# Magnetic Fields; effect on unknown particle?

1. Jul 31, 2007

### exi

1. The problem statement, all variables and given/known data

A charged particle enters a uniform magnetic field and follows this circular path:

Questions:

1: Is it positively or negatively charged, or is it electrically inert?
2: Why?

2. Relevant equations

None.

3. The attempt at a solution

Our class time has mostly focused on formulas and use of such; I'm a little unsure as to what I'm looking at here.

Would much appreciate it if someone could Cliffs Notes what exactly is represented in the above figure.

2. Jul 31, 2007

### Staff: Mentor

Here you go:

http://en.wikipedia.org/wiki/Lorentz_force

The B field is pointing at you in the figure that you posted, and the particle is deflected in a circular manner to the left. The vector equations at the wikipedia page should help you answer the questions.

3. Jul 31, 2007

### exi

Really not sure what I'm not understanding here. The Lorentz article does make mention of the right hand rule, which - if I'm doing it correctly - tells me that the net force on a positively-charged particle would be directed straight downwards. Is there any significance to this?

edit: WP says "a positively charged particle will curve perpendicularly to both the instantaneous velocity vector v and the B field according to the right-hand rule." Wouldn't that make this a positive particle?

Last edited: Jul 31, 2007
4. Jul 31, 2007

### Staff: Mentor

To apply the righthand rule, point your right fingers in the direction of qv (which if the particle is positive in the drawing, would be up toward the top of the screen), then curl your fingers in the direction of B (that's forming the qv X B cross product using the righthand rule), and your thumb now points in the direction of the force. Which way is it pointing? So was the initial assumption about a + charge correct?

5. Jul 31, 2007

### exi

If I do that, my thumb's pointing upward, and the particle's still apparently curving perpendicular to that, so...

6. Jul 31, 2007

### Staff: Mentor

No, no, no. This is hard to do by written word. Let me find a drawing...

7. Jul 31, 2007

### Staff: Mentor

Wikipedia to the rescue again:

http://en.wikipedia.org/wiki/Right_hand_rule

In the problem in your original post (OP), aim your arm, hand and fingers running up the display (pointing at the ceiling in the room where you are). Call that the x direction. Now curl the fingers only, so that they point in the direction of the B field coming out of the page at you. Your arm hand are still pointing up at the ceiling, but now your fingers are curled over pointing at your face. There is only one way that your thumb can be pointing....to the _____ .

8. Jul 31, 2007

### Staff: Mentor

I have to bail to ride to some meetings and then to home. I'll try to check back in a few hours to be sure you got it right.

9. Jul 31, 2007

### exi

Ahh, without keeping your thumb at your index finger's side and just left out naturally? Pointing to the right.

10. Jul 31, 2007

### mpswee2

exi,
another form of the RH rule which you may find easier is to point your thumb in the direction of the charge's velocity vector. The point your fingertips in the direction of the B field. Finally, the force on a positive charge is in the direction your open palm is facing. If the charge is negative, the force points in the opposite direction.

From the drawing you posted, if you point your thumb toward the ceiling, and point your fingertips out of the page, you'll see your palm is opening to the right-- which indicates the F on a positive charge would accelerate the charge in a clockwise circle. Here, the force accelerates the charge to the left, so we know the charge is negative.

There are several RHRs...find one that works for you.

11. Jul 31, 2007

### Staff: Mentor

Correct-amundo. Glad you got it figured out.

12. Aug 1, 2007

### exi

Thanks, berkeman/mps. Makes sense now.