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# Question I dont understand... watch

1. (Original post by 2710)
Yes, I think if the 6muC charge was -ve, then it would have worked.
Im moigh tbe missing something , but this is what I did ..
they want you to find the point x=0 , so when the potential between point P and Q are equal

so

2. Isnt r^2 ???
3. (Original post by 2710)
Mm OK, thanks. What do you mean when you say be careful of the units of charge? Do you mean the polarity of the charges?

Thanks

PS am I right in also saying that there is also no potential 0 with two negative charges?
Although it gives the right answer here, you need to be careful about the micro coulombs. In most questions you would need to multiply by 10^-6. Here it cancels out because it actually depends on the ratio of the two charges.

Yes, same for two negative charges. No zero potential point.
4. (Original post by rbnphlp)
Im moigh tbe missing something , but this is what I did ..
they want you to find the point x=0 , so when the potential between point P and Q are equal

so

Thats what I thought as well. but isnt the formula for resultant potential:

V1 + V2 = Resultant potential?

But then when I think about it, can't you say that the potential of V1, is the same as the potential of V2? But then that would contradict the formula... >__>
5. Ok another question.

Motor Rule uses current as conventional current right?

Say I have a Magnetic field going from North to south, and I have an electron going in perpendicularly (going east), will it move up or down?

I am pointing my middle finger west, because that's conventional current right? Meaning the electron will want to move up into the sky.

Or am I wrong? Am i meant to point my middle finger right?

Thanks
6. (Original post by 2710)
Thats what I thought as well. but isnt the formula for resultant potential:

V1 + V2 = Resultant potential?

But then when I think about it, can't you say that the potential of V1, is the same as the potential of V2? But then that would contradict the formula... >__>
no we cant , we can say that for electrostatic force but not for potential ..they are only zero at infinite distances from the charges
7. (Original post by 2710)
Ok another question.

Motor Rule uses current as conventional current right?

Say I have a Magnetic field going from North to south, and I have an electron going in perpendicularly (going east), will it move up or down?

I am pointing my middle finger west, because that's conventional current right? Meaning the electron will want to move up into the sky.

Or am I wrong? Am i meant to point my middle finger right?

Thanks
I think so , the electrons are moving toward me
8. I mean, like this:

I thought Flemming's left hand rule's middle finger is measuring conventional current...doesn't that mean you point ur middle finger left?

Ie, the electron will be moving towards YOU, rather than into the page.

Thanks
9. (Original post by 2710)
I mean, like this:

I thought Flemming's left hand rule's middle finger is measuring conventional current...doesn't that mean you point ur middle finger left?

Ie, the electron will be moving towards YOU, rather than into the page.

Thanks
after twisting my hand so badly , it looks its the motion of the wire is into the page , so what is the question asking you to find the motion of the wire or the force on the wire? if the former you need to use flemming's right hand rule, which is what I used..
my middle finger is pointing towards the left
10. (Original post by rbnphlp)
after twisting my hand so badly , it looks its the motion of the wire is into the page , so what is the question asking you to find the motion of the wire or the force on the wire? if the former you need to use flemming's right hand rule, which is what I used..
Omg I need stone bridge lol!

Basically, this is an electron moving right, and I am trying to find the force on it. I am not using the right hand rule.

The above diagram is the same as this one:

So how will this wire move?

(I think there is confusion with conventional current)

EDIT:

Copy and pasted from site:

To work out the direction of force experienced we use Fleming's Left Hand Rule.

Your first finger points in the direction of the magnetic field (North to South).
Your second finger points in the direction of conventional current (positive to negative).
Your thumb points in the direction of the thrust or force on the conductor.

Since the 1st diagram is electron flow (opposite of conventional currrent) then that is why you have to point your middle finger opposite to it, ie left. Right?
11. (Original post by 2710)
Omg I need stone bridge lol!

Basically, this is an electron moving right, and I am trying to find the force on it. I am not using the right hand rule.
you ddint mention that in the beginning so how am I supposed to know?

