HSC Physics Question Thread

[sidzeman]

1. The GPE is halved use this equation E=-G(m1*m2)/R to see that the radius is doubled, then plug into the force equation F=Gm1m2/r^2 to see that the force will become a quarter of the initial force.

2. Use the right hand grip rule with your thumb pointing down to Y to see the direction of the force.

3. The force is the calculated from the angle between the wire and the magnetic field, you can see that WX is perpendicular to the poles of magnets. The torque is read off the information because it is the angle of the plane and the magnetic field.

So for 2 the rotation is clockwise right?
I understand the angle  between the wire and the mag field for force - but what do you mean by "the plane" for torque?

[Savas_P]

Hey guys!

The answer listed for Q7 is B and I was 99% sure that the answer was D when I was answering..
Also for Q16, I just wanted some clarification. When is the angle that they've given you a distractor? Is it only when the field is into/out of the page. And when, in this situation, do they give you the wrong angle? What does that look like?

Would love some help on these, thank you

1. The experiment had a null result, so the answer should definitely be D lol.

2. Think you need to look at whether the electron is perpendicular to the field, in your example the electron is entering at 30° which is what you put in F=qvBsinθ and in the equation where you put in 90° is when the magnetic field lines are going into or out of the page.

[Savas_P]

So for 2 the rotation is clockwise right?
I understand the angle  between the wire and the mag field for force - but what do you mean by "the plane" for torque?

yep clockwise, you put your thumb down and then your fingers curl clockwise.

sorry, I meant the torque depends on the angle of plane WXYZ and the magnetic field. It will change as the coil rotates but in the picture they have marked it at 30°. for the force imagine XY as a conductor and it will always be 90° because it will always be perpendicular.

[sidzeman]

yep clockwise, you put your thumb down and then your fingers curl clockwise.

sorry, I meant the torque depends on the angle of plane WXYZ and the magnetic field. It will change as the coil rotates but in the picture they have marked it at 30°. for the force imagine XY as a conductor and it will always be 90° because it will always be perpendicular.

Hmmmm sorry this is the one part of physics I always struggle with. So the force individual sections of the coil will always  be a max because it will always be perpendicular really?

[Savas_P]

Hmmmm sorry this is the one part of physics I always struggle with. So the force individual sections of the coil will always  be a max because it will always be perpendicular really?

hahaha I am confused as well, hopefully someone can confirm. would have thought that is right, then the force of the rotation produces the toque. the torque considers the area of the plane WX*XY so how much flux lines the shape cuts would change the torque as it rotates.

[SesquipedalianIndividual]

first time posting so im not entirely sure how its supposed to go but anyways
I have a lot of trouble with understanding torque and back emf and i dont want it to hold me back
can i get another attempt at an explanation please

[Savas_P]

first time posting so im not entirely sure how its supposed to go but anyways
I have a lot of trouble with understanding torque and back emf and i dont want it to hold me back
can i get another attempt at an explanation please

I am so glad I am not the only person that struggled with this, I also have no idea how to graph the emf, sidzeman I only just realized that the force is a flat line because it is always perpendicular. I still don't understand how to tell when is the emf/force on the graph positive or negative if someone could explain this please.

[bsdfjnlkasn]

Hey there,

Was hoping to get some explanation on these two multiple choice questions

Thank you!!

[bsdfjnlkasn]

first time posting so im not entirely sure how its supposed to go but anyways
I have a lot of trouble with understanding torque and back emf and i dont want it to hold me back
can i get another attempt at an explanation please

Hey there!

Welcome to the forums What in particular do you not understand about torque and back EMF?

I can give you some quick definitions, and if there are some conceptual things you want me to expand on, let me know.

Torque: Turning force which acts a certain distance from the axis of rotation. Has two formulas (both of which are listed on the data sheet) 

Back EMF: Induced EMF which seeks to oppose the change that caused it as a consequence of the law of conservation of energy (otherwise known as Lenz's Law)

[bsdfjnlkasn]

I am so glad I am not the only person that struggled with this, I also have no idea how to graph the emf, sidzeman I only just realized that the force is a flat line because it is always perpendicular. I still don't understand how to tell when is the emf/force on the graph positive or negative if someone could explain this please.

Hey there

In order to graph EMF, you will need to graph the change in magnetic flux over time. In exam questions, this is usually given to you. A quick trick (if you do 2 unit maths or higher) would be to just sketch the derivative because the gradient of the flux/time graph is our definition for induced EMF!

Force is defined as positive when it is acting upwards, and yes the lines are always straight because the force acts in a constant direction. This is why we need a split ring commutator to ensure that the current's direction changes allowing a constant force to act down, hence allowing the coil to rotate in a consistent direction. 

[Savas_P]

Hey there

In order to graph EMF, you will need to graph the change in magnetic flux over time. In exam questions, this is usually given to you. A quick trick (if you do 2 unit maths or higher) would be to just sketch the derivative because the gradient of the flux/time graph is our definition for induced EMF!

Force is defined as positive when it is acting upwards, and yes the lines are always straight because the force acts in a constant direction. This is why we need a split ring commutator to ensure that the current's direction changes allowing a constant force to act down, hence allowing the coil to rotate in a consistent direction.

wow thanks!! you are legend. that's a good trick, now i just need to practice my derivatives.

[winstondarmawan]

Would appreciate help with the following MCs:
13. https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/22414729_1347448845380635_323509929_n.jpg?oh=134c4ab9f911a5324356ef6198cfc21c&oe=59DFE736
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/22386637_1347448898713963_942589714_n.jpg?oh=2ea0b0e92ae1cdc8e1af5165ca2a0c3c&oe=59DFB08E
14. https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/22447103_1347448998713953_512491506_n.jpg?oh=356849cd6400b6bee39669ea6fc0fb11&oe=59E0A675
And also, can someone please tell me that this question is impossible (if not how do I do it):
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/22471704_1347448518714001_1038035986_n.jpg?oh=18c9c49a3b603443015d3a57eddae686&oe=59DFA088
Thanks in advance!

[jamonwindeyer]

Hey there,

Was hoping to get some explanation on these two multiple choice questions

Thank you!!

For the first one, the disc spinning in the magnetic field will induce a current in the disc. The discs movement is in the one direction, so it is a direct current - So the question is, what direction does it flow? If you consider the right hand slap rule, the disc is moving into the page, and the field is directed to the right - This results in a current flow directed downwards, through Y and to X! So I'm pretty sure the answer is B

For your second, it is definitely the wire that will move - The solution moving makes no sense and the magnet definitely isn't going anywhere either. Use the right hand slap rule for the force on a current carrying conductor - Field is sort of directed perpendicular to the wire, to the upper left corner. Current goes down and to the left. Force will be into the page, which would cause the wire to rotate

Hard to stuff to explain in text, but I hope this helps - You might need to just try the rules a bit and work through it a few times

[3unitgang]

Hi, I have just recently bought the physics atar notes textbook and it had the question, "A projectile has a range of 1240 metres and a flight time of 5 seconds. At what velocity is it projected?" but when finding the initial velocity in terms of y, is used the formula r=u+1/2at^2 isn't it r=ut+1/2at^2?

[jamonwindeyer]

Hi, I have just recently bought the physics atar notes textbook and it had the question, "A projectile has a range of 1240 metres and a flight time of 5 seconds. At what velocity is it projected?" but when finding the initial velocity in terms of y, is used the formula r=u+1/2at^2 isn't it r=ut+1/2at^2?

Hey! Great catch, that looks like an error! That working should be:



So your final answer should in fact be the answer given, divided by 5 -> 24.5 metres per second sorry for the error! First one I know of in those tests, I'll make sure it is fixed in the future