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My textbook(halliday) talks about torque and potential energy of a dipole placed in an electric field. enter image description here

It doesn't say anything about its applications. I'm pretty sure there must exist some practical limitations. Theoretically it seems plausible that we can make the dipole spin continuously by switching the direction of external electric field. I'd like to know why this torque cannot be used to do mechanical work. Thanks!

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  • $\begingroup$ Would the downvoter kindly let me know why so that it helps post a better question next time ? $\endgroup$ – rsadhvika Apr 5 '18 at 15:34
  • $\begingroup$ -1 Not useful as a physics question. Nobody is claiming that this cannot do mechanical work. You are asking for applications, which is an engineering question. $\endgroup$ – sammy gerbil Apr 5 '18 at 16:05
  • $\begingroup$ I see how this is an engineering question and not physics. Ty for responding :) I should delete the question ? @sammygerbil $\endgroup$ – rsadhvika Apr 5 '18 at 16:22
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    $\begingroup$ I think this is definitely a Physics question. The first electric motors were made by physicists when electrical engineers did not exist yet. See History of the Electric Motor: en.wikipedia.org/wiki/Timeline_of_the_electric_motor Furthermore, Physics comprises also the field of Applied Physics en.wikipedia.org/wiki/Applied_physics . $\endgroup$ – freecharly Apr 5 '18 at 16:34
  • $\begingroup$ @freecharly While physics enables good engineering, and engineering could be colloquially described as "applied physics," the question still isn't about the underlying physical concept but about how to power a machine with it. $\endgroup$ – Asher Apr 5 '18 at 17:17
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It can and this kind of thing is used in MEMS micromechanical technology. The magnetic force on a current is a much more practical effect for electromechanical work production. One doesn't have to accumulate nett unbalanced charges on the rotor and the figures turn out to be much more practicable. It is only when the motor is very tiny and the torque needed very small that electrostatic motors become practicable. Also, the electromagnetic motor can use a split ring commutator to switch the field direction; there is no analogue for electric field. These days, however, with all kinds of technology to sense the rotor's angular position available, this last is a minor point.

You should calculate the torque you would get for practicable electric fields and charges and compare these with the Lorentz force on currents from practicable magnetic fields. That exercise more than any answer will show you the reasons for the lack of use of the idea aside from specialist fields, such as MEMS microactuation.

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  • $\begingroup$ Ahh ty :) I guess its not that east to put together lots of charge as they repel and also there is a limit on max electric field due to electric breakdown. I'll look up MEMS. Seems very interesting... $\endgroup$ – rsadhvika Apr 5 '18 at 15:40
  • $\begingroup$ @rsadhvika MEMS in general are extremely interesting IMO. Maybe it's just because I'm a sucker for how "futuristic" they seem compared to other common electronics. $\endgroup$ – JMac Apr 5 '18 at 16:28
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Electrostatic motors have been demonstrated since the 1700s (1st by Andrew Gordon and Benjamin Franklin). However, they usually need high voltages and are much more cumbersome to use than magnetic motors. See Wikipedia Electrostatic motors.

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