HSC Physics Question Thread

[Aliceyyy98]

Hi guys!

Could someone plz explain how cathode rays work as in like the electron guns and deflection plates etc? The dot pt says in tv and oscilloscopes so do they work differently in different things??

Cheers

[jamonwindeyer]

Hi guys!

Could someone plz explain how cathode rays work as in like the electron guns and deflection plates etc? The dot pt says in tv and oscilloscopes so do they work differently in different things??

Cheers

Hey Alice!

So they are applied ever so slightly differently. Are you okay with the Physics behind how the deflection plates actually work? Let me know if you aren't, but assuming you are, it's all about what external signal the horizontal and vertical deflection plates are aligned to.

In Oscilloscopes, the horizontal deflection is controlled by a time varying signal. This causes the beam to sweep horizontally across the screen, from left to right, at a constant pace. The vertical deflection is controlled by some other signal, a measurement (for example, a microphone). What this creates is a vertical axis dependent on signal strength of some stimuli, and a horizontal axis dependent on time. Think of your typical heartbeat monitor in the hospital drama shows, that is exactly what I'm talking about

In Cathode Ray Televisions, both the horizontal and vertical deflection plates have time varying strengths. The idea here, put simply, is that it makes the electron beam sweep across the entire television screen. It sweeps along the top row of phosphor dots (kind of like pixels on your laptop), then the second row, then the third row, so quick that you just see the solid image.

There are 3 electron guns in Colour CRT's, each corresponding to either Red, Green, or Blue. Each is tasked with creating a specific colour on the screen (there are different phosphors, one for each colour). The intensity of each is controlled with a separate accelerating anode 

This explanation is definitely lacking a bit in certain areas, there is some pretty complex stuff involved, but this is a good overview. Is there any specific part of this that is a bit iffy for you?

[Neutron]

Hey Alice!

So they are applied ever so slightly differently. Are you okay with the Physics behind how the deflection plates actually work? Let me know if you aren't, but assuming you are, it's all about what external signal the horizontal and vertical deflection plates are aligned to.

In Oscilloscopes, the horizontal deflection is controlled by a time varying signal. This causes the beam to sweep horizontally across the screen, from left to right, at a constant pace. The vertical deflection is controlled by some other signal, a measurement (for example, a microphone). What this creates is a vertical axis dependent on signal strength of some stimuli, and a horizontal axis dependent on time. Think of your typical heartbeat monitor in the hospital drama shows, that is exactly what I'm talking about

In Cathode Ray Televisions, both the horizontal and vertical deflection plates have time varying strengths. The idea here, put simply, is that it makes the electron beam sweep across the entire television screen. It sweeps along the top row of phosphor dots (kind of like pixels on your laptop), then the second row, then the third row, so quick that you just see the solid image.

There are 3 electron guns in Colour CRT's, each corresponding to either Red, Green, or Blue. Each is tasked with creating a specific colour on the screen (there are different phosphors, one for each colour). The intensity of each is controlled with a separate accelerating anode 

This explanation is definitely lacking a bit in certain areas, there is some pretty complex stuff involved, but this is a good overview. Is there any specific part of this that is a bit iffy for you?

(Image removed from quote.)

On top of Jamon's flawless answer, it might also be worth mentioning that Cathode Ray Oscilloscopes (CRO) use electric fields as their deflection plates and TVs use magnetic fields. CROs use electric fields as they can switch at higher frequencies than magnetic fields (so it can basically respond to the stimuli faster whereas magnetic fields take a bit longer) and TVs use magnetic fields because when they deflect the electron beam, they deflect it in a circular path (the force is perpendicular to the direction of motion of the electron beam (from right hand palm rule), which means it provides a centripetal force and therefore, circular motion) whereas electric fields deflect in a parabolic path. So then the radius of curvature is greater and therefore, the TV can have a bigger screen (CROs don't need a massive screen) !! But yeah, the electron gun functions the same way for both (i.e. both have heating filament at cathode, which 'boils' off electrons through thermionic emission, which then goes through control electrode, focussing anode and accelerating anode before reaching the deflection plates) and its just the different fields used by the deflection plates

 If you wanted a few more advantages and disadvantages between electric plates and electromagnetic coils (magnetic fields), here they are:
Electric plates
Advantages
-Can respond at higher frequencies (mentioned above)
-Less power required for deflection
Disadvantages
-Needs to be inside the tube in order for the field to be strong enough (this complicates the tube design)
-Deflects in parabolic path (lower radius of curvature)
-Larger defocussing during deflection (longer tube is required)
Electromagnetic coils
Advantages
-Smaller defocussing of the electron beam when deflecting
-Deflects in a circular path, therefore greater radius of curvature (reduces length of cathode ray tube needed for a bigger screen size)
-Can be positioned outside the tube (simplifies tube design)
Disadvantages
-Requires more power to operate
-Harder to get uniform magnetic field

I was just studying this yesterday so feel free to correct me if I stuffed up somewhere!!

