# Q1S - x-axis where the total E-Feld vanishes[Examine points...

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Quiz 1 Solutions Check the best answer. 1. At all points on a certain closed surface the E-field points outward (away from the enclosed region). Concerning the sign of the net charge in the enclosed region: It is positive. [Gauss’s law] It is negative. It is zero. One cannot tell from the given information. Check T or F depending on whether the statement is true or false. 2. A charged rod brought near an object with zero total charge can exert no electric force on it. T F Physics 54 Summer 2011 1

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3. A charge + q is placed at the origin, while charge –4 q is to the left of it a distance d as shown. a. Show by argument (not calculation) that the total E- field vanishes for some value x > 0 on the axis. b. Find the value of x for which this happens. c. Show by argument that there is no other point on the finite
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Unformatted text preview: x-axis where the total E-Feld vanishes. [Examine points between the charges and points to the left of the negative charge.] a. ±or x > the Felds of the two charges are in opposite directions. Since the larger one is farther away, there is some point for which the two magnitudes are equal, making the Felds cancel. b. We have, setting the two Feld magnitudes equal: 4 ( x + d ) 2 = 1 x 2 , or 2 x = x + d . This gives x = d . c. ±or points between the charges, the two Felds are in the same direction and cannot cancel. ±or points where x < − d the larger charge is closer than the smaller charge, so the magnitudes cannot be equal. x − 4 q q d Physics 54 Summer 2011 2...
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