Teaching Modern Physics - QuarkNet - Fermilab

Particle Quiz 2
Key
Answer each question on a separate sheet of paper showing as much work as possible. When you
are finished, add an extra question at the end of the quiz. If you ask about this at all, you will not
receive credit for this question.
1. A uniform electric field exists in a region between two oppositely charged plates. An electron
is released from rest at the surface of the negatively charged plate and strikes the surface of
the opposite plate 2 cm away, 1.5x10 -8 seconds later.
a. Knowing that the electron will accelerate in the field, choose an equation from the board
that will determine the final velocity of the electron.
vi = 0
vf = ? d = vit + 1/2at 2
d = 0.02m 0.02m = 0 + 1/2a (1.5 x 10 -8 s) 2
a = ? a = 1.8 x 10 14 m/s 2
t = 1.5 x 10 -8 s
b. Calculate the net force on the electron.
F = ma = (9.11x10 -31 kg) (1.8x10 14 m/s 2 ) = 1.6x10 -16 N
c. Determine the weight of an electron.
W = mg = (9.11x10 -31 kg) (9.8 m/s 2 ) = 8.9x10 -30 N
d. Determine the electric force on the electron and then calculate the electric field strength.
F = E-W E = F+W = 1.6x10 -16 N
e. Determine the electric potential difference between the plates.
V = Ed = 1000 N/C (.02m) = 20 V
2. An electron moving westward with a speed of 10000 m/s enters a uniform magnetic field of
strength of 3T directed north.
a. What is the magnetic force felt by the electron?
FB = qvB = -1.6x19 -19 C (10000 m/s) (3T) = -4.8x10 -15 N
37