# Flemming left hand rule !

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#1
I get really confused with the words such as :

In to the page
From left to right
Down the page
Out of the page
Up the page

can someone explain me how it works with the Flemming's left hand rule with an example ?

A beam of electrons enters a region of uniform magnetic field, moving at right angles to the field, and is deflected in to circular path. Draw a diagram to show the path of electrons including directions of the electron velocity V and magnetic field B .
How do you do the question above? :s
0
10 years ago
#2
well to show a magnetic field coming out of the board you usualy draw a/a few 'o's. You can always just write it on the side.

thumb is force
index is magnetic field
midle finger is current.

In the example since it's an electron, the current is the opposite to the direction of motion of the electron, since it is conventional current.

then point your index into the page, and if the electron is moving to the right on the page, point your middle finger to the left. your thumb will show you which way the path of the electron will curve

hope it helps
1
#3
(Original post by jonasdb)
well to show a magnetic field coming out of the board you usualy draw a/a few 'o's. You can always just write it on the side.

thumb is force
index is magnetic field
midle finger is current.

In the example since it's an electron, the current is the opposite to the direction of motion of the electron, since it is conventional current.

then point your index into the page, and if the electron is moving to the right on the page, point your middle finger to the left. your thumb will show you which way the path of the electron will curve

hope it helps
Hey, I got the answer for the example ! Yay! Thanks! How will it work if it was a proton instead of an electron ?

Btw, I don't get what you mean by 'you usualy draw a/a few 'o's. You can always just write it on the side'. Could you explain further please?
0
10 years ago
#4
(Original post by warkan)
Hey, I got the answer for the example ! Yay! Thanks! How will it work if it was a proton instead of an electron ?

Btw, I don't get what you mean by 'you usualy draw a/a few 'o's. You can always just write it on the side'. Could you explain further please?
No problem

conventional current is the movement of positive charge in a circuit. We use it because at first, physicists thought the actually particles moving were positively charged, which they were wrong about. Since they had already made losts of equations and things using this 'positive' current, they stuck with it, making it awkwards for everyone learning it hehe

So basically since a proton is the opposite charge to an electron, you would point your middle finger in the direction the proton is moving in.

Pictures-speak-a-thousand-words strikes again:

ignore the actualy diagram, but the 'x's on the page mean the magnetic field is acting into the page. 'o's means that opposite. It wouldn't hurt to mention it on the side of the diagram though

Hope that clears it up
1
#5
(Original post by jonasdb)
No problem

conventional current is the movement of positive charge in a circuit. We use it because at first, physicists thought the actually particles moving were positively charged, which they were wrong about. Since they had already made losts of equations and things using this 'positive' current, they stuck with it, making it awkwards for everyone learning it hehe

So basically since a proton is the opposite charge to an electron, you would point your middle finger in the direction the proton is moving in.

Pictures-speak-a-thousand-words strikes again:

ignore the actualy diagram, but the 'x's on the page mean the magnetic field is acting into the page. 'o's means that opposite. It wouldn't hurt to mention it on the side of the diagram though

Hope that clears it up
Right, lets see if I have actually learned something ! I am getting there!!

So if it is an electron, you point your middle finger in the opposite direction of its path. The magnetic field is usually 'into the paper' if it is an 'X' or 'out of the paper' if its an 'o' and it changes every question .Right ? or does it always point in to the paper for an electron ? So by pointing your index finger and your second finger in the right direction, it will give you the motion of the electron(thumb ) right ? Using that motion , you would be able to tell if its moving to the right or left or up or down ?

If its an proton , you just point your middle finger in the direction of the path. And it follows the same rules as mentioned for an electron above right ?

We just looked how the LHR would work for a positive and a negativity charged particle. What if the particle is uncharged or unknown charge ?
0
10 years ago
#6
(Original post by warkan)
Right, lets see if I have actually learned something ! I am getting there!!

So if it is an electron, you point your middle finger in the opposite direction of its path. The magnetic field is usually 'into the paper' if it is an 'X' or 'out of the paper' if its an 'o' and it changes every question .Right ? or does it always point in to the paper for an electron ? So by pointing your index finger and your second finger in the right direction, it will give you the motion of the electron(thumb ) right ? Using that motion , you would be able to tell if its moving to the right or left or up or down ?

If its an proton , you just point your middle finger in the direction of the path. And it follows the same rules as mentioned for an electron above right ?

We just looked how the LHR would work for a positive and a negativity charged particle. What if the particle is uncharged or unknown charge ?

