The vertical deflecting plates of a typical classroom oscilloscope are a pair of
ID: 1331685 • Letter: T
Question
The vertical deflecting plates of a typical classroom oscilloscope are a pair of parallel square metal plates carrying equal but opposite charges. The potential difference between the plates is 25.0 V Typical dimensions are about 3.9 cm on a side, with a separation of about 5.4 mm . The plates are close enough that we can ignore fringing at the ends.
Part A
Under these conditions. how much charge is on each plate?
Part B
How strong is the electric field between the plates?
Part C
If an electron is ejected at rest from the negative plates, how fast is it moving when it reaches the positive plate?
Explanation / Answer
A) Capacitance of parallel plate cpacitor, C = A*epsilon/d
= 0.039^2*8.854*10^-12/(5.4*10^-3)
= 2.49*10^-12 F
charge on capacitor plates, Q = C*V
= 2.49*10^-12*25
= 6.23*10^-11 C or 62.3 pC
B) E = V/d
= 25/(5.4*10^-3)
= 4629.6 N/c
C) Workdone on the electron = gain in kinetic enrgy
q*V = 0.5*m*v^2 (here v is final speed of electron and V is potentail diffrenece)
v = sqrt(2*q*V/m)
= sqrt(2*1.6*10^-19*25/(9.1*10^-31))
= 2.96*10^6 m/s
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