Torque Question Calculation 2009 HSC (1 Viewer)

[blackratpoo]

http://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2009exams/pdf_doc/2009-hsc-physics.pdf

Hi, I need help with 21 c).
i get a) and b).

 

[kaz1]

Equate the torque from b) with torqure=BANI

 

[helper]

Remember to only use one half of the coil as it is a single pole and not across the whole coil.

A better way to do this is a force balance on the side.

 

[adomad]

c) for this, i would say that you calculate the force of the mass (weight force). since the see-saw needs to balance, you then use F=nBIl and weight = force applied by the magnetic field...

n=1, B= x , I=20, l=0.2m

weight of mass = 0.04 kg X 9.8=0.392N

F=nBIl
.'. 0.392=B X 20X0.2m.... .'. B=0.098T...

 

[cutemouse]

Remember to only use one half of the coil as it is a single pole and not across the whole coil.

Yeah I did this in the HSC exam and got it right.

I don't understand though why I had to only use half the area of the coil...