Torque & Force (1 Viewer)

[omgmynameislong]

Torque is like an |cosx| graph while Force is a constant (straight line)?

Teacher says that force is the same as the torque graph but shouldn't it be constant as the wire will always be perpendicular to the magnetic field no matter how it turns?

 

[ilikebeeef]

Torque is like an |cosx| graph while Force is a constant (straight line)?

Teacher says that force is the same as the torque graph but shouldn't it be constant as the wire will always be perpendicular to the magnetic field no matter how it turns?

I think you're getting Torque and Force mixed up. Force acts only in one direction, in this case, there are two forces acting in opposite directions in opposite sides of the rotor. Torque is the rate of change in force.

Force is not constant. Using the right hand palm rule (or Lorentz Force Law), you can tell that force is max when the wire is perpendicular to the magnetic field. Hence we have the formula F=nBIlsin(theta), which will create a sine curve in respect to time.

As for torque, since it is the change in force, its graph will be the derivative of the force graph, hence it is a cos curve (NO absolute values sign). Torque is max when the wire is parallel to the magnetic field, and is zero when perpendicular. If you want to make sure, just use the formula T=nBIAcos(theta). Anything with cos or sin in it is not constant when the angle changes.

In short, the torque of the rotor is not constant, and so neither will the force. Either you've misheard your teacher, or your teacher's wrong.

 

[omgmynameislong]

Ok I understand torque but isn't force always perpendicular to the magnetic field no matter how the motor spins?


-|---------->
-|---------->
-|---------->

Current is going from bottom to top so force is into page. If that spins into:
----------|->
----------|->
----------|->
Force is still the same since it's still perpendicular or am I missing something >_<.

 

[ilikebeeef]

Ok I understand torque but isn't force always perpendicular to the magnetic field no matter how the motor spins?


-|---------->
-|---------->
-|---------->

Current is going from bottom to top so force is into page. If that spins into:
----------|->
----------|->
----------|->
Force is still the same since it's still perpendicular or am I missing something >_<.

Right, think of the wire frame as a coil wrapped around an iron core, and assume that the coil has no width. I tend to imagine it like an "infinitely thin" flat piece of glass plane.

Visualise this plane. When it is parallel to the magnetic field, NO flux passes through the it. Because there is no flux passing through the plane, there is no force acting on it!

On the other hand if the plane is perpendicular to the magnetic field, then max flux passes through it, therefore force is max at that point.

What you were thinking was that the wire has width. It does in the real world indeed. However in HSC Physics, we disregard this, and assume that everything's "ideal", as one piece of wire is negligible when compared to the area of the coil.

 

[ilikebeeef]

^ updated answer

 

[omgmynameislong]

Ahh I get it lol, so it's just a sine curve?

How do I give rep points LOL ?

 

[ilikebeeef]

Ahh I get it lol, so it's just a sine curve?

Yep. Look at the formula. F=nBIlsin(theta). There's a sine in there.

How do I give rep points LOL ?

Lol, you click the

under the user's name.