17.71.Set Up:The ball is in equilibrium, so for it and The force diagram for the ball isgiven in Figure 17.71. is the force exerted by the electric field. Since the electric field is horizontal,ishorizontal. Use the coordinates shown in the figure. The tension in the string has been replaced by its xand ycomponents.Figure 17.71Solve:gives and gives so qis negative and is to the right, so is to the left in the figure.Reflect:The larger the electric field Ethe greater the angle the string makes with the wall.17.72.Set Up:is toward a negative charge and away from a positive charge. At the origin,due tothe charge is in the direction, toward the charge.Solve: (a)so is away from Qso Qis positive. gives (b)so is toward Qso Qis negative. 17.73.Set Up:The gravity force (weight) has magnitude and is downward. The electric force is Solve: (a)To balance the weight the electric force must be upward. The electric field is downward, so for an upwardforce the charge qof the person must be negative. gives and(b)
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