When to use F=BIL and F=BILsin(theta)? (1 Viewer)

[benji_10]

I am very confused as to which situations call for which formula. I would greatly appreciate it if anyone could tell me the answer in relation to the orientation of the conductor within the magnetic field. So perpendicular to magnetic field, field going into/out of page, etc.

For an example problem, Q11 of the 2009 HSC paper: http://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2009exams/pdf_doc/2009-hsc-physics.pdf
The answer was b)

The main beef I have with it is why there is no angle created with the field as AB rotates.

Cheers

 

[Fizzy_Cyst]

I am very confused as to which situations call for which formula. I would greatly appreciate it if anyone could tell me the answer in relation to the orientation of the conductor within the magnetic field. So perpendicular to magnetic field, field going into/out of page, etc.

For an example problem, Q11 of the 2009 HSC paper: http://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2009exams/pdf_doc/2009-hsc-physics.pdf
The answer was b)

The main beef I have with it is why there is no angle created with the field as AB rotates.

Cheers

As AB rotates, it is still perpendicular to the magnetic field, so the angle it is creating with the magnetic field is always 90 throughout its entire rotation, however, although the force is constant in magnitude throughout its entire rotation, the force is always acting in the same direction (which should be down, and is wrongly noted as being up, by the arrow shown in the diagram) until SRC makes contact with the carbon brushes , as the coil rotates, the perpendicular distance between the side of the coil and the axle changes, hence torque changes as a result of this.
For example, after the coil has rotated 90 degrees, the perpendicular distance is 0, hence using torque = Fd, torque = 0. Although Force is still maximum as the side AB is still oriented perpendicualr to the magnetic field.

Always use F=BIlsin theta, but for scenarios like the rotation of a motor in a mag field, the force acting on the section of the coil experiencing a torque is always constant as theta = 90, as the motor is always perpendicular to the mag field

 

[benji_10]

Thank you very much Fizzy. You've been a great help. Now I can solve the backlog of M&G questions I've piled up because of this.

Cheers!