17. An opera singer in a convertible sings a note at 600 Hz while cruising down the highway at 90km/hr. What is the frequency heard by:

 

     a) A person standing beside the road in front of the car.

 

     b) A person standing beside the road behind the car.

Two test charges are brought separately into the vicinity of acharge +Q. First, test charge +q is brought to point A a distance rfrom +Q. Next, +q is removed and a test charge +2q is brought topoint B a distance 2r from +Q. Compared with the electrostaticforce on the charge at A, the force on the charge at B is
1
a factor of 4 weaker.
the same.
a factor of 4 stronger.
a factor of 2 stronger.
a factor of 2 weaker.
.

(a) Find the electric field at the center of the triangular configuration of charges in Figure 18-54., given that q a =+ 2.50 nC, q b =-8.00 nC, and q c =+ 1.50 nC.

(b) Is there any combination of charges, other than q a = q b = q c, that will produce a zero strength electric field at the center of the triangular configuration?

(a) Twelve equal charges, q, are situated at the corners of a regular 12-sided polygon (for instance, one on each numeral of a clock face). What is the net force on a test charge Q at the center?

(b) Suppose one of the 12 q’s is removed (the one at “6 o’clock”). What is the force on Q? Explain your reasoning carefully.

(c) Now 13 equal charges, q, are placed at the corners of a regular 13-sided polygon. What is the force on a test charge Q at the center?

(d) If one of the 13 q’s is removed, what is the force on Q? Explain your reasoning.