Question 36

[yfangl097543]

In order for the forces to cancel each other, wouldn't they have to point in opposite directions? The electric field points down, which makes the electron go up. If the magnetic field point down also, the electron would again be propelled up, so the two forces don't cancel.

[admin]

Actually, the electric field force FE does point downward as you mentioned but the magnetic field force FB points upward (see question 35). The magnetic field B points out of the page. Recall (information provided):

FB = ev x B

Because this is a cross product, the magnetic induction vector B must be perpendicular to both FB (which points up) and the velocity of the electron ev (points in the positive x direction) which means that B comes out of the page.

To see a video of the use of the Right Hand Rule:

http://www4.ncsu.edu/~rwchabay/emimovies/moving1.html

When dealing with current through a wire (not the case in this question), use the following Right Hand Rule:

http://www4.ncsu.edu/~rwchabay/emimovies/right-ha.html

[yfangl097543]

It makes sense that th magnetic field has to point upward in order for the electron to be pushed down. However, when I used the right hand rule, I got that the magnetic field points into the page. I pointed my fingers in the direction of the electron's velocity, then I pointed my thumb upward, and my fingers then can only curl into the page. Am I doing something wrong?

[admin]

I think that the problem is the direction that you are pointing your fingers (ie, the direction of the velocity vector). Recall that when using the right hand rule, you must use the movement of positive charge meaning the opposite of what you did which would yield, of course, the opposite result: B points out of the page.

Just in case someone else got confused about the way you are describing your use of the right hand rule, I must underline that the magnetic field is not circular in this instance. If it were the case, the field points into the page and, because it is a circle, eventually it will point out of the page.

The magnetic field is indeed a circle but for a current carrying wire. This is not the case in this problem. I put the links above to show how the right hand rule applies for a moving charge (Q36) vs a current carrying wire.

Here is another good link to show the difference: www.physics.sjsu.edu/becker/physics51/mag_field.htm

[yfangl097543]

I'm sorry, but I'm still confused.

If I treated the electron as a positive charge and pointed my fingers to the left, then indeed the field would be out of the page and the force would be up. But in this case, since you already accounted for the negative charge of the electron by pointing the opposite way for the velocity, the force derived would push the electron upward in the same direction as the electric field, so they wouldn't cancel.

Furthermore, I consulted some texts, which said that if you apply the right hand rule to negative charges by treating them as positive, all you have to do is flip the direction of the force thus derived. Using this method, I still got that a into the page field is needed to propel the electron downward against the electric field.

I'm pretty confused right now.

[wallstng1442]

I think the answer for mag field B is backwards. Question #35 states that if the forces cancel, Fe points UP and Fb points DOWN. No problem there.

Now, for the direction of the two fields (question 36), we see that E will point DOWN (b/c charge will go from + to -). No problem here either.

For mag field B, I keep getting "into the page." The charge q is negative, so I used the left hand rule. The thumb of your left hand points in the direction of V, which is to the right. The palm faces perpendicular to Fb, which is DOWN. Now, your fingers will point in the direction of B, which is into the page. Am I doing something wrong here? I've also checked with other texts and I can't figure out how B is coming out of the page. Thanks!

[susu6520]

I'm getting the same thing! (That to cancel, B should be going into page)

[admin]

It is true, E is down, B points into the page: answer choice D. This has been updated. Sorry for the delay in the response but I missed this question when going through the Forum.

[guessone]

In regards to this question, E is downward which causes the electron to move towards the top plate (FE); shouldn't B be coming out of the page so that FB would be pulling downward on the electron canceling the forces?

[jvuofm1709]

100% yes. The answer is B. Force vectors are always flipped for electrons, along with cross product vectors and Right Hand Rules (become left hand rules).

[jellywing_2058]

I think you are taking it to be a propagating electromagnetic wave, which it is not. It is a situation where we are imposing an electric field and a magnetic field on to a moving charge, both these fields are static unchanging fields.

The result of the influences on the electron should be such that the electron goes undeflected; that is the influence of the Electric field - the electric force, and that of the magnetic field - the magnetic force, cancel each other out.

To do so the correct choice is D, not B.
B says E points downward, so the E force on an electron will be upwards.
B points out of the page - so the B force on the electron will be upwards too. Cross product FB = -e x v x B

Therefore they can't cancel out.
Correct choice therefore is D.

[dltkdgn896512]

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