HSC Physics Question Thread

[Neutron]

And also, could someone please explain how a coloured TV works? Like is it basically the same as a cathode ray tube only with three electron guns? :O Sorry for the bombardment of questions, have trials in two days ;;

[JennHen]

Hey! Just a question about back EMF - i'm a little confused as to how it is caused, what it actually is, and how it affects the motor. Would be awesome if someone could give an explanation. Cheers!

[RuiAce]

And also, could someone please explain how a coloured TV works? Like is it basically the same as a cathode ray tube only with three electron guns? :O Sorry for the bombardment of questions, have trials in two days ;;

From memory the answer to this question is yes. One electron gun for each of the three digital colours. Each gun makes one of the three metals in one pixel glow blue/red/green.

Hey! Just a question about back EMF - i'm a little confused as to how it is caused, what it actually is, and how it affects the motor. Would be awesome if someone could give an explanation. Cheers!

So a bit of background information:
We know that by Faraday's law, if an electrical conductor is subject to a change in magnetic flux, some EMF is induced.
We also know that by Lenz's law, the induced EMF always seeks to oppose the cause of induction.

But here's the thing. As a consequence of Faraday's law, it doesn't matter whether we move the conductor or the magnetic field. If the magnetic field is moved then obviously an EMF will be induced. However, if a conductor is moved through a magnetic field, that also triggers electromagnetic induction.

So keep in mind that for your motor, the armature (coils) are basically conductors moving through a magnetic field! This is the EMF induced into the motor.

Applying Lenz's law, the cause of induction is the current forcing the coils to rotate in the magnetic field. The reason why it's "back" EMF is that well, by Lenz's law it's going to try to make the coils go back the other way.

What does this do? There will be a point where this EMF induced cannot oppose the actual voltage of the circuit anymore. So this effectively puts a limiting (maximum) velocity on what a motor can achieve.

[imtrying]

Hey:) Just doing some summaries for Medical Physics and a little stuck on "Explain that the amplitude of the signal given out when precessing nuclei relax is related to the number of nuclei present"
Can't seem to make sense of my notes 

[jamonwindeyer]

:O (just a bunch of maths errors hahaha)

The solutions found the initial kinetic and gravitational potential energy of the projectile, and then they let that value be equal to -6.67x10^-11 x 1150 x1.1 x 10^20 all over r (think this is like the gravitational potential energy formula but I have no idea why)

Oh that's an interesting method!! Jake's is what I would do, but let me explain this as well. Basically, it is using the conservation of energy. The energy of the projectile must remain conserved (it acts as a closed system in this example, we don't add any energy after launch).

By calculating the initial kinetic and potential energies and summing them, we are calculating the initial total energy of the projectile. This cannot change. How it is distributed, however, can change.

Consider a projectile launched vertically. It will go up, slow down, and at the peak of its motion it will stop. At THIS precise moment, at the highest point, it has no kinetic energy. It has stopped and is about to turn around to plummet back to ground. At this point, we can equate the initial total energy to the gravitational potential energy of the projectile, since this is the only form of energy it has at that point. So, the total initial energy is equated to GPE with the radius unknown: That is what we find to get our answer 

I hope that helps!

[jamonwindeyer]

And also, could someone please explain how a coloured TV works? Like is it basically the same as a cathode ray tube only with three electron guns? :O Sorry for the bombardment of questions, have trials in two days ;;

Just confirming Rui's answer above! Three separate electron guns, three separate electron beams, and the screen of a colour TV has three different phosphors arranged in groups of three, one for each colour. Each electron beam is focused on illuminating only one colour phosphor 

Good luck for your Trials! Be sure to shoot any more questions our way!

[conic curve]

Me and physics maths don't work together 

Question is

a) If marble B rebounds to the right with a velocity of 3ms^-1, what happens to marble A? (2 marks)

b) Is this an elastic or inelastic collision? Using calculations, justify your answer (2 marks)

[jamonwindeyer]

Hey! Just a question about back EMF - i'm a little confused as to how it is caused, what it actually is, and how it affects the motor. Would be awesome if someone could give an explanation. Cheers!