(Original post by 2710)
Your first finger points in the direction of the magnetic field (North to South).
Your second finger points in the direction of conventional current (positive to negative).
Your thumb points in the direction of the thrust or force on the conductor.[/COLOR]

Since the 1st diagram is electron flow (opposite of conventional currrent) then that is why you have to point your middle finger opposite to it, ie left. Right?
..it is pointing to the right..
12. (Original post by rbnphlp)
you ddint mention that in the beginning so how am I supposed to know?
Yes I did

(Original post by 2710)
Ok another question.

... perpendicularly (going east), will it move up or down?

I am ...
(Original post by rbnphlp)
..it is pointing to the right..
instead of just stating that I am wrong, would you care to explain

1) Why you are right
2) Why I am wrong

I have clearly stated that Fleming's Left hand rule uses CONVENTIONAL current. So why would it still be pointing in the SAME direction as electron flow?
13. (Original post by 2710)
I mean, like this:

I thought Flemming's left hand rule's middle finger is measuring conventional current...doesn't that mean you point ur middle finger left?

Ie, the electron will be moving towards YOU, rather than into the page.

Thanks
Correct.
LH Rule for the force on a current or moving charge in mag field.
Field downwards - first finger
current to the left (conventional current opposite to electrons) - centre finger
Force out of page - thumb
14. (Original post by Stonebridge)
Correct.
LH Rule for the force on a current or moving charge in mag field.
Field downwards - first finger
current to the left (conventional current opposite to electrons) - centre finger
Force out of page - thumb
Thank you!
15. (Original post by 2710)
Thank you!
after twisting my hand so badly , it looks its the motion of the wire is into the page , so what is the question asking you to find the motion of the wire or the force on the wire? if the former you need to use flemming's right hand rule, which is what I used..
my middle finger is pointing towards the left

Isnt that what I exactly said in post 29?
16. (Original post by rbnphlp)
after twisting my hand so badly , it looks its the motion of the wire is into the page , so what is the question asking you to find the motion of the wire or the force on the wire? if the former you need to use flemming's right hand rule, which is what I used..
my middle finger is pointing towards the left

Isnt that what I exactly said in post 29?
There is no wire in the question. It is an electron.
There is a difference between an electron moving in a field (LHR), and a wire moving in a field (RHR). The directions are perpendicular.
17. (Original post by rbnphlp)
after twisting my hand so badly , it looks its the motion of the wire is into the page , so what is the question asking you to find the motion of the wire or the force on the wire? if the former you need to use flemming's right hand rule, which is what I used..
my middle finger is pointing towards the left

Isnt that what I exactly said in post 29?
I am not moving the wire. I am trying to find the force on the wire. Your posts are very confusing. I am not using the Dynamo rule, but the Motor Rule. I do not know why you are using your right hand, unless you was taught a different way.

And then with your next posts, you say that it is pointing right...so, meh, it doesn't matter anymore. I got it.

EDIT:

StoneBridge, can I just ask:

Is the same as this:

Right? (when talking about the force)

Ie: Both move out of the page towards you?
18. (Original post by Stonebridge)
There is no wire in the question. It is an electron.
There is a difference between an electron moving in a field (LHR), and a wire moving in a field (RHR). The directions are perpendicular.
I qoute my text book
" Flemming's LHR is a good way to work out the direction of the force on a wire or a charged particle .T"
19. (Original post by rbnphlp)
I qoute my text book
" Flemming's LHR is a good way to work out the direction of the force on a wire or a charged particle .T"
I think there is confusion with the dynamo and motor rule >__>

Dynamo = RHR

Motor = LHR
20. (Original post by rbnphlp)
I qoute my text book
" Flemming's LHR is a good way to work out the direction of the force on a wire or a charged particle .T"
Yes, and the wire is stationary, but the charged particle is moving.
The LHR is used for a stationary wire carrying current. The wire experiences a force.
If the wire is moving you use the RHR to find the induced emf/current direction.
With the LHR the current is already flowing and the rule finds the force direction.

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