HSC Physics Question Thread

[winstondarmawan]

Hello!
Can someone help explain the 1st and 3rd point in the answers? I'm not used to seeing DC motors like this so I am very confused.
Q: https://scontent-syd2-1.xx.fbcdn.net/v/t35.0-12/22851172_1362180510574135_1756100432_o.jpg?oh=c99e6aa9f3e2042b617df10595f4c77b&oe=59F488F0
A: https://scontent-syd2-1.xx.fbcdn.net/v/t35.0-12/22906523_1362180517240801_1608008686_o.jpg?oh=63611363f57c0174f90fb5a1afcdc067&oe=59F56D36
https://scontent-syd2-1.xx.fbcdn.net/v/t35.0-12/22851579_1362180543907465_554963234_o.jpg?oh=cc8950f62400f91ef10184ef0c67b351&oe=59F48DC8
Thanks!

[Bubbly_bluey]

Hello!
Can someone help explain the 1st and 3rd point in the answers? I'm not used to seeing DC motors like this so I am very confused.
Q: https://scontent-syd2-1.xx.fbcdn.net/v/t35.0-12/22851172_1362180510574135_1756100432_o.jpg?oh=c99e6aa9f3e2042b617df10595f4c77b&oe=59F488F0
A: https://scontent-syd2-1.xx.fbcdn.net/v/t35.0-12/22906523_1362180517240801_1608008686_o.jpg?oh=63611363f57c0174f90fb5a1afcdc067&oe=59F56D36
https://scontent-syd2-1.xx.fbcdn.net/v/t35.0-12/22851579_1362180543907465_554963234_o.jpg?oh=cc8950f62400f91ef10184ef0c67b351&oe=59F48DC8
Thanks!

Hey! I'll give a crack at explaining the 3rd one. So on the diagram on one side the coil is wrapped  around. Lets look at the left side first So you have 4 things going on:
1.The wire (from where it begins) starts to wrap vertically up. Using the right hand palm rule it goes down towards the pin direction. (not possible in motors)
2.Then the wire is wrapped horizontally at the top which current going in. Resultant force should be up, so thats possible
3. The current then wraps vertically down. Again observation (1) applies
4. The wire wraps horizontally at the bottom of the rotor so current going towards the pin. Resultant force would be down.
Now looking at 2 and 4, when a wire is wrapped just once, you have two opposing forces up and down. And so thats why I think its a mistake. The same goes for the wrapped coil on the right. Hope I'm making sense and someone corrent me if I'm wrong

[Bubbly_bluey]

hey guys! if a q says to describe how g-forces affects of astronaunts in launch: what exactly do you have to write? They experience an increasing force due to acceleration but I don't know how to describe it. Is it refering to multistage rockets?
Thanks 

[jaskirat]

Hey guys, need help with these 3 questions

[hinakamishiro]

Hey guys i'm just having trouble understanding/ working out these questions and i would really appreciate if anyone could give me a hand! I know it looks like a lot, but they are all just multiple choice questions which didn't have any explanations that i could find. Cheers

[imda.beast]

hi,
i don't quite understand how this question is done. can you help? thanx in advance

[pikachu975]

hi,
i don't quite understand how this question is done. can you help? thanx in advance

Boron doping means it is p type so extra holes.

Positive flow goes left in the silicon block hence holes move left, so therefore electrons move right in the valence band as they causing the apparent hole movement, thus A.

[Bubbly_bluey]

Hey guys, need help with these 3 questions

Hey! So q14 is converting the magnetic flux graph to emf. (My explanation is using maths so I'm sorry if it doesn't make sense but I'll try) These two are linked together with the E= -change flux/ change time so like a differentiation q (you have to differentiate magnetic flux to get emf. Solooking at the magnetic flux graph at the origin is a point of inflection (POI) with positive graident, meaning on the emf graph should start above the x axis. BUT, remember the formula has a negative sign so its the opposite and actually starts at the bottom. Leaving options C and D. However when differentiating a POI you get a stat point so it should be D.
To confirm you can look back at the magnetic flux graph at the next time and you have a stationary point, gradient =0. Therefore that is an indication that you should have an x-int next.
q16 Im a bit iffy on back emf. But i think maybe because has the DC motor gain speed until it reaches constant velocity when back emf=supply emf. So there would not be any extra work/ no net force done because the induced emf would balance with the supply emf?? dont quote me on this but hence why I would pick C?
q20 I would find the energy of the photon at the peakof the graph so maybe around 8-9micrometres in wavelength. Use E=hf to find that energy. Then convert it to eV by dividing it by (1.602x10^-19). So I worked that out to be 0.155 eV which is how much you need. So 0.17eV would be enough to provide this.
Hope this help
 

[winstondarmawan]

hey guys! if a q says to describe how g-forces affects of astronaunts in launch: what exactly do you have to write? They experience an increasing force due to acceleration but I don't know how to describe it. Is it refering to multistage rockets?
Thanks 

I think it's best to talk about the Law of Conservation of Momentum as well as F=ma to prove that the spacecraft is accelerating. From there you talk about how g-forces are directly proportional to acceleration.

