torque and force (1 Viewer)

[xeriphic]

for a dc motor the torque is maximum when the plane is parallel to the field and zero when it is perpendicular to the field, what about the magnitude of the force on each side of the coil which turns the motor, does it change ??

 

[wogboy]

what about the magnitude of the force on each side of the coil which turns the motor, does it change ??

If you keep the current and magnetic field constant, then the force on each side of the coil is unchanged. The force is given by: F = (I x B)L ('x' means vector cross product). I and B are constant and are always perpendicular, and the length L is constant, so the force F is constant. The only thing is that as the coil rotates around, the direction of the force will not always point perpendicular to the plane of the coil, so the torque will vary as the coil rotates reaching a maximum torque point and zero torque point. But the force remains constant.

 

[xeriphic]

wow thanks I understand it now

 

[govich]

the force remains constant on the long sides of the loop only... the short sides vary between being parallel and perpendicular to the field and they are 'in-line' with the field so the force on them changes constantly.. but that force doesnt really do anything

 

[Steven12]

i dont think there is any force exerting on the short side
since even if it is perpendicular to the Mg field. it is still in line with the field. it does not cut across any field lines so there is no force.

 

[xeriphic]

apparently there is forces acting on the shorter sides and is at the greatest when the plane is perpendicular to the field, consider the formula F = BILsinQ when Q is zero no force is acting, however when Q is at 90 greatest force applies

I have another question

this is an extract on galvanometer

"The pole faces of the magnet are curved to surround the coil and there is a soft iron core inside the coil, these features ensure that the magnetic field is perpendicular to the coil thus the torque on the coil remains proportional to the current as the coil rotates."

okai in the red section is says the coil is perpendicular to the magnetic field, shouldn't it be parallel in order to continuoesly produce the greatest torque, that is when coil means the same as the plane

also

"If you keep the current and magnetic field constant, then the force on each side of the coil is unchanged. The force is given by: F = (I x B)L ('x' means vector cross product). I and B are constant and are always perpendicular, and the length L is constant, so the force F is constant. The only thing is that as the coil rotates around, the direction of the force will not always point perpendicular to the plane of the coil, so the torque will vary as the coil rotates reaching a maximum torque point and zero torque point. But the force remains constant."

from Wogboy's quote since always constant so force is also constantly at its greatest right

 

[wogboy]

i dont think there is any force exerting on the short side
since even if it is perpendicular to the Mg field. it is still in line with the field. it does not cut across any field lines so there is no force.

Govich is correct, there is a non-constant (sometimes zero) force acting on the "short" sides, but these forces do nothing other than creating a tiny bit of vertical tension in the coil (they don't contribute to the rotation of the coil).

The pole faces of the magnet are curved to surround the coil and there is a soft iron core inside the coil, these features ensure that the magnetic field is perpendicular to the coil thus the torque on the coil remains proportional to the current as the coil rotates..

I assume you mean some setup like the top diagram in the following page:
http://www.kangflatsc.vic.edu.au/home/solar/electric_motors.htm

Sure, the magnetic field is always perpendicular to the coil (the sides which make it move), but this does NOT mean the torque will be constant as the coil rotates around. Consider what happens when the coil from that top picture in that web page, lies in the vertical plane (i.e. rotated 90 degrees from the position shown). Then if current flows in the coil, sure there will be the F = (I x B)L force present, but there'll be no torque because the direction of the force is purely radial (directly away from the hinge or pivot line). The torque is given by T = R x F, and if R and F are in the same direction then T = 0 (the cross product of any two parallel/antiparallel vectors is zero).

okai in the red section is says the coil is perpendicular to the magnetic field, shouldn't it be parallel in order to continuoesly produce the greatest torque, that is when coil means the same as the plane

You are correct about the coil plane being parallel to the magnetic field (perhaps they means the normal to the coil plane?), but it's impossible to continuously produce maximum (i.e. constant) torque with such a system (see above).

 

[xeriphic]

hmm I understand it now thanks for the explaination dude

why does the polarity of both ac and dc generators change every half cycle thanks

 

[abdo]

hmm I understand it now thanks for the explaination dude

why does the polarity of both ac and dc generators change every half cycle thanks

maybe because it turns or cuts through the B field at a different direction when it passes a half cycle hence the induced current has a different polarity, if that is what you mean by polarity.

should just draw a diagram of a rotor in a B field and derive the equation t=nBIAcos@. if you do that everything becomes clear, trying to learn this stuff by word reasoning is quite hard...

 

[:: ck ::]

umm let me haf a go ...

AC - the slip rings have constant contact to the coils and say we have sides AB CD of the coil... say we turn it clockwise so AB is moving upwards...by the time the entire coil is perpendicular to the magnetic field... the direction of force applied is going in the opposite direction (first goin up .. then down) ... use RHPR or wotever its called and ull see the direction of current changes thus the change in polarity

DC - does it actually change polarity? i thought every half turn the split ring thing changes the dir of current so when when u change the dir of the force the polarity of each terminal stays the same?

 

[abdo]

DC - does it actually change polarity? i thought every half turn the split ring thing changes the dir of current so when when u change the dir of the force the polarity of each terminal stays the same?

of course it doesn't, hahaha... why would it? its direct current!!! lol

i think its the rotor's polarity mate

 

[:: ck ::]

hehe yeah i was wonderin why he was askin abt dc gens =P