# Homework Help: What are the directions of the forces due to the electric field and to the mag field

1. Jul 10, 2009

### jlmessick88

1. The problem statement, all variables and given/known data
Assuming the particle in Figure P24.62 is positively charged, what are the directions of the forces due to the electric field and to the magnetic field?
a. The force due to the electric field is directed up (toward the top of the page); the force due to the magnetic field is directed down (toward the bottom of the page).
b. The force due to the electric field is directed down (toward the bottom of the page); the force due to the magnetic field is directed up (toward the top of the page).
c. The force due to the electric field is directed out of the plane of the paper; the force due to the magnetic field is directed into the plane of the paper.
d. The force due to the electric field is directed into the plane of the paper; the force due to the magnetic field is directed out of the plane of the paper.

3. The attempt at a solution
Now if i use the right hand rule where my thumb is the velocity, my index finger is the mag. field, and my middle finger is the force, then the mag. field is pointed up and the force on the proton is pointed out of the plane of the paper...but i'm not sure how i'm supposed to solve for the electric field??
any suggestions??

#### Attached Files:

• ###### Q3.doc
File size:
31.5 KB
Views:
233
2. Jul 10, 2009

### diazona

Re: what are the directions of the forces due to the electric field and to the mag fi

The force that an electric field exerts on a positively charged particle is parallel to the field. (For a negatively charged particle, it's in the opposite direction, antiparallel to the field.)

3. Jul 10, 2009

### jlmessick88

Re: what are the directions of the forces due to the electric field and to the mag fi

so with that in mind, the electric field would be directed out the plane of the paper???

4. Jul 10, 2009

### Maxwellkid

Re: what are the directions of the forces due to the electric field and to the mag fi

That's a right hand rule I haven't seen before, but it seems to work. I usually point my four fingers in direction of the Velocity vector of the particle, curl my fingers in the direction of the Magnetic Field Vector, and my thumb points in the direction of the Mag. Force.

For your question, you should first think about how Electric Fields are defined starting from Coulomb's Law. That ought to take you into the correct direction on figuring out the Electric Force on the positive particle due to the Electric Field.

5. Jul 10, 2009

### diazona

Re: what are the directions of the forces due to the electric field and to the mag fi

It's the way I first learned it (I mean the RHR that jlmessick88 described), and still my favorite variation of the right-hand rule. Partially because it doubles as a frisbee grip but I digress.

6. Jul 10, 2009

### Maxwellkid

Re: what are the directions of the forces due to the electric field and to the mag fi

how can you get your middle finger to point down if you place your palm facing upwards? There are 2 ways you can position your index finger to point out of the xy plane.

I don't think that's a sound version of the right hand rule. try it the other way. Also, if you are using electrons as your particle, then you can switch hands to form the LEFT hand rule!

7. Jul 12, 2009

### jlmessick88

Re: what are the directions of the forces due to the electric field and to the mag fi

i guess i'm still not getting it...everything that i read shows that the electric field and the mag. field are always perpendicular...all of my answers indicate that they are opposite from eachother...how??