Particle travelling in magnetic field unit vectors

[bmxicle]

Homework Statement

a particle with charge -2.00 C initially moves at v = 1.00 i^hat + 7.00 j^hat. If it encounters a magnetic field B = 60.00 k^hat T, find the force on the particle

Homework Equations

F = Q (v x B)

The Attempt at a Solution

I set up a matrix to find the cross product.

v x B = det i j k
1 7 0
0 0 60

If you expand along row three you get (-1)^(3+3) * (60) det i j
1 7

= 60(7i-1j)

===> F=(-2.0 C)(60)(7i - 1j) = -840i + 120 j

The answer i was given has the signs reversed, and i can't figure out why. I've tried it drawing diagrams too and i still got the same answer.

 

[Apphysicist]

Homework Statement

a particle with charge -2.00 C initially moves at v = 1.00 i^hat + 7.00 j^hat. If it encounters a magnetic field B = 60.00 k^hat T, find the force on the particle

Homework Equations

F = Q (v x B)

The Attempt at a Solution

I set up a matrix to find the cross product.

v x B = det i j k
1 7 0
0 0 60

If you expand along row three you get (-1)^(3+3) * (60) det i j
1 7

= 60(7i-1j)

===> F=(-2.0 C)(60)(7i - 1j) = -840i + 120 j

The answer i was given has the signs reversed, and i can't figure out why. I've tried it drawing diagrams too and i still got the same answer.

I think you're right. Whatever answer you were given probably did not take into account the negative charge.

Using the right hand rule (and then flipping your hand 180-deg because a negative particle) confirms your answer qualitatively.

 

[bmxicle]

thanks, i was wondering if that was the case.