Welcome to the forums JennHen! Hope Rui's answer helped, be sure to let me know if you need any help finding anything 

[jakesilove]

Hey:) Just doing some summaries for Medical Physics and a little stuck on "Explain that the amplitude of the signal given out when precessing nuclei relax is related to the number of nuclei present"
Can't seem to make sense of my notes 

Hey! So basically, when a precessing nucleus is 'given' more energy, it will flip (ie. move to a anti-parallel state). When the energy source stops (the signal stops being sent into the body), the nuclei will want to flip back to a parallel position. This means it has to give off the extra energy it had been given. The more nuclei in a region, the more energy that will be given off. Therefore, the amplitude/amount of signal/energy given out when the nuclei relax is related to the number of nuclei present! Does that make sense?

Jake

[Neutron]

Just confirming Rui's answer above! Three separate electron guns, three separate electron beams, and the screen of a colour TV has three different phosphors arranged in groups of three, one for each colour. Each electron beam is focused on illuminating only one colour phosphor 

Good luck for your Trials! Be sure to shoot any more questions our way!

Thank you so much Jamon you're too nice D: 
This is probably really stupid but can someone please show me what the eddy currents look like in that experiment with the magnet rotating around a non-magnetic metal disk? It's the one with that stimulates an AC induction motor.. Like do two eddy currents form like in electromagnetic braking or only one..? Idk why I keep getting confused with old concepts now, maybe I never understood it properly :/

[jakesilove]

Thank you so much Jamon you're too nice D: 
This is probably really stupid but can someone please show me what the eddy currents look like in that experiment with the magnet rotating around a non-magnetic metal disk? It's the one with that stimulates an AC induction motor.. Like do two eddy currents form like in electromagnetic braking or only one..? Idk why I keep getting confused with old concepts now, maybe I never understood it properly :/

As far as I remember, the eddy currents formed will just be concentric circles that "chase" the movement of the magnet (or move in the opposite direction, depending on which pole is being held towards the non-magnetic plate!).

[jamonwindeyer]

Me and physics maths don't work together 

Question is

a) If marble B rebounds to the right with a velocity of 3ms^-1, what happens to marble A? (2 marks)

b) Is this an elastic or inelastic collision? Using calculations, justify your answer (2 marks)

Hey! Okay, so for the first one, we first need to calculate the total initial momentum of the system. Remember that the marbles are moving in opposite directions, so one will have positive momentum, one will have negative. I'll take A's momentum/direction as positive:



So after the collision, this momentum MUST be the same as before the collision. That's the conservation of momentum. So, we know the momentum of B after the collision:



So the momentum of A must be equal to the difference:



So Marble A continues to the right with a reduced velocity of 6 metres per second

For the next question, we need to understand that collisions are elastic if kinetic energy is conserved. The principal here is very similar, we first calculate the kinetic energy initially:



I'll leave you to finish this, calculate the kinetic energy after the collision with the values above!! If it matches, your collision is elastic, if not, you've lost energy and so it is inelastic 

[imtrying]

Hey! So basically, when a precessing nucleus is 'given' more energy, it will flip (ie. move to a anti-parallel state). When the energy source stops (the signal stops being sent into the body), the nuclei will want to flip back to a parallel position. This means it has to give off the extra energy it had been given. The more nuclei in a region, the more energy that will be given off. Therefore, the amplitude/amount of signal/energy given out when the nuclei relax is related to the number of nuclei present! Does that make sense?

Jake

Thanks so much, that does make sense. One dot point down, only 80 more to go haha

[jakesilove]

Thanks so much, that does make sense. One dot point down, only 80 more to go haha

Feel free to post any other questions here! Been a while since I've been able to go over Medical Physics, but I remember it being a really interesting option

[jamonwindeyer]

Thanks so much, that does make sense. One dot point down, only 80 more to go haha

Let us know if you need any more help with MRI stuff! Probably the most complex thing I learned in Physics!