Where does the magnetic field go for the electron?

  • #1
146
1

Homework Statement


iVXeS.png

**Question** Is the magnetic field directed into the page or out of the page?


Homework Equations


Right Hand Rule

The Attempt at a Solution


My index finger is pointing downwards, and since the magnetic force is towards the right and this is an electron (negative) my thumb should point left.

But then my middle finger [the magnetic field] points towards the page. But the actual answer is that the magnetic field is coming out the page?

Where am I wrong?
 

Answers and Replies

  • #2
berkeman
Mentor
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Homework Statement


iVXeS.png

**Question** Is the magnetic field directed into the page or out of the page?


Homework Equations


Right Hand Rule

The Attempt at a Solution


My index finger is pointing downwards, and since the magnetic force is towards the right and this is an electron (negative) my thumb should point left.

But then my middle finger [the magnetic field] points towards the page. But the actual answer is that the magnetic field is coming out the page?

Where am I wrong?
What is the sign of the charge of an electron? :smile:
 
  • #3
146
1
The charge is negative, but I already accounted for that by flipping the force and pointing my thumb to the left?
 
  • #4
berkeman
Mentor
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9,105
The charge is negative, but I already accounted for that by flipping the force and pointing my thumb to the left?
I didn't quite understand your description of how you used your right hand to figure out the direction of the B-field and force.

For me, I point my fingers straight in the direction of qv, then curl them in the direction of B, and my thumb points in the direction of F = qv X B. So my fingers start pointing up the page first in the opposite direction of the electron velocity, I curl my fingers up out of the page toward a vertical B to get the force to the right to deflect the electrons.

Does that help?
 
  • #5
146
1
I didn't quite understand your description of how you used your right hand to figure out the direction of the B-field and force.

For me, I point my fingers straight in the direction of qv, then curl them in the direction of B, and my thumb points in the direction of F = qv X B. So my fingers start pointing up the page first in the opposite direction of the electron velocity, I curl my fingers up out of the page toward a vertical B to get the force to the right to deflect the electrons.

Does that help?
I am actually very unfamiliar with that right hand rule. I use this one:

jykOLZyTcOjR9MBPCwVA.png


So using this, I had the index pointing down and thumb pointing left, but that still had my middle finger pointing into the page?
 
  • #6
berkeman
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That's the same as what I use. Just instead, I start with my fingers all pointing in the direction of the positive current (qv) and then curl them in the direction of the B field, and look where my thumb is pointing to get the direction of the force F = qv X B. If you put your hand like you show above, you should get the right answer for this problem.
 
  • #7
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51
I am actually very unfamiliar with that right hand rule. I use this one:

jykOLZyTcOjR9MBPCwVA.png


So using this, I had the index pointing down and thumb pointing left, but that still had my middle finger pointing into the page?
You just have to flip the magnetic field direction if you use the method above. So the current is downward the force is to the right and the magnetic field is going in the paper, However as it is a flow of electrons your reverse the magnetic field direction.

Another way is, when you have an electron. Use your left hand instead much much better and less painful XD
 
  • #8
berkeman
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So using this, I had the index pointing down and thumb pointing left, but that still had my middle finger pointing into the page?
Why are you doing that? Lay the back of your hand on the page. That has your index finger pointing up in the opposite direction of the electron flow, your middle finger with the B field pointing up out of the page, and your thumb in the direction of the force to push the electrons to the right towards the "X" location.
 
  • #9
146
1
Why are you doing that? Lay the back of your hand on the page. That has your index finger pointing up in the opposite direction of the electron flow, your middle finger with the B field pointing up out of the page, and your thumb in the direction of the force to push the electrons to the right towards the "X" location.
Okay. I see this now.

So in general, should I always put the back of my hand on the page? For any problem?

And then for the electron, just flip the direction while keeping the others same?
 
  • #10
berkeman
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So in general, should I always put the back of my hand on the page? For any problem?
Not necessarily -- just start with your fingers pointing in the direction of qv and go from there.
And then for the electron, just flip the direction while keeping the others same?
Same answer. :smile:
 
  • #11
146
1
I see. qv is negative right? So that is why you flip the direction?
 
  • #12
berkeman
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I see. qv is negative right? So that is why you flip the direction?
Yes, the charge q of the electron is negative, so that's why qv is in the opposite direction of the velocity v. :smile:
 
  • #13
TSny
Homework Helper
Gold Member
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The Attempt at a Solution


My index finger is pointing downwards, and since the magnetic force is towards the right and this is an electron (negative) my thumb should point left.

But then my middle finger [the magnetic field] points towards the page. But the actual answer is that the magnetic field is coming out the page?

Where am I wrong?
Seems to me this should have given you the right answer. Make sure you use the right hand.

Of course berkeman's and Biker's ways are good, too.
 
  • #14
berkeman
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You just have to flip the magnetic field direction if you use the method above. So the current is downward the force is to the right and the magnetic field is going in the paper, However as it is a flow of electrons your reverse the magnetic field direction.

Another way is, when you have an electron. Use your left hand instead much much better and less painful XD
In my experience, this is bad advice, IMO. Using your right hand to trace out the cross product is very reliable. You just need to understand the vectors that you are forming the cross product from.
 

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