# Homework Help: Magnetic Field / Force problem

1. Mar 4, 2012

### bdh2991

1. The problem statement, all variables and given/known data
a particle with a charge of -1.24 x 10^-8 c is moving with instantaneous velocity v= 4.19 x 10^4 i + -3.85 x 10^4 j (both in m/s). what is the force exerted on this particle by a magnetic field a) 1.4 T i and b) 1.4 T k ?

2. Relevant equations

F = qv x B (cross product not multiplication)

3. The attempt at a solution

I tried using a determinate for part a where you put i j k and the velocity vector multiplied by q and then the magnetic field vector underneath that but i got -6.68 x 10^-4 i, however the books answers says that it is in the -k direction.

I don't understand the way my physics teacher does the cross multiplication that's why i tried using a determinate like in calculus....please help?!?!

2. Mar 4, 2012

### SammyS

Staff Emeritus
Can you show your work ?

3. Mar 4, 2012

### bdh2991

sure, this is what i was trying to do: for part a)

i , j , k
q(4.19x10^4) , q(-3.85x10^4) , 0
1.4 , 0 , 0

(i added commas in-between to not confuse)

i did the cross product using this determinate thinking it would give me the right answer and i got the right number just the wrong direction....

q(-3.85x10^4)(1.4) in the i direction, the book says -k direction....

4. Mar 4, 2012

### SammyS

Staff Emeritus
Use the code icon, https://www.physicsforums.com/Nexus/editor/code.png [Broken] , to display items with lots of spaces, like your determinant ... or simply use [CODE ] [/CODE ] tags.

Code (Text):

i          ,                j            ,        k

q(4.19x10^4)  ,  q(-3.85x10^4)  ,    0

1.4           ,            0              ,     0
You are calculating your determinant incorrectly.

Show
what you multiply i by,
what you multiply j by,
what you multiply k by;​
including zeros, & prior to simplifying anything.

Last edited by a moderator: May 5, 2017
5. Mar 5, 2012

### bdh2991

I just figured out what i did wrong....i crossed out the columns and rows backwards....i didnt realize until you told me to write everything out lol thanks ....stupid mistake on my part