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# Electrostatics and potential differences

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#1 A charge of -3.00 nC and a charge of -5.80 nC are seperated by a distance of 50.0 cm. Find the position at which a third charge of +7.50 nC can be placed so that the net electrostatic force on it is zero.

#2. A 7.5-nC charge is located 1.8 m from a 4.2-nC charge. Find the magnitude of the electrostatic force that one charge exerts on the other. Is the force attractive or repulsive?

#3 A uniform electric field of magnitude 375 N/C pointing in the positive x-direction acts on an electron, which is initially at rest. After the electron has moved 3.20 cm, what is (a) the work done by the field on the electron, (b) the change in potential energy associated with the electron, and (c) the velocity of the electron?

#4 A potential difference of 90mV exists between the inner and outer surfaces of a cell membrane. The inner surface is negative relative to the outer surface. How much work is required to eject a positive sodium ion (Na+) from the interior of the cell?