- #1
Matt Q.
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Homework Statement
An electron enters a region between two capacitor plates with equal and opposite charges. The plates are L=0.1 m by d=0.05 m, and the gap between the plates is h=0.002 m. During a short time interval of dt=0.002 s, while between the plates and far from the edges, the change of momentum is dp=<0,-9x10-17,0> kg m/s. Ignore gravitational effects here since they are so weak compared to electric effects.
(a) What is the electric field vector between the plates (far from the edges)? (hint: momentum principle)
(b) What is the charge (magnitude and sign) on the upper plate?
(c) What is the potential difference dV = Vtop - Vbottom ?
I figured out how to solve the problem, however, I have some questions.
1) Do I use 1.6x10-19 or do I use -1.6x10-19 ? I know the negative makes more sense since it's an electron but my friend were arguing that signs does not matter.
2) How do I tell if the charge I got is for the upper plate or lower plate? I used the equation: E = Q / A epsilon0 to find the charge Q.
3) I'm not entirely sure that my work is correct. I might have done something wrong somewhere.
Homework Equations
a)
dp = F dt => F = dp / dt
F = qE => E = F / q
b)
E = Q / A epsilon0 => Q = E A epsilon0
c)
dV = -E dl
The Attempt at a Solution
a)
F = -9x10-17 / 0.002 = -4.5x10-14
E = 4.5x10-14 / -1.6x10-19 = 2.8x105 V/m (not sure if this should be plus or minus because the sign of charge of electron)
b)
Q = 2.8x105x0.1x0.05xepsilon0 = 1.24x10-8 ( not sure if this charge belongs to upper plate or lower plate )
c)
dV = -2.8x105 x 0.002 = -562.5 Volt