Posted by **Mary** on Saturday, September 29, 2007 at 9:32am.

The electrons in the beam of a television tube have a kinetic energy of 2.10 10-15 J. Initially, the electrons move horizontally from west to east. The vertical component of the earth's magnetic field points down, toward the surface of the earth, and has a magnitude of 5.00 10-5 T.

(a) In what direction are the electrons deflected by this field component?

due south (correct)

due west

due east

due north

(b) What is the acceleration of an electron in part (a)?

I have tried everything and all of my answers are wrong. In part a the correct answer is due south. Please help with part (b).

- Physic please help! -
**drwls**, Saturday, September 29, 2007 at 9:57am
Force = q V X B

where V and B are vectors and "x" denotes the vector "cross product". For an electron, q is negative. In (a), V is towards the east (unit vector i) and B is down (unit vector -k). The vector V x B is north (according to he right hand rule) and the minus sign of q makes the force to the south.

(b) The acceleration is F/m, in the same direction as the force F. m is the mass of the electron

- Physic please help! -
**Mary**, Saturday, September 29, 2007 at 12:19pm
I am so confused! Can you please define "cross product". I interpret it to mean that you multiply the value of V and B. Obviously this is incorrect.

Will my answer have a negative value because of the mass of the electron?

This is what I did:

Acceleration = [(q V X B)/ m

(-1.60x10^-19)(6.789932x10^12)(X)(5.00x10^-5)] / 9.11x10^-31)

I got the value of V from the equation for KE

KE = 1/2m V^2

2.10x10^-5 J = 1/2(9.11x10^-31)V^2

(2.10x10^-5)/(4.555x10^-31) = V^2

4.610318x10^25 = V^2

6.789932x10^12 = V

- Physic please help! -
**drwls**, Saturday, September 29, 2007 at 1:25pm
Your answer for the magnitude of the acceleration will not have a negative sign. However, you must specify the direction.

You should be familiar with the term "cross prodct" if you are studying the forces of magnetic fields. It is a vector concept and vectors are usually tught in introductory calculus and analytic geometry. Awhat a cross product A X B means is |A|*|B| sin theta, where |A| amd |B| are the magnitudes of two vectors and theta is the angle between them. The direction of A X B is perpendicular to the plane containing A and B

- Physic please help! -
**drwls**, Saturday, September 29, 2007 at 1:27pm
I suggest you learn more about vector cross products here, if a description is not in your textbook somewhere:

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

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