Question

A beam of protons is moving in the +x direction with a speed of 28.0 km/s through a region in which the electric field is perpendicular to the magnetic field. The beam is not deflected in this region. If the magnetic field has a magnitude of 0.719 T and points in the +z direction, what is the magnitude of the electric field?

What is the direction of the electric field?
Correct: +y
Incorrect +x
Incorrect -x
Incorrect -y
Incorrect -z
Incorrect +z

Review your notes about the velocity selector.

Would electrons that have the same velocity as the protons be deflected by these fields? If so, in what direction would they be deflected?
Yes, +z
Yes, +x
Yes, +y
Yes, -y
No
Yes, -x
Yes, -z

NEED HELP PLEASE!!

Answer 1

Velocity of proton,V=28km/s=28*10³m/s

Magnetic field,B=0.719T

Charge,q=1.6*10^-19C

So magnetic force on proton is

Fm=q *(Vi x Bk)

Fm=1.6*10^-19*28*10³*0.719*(i x k)

Fm=32.21*10^-16 N in (-j) direction or negative y axis.

For beam to go undeviated there must be a force in positive y direction to compensate the magnetic force.

Thus we have Fmagnetic=F electrostatic

Fe=32.21*10^-16

Fe=qE

1.6*10^-19*E=32.21*10^-16

E=32.21*10^-16/(1.6*10^-19)

E=20.13*10³N/C.

As force on positive charge is in the direction of electric field, thus electric field E must be opposite to that of magnetic force i.e in positive y direction.

B) ;as we have magnetic field in z direction, thus magnetic force on electron is

Fm=(-q) (Vi x Bk)

Fm= - qVB(-j)

Fm = qVB j i.e in positive y direction

And as electric field is in positive y direction, the electric force on electron(negatively charged) will be in opposite direction of electric field i.e in negative y direction. AS the two forces on electron are equal and OPPOSITE, electron will go undeviated.