Question

1) a negatively charged rod is brought near a second rod that is neutral and suspended from a non conducting spring. The second rod begins to move toward the negative rod, showing attraction of the two rods. after the first rod is removed, the second rod... a) has no net charge. b) has a positive net charge. c) has a negative net charge. d) is polarized with one positive and one negative end

2) An isolated solid metal sphere that sits on an insulating stand is given a net charge of -10 uC. Which of the following statements best describes the charged sphere? A) the net charge will be evenly throughout the volume (B) The net charge will be distributed evenly over the surface of the sphere (C) The net charge will concentrate on the side of the sphere near the insulating stand D) The net charge will be distributed evenly, with half the charge on the outside of the sphere, and half the charge on the inside.

3) Four equally charged positive particles are held in position in a square arrangement, so that each side of the square has a length R. What is the net force on an electron placed in the center of the square? .a) F= 3kq^2/R2 (B) F=4kq^2/R2 C) F=sqrt2 Kq^2/r^2 (D) 0

4) In a laboratory, an oil droplet carrying a two-electron charge is observed to hover between electrically charged plates so that the electric force upward is equal to the gravitational force downward. Which of the following expressions can be used to determine the mass of the oil droplet?

5) The theoretical distance of an electron (in its ground state) from the proton nucleus of a hydrogen atom is called the Bohr radius, which is approximately 5.29 X 10-11 m. What is the electric force of the proton on the electron at this distance?

Answer 1

SOLUTION

(1)

The second rod is charged by unduction, the electrons of second rod are repelled by the electrons of the rod charged negatively. Therefore, second rod is polarized.

Option (d). Is polarized with one positive and one negative end

(2)

Because the sphere is conductive and isolated, all the charges it receives are distributed evenly over the entire surface of the sphere.

Option (b) The net charge will be distributed evenly over the surface of the sphere

(3)

According to the following figure:

The electron "" is equidistant from all positives charges; aditionally the four charges has the same magnitude.

The force exerted on the electron " " by the charge 1 is   and points at the opposite direction of the force exerted on the electron " " by the charge 4. Similarly, The force exerted on the electron " " by the charge 3 is   and points at the opposite direction of the force exerted on the electron " " by the charge 2. This means that:

and

Therefore, the net force on an electron placed in the center of the square can be obtained adding equations (1) and (2):

OPTION D) 0 N

(4)

The magnitude of the electric force is defined as:

Where, in this problem the charge is :

Replacing (2) into equation (1) we have:

Now, the electric force upward is equal to the gravitational force downward:

Combining equations (3) and (4) we have:

isolating we obtain:

(5)

The magnitude of the electric force is defined as:

Replacing data given values:

Solving we obtain: