• Best of luck to the class of 2019 for their HSC exams. You got this!
    Let us know your thoughts on the HSC exams here
  • Help the next generation of students in the community for the new syllabus!
    Share your notes and exam papers on our Notes & Resources page

HSC Physics MC Thread (1 Viewer)

JavaScript

Member
Joined
Oct 16, 2015
Messages
84
Gender
Male
HSC
2015


Why is it B?

i thought it would be an INCREASING straight line.

Because it is a radial magnetic field and THETA is always 0 degrees it must be some form of straiht line, but i thought if it speeds up then the torqu needs to be larger
Because a motor has almost instant torque and the torque doesn't decrease if its radial. The motor effect happens at every angle (in a perfect motor).
That's why its a straight line.
 

Mr_Kap

Well-Known Member
Joined
Mar 24, 2015
Messages
1,131
Gender
Male
HSC
2015
wait, so why is the answer a straight line with NEGATIVE gradient.
 

InteGrand

Well-Known Member
Joined
Dec 11, 2014
Messages
6,104
Gender
Male
HSC
N/A
Because a motor has almost instant torque and the torque doesn't decrease if its radial. The motor effect happens at every angle (in a perfect motor).
That's why its a straight line.
Answer's not actually (B), he misread it.
 
Joined
Sep 17, 2015
Messages
71
Gender
Male
HSC
2015
more explanation please? (if possible)
Remember Faraday's law which states that relative movement of a conductor in a magnetic field results in induced EMF. Lenz's law further refines this by stating that the induced EMF produces a magnetic field that opposes the initial change in flux.

Thus as a motor is spinning, the coils in the rotor cuts through the magnetic field in the stator. This creates induced EMF by Lenz's law. The magnitude of this opposes the initial change, so by necessity, this results in the reverse of the supplied current. (Just use right hand rule on one side of the coil, and flip the forces). This is back EMF.

Lenz's also states that the induced EMF is proportional to rate of change of flux, (negative change in flux over change in time). Thus, as the motor spins faster and faster, the rate of change of flux increases, and the magnitude of the back EMF increase. Thus the supplied current and the induced current cancels each other out, reducing the total current.

Since torque = nBIA cos thetha and I is the decreasing variable, this represents a linear decrease as rotational speed goes up and back emf increases.


Jk already explained above
 
Last edited:

Crisium

Pew Pew
Joined
Feb 17, 2014
Messages
2,022
Location
Australia
Gender
Male
HSC
2015
Remember Faraday's law which states that relative movement of a conductor in a magnetic field results in induced EMF. Lenz's law further refines this by stating that the induced EMF produces a magnetic field that opposes the initial change in flux.

Thus as a motor is spinning, the coils in the rotor cuts through the magnetic field in the stator. This creates induced EMF by Lenz's law. The magnitude of this opposes the initial change, so by necessity, this results in the reverse of the supplied current. (Just use right hand rule on one side of the coil, and flip the forces). This is back EMF.

Lenz's also states that the induced EMF is proportional to rate of change of flux, (negative change in flux over change in time). Thus, as the motor spins faster and faster, the rate of change of flux increases, and the magnitude of the back EMF increase. Thus the supplied current and the induced current cancels each other out, reducing the total current.

Since torque = nBIA cos thetha and I is the decreasing variable, this represents a linear decrease as rotational speed goes up and back emf increases.


Jk already explained above
LOL don't worry the same thing has happened to Zlatman heaps of times recently

InteGrand's LaTex skills are too OP
 

Mr_Kap

Well-Known Member
Joined
Mar 24, 2015
Messages
1,131
Gender
Male
HSC
2015



In this question, shouldn't the current made in the coil generate a magnetic field such to oppose the magnet, hence the magnetic field in the coil will be down (as the bottom of the coil needs to be south to oppose the south end of the magnet).

This means when i use the right hand curl rule, the current is CLOCKWISE. So the answer should be D.

HOWEVER the answer is actually B. Why??? What am i doing wrong??
 

Mr_Kap

Well-Known Member
Joined
Mar 24, 2015
Messages
1,131
Gender
Male
HSC
2015



In this question, shouldn't the current made in the coil generate a magnetic field such to oppose the magnet, hence the magnetic field in the coil will be down (as the bottom of the coil needs to be south to oppose the south end of the magnet).

This means when i use the right hand curl rule, the current is CLOCKWISE. So the answer should be D.

HOWEVER the answer is actually B. Why??? What am i doing wrong??
anyone??
 

DattMyball

New Member
Joined
Nov 4, 2014
Messages
5
Gender
Male
HSC
2015
In this question, shouldn't the current made in the coil generate a magnetic field such to oppose the magnet, hence the magnetic field in the coil will be down (as the bottom of the coil needs to be south to oppose the south end of the magnet).

This means when i use the right hand curl rule, the current is CLOCKWISE. So the answer should be D.

HOWEVER the answer is actually B. Why??? What am i doing wrong??

You might have the right hand rule the wrong way. I think the fingers point in the north direction of the b-field so the field goes up
 
Last edited:

Users Who Are Viewing This Thread (Users: 0, Guests: 1)

Top