Torque on a Coil (1 Viewer)

[MrLuvable]

A square coil of PQRS with sides 0.05m and 250 coils carries 1.6A and lies in a 0.4T magenetic field directed from left to right. The plane of the coil is parallel to the field with PQ perpendicular to the field.
a) Calculate the magnitude of the force on side PQ.
b) Calculate the magnitude of the force on side QR.
c) The coil rotates so that PQ movies upwards. Deduce the direction of the current in it.
d) Calculate the maximum torque on the coil.

Answers from Surfie:
a) 8 N
b) 0
c) Out of page
d) 0.4Nm

These answers seem wrong to me as it says that PQ is perpendicular to the mag. field, thus it should be cos90 right? Meaning the answer to a) should be 0 ? Or am I mistaken. Thanks

 

[P!xel]

Force acting on a current carrying conductor is given by:



B = 0.4T
I = 1.6A
L = 0.05
n = 250
= 90

Note that

Question a)
F = 250 x 0.4 x 1.6 x 0.05 x sin90
F = 8N

Question b)
F = 0
Because the current carrying conductor is parallel to the magnetic field, and therefore

Question c)
Draw a diagram to better understand this question.
Using the right half palm rule, the current will be depicted to flow from Q to P.

Question d)

Yeh for clarification the Ө in F=BIl sin Ө is angle between current and magnetic field, Ө in BIA cos Ө is angle between the coil and the magnetic field

Torque is given by:



There will be maximum torque when the coil makes an angle of 0 with the magnetic field:



Therefore the equation is given by:



T = 250 x 0.4 x 1.6 x (0.05 x 0.05)
T = 0.4Nm

EDIT: So many mistakes! Sorry guys~
You can say that it's my first time answering a question on this board.

 

[helper]

P!xel, you need to fix your area in the last question

MrLuvable, remember the has a different meaning in the two equations.

 

[JasonNg1025]

Yeh for clarification the Ө in F=BIl sin Ө is angle between current and magnetic field, Ө in BIA cos Ө is angle between the coil and the magnetic field

 

[P!xel]

Oh sorry, I obviously read a misleading quote.
Thanks for the heads up.

 

[youngminii]

Pixel, you forgot the n in F = nBIlsin(theta).
0.032 x 250 = 8N, as stated in the answers.

Also, you used 0.5 as the length of the sides of the coil, whereas it is 0.05.
Hence:
T = 250 x 1.6 x 0.4 x 0.05 x 0.05
= 0.4 Nm, as states in the answers.

@ OP: When it says calculate the MAXIMUM torque/force, it means when the coil is at an angle that will produce the said maximum. So don't calculate the current torque, calculate the maximum torque that's possible for the coil to experience during it's rotation.
If you don't know what I mean, just put theta = 0 for cos(theta) ALL THE TIME (when it says maximum) and theta = 90 for sin(theta).

 

[P!xel]

Pixel, you forgot the n in F = nBIlsin(theta).
0.032 x 250 = 8N, as stated in the answers.

Also, you used 0.5 as the length of the sides of the coil, whereas it is 0.05.
Hence:
T = 250 x 1.6 x 0.4 x 0.05 x 0.05
= 0.4 Nm, as states in the answers.

@ OP: When it says calculate the MAXIMUM torque/force, it means when the coil is at an angle that will produce the said maximum. So don't calculate the current torque, calculate the maximum torque that's possible for the coil to experience during it's rotation.
If you don't know what I mean, just put theta = 0 for cos(theta) ALL THE TIME (when it says maximum) and theta = 90 for sin(theta).

Thanks again.
I think the OP would have been better off doing it himself -.-
At this rate, I've created more errors than him.

 

[youngminii]

Lol dw, at least you're trying to help. You'll get better.