Author Question: A beam of electrons moves at velocity . The number of particles per unit volume in the beam of area ... (Read 339 times)

saliriagwu · **on:** Jul 28, 2018

A beam of electrons moves at velocity

. The number of particles per unit volume in the beam of area A is . If we imagine a cylindrical Gaussian surface of radius r and length

centered on the beam, the electron flux through the surface is

a.
0.

b.
v_fA.

c.
2v_fA.

d.
v_f(A+2r

).

e.
2v_f(A+r

).

Question 2

Gino says that the analog of Gauss's law for the flow of an incompressible fluid of density at constant velocity

is

for an imaginary surface within the fluid. Lorenzo says that it is true only if the area where the fluid enters the surface and the area where it leaves the surface are both perpendicular to the velocity of the fluid. Which one, if either, is correct?

a.
Gino, because as much fluid leaves as enters.

b.
Lorenzo, because

is not equal to zero if the fluid enters or exits at angles other than 90.

c.
Lorenzo, because this is true only when the fluid executes rotational motion.

d.
Gino, because it is true only when the fluid is enclosed on all sides, not when it is flowing.

e.
Lorenzo, because it is true only when the fluid is enclosed on all sides, not when it is flowing.