[statues]

Could Someone please help explain this to me
What I understand is that it's a DC source which means that instead of trying to minimise the Voltage as we would if it were an AC source in order to maximise the current - we are instead trying to maximise the voltage as this corresponds to an increase in emf which corresponds to an increase in current? I don't understand why in this case a high voltage in the primary coil corresponds to a high current.
This is from the 2015 hsc q18. The answer is A.
please help

Many Thanks

[Bubbly_bluey]

Hey guys i'm just having trouble understanding/ working out these questions and i would really appreciate if anyone could give me a hand! I know it looks like a lot, but they are all just multiple choice questions which didn't have any explanations that i could find. Cheers

hey! q12: So it says centripetal force that means we have to apply F=mv^2/r. We have mass and orbital radius but not velocity. So note that its orbiting so we apply the orbital velocity remembering that mass (M) is the earth!. You sub it back into the centripetal force and you should get D.
8 ) maximum force would always be at 90degrees. b/c sin90=1. You can elimate B and D. But moving charges in magnetic fields only experience a force by F=qvbsin(theta). Hence C
7) For torque to be proportional to current, there must be a uniform change in magnetic flux. Hence a radial magnetic (B) because it would give maximum torque at every direction and so magnetic flux would always change to induce current.
8 ) Use F=nBilsin(theta). Working out the angle first that should be between the magnet and the wire (hence not 30 degrees). A trick is to hold a pen vertically down into the page for flux and another pen for the wire. you will see that it always makes 90 degrees with the magnetic field. To find length you sin rule. hence A.
3) Current moving down X-Y. From birdseye field lines are cutting the wire horizontally. Use Right hand rule and force is into the page so it must go clockwise.
20) I would find the energy of the photon at the peakof the graph so maybe around 8-9micrometres in wavelength. Use E=hf to find that energy. Then convert it to eV by dividing it by (1.602x10^-19). So I worked that out to be 0.155 eV which is how much you need. So 0.17eV would be enough to provide this.
19) idk too XD
17) DC is direct current so has it is turned on there is changes in flux>induced current>induced force> therefore torque. But DC motors have back emf. As it increases supply emf=back emf eventually. Hence the current decrease.But current is proportional to torque by T=NBIA cos, so torque decreases too.
20) So first convert the frequency of the 450nm wavelength to see if it surpasses any of the thresholds of X or Y. I got 6.66x10^14 Hz. So only x would emit. Intensity only introduces more photons so more electrons can be emmitted not increasing KE. Hence B
15) Angle for torque is the angle between the armature plane with the magnetic field hence 30. Angle for force on wire always the angle the wire (XW or YZ) makes with the magnetic field, 90. You can try use the pen trick I said earlier to visualise.
14) For projectile motion to happen you need two components and horizontal and vertical. Accelerating would not projuce a projectile curve so its not B or D.The horizontal from the pushing of the cue against the ball. Now to find the vertical start drawing vector coponents. You can see vertical is down. (from your view on the page) Now if you look the front of the train its to the left so try to imagine: what would happen if you turned left or right IF the ball was just stationary? If you turn right the ball will go opposite and go left (from the train driver's   view) But to our view its equivalent to moving down as shown in the vector. Hence C 

[CyberScopes]

Hey guys i'm just having trouble understanding/ working out these questions and i would really appreciate if anyone could give me a hand! I know it looks like a lot, but they are all just multiple choice questions which didn't have any explanations that i could find. Cheers

Ok ill see what I can do

1. Remembering that centripetal force is the same as gravitational force, so use Newtons Gravitation Law:
F = GMm/d²
= (6.67*10^-11 * 6*10²⁴ * 7.35*10²²) / (3.83*10⁸)²
= 2*10²⁰N
= A

2. Using F = qvBsinθ:
-> For A, v = 0, therefore F = 0, so it cant be A.
- > For B, the particle is moving parallel to the magnetic field lines: sin0 = 0 which means F = 0, so cant be B.
-> For C, F = q(40)B*sin90
Since we're comparing, we can assume q and B are just 1, so: F = 40
-> For D, considering B and q as 1, F = 50sin45 = 35 < 40
Therefore answer must be C

3. A radial magnetic field assures that the plane of the coil is always flat at 0 degrees, and cos0 = 1, so this is what makes the torque directly proportional to the current (since T = nBIAcosθ and n, B, I and A are constant)
So answer must be B

4. F = BILsinθ
B = 0.05
F = 0.03
θ = 90 degrees (since its perpendicular to the magnetic field lines)
L = 0.4 (using trig to find the length of just the wire)
Plug in and rearrange for I and you get 1.5
Answer is A

5. This was answered previously in the thread a few posts back

6. Not sure

7. Not sure, remember having troubles with this one before

8. A does not make sense and B does not consider back EMF.
I think it may be D since it considers that after a while, back EMF impacts the torque and slow it down, but I am not completely sure.

