Physics right hand rule

[ScarletKnights]

This is from examkrackers

A charged oil drop falls into a magnetic field made by two plates, positively charged plate on the left and negatively on the right. If the particle is to fall in a straight line where will the force of the magnetic field be directed?


the answer is out of the page but i keep getting into the page

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[ilovemcat]

This is from examkrackers

A charged oil drop falls into a magnetic field made by two plates, positively charged plate on the left and negatively on the right. If the particle is to fall in a straight line where will the force of the magnetic field be directed?


the answer is out of the page but i keep getting into the page

Basically, by mentioning that the particle is to fall in a "straight line" they are essentially telling you that the magnitude of Magnetic Force needs to equal the Electric Force but in exactly opposite direction - this is what will allow the charge to fall straight through undeflected between the capacitor plates.

So first, you have to ask yourself - Which direction is the Electric Field Pointing? E-Field always points away from the Positive Plate and towards the Negative Plate. So in this scenario, it's pointing from Left to Right. Now ask yourself, in the absence of a magnetic field - will the charge (which in this case is an electron immersed in oil) be attracted to the positive plate or negative plate. (Hopefully you said positive) Therefore, we can say that the Electric Force points Left towards the positive plate.

Now we need the Magnetic Force to point exactly opposite that. Using the right hand rule, point your thumb in the direction the charged particle is traveling (down the page). Now face your palm in the direction of you want the force to point (we want it to point right so it balances the electric force). Where your fingers point is where the magnetic field points - and if you did this right, they'll be pointing out of the page.

 

[Rabolisk]

Basically, by mentioning that the particle is to fall in a "straight line" they are essentially telling you that the magnitude of Magnetic Force needs to equal the Electric Force but in exactly opposite direction - this is what will allow the charge to fall straight through undeflected between the capacitor plates.

So first, you have to ask yourself - Which direction is the Electric Field Pointing? E-Field always points away from the Positive Plate and towards the Negative Plate. So in this scenario, it's pointing from Left to Right. Now ask yourself, in the absence of a magnetic field - will the charge (which in this case is an electron immersed in oil) be attracted to the positive plate or negative plate. (Hopefully you said positive) Therefore, we can say that the Electric Force points Left towards the positive plate.

Now we need the Magnetic Force to point exactly opposite that. Using the right hand rule, point your thumb in the direction the charged particle is traveling (down the page). Now face your palm in the direction of you want the force to point (we want it to point right so it balances the electric force). Where your fingers point is where the magnetic field points - and if you did this right, they'll be pointing out of the page.

Maybe this is just my Mardi Gras alcohol talking, but I don't believe you are entirely correct about this. First you are absolutely correct that the magnetic force needs to balance the electric force. But I don't think you are right to assume that the oil drop is negatively charged. Imagine if it was, then the e-force points to the left. In order for the magnetic force to point to the right, then qv points towards TOP. So the field has to point out of the page. If the drop is positively charged, then the magnetic force needs to point to the left. Since qv points towards bottom, the field has to point out of the page. This implies that the drop can be either + or -.

 

[NYR56]

The particle is assumed to be positively charged, so the E field pushes it to the right, thus we need the M field to point to the left. Point your thumb downwards, with your palm facing the left, and your fingers will face out of the page, hence the M field is out of the page.

 

[ilovemcat]

Maybe this is just my Mardi Gras alcohol talking, but I don't believe you are entirely correct about this. First you are absolutely correct that the magnetic force needs to balance the electric force. But I don't think you are right to assume that the oil drop is negatively charged. Imagine if it was, then the e-force points to the left. In order for the magnetic force to point to the right, then qv points towards TOP. So the field has to point out of the page. If the drop is positively charged, then the magnetic force needs to point to the left. Since qv points towards bottom, the field has to point out of the page. This implies that the drop can be either + or -.

Yikes, you're right Rabolisk! For some reason, I was thinking of the Millikan Oil Drop experiment, but either way it was an assumption I should of avoided.

To OP: I also forgot to mention if you're using the "RIGHT" hand rule for an electron (which by convention is intended for positive charges only), you should remember that magnetic force points in the opposite direction. So using the right hand rule, you'd find that the Magnetic Force would point Left but because this is a negative charge, we reverse the direction and find that Magnetic Force points to the right. The direction of the moving charge and Magnetic Field still points in the same direction however.

If the traveling particle were positively charged, electric field would still point Left to Right. But, this time the Electric Force would point in the same direction as the E-field (Right). Magnetic Force would need to point Left. Using the right hand rule, you'd find that still Magnetic Field would point out of the page.

So either way really, you get the same answer. In both scenarios, Magnetic Field points out of the page.