Maximum torque question. (1 Viewer)

[Shoom]

Hi
When is torque on a coil maximum, when its perpendicular to field or parallel to the field?

Because when we use T=nBIACos its maximum when its parallel, however when we use T=nBIASin its maximum when the field is perpendicular.


Which do I use, when is torque maximum?

 

[Leo 100]

i am pretty sure perpendicular... oh god i really should know that
sometimes you have to change the angle measurement

 

[obimoshman1234]

T=nBIACos theta

 

[SkimDawg]

I think its parallel, as cos0 = 1 and cos90 = 0

 

[dolbinau]

It's not t=NBIAsin(theta) that's F=nBIAsin(theta). Different equation (not torque), torque is as stated above . Misread the formula sheet perhaps?

If you think about it logically, when the coil is 90 degrees in the field the force on the wires on the outside will be up/down - how can there be torque if this is so?

Maximum torque when direction of force on each wire is perpindicular to the distance to the axis, but when it is 90 degrees in the field the force will be towards the axis not perpindicular to it.

 

[independantz]

It's not t=NBIAsin(theta) that's F=nBIAsin(theta)

Do you mean F=BILSin(theta), never seen the equation you mentioned before, the sin(theta) one i mean.

 

[dolbinau]

Yes sorry, getting it wrong myself LOL

 

[Shoom]

OOOOOOHHHHHHHHHHHHHH

The one with Sin is for the force on the coil, the one with cos is for torque.


HOW COULD I BE SOP STUPID?


Define the motor effect please.

So to get a motor started easier is it better to have Force maximum ( coils perpendicular ) or have torque maximum ( coils parallel )

 

[dolbinau]

The one for Sin is the force on a current carrying conductor (F=BilSin(theta) as someone corrected above). YOu could use it to find the force on say one side the coil, or a wire in a magnetic field etc...

You could think that when the coil is flat in the field, the wires on each side are perpendicular to the magnetic field, but the coil is parallel with the field. Does that make sense?

The wires on the sides of the coil will always have a maximum force on them as they are perpendicular to the field.

It's best just to forget about F=BILSin(theta) with coils as it would only be used to find the force on one side, for example. If it had multiple coils in one side you would also use F=nBILsin(theta) [2008 CSSA trial question IIRC]

The best position for the motor to start in, is when the coil(s) are flat in the field (parallel with the field) - torque will be a maximum as the direction of the force on the sides of the coil will be perpendicular to the distance of the turning axis.

maybe I should draw a diagram, I don't know.

 

[ashbashness]

There's no point in remembering cos θ or sin θ if you don't know what angle θ stands for. In order to simplify our equations we always make θ the angle between the current and the magnetic field - watch out, sometimes they give you another related angle, subbing that into the equation will give you the wrong answer (generally).

Another way to think about torque is that it is Force x Perpendicular Distance from the line of action to the pivot point. Drawing a triangle should help you understand which angle to use and whether to use cos or sin.

 

[dolbinau]

There's no point in remembering cos θ or sin θ if you don't know what angle θ stands for. In order to simplify our equations we always make θ the angle between the current and the magnetic field - watch out, sometimes they give you another related angle, subbing that into the equation will give you the wrong answer (generally).

Can you provide an example where the angle isn't between conductor+magnetic field?

 

[ashbashness]

2003 Multiple choice question 9
2006 Multiple choice question 7 (this one's a bitch)

EDIT: These are for force though, not for torque, but the principle still applies. Check the angle!

 

[Shoom]

Hold on

T= FDSIN aswell WTF


THIS IS SO CONFUSING.


WHICH DO I USE T=FDSin or T=naBiCOS

 

[independantz]

Hold on

T= FDSIN aswell WTF


THIS IS SO CONFUSING.


WHICH DO I USE T=FDSin or T=naBiCOS

Lol wtf have you been doing all year?

You can use either one depending on the given data...

 

[Shoom]

But Sin90=1
Cos0=1


So how can something be at maximum torque and yet you can use both equations.

Honestly HSC coming up, if I cant understand this by tonight I am jumping infront of a train.

 

[adnan91]

But Sin90=1
Cos0=1


So how can something be at maximum torque and yet you can use both equations.

Honestly HSC coming up, if I cant understand this by tonight I am jumping infront of a train.

independentz guy is wrong! F= BILsin@ this is the FORCE on the conductor in a magnetic field. Imagine a wire in a magnetic by lying at a 30 degree angle. Then u put sin 30

T=nBIAcos@ this is torque! torque is the turning effect of a force and it equals F x D where D= perpindicular distance.

You cannot use both equations for torque

 

[Shoom]

but Jacaranda has T = fd SIN aswell

QUICK ITS ALMOST TIME FOR THE TRAIN TO PASS

 

[helper]

but Jacaranda has T = fd SIN aswell

QUICK ITS ALMOST TIME FOR THE TRAIN TO PASS

You need to look at the angles they are talking about. It depends on which angle, they supply to how you use it.

 

[Shoom]

If they give is force and distance only with an agnle I use

t = fd Sin

when they give us B etc I uiset t=nbiacos

 

[helper]

Once again, it depends on the angle given. You are currently doing exactly what they wanted as a distractor in Eg Q7 in the 2006 paper.
http://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2006exams/pdf_doc/physics_06.pdf

The angle is not the one represented by the one in the formula. You needed to think about what angles and lengths were needed before substituting in.
In that question theta was 90 for the equation but you had to use the angle given to calculate the length.

If using T=Fd, you need to work out what the perpindicular distance is and go from there.

For T=BIAn
the angle between the plane of the coil and the direction of the magnetic field

You need to check if that is the angle given.