Right hand rule!!!

[peacefulheart]

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A charged oil drop is allowed to fall through the electric
field created by the plates as shown. In order to give the
oil drop a straight trajectory, a magnetic field should be
established with field lines pointing: ( the electric field pointing from left to right)

A. left to right.
B. right to left.
C. out of the page.
D. into the page.

1. I think the problem has a flaw since it does not mention if the the charged oil drop is positive or negative.

2.thanks a lot

 

[PlumbLine]

Wouldn't it not matter whether it's + or -? They will just move in opposite directions, but they will both move straight.

 

[chem4ever]

A charged oil drop is allowed to fall through the electric
field created by the plates as shown. In order to give the
oil drop a straight trajectory, a magnetic field should be
established with field lines pointing: ( the electric field pointing from left to right)

A. left to right.
B. right to left.
C. out of the page.
D. into the page.

1. I think the problem has a flaw since it does not mention if the the charged oil drop is positive or negative.

2.thanks a lot

The charge on the oil drop will not matter. The answer is C.

Here's why: if the oil drop is positive, the electric field will give it a force to the right because the electric field direction tells us where a positive charge would go. This means that the magnetic field would need to give it a force to the left to counteract the electric force. Since F = qv x B, and the v is going down, if the B is coming out of the page, you would get a force to the left (using right hand rule).

Now let's see what happens if the oil drop is negative: The electric field will give it a force to the LEFT (antiparallel to the electric field lines because it is negative) so we need a magnetic force to the right. However, since it's a negative charge, a magnetic field will give a force that is antiparallel to what you would get with the right hand rule (basically just use your left hand instead). If v is down, then a magnetic field out of the page would give a negatively charged drop a force to the right.

So regardless of charge on the oil drop, it would require a magnetic field out of the page to counteract the electric force.

Hope this helps. Good luck!