9.  X hits at around 4 and Y hits at around 9
First you need to check which ones actually can eject electrons:

It needs to be within the threshold frequency of the light:
f = c/λ = 3*10⁸ / 450*10^-9 = 6.67*10¹⁴Hz

X has around 4*10¹⁴Hz so its within the frequency to emit electrons, but Y is above it so it cannot eject electrons, so C and D are eliminated.

Knowing how the photoelectric effect occurs, more intensity only increases the photo-current that is ejected, not the energy of the electrons.
So the answer is B.

10. The angle for torque is the plane of the coil to the magnetic field lines, which is 30 degrees.
The angle for force, for example by taking line WX, is 90 degrees since its always perpendicular to the magnetic field lines (same as for YZ)
So answer is A

11. This one is sort of by using "common sense" or just sort of understanding physics environments. Imagine leaving a ball in a box that cannot move, how can u make it so the ball is touching the left side of the box by moving the box? This can only be done by rotating, or moving, the box to the right. You can't "shift" the train to the right but it can turn to the right.
Answer is C

I did most of these on the spot so I may have made mistakes, just let me know if I did.

[pikachu975]

Could Someone please help explain this to me
What I understand is that it's a DC source which means that instead of trying to minimise the Voltage as we would if it were an AC source in order to maximise the current - we are instead trying to maximise the voltage as this corresponds to an increase in emf which corresponds to an increase in current? I don't understand why in this case a high voltage in the primary coil corresponds to a high current.
This is from the 2015 hsc q18. The answer is A.
please help

Many Thanks

This is out of HSC so it's dumb that they put it in, since we only work with AC in HSC

[beau77bro]

could someone please explain T1 and T2 relaxation times simply, as well as in detail/explanation hard out. im seriously struggling with this and it seems both overly complicated and lackingly explained all at once. thanks and pliz.

[blasonduo]

Hey guys i'm just having trouble understanding/ working out these questions and i would really appreciate if anyone could give me a hand! I know it looks like a lot, but they are all just multiple choice questions which didn't have any explanations that i could find. Cheers

Hey!!!!! Well we are in for a looooooong ride

12) For this question we must firstly find the orbital velocity:



=











8.) This whole question is going off the formula:



For A, the velocity is 0, so the force is 0, so that's wrong.
For B, the velocity is 60, but it is travelling Parallel to this magnetic field (ie sin0 = 0)
For C, everything works out, so the force is 40
For D, everything works out, the velocity is 50, but due to sinϑ = 45, the force is half, and thus 25

This means C will have the highest force

7) This is something that you really just need to know, In a galvanometer, this is the reason why the radial magnet is so vital, to get accurate measurements! B!

8.) Big note here, the wire is still PERPENDICULAR to the field, so sinϑ = 90.

To find the length, just use Pythagoras, and you should find length to be 0.4m









3) This was on the previous page This is more applying your knowledge of the Motor Effect.

For this, we have to use our good ol' right hand!

We know the current is going down the page (thumb)
We know its a north pole and the magnetic field lines are going away from the magnet (fingers)

We then can see where our palm is facing, (which is into the page!) but as we move backwards our direction of magnetic field changes VERY slightly! (as we see in the diagram) It is this slight chance which causes the wire to rotate CLOCKWISE around the magnet A

20 Bubbly_bluey Just made a very good explanation about this above! If you need more help, feel free to ask B

19) This is MUCH easier if we rule out the incorrect ones, For A, the force of gravity is NOT negligible, as that's what keeps objects in orbit! so it is wrong. While B is true, this is not answering the question, it is just a statement.

We now have it to C or D, for C, IF the forces were to be the same, (ie F = ma) since, the question claims F is the same, while mass is different, their accelerations must be different, and well, if their accelerations are  different, how on earth can they travel at the same speed? So C is incorrect, leaving only D as the answer (if you would like me to explain why i'll be happy to!)

17) again, Bubbly_bluey pretty much explained this above! (Well done btw!!) The work done (which is torque) once hitting it's fixed maximum speed will be 0, so it must decrease! D

20) We here are given wavelength, but the graph shows frequency, so we must change the wavelength to frequency.



As this is before Line Y, this means NOTHING will happen to Y, so C and D are out!

The next part is all about the properties of photons, where INTENSITY means more electrons, not more energy, so B

15) We know that from the motors, the force is ALWAYS perpendicular to the coils, so it has to be A or C, but as torque varies with the angle, we need to know where torque = 1, (cos 0) which is where the coil is "flat" meaning that moving it 30 will make cos 30 and thus it is A

14) 14) For these questions especially, I'd like to picture myself in these situations and go from there. Imagine you are driving a car, speeding it up or slowing it down will NOT make you experience a force to the left or right (there is no circular motion!), so this eliminates both B and D. Now referring back to the car analogy, when you turn the car, you'd always experience a force opposite to the way you are turning. The car is beginning its turning motion while you continue in a straight line path. This is because of Newton's first law.

So back to the question, the ball began to roll down the page, and from what was said before, it must mean the train was turning up the page, or to the RIGHT, so it is C.

WHEWWWWWWWWW That WAS a ride!!!! I hope this helps! Ask away if any of this made no sense, I'll be happy to help!!!