A small 12.3 g plastic ball is tied to a very light 28.6 cm string that is attached to the vertical wall of a room (see Figure). The wall is charged, creating a horizontal electric field. When the ball has been given an excess charge of -1.11 mC, you observed that it remains suspended, with the string making an angle of 17.4° with the wall.
(a) Identify the sign of the charge on the wall.
(b) Find the magnitude and direction of the electric field in the room.