20. Two charged particles are fixed to the x-axis: Particle 1 of charge Y1 = 2.1

Question

20. Two charged particles are fixed to the x-axis: Particle 1 of charge Y1 = 2.1 × 10-8 C is at position (coordinate) = 20 cm and particle 2 ofcharge q2--4.00q1 is at position 70 cm. At what coordinate on the x-axis (other than infinity) is the net electric field produced by the two particles equal to zero? .4XO-A)-30 cm B)-20 cnm O -10 cm D) 0 cm E) +10 cm 20 5e 2 21. If the area of the plates of a parallel plate capacitor is halved and the separation between the plates tripled, while the charge on the capacitor remains constant, then by what factor does the energy stored in the capacitor change? A) 1/6 times the original energy stored in the capacitor B) 2/3 times the original energy stored in the capacitor 3/2 times the original energy stored in the capacitor D) 2 times the original energy stored in the capacitor E) 6 times the original energy stored in the capacitor 22. The voltage between the cathode and the screen of an old television set is 22 kV. If we assume a speed of zero for an electron as it leaves the cathode, what is its speed just before it hits the screen? A) 2.8 x 106 m/s 896.2 x 106 m/s C) 5.3 × 107 m/s D) 7.1 × 107 m/s E) 8.8 x 10 m/s

Explanation / Answer

that point will left of q1.

say at distance r left to q1.

Enet = k q1 / r^2 + k q2 / (r + r')^2 = 0

k (q1) / (r^2) - k (4 q1) / (r + 50)^2 = 0

(r + 50 ) / r = sqrt(4) = 2

r = 50 cm

x = 20 - 50 = - 30 cm ........Ans(A)

21. C = e0 A / d

C' = e0 (A/2) / 3d = (e0 A /d) / 6 = C / 6

U = Q^2 / 2 C

U' = Q^2 / (2 C / 6) = 6 ( Q^2 / 2 C) = 6 U

Ans(E)

22. KE = q deltaV  

(9.109 x 10^-31) v^2 / 2 = (1.6 x 10^-19)(22 x 10^3)

v = 8.8 x 10^7 m/s

Ans(E)

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