[jakesilove]

On top of Jamon's flawless answer, it might also be worth mentioning that Cathode Ray Oscilloscopes (CRO) use electric fields as their deflection plates and TVs use magnetic fields. CROs use electric fields as they can switch at higher frequencies than magnetic fields (so it can basically respond to the stimuli faster whereas magnetic fields take a bit longer) and TVs use magnetic fields because when they deflect the electron beam, they deflect it in a circular path (the force is perpendicular to the direction of motion of the electron beam (from right hand palm rule), which means it provides a centripetal force and therefore, circular motion) whereas electric fields deflect in a parabolic path. So then the radius of curvature is greater and therefore, the TV can have a bigger screen (CROs don't need a massive screen) !! But yeah, the electron gun functions the same way for both (i.e. both have heating filament at cathode, which 'boils' off electrons through thermionic emission, which then goes through control electrode, focussing anode and accelerating anode before reaching the deflection plates) and its just the different fields used by the deflection plates

 If you wanted a few more advantages and disadvantages between electric plates and electromagnetic coils (magnetic fields), here they are:
Electric plates
Advantages
-Can respond at higher frequencies (mentioned above)
-Less power required for deflection
Disadvantages
-Needs to be inside the tube in order for the field to be strong enough (this complicates the tube design)
-Deflects in parabolic path (lower radius of curvature)
-Larger defocussing during deflection (longer tube is required)
Electromagnetic coils
Advantages
-Smaller defocussing of the electron beam when deflecting
-Deflects in a circular path, therefore greater radius of curvature (reduces length of cathode ray tube needed for a bigger screen size)
-Can be positioned outside the tube (simplifies tube design)
Disadvantages
-Requires more power to operate
-Harder to get uniform magnetic field

I was just studying this yesterday so feel free to correct me if I stuffed up somewhere!!

Neutron mate, you're a star

[jakesilove]

Neutron mate, you're a star

See what I did there

[jakesilove]

See what I did there

[jamonwindeyer]

On top of Jamon's flawless answer, it might also be worth mentioning that Cathode Ray Oscilloscopes (CRO) use electric fields as their deflection plates and TVs use magnetic fields...

Ahh I forgot about this, VERY good point, and probably one of the main things to mention,  aha thanks Neutron! 

[Loki98]

Hey guys,
In the question attached, for part ii) Could someone plz explain why the LED would not light up. Wouldn't there still be a change in flux causing an emf to be produced which should light up the LED but slightly weaker than before since the rate of change in flux is slower than before?

[Spencerr]

Hey guys,
In the question attached, for part ii) Could someone plz explain why the LED would not light up. Wouldn't there still be a change in flux causing an emf to be produced which should light up the LED but slightly weaker than before since the rate of change in flux is slower than before?

Hey, my interpretation of it is that the magnitude of the emf induced via Faraday's law is proportional to the rate of change in flux. The question mentions that the change in shape is "much more slowly", I would assume that the emf induced would be too small to cause the light bulb to light up (V = IR), if V is too small, then there will not be enough current to create the light. (Any change in flux will always cause emf to be produced)

[Loki98]

Hey, my interpretation of it is that the magnitude of the emf induced via Faraday's law is proportional to the rate of change in flux. The question mentions that the change in shape is "much more slowly", I would assume that the emf induced would be too small to cause the light bulb to light up (V = IR), if V is too small, then there will not be enough current to create the light. (Any change in flux will always cause emf to be produced)

So the only way it can be justified is by saying the current would be insufficient to turn the LED on?

[jakesilove]

So the only way it can be justified is by saying the current would be insufficient to turn the LED on?

I think these two answer cover it pretty well! You're absolutely right, an EMF is produced, however it is much lower. This will be insufficient to turn the LED on. Since the question SAYS that it doesn't turn on, that's really all you can say! Great answers

[MysteryMarker]

Hey guys,

How does electron density affect the resistance of a conductor? Like would an increase in electrons result in more collisions, or allow for the movement of more charge?

Cheers guys.

[conic curve]

What is the action and reaction force(s) when it comes to running, swimming and rowing a boat?

Does the water affect this at all?

[jamonwindeyer]

I think these two answer cover it pretty well! You're absolutely right, an EMF is produced, however it is much lower. This will be insufficient to turn the LED on. Since the question SAYS that it doesn't turn on, that's really all you can say! Great answers

So the only way it can be justified is by saying the current would be insufficient to turn the LED on?

That's definitely the answer!! The rate of change of magnetic flux density (magnetic field strength) is much less in the second scenario, and thus, the induced EMF will not be sufficient to light the LED. To give an electrical engineers perspective, an LED requires a minimum forward operating current to turn on, anything below that can't induce electroluminescence in the junction (an LED is, speaking very roughly, essentially the reverse of a simple solar cell, converting electrical energy into visible light )

[jamonwindeyer]

Hey guys,

How does electron density affect the resistance of a conductor? Like would an increase in electrons result in more collisions, or allow for the movement of more charge?

Cheers guys.

Hey there!! The mathematics behind proving this is a little bit intense (even I don't quite get it tbh) but we can express the electrical resistance of something in the following way:



The 'n' in the denominator is your free electron density. The free electron density being larger increases your conductivity, and thus decreases resistance. More electrons equals more current flow