Torque, Force and EMF graph questions (1 Viewer)

[Viru10]

Hey guys,

I was looking over some past papers and I wanted to clarify something with a DC/AC motor.

With torque on a DC motor, the graph looks like a cosine graph right? as it is max at cos(0) and then decreases to 0 at cos(90) etc...

With the Force on a single wire in the coil (for simplicity's sake take a wire AB which is perpendicular to the magnetic field), the graph should be like a sine curve? where it is 0 at sin(0) and so on..

With EMF im a tad confused. Where/When is it on a motor where the EMF is greatest or lowest? I saw a q similiar to this in the 2009 physics paper Q. 8
http://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2008exams/pdf_doc/2008HSC-physics.pdf I know its B...but im not 100pc sure why...

Thanks in advance!

 

[cutemouse]

Where/When is it on a motor where the EMF is greatest or lowest?

When the rate of change of flux is the greatest.

 

[whitnall8]

EMF = Δ(BA)/Δt: Rate of change in magnetic flux

 

[cutemouse]

EMF = Δ(BA)/Δt: Rate of change in magnetic flux

or EMF = Blvsinθ

 

[helper]

Hey guys,

I was looking over some past papers and I wanted to clarify something with a DC/AC motor.

With torque on a DC motor, the graph looks like a cosine graph right? as it is max at cos(0) and then decreases to 0 at cos(90) etc...

With the Force on a single wire in the coil (for simplicity's sake take a wire AB which is perpendicular to the magnetic field), the graph should be like a sine curve? where it is 0 at sin(0) and so on..!

No the graph of the force would be a a flat line because the angle always remains perpindicular