# Urgent question related to induced e.m.f

1. Jun 10, 2012

### ehabmozart

1. The problem statement, all variables and given/known data
A student is experimenting with a bicycle wheel. He turns the bicycle upside down and spins the wheel in a vertical plane at a constant rate. The diagram shows the wheel. At the place where the experiment is performed, the Earth‟s magnetic field is in a horizontal direction. It acts into and perpendicular to the paper.

A constant e.m.f. is induced across the length of each spoke.
(i) Label the hub and rim either plus or minus to show the polarity of the e.m.f.

The figure shows a wheel with the direction of motion clockwise. The hub is the center and the rim are the outerpart

2. Relevant equations

3. The attempt at a solution
Well, in this problem, it is said that the magnetic field lines point inside the paper. So if this is cut, it tries to oppose it... Hence, magnetic field lines should be outside. We should use flemings right hand rule but i have no idea about what would be the new direction of the spokes of the wheel (i.e the motion) and indeed upon that i don't know the direction of E.M.F..... I need help urgently.. Thanks!.. By the way, in the replies i would like the person to stick to a certain rim.. Let us say, the horizontal rim that is about to move downwards.. Please HELP.. PLEASE! "D

2. Jun 10, 2012

### tiny-tim

hi ehabmozart!
forget the rim, the rim has nothing to do with it (the rim isn't cutting any field lines)

concentrate on the horizontal spoke that is about to move downwards..

(i don't understand what you mean by "the new direction of the spokes" … the whole bike is fixed, and the spokes are rotating as usual)

3. Jun 10, 2012

### ehabmozart

OH.. Thanks for replying dude.. Am dying right now to find the solution.. OK.. I will tell u in a different way.. I mean i will use my logic.. There is an emf induced because the magnetic flux of the earth is cut.. My question is what cause the change in flux.. My logic says it is the rims.. or the full cycle as a whole.. Btw, visualize it as a single tyre and the student keeps on rotating it clockwise. Anyway, if the rim cuts the flux, it will induce emf which will inturn induce magnetic field which will oppose the original. i guess.. am i right till here?? So, we got one clue.. Magnetic force out of the paper... What bout the direction of the tyre.. Wouldn't this induced emf try to slow it down.. This part is making me crazy.. How would the current oppose the change.. By opposing a different magnet direction trying to make the tyre move in the opposite direction or both of them... I need the reply asap and am dying for it.. So i am thankfull for any kind of contribution. THanks!

4. Jun 10, 2012

### tiny-tim

5. Jun 11, 2012

### ehabmozart

OK dude, tell me the final answer .. From the that, i can build my knowledge cs i am totally confused in this law... Give me the answer! :D

6. Jun 11, 2012

### tiny-tim

call the strength of the magnetic field "B", the radius "r", and the angular velocity "ω"

how much area is swept out by one spoke in one second?

how much flux is swept out by one spoke in one second?​

7. Jun 11, 2012

### ehabmozart

Well I know this formula.. My question is what will be the direction of the induced emf.. negative or positive in the hub (center)... How DOES THE LENZ RULE APPLY HERE?? It will try to minimise the flux change.. HOW?? Common.. Gimme a direct REPLY.. I swear i gotta test in 2 hours... Please tiny tim..

8. Jun 11, 2012

### ehabmozart

9. Jun 11, 2012

### SammyS

Staff Emeritus
Look at the Homework Help section of the rules for this Forum !

10. Jun 11, 2012

### ehabmozart

OMG Guyes.. This is not even a homework.. It's an extra question.. I have an exam in 2 hours.. I need to know the answer to actually know the concept.. OK?? Atleast tell me the concept in this question... I swear i am dying here

11. Jun 11, 2012

### tiny-tim

if you must apply the Lenz rule, the circuit is the present position of the spoke, together with any previous position, and the arc joining them

12. Jun 11, 2012

### ehabmozart

Tiny tim.. Have u seen the link i've given U .............. http://www.thestudentroom.co.uk/show....php?t=1141210 [Broken]
I need to know whether the hub will be positive or negative.. and WHY?????

Last edited by a moderator: May 6, 2017
13. Jun 11, 2012

### tiny-tim

if you must apply the Lenz rule, the circuit is the present position of the spoke, together with any previous position, and the arc joining them

14. Jun 11, 2012

### ehabmozart

Geez... Is the hub positive or NEGATIVE?? Simple as that with a reason.. DOn't play with my nerves.. I have an exam after an hour