We were assigned a project in my Physics 2 class that asked us to build a homema

Question

We were assigned a project in my Physics 2 class that asked us to build a homemade electro static motor and then calculate a theoretical RPM based on the given quantities of the power source. The power source is a Van De Graaff Generator and we are given that it has 400kV for voltage and 6A of current. The motor we built was made from two tin soda cans that had a metal paperclip attached to them to direct the charge towards a plastic bottle cap that we stripped with metallic tape and balanced on a wooden skewer at its center point. When tested the motor and found that the cap rotated at around 300RPM. What method should I use to come up with a theoretical RPM that is somewhat close to the measured RPM? My current efforts up to now ended with at a low estimate of 92 RPM and the work being posted below:

Explanation / Answer

given power source Voltage = V = 400 kV
current i = 600 micro A = 600*10^-6 A

now,power delivered by the voltage source P = Vi = 400*1000*600*10^-6 = 240 W
also Power used by the rotating cap = Tw ( where T is torque on the cap and w is its angular speed)

now torque = F*d/2 ( where d/2 is the radius of the cap)
F = kq^2/r^2
and V = kq/r
hence
F = V^2/k = (400,000)^2/8.98*10^9 = 17.8173 N
hence
T = F*d/2
d = 0.028 m
so T = 17.8173*0.028/2 = 0.24944 Nm
hence w*0.24944 = 240
w = 962.155 rad/s
w = 153.1317 rpm

this is the rpm generated when we use just on e clip
if we use two clips, we are giving twice the momentum to the cap to rotate hence
rpm = 2*w = 306.263 rpm

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