Back EMF Question (1 Viewer)

[sasquatch]

My physics textbook states: "When there is nothing attached to the motor to slow it down, the speed of the armature increases until the back emf is equal to the external emf. When this occurs there is no voltage across the coil and therefore no current flowing through the coil. With no current through the coil there is no net force acting on it and the armature rotates at a constant rate."

If there is no current flowing through the coil, how the hell can the armature rotate. Isnt the point of the motor effect "a CURRENT CARRYING conductor experiences a force in an external magnetic field". This force causes the rotation of the coil. So if there is no current and no force on the ends of the wire (i understand there is no NET force), how the hell does the motor rotate at all, let alone "a constant rate".

 

[Mountain.Dew]

this is my 2 cents...just my opinion.

i think that the textbook assumes that the momentum of the coil rotating will be constant in a 'perfect' world, where there is no air resistance and friction. when there is net voltage, the momentum of the coil rotating builds up, until the back emf = supply emf ==>no more net voltage, no current. this, under the assumptions of the perfect world, doesn't stop the coil from rotating forever, abiding the law of conservation of momentum. it will continue to rotate forever, unless, friction or air resistance is involved. that is why in the real world, supply emf MUST > back emf so that the net emf can overcome the friction forces and air resistance of the coil on the shaft, and on the armature.

mind you, when the motor first begins to rotate, back emf is basically negligible until it begins to rotate at higher rate.

personally, i would ignore what the textbook says entirely...has no relevance to any syllabus dot pt, so no relevance in the HSC.

 

[billeh]

yeah .. i did hsc physics 2005, and i noticed that in some textbooks they r just WRONG!
but there's a really good website u can use to get accurate information on hsc physics topics
http://www.phys.unsw.edu.au/hsc/index.html

 

[sasquatch]

thanks people... it did sound stupid..

and the book was jacaranda.......i hate it so much (for other reasons)

 

[acullen]

Back when I did the HSC, I used the textbook "Physics in Context: The Forces of Life" it was a great book. It was written by Bill Zealey, an Assoc. Prof in Physics at UoW (and a nice guy).

I don't think it cost all that much either. When you go to uni, you'll find out how high publishers can charge. I've got a small ~450pg book next to me on Thermodynamics, Kinetic Theory and Statistical Thermodyanmics that cost $140.

 

[del pietro]

$140 ! wow i better start saving. haha