When Dr. Bennett was younger less old. vacuum tubes were widely used in radios a

Question

When Dr. Bennett was younger less old. vacuum tubes were widely used in radios and other devices. Suppose that in a particular vacuum tube the potential of the anode is 337 V higher than that of the cathode. If an electron leaves the surface of the cathode with zero initial speed, what is its speed when it strikes the anode? (include units with answer) B. -A small sphere with a mass of 2.2 g hangs by a thread centered between two parallel vertical plates 5.0 cm apart as shown above. The plates have uniform surface charge densities of +sigma and -sigma. The charge on the sphere is q = 9.1 time 10^-6 C. What potential difference between the plates will cause the thread to assume an angle of 21degree with the vertical? (include units with answer)

Explanation / Answer

B.

The force needed to keep the sting at angle 21° is given by
F = mg tan 21° = 2.2*10-3* 9.8*0.38 =8.276*10-3 N

Electric field E = F/q = 8.276*10-3 / 9.1*106 =909.46 N/C

V = E*d = 909.46 *5*10-2 = 45.47 V

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