For the first part yes. Always remember you point in the direction of movement of positive charge

In bold: You need to remember these are two completely different aspects of a situation. The question will always tell you if the field is moving into or out of the page (btw I have never seen one where it wasn't into or out of the page, but if it went, say, to the left, just apply the left hand rule as you would normally, don't let things like that put you off )

in italic: It's not the motion as such but the resultant force. remember that using the left hand rule will imply that the particle (electron/proton/etc) is now following a circular path. Your thumb points to the center of that circle so in other words, the direction in which it 'bends'

underlined: I don't know how they do that I've never thought about it. Would be interesting to know
But for 'unknown charged particles' surely they would know the charge of the particle they are accelerating?

btw are you doing edexcel paper 4 next Thursday?
0
#7
(Original post by jonasdb)
For the first part yes. Always remember you point in the direction of movement of positive charge

In bold: You need to remember these are two completely different aspects of a situation. The question will always tell you if the field is moving into or out of the page (btw I have never seen one where it wasn't into or out of the page, but if it went, say, to the left, just apply the left hand rule as you would normally, don't let things like that put you off )

in italic: It's not the motion as such but the resultant force. remember that using the left hand rule will imply that the particle (electron/proton/etc) is now following a circular path. Your thumb points to the center of that circle so in other words, the direction in which it 'bends'

underlined: I don't know how they do that I've never thought about it. Would be interesting to know
But for 'unknown charged particles' surely they would know the charge of the particle they are accelerating?

btw are you doing edexcel paper 4 next Thursday?
Well that clears quite a few doubts ! Thanks for the help !!
How do we know which one is the direction of positive charge again ? I am clear about the thumb and the first finger now ! yay !

Well , I came across a question recently regarding a particle which had an unknown charge and you were given the magnetic field and asked for the charge. I pointed my index finger in to the page and my thumb towards the center of the circle . This left my middle finger pointing in the direction of the motion of the particle. What does this suggest? Positive or negative?

I just thought about the uncharged particle. They cant give us a track of an uncharged particle and ask questions about it since, It wont leave any trail in a bubble chamber

Yup, I am taking U4phy edexcel this Thursday ! My last exam ! wuuhoo !! I hate the first chapter about banking , those circular rides and stuff. Do you get them ? and I feel particle physics is the only okayish bit :/ lol Are you ready for the exam ? Me deffo not !
0
10 years ago
#8
(Original post by warkan)
Well that clears quite a few doubts ! Thanks for the help !!
How do we know which one is the direction of positive charge again ? I am clear about the thumb and the first finger now ! yay !

Well , I came across a question recently regarding a particle which had an unknown charge and you were given the magnetic field and asked for the charge. I pointed my index finger in to the page and my thumb towards the center of the circle . This left my middle finger pointing in the direction of the motion of the particle. What does this suggest? Positive or negative?

I just thought about the uncharged particle. They cant give us a track of an uncharged particle and ask questions about it since, It wont leave any trail in a bubble chamber

Yup, I am taking U4phy edexcel this Thursday ! My last exam ! wuuhoo !! I hate the first chapter about banking , those circular rides and stuff. Do you get them ? and I feel particle physics is the only okayish bit :/ lol Are you ready for the exam ? Me deffo not !
In bold: think about what you just asked hehe, what particle has a positive charge or a negative charge?

in italic: refer to what i've already told you in the first post seems you got the left hand rule down now though nice one

underlined: yea i was wondering how they would do it, since the LHC is all magnets

And about the circular motion banking I find them quite easy, but I think the fact that I have an M3 exam the next day is definitely helping ^^ But yea I quite like particle physics. i always find the electronic stuff the hardest, circuits are so hard to think about!
Overall it should be ok, but i seriously need an A in it so i feel some revision is in order this week
0
#9
(Original post by jonasdb)
In bold: think about what you just asked hehe, what particle has a positive charge or a negative charge?

in italic: refer to what i've already told you in the first post seems you got the left hand rule down now though nice one

underlined: yea i was wondering how they would do it, since the LHC is all magnets

And about the circular motion banking I find them quite easy, but I think the fact that I have an M3 exam the next day is definitely helping ^^ But yea I quite like particle physics. i always find the electronic stuff the hardest, circuits are so hard to think about!
Overall it should be ok, but i seriously need an A in it so i feel some revision is in order this week
Electron- negative - Direction of current opposite to motion.
Proton - Positive -Direction of current in the direction of its motion.

Okay after all this I tried few questions and still got problems.LHR sucks! Help me out on these two questions below please !

[IMG][/IMG]

[IMG][/IMG]
0
10 years ago
#10
(Original post by warkan)
Electron- negative - Direction of current opposite to motion.
Proton - Positive -Direction of current in the direction of its motion.

Okay after all this I tried few questions and still got problems.LHR sucks! Help me out on these two questions below please !

[IMG][/IMG]

[IMG][/IMG]
for image 1:
(that paper looks familiar)
current is going to the right, and force is going down. This should suggest that the field is going out of the plane of paper. is that not he answer?

Image 2:

pretend all particles are positively charged, then try out the left hand rule. For me it is giving me the right answer

using:
thumb upwards
and
index finger into the plane. That should show you that the right side is the direction of current hence, they are positive particles. no?
0
#11
(Original post by jonasdb)
for image 1:
(that paper looks familiar)
current is going to the right, and force is going down. This should suggest that the field is going out of the plane of paper. is that not he answer?

