Magnetic Field / Force problem

[bdh2991]

Homework Statement

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 ?

Homework Equations

F = qv x B (cross product not multiplication)

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??

 

[SammyS]

Homework Statement

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 ?

Homework Equations

F = qv x B (cross product not multiplication)

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??

Can you show your work ?

 

[bdh2991]

Can you show your work ?

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...

 

[SammyS]

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...

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

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.

 

[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