises about the striation patterns. Basically when you see gaps between stripes, it is a brief region of dark space where no collisions occur because the cathode ray is accelerating without being interfered by any air molecules along the day.

[RuiAce]

I think Rui you are talking about violet streamers appearing in the cathode ray tube. This indicates a high pressure (20mm of Hg) inside the discharge tube. This high pressure indicates a high number of gas particles inside the tube. The reason why you see violet streamers is because collision is occurring everywhere inside the tube. We talking about how when energetic collision between cathode ray and air molecules occur, fluorescence occurs. So when collision occurs everywhere, there will be fluorescence occurring everywhere, forming violet streamers (or purple lines as you call it). These might not be "lines", but our eyes perceive it to be purple lines because collisions are happening at spots very close to one another inside the tube.

You're right I forgot that the correct term was "streamers". But he did provide quite a fair bit of info on what I needed - I just wanted to deduce what pressure exactly meant. Collisions as a word by itself doesn't explain the change in what's observed to me

[jamonwindeyer]

You're right I forgot that the correct term was "streamers". But he did provide quite a fair bit of info on what I needed - I just wanted to deduce what pressure exactly meant. Collisions as a word by itself doesn't explain the change in what's observed to me

"Collisions" is probably better described as electrons 'interacting' with positive ions with missing electrons lost to the flow of charge in the tube. The atoms regaining their electron is what causes fluorescence. 'Pressure' corresponds to the spacing between the atoms for which this occurs, thus changing the patterns. At least that's how I would explain it 

[jamonwindeyer]

Love how this was prompted by what is actually an "Identify/Recognise" part of the syllabus!

[smiley2101]

why is the answer C not D? thank you so much

[smiley2101]

why is the answer B?

thank you!

[smiley2101]

sorry I have so many questions!!!
I just don't get it haha

why is the answer D and not c?

[smiley2101]

I always stuff up the frame of reference like in this question. Do you have any tips or tricks that you used to know what tv or to should be substituted by?