Image 2:

pretend all particles are positively charged, then try out the left hand rule. For me it is giving me the right answer

using:
thumb upwards
and
index finger into the plane. That should show you that the right side is the direction of current hence, they are positive particles. no?
For image 1, what about the track which is moving upwards? don't we have to consider that as well ? I understand the current bit . But why does the force go down ? it can go up as well right? And if the force does go up, the field becomes in to the paper. lol .

Can the magnetic field ever be from left to right or the other way round?

Second image,the thumb has to point either to left or right side right? how can you point in upwards when there is no track going upwards? Two of them are going left and other two are going to the right. Try pointing your index finger in to the page and thumb to the left . It should show you that the particle on the left is positively charged! But that's not the answer!!!
0
10 years ago
#12
(Original post by warkan)
For image 1, what about the track which is moving upwards? don't we have to consider that as well ? I understand the current bit . But why does the force go down ? it can go up as well right? And if the force does go up, the field becomes in to the paper. lol .

Can the magnetic field ever be from left to right or the other way round?

Second image,the thumb has to point either to left or right side right? how can you point in upwards when there is no track going upwards? Two of them are going left and other two are going to the right. Try pointing your index finger in to the page and thumb to the left . It should show you that the particle on the left is positively charged! But that's not the answer!!!

(Original post by jonasdb)
in italic: It's not the motion as such but the resultant force. remember that using the left hand rule will imply that the particle (electron/proton/etc) is now following a circular path. Your thumb points to the center of that circle so in other words, the direction in which it 'bends'
The magnetic field can be set up in any direction the question wants it to be

And the last bit. Why are you now telling me that the force is in the direction of the particle? It isn't. refer to my quote ^. for the particles on the right (assuming it is positively charged) middle finger points to the right, thumb up and index into the plane. it all works
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#13
(Original post by jonasdb)

The magnetic field can be set up in any direction the question wants it to be

And the last bit. Why are you now telling me that the force is in the direction of the particle? It isn't. refer to my quote ^. for the particles on the right (assuming it is positively charged) middle finger points to the right, thumb up and index into the plane. it all works
Dude, that paper didn't have a single question where I had to use FLR. -_- . Thanks for your help through out though! how did you find the paper?
0
10 years ago
#14
(Original post by warkan)
Dude, that paper didn't have a single question where I had to use FLR. -_- . Thanks for your help through out though! how did you find the paper?
waaay to little calculation questions in my opinion, too many describe/explain ones.

It always difficult to tell how well you did with explainy questions though, since you can't really compare answers with others etc that easily

There was one question where you could use FLR: the one with the electric motor, use it to work out which way the motor would spin. But pointing that out wasn't crucial

No probs, happy to help a fellow physicist out

How did you find it?
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#15
(Original post by jonasdb)
waaay to little calculation questions in my opinion, too many describe/explain ones.

It always difficult to tell how well you did with explainy questions though, since you can't really compare answers with others etc that easily

There was one question where you could use FLR: the one with the electric motor, use it to work out which way the motor would spin. But pointing that out wasn't crucial

No probs, happy to help a fellow physicist out

How did you find it?
Yeah, totally agree on that. Too many wordy questions. After the first two parts, I was like 'oh god please tell me a calculation please!!' and of-coarse it was yet another big wordy question . In the end too, all words questions . But , I guess I have done okayaish because those questions on X-rays were just lol ?

And yeah, that question about the motor I did use the FLH and I was confused with the arrow of the current :s

There were some explanation bits which were hard . Other than that, a very fair paper from edexcel. Just didn't have enough number work.
0
10 years ago
#16
Come on, give jonasdb a rep!
0
10 years ago
#17
(Original post by warkan)
Yeah, totally agree on that. Too many wordy questions. After the first two parts, I was like 'oh god please tell me a calculation please!!' and of-coarse it was yet another big wordy question . In the end too, all words questions . But , I guess I have done okayaish because those questions on X-rays were just lol ?

And yeah, that question about the motor I did use the FLH and I was confused with the arrow of the current :s

There were some explanation bits which were hard . Other than that, a very fair paper from edexcel. Just didn't have enough number work.
Yeah the questions on x-rays were quite out of the blue, but hopefully I made a reasonable attempt at explaining myself!

It's only very near the exam that i actualy realised that explaining and describing questions, although sometimes have ******ed mark schemes, do usually require you to know the subject much more than simply doing calculations, where most of the time you just multiply all the numbers they give you

But yeah, it was an ok paper, better thank the sample paper anyway!
0
10 years ago
#18
(Original post by warkan)
Hey, I got the answer for the example ! Yay! Thanks! How will it work if it was a proton instead of an electron ?

Btw, I don't get what you mean by 'you usualy draw a/a few 'o's. You can always just write it on the side'. Could you explain further please?
for an electron, the direction of the current would be opposite. So still using fleming's left hand rule, the force acting(magnetic force) on the electron is downwards.
0
5 years ago
#19
hi everyone!
i need to ask an important question if anyone could help
when the give us a picture from a bubble chamber for example
and there are two paths from one, one curves to the right the other to the left( the spiral kind things)
and they ask which one is the electron
how to do this, i have my edexcel ppr day after tmrw and i need someone to answer this please
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