Vector cross products involving magnetic forces

[ACLerok]

magnetic forces and torque

The plane of a rectangular loop of wire with a width of 5.00cm and a height of 8.00cm is parallel to a magnetic field of magnitude 0.200T. The loop carries a current of 6.60A.

What is the maximum torque that can be obtained with the same total length of wire carrying the same current in this magnetic field?

I found the torque that acts around the loop to be 5.28*10^-3 N*m. I figured that since the torque = IBA sin(theta) and that the length, area, and magnetic field must the same, the maximum torque must be when sin(theta)=1. Wouldnt' this mean that the torque I found is the max amount?

Thanks!

 

[ACLerok]

can anyone help me out?

 

[Tide]

The question is somewhat ambiguous. In any case, if the wire is planar the maximum torque on it will be when the normal to the cross section is perpendicular to the magnetic field. I think the question is what combination of length and width must the wire have in order to maximize the torque - subject to the constraint that the total length of the wire is fixed.

I say it's ambiguous because it's possible that shapes other than rectangular might lead to yet greater torque - such as a wire formed into a circular loop or something with many loops! My guess would be they are asking for the rectangular case.

 

[ACLerok]

I don't think i understand what they are asking for. I tried changing the area of the rectangle by making one side .07 and the other 006 to find the maximum area possible but i don't know if its rihgt. Anyone?

 

[ACLerok]

help please!?

 

[Tide]

The torque depends on the product of the IL (force = current times length) and W (moment). If L + W is fixed then how can you maximize the product of ILW? (I.e. how do you maximize the AREA of a rectangle if its perimeter is held constant?)

 

[ACLerok]

The torque depends on the product of the IL (force = current times length) and W (moment). If L + W is fixed then how can you maximize the product of ILW? (I.e. how do you maximize the AREA of a rectangle if its perimeter is held constant?)

does it involve some method of calculus? maybe taking the derivative and setting it to zero? Sorry, i am totally stuck on this problem.

 

[Tide]

You can do it without calculus. You want to maximize the area A = LW with fixed perimeter. Let P be the perimeter P = 2(L+W). Then A = L(P/2 - L) which you will recognize as the equation for a parabola. You can find the vertex of the parabola by completing the square so that

$$A = \left( \frac {P}{2} \right)^2 - \left( L - \frac {P}{4} \right)^2$$

so that the maximum occurs when L = P/4 (and therefore W = P/4) which makes the shape a square!

 

[ACLerok]

ok, i found the perimeter to be .26m so each side of the square would be .26/4= .065m. i then found the area of the square with the length and multiplied it by the magnitude of the magnetic field and the current and got .00557 and that is still wrong i am told.

 

[Tide]

You should always write out the units when referring to a physical quantity.

Who told you it's "wrong" and what is the answer "supposed" to be?

 

[ACLerok]

its a problem on my physics webassign. it only gives my a couple chances before i get the problem wrong.

 

[Tide]

Hmm. That's strange. Have you checked your units? I don't know what else to tell you.

 

[Spectre5]

Are you sure that you must keep the shape to be a rectangle...because a single circle would give you more torque than a square...I would guess that the ambiguous question is asking for the shape that is planar and makes a single loop. in tis case, a circle will provide you with the maximum torque.

 

[ACLerok]

Are you sure that you must keep the shape to be a rectangle...because a single circle would give you more torque than a square...I would guess that the ambiguous question is asking for the shape that is planar and makes a single loop. in tis case, a circle will provide you with the maximum torque.

if that's the case, the length of the wire would just be the perimeter correct?

 

[Spectre5]

yea...the length of the wire would be the circumference...which is 2(pi)r

So the perimeter is equal to that...divide by 2(PI) and then you know r...then use area of a triangle to get its area

 

[ACLerok]

thanks a lot! i didnt use the triangle equation. i just found r and then used the area of a circle equation.