Motors and Generators - Success one (1 Viewer)

[Premus]

Hey

i have a few questions from this chapter in success one......especially the ones with the diagrams! ( i dont have a scanner) sorry
could u guys plss explain the following questions
in multiple choice :
1
2
6 (how can u tell which is positive? and whether it slows down or not?)
11
23 - (there will be a south pole induced....but how do u know which direction the induced current will flow)
25 - ( i think its C ?)

Long answers : question 22 b) - how can u tell whether X or Y is positive?

Thank you soo much for your help!

 

[Jase]

1. I dont know lol.. i guessed A for some reason. You see, my logic was, grip around bar magnet with thumb pointing up to have anticlockwise current. So theres an anti-clockwise conventional current flowing around in the reservoir. then the wire is also positively charged, so it's pushed away and somehow goes in the opposite direction. please don't take that as the answer, this was just a in the moment have a guess answer.

2. Okay, you know that B is proportional to current. So let the wire with 10A = B
20A = 2B

by using the right hand grip rule around each wire, youll see that its wire X and Y point into the page at O and out of the page from wire Z at O. So wire Y and wire Z's fields cancel each otehr out, and theres only the WireX's B field left, going into the page. answer D.

6. The diagram is really really dodgey. Think of the wire as moving perpendicular in all forms against the field, its not sloped as it looks. So using lenz's law, the force opposes to the left, so palm left, and the mag field up, making the thumb point to Y. Positive charge accumulates at Y making Y positive. So its eitehr B or D. now if it moves at a constant speed- AFTER the intial change in flux (from zero to a value), the flux will remain constant. So there's initial resistance and then constant speed.

11. i have no idea. Sound's like a patented stupid question to me.

23. This is using the right hand GRIP rule. You know that solenoid thing? with the fingers wrapped around the bunch of coils to find the North pole. This is the same thing. As the magnet moves in... the coil needs to create a south pole to oppose the magnet. Thumb point in = fingers curled clockwise. Then as it moves out, it creates a south pole at the otehr side in order to attract the north pole. So thumb out = fingers curled anti-clockwise.
hence its A.

25. This is another stupid question. It could be B or C. but yes, i'd say its C. Component of the parralell field is now more instead of less..

22b) When the field is reduced, the flux lines start to decrease. By Lenz's law, the loop then tried to oppose that decrease by increasing it, effectively "reinforcing" the weakening field. So using the GRIP rule, point your thumb in the direction it needs to reinforce, which is up.
The fingers curl anti-clockwise, which means the conventional current goes from Y to X. hence 'positive' charge accumulates at X.

 

[aim54x]

isn't ther explantaions with the answers??? i don't have the book but i have done the stuff in it, but i don't remember specifics i would have to dig out the questions. SORRY

 

[Heinz]

2. the fields produced by wires Z and Y cancel eachother. So your left with wire X only which has field lines going into the page at point O (and a strength of B teslas which is given).

 

[Premus]

Thanks a lot for ur help guys!
But just two questions regarding ur explanation Jase,

"23. This is using the right hand GRIP rule. You know that solenoid thing? with the fingers wrapped around the bunch of coils to find the North pole. This is the same thing. As the magnet moves in... the coil needs to create a south pole to oppose the magnet. Thumb point in = fingers curled clockwise. Then as it moves out, it creates a south pole at the otehr side in order to attract the north pole. So thumb out = fingers curled anti-clockwise.
hence its A."

ok i understand the solenoid rule...where the thumb points to the North Pole.
so as the magnet moves in....it induces a SOUTH pole in the loop and so the thumb points into the coil. then as it comes out the other side, it induces a south pole again....and so, wouldnt the thumb be pointing into the coil again?

------------------

"22b) When the field is reduced, the flux lines start to decrease. By Lenz's law, the loop then tried to oppose that decrease by increasing it, effectively "reinforcing" the weakening field. So using the GRIP rule, point your thumb in the direction it needs to reinforce, which is up.
The fingers curl anti-clockwise, which means the conventional current goes from Y to X. hence 'positive' charge accumulates at X."

Yeap i understand that according to Lenz's law the loop tries to INCREASE the field. what i dont understand is how u used the right hand grip rule?? and how u got UP?( the grip rule , is the same as the solenoid right?) where and how did u used the rule? im really confused here

Thank you very much!

 

[Jase]

but it's inducing a south pole on the OTHER side of the loop, like not at the front. So the poles are reversed.

for 22 b) imagine that the loop is a solenoid with only one turn. So put it on its side. Now imagine that the magnetic field is the north pole of a bar magnet. By reducing that field youre doing the same thing as pulling the magnet away. so its the same case. just turned on its side.

 

[mojako]

the way I do 1:
-> inside the bar magnet field goes up.
the current wants to minimise this by making a field going down
using the grip rule it flows clockwise
-> when I did this Q I actually used the right hand palm rule (some call it the FBI gun thing)
current flows from left to right in the solution, so point thumb to right
field created by bar magnet goes down on the left side of the magnet, so fingers down
force is now into the page so its clockwise
this shud be the force on the charges in the solution, not the wire... but somehow it works

for 6 u take the perpendicular component of the wire.. or u can argue that wire is a solid thing and not a vector and u can take the perpendicular component of the field.. it gives the same thing by geometry / trigonometry.

for 11, I dont understand it at all
I guess since theres a load then it slows down to conserve input and output energy lol

for 22b u can also use the right hand palm rule

for 25, the key is on the word "current's magnetic field", not the current
the field wll be sorta perpendicular to the current.. think of a vertical wire with current going up... field will be anticlockwise.. at the back/front of the wire its going left/right which is perpendicular to up/down.

 

[Premus]

ahh i see....so when u said UP u meant out of the page yeah?

so instead if the question said...INCREASING the magnetic field instead of reducing....
then according to Lenz's law, the field will point into the page and the current will flow clockwise right?

Thanks

 

[Jase]

yup. and yup.