SAT Physics Subject Test

Chapter 8 Electric Forces and Fields

Chapter 8 Review Questions

See Chapter 17 for solutions.

  1. If the distance between two positive point charges is tripled, then the strength of the electrostatic repulsion between them will decrease by a factor of

(A)  3

(B)  6

(C)  8

(D)  9

(E) 12

  2. Two 1 kg spheres each carry a charge of magnitude 1 C. How does FE, the strength of the electric force between the spheres, compare with FG, the strength of their gravitational attraction?

(A) FE < FG

(B) FE = FG

(C) FE > FG

(D) If the charges on the spheres are of the same sign, then FE > FG ; but if the charges on the spheres are of opposite sign, then FE < FG.

(E) Cannot be determined without knowing the distance between the spheres

  3. The figure below shows three point charges, all positive. If the net electric force on the center charge is zero, what is the value of  ?

(A) 

(B) 

(C) 

(D) 

(E) 

  4. The figure above shows two point charges, +Q and –Q. If the negative charge were absent, the electric field at point P due to +Q would have strength E. With –Q in place, what is the strength of the total electric field at P, which lies at the midpoint of the line segment joining the charges?

(A) 0

(B) 

(C) 

(D) E

(E) 2E

  5. A sphere of charge +Q is fixed in position. A smaller sphere of charge +q is placed near the larger sphere and released from rest. The small sphere will move away from the large sphere with

(A) decreasing velocity and decreasing acceleration.

(B) decreasing velocity and increasing acceleration.

(C) decreasing velocity and constant acceleration.

(D) increasing velocity and decreasing acceleration.

(E) increasing velocity and increasing acceleration.

  6. An object of charge +q feels an electric force FE when placed at a particular location in an electric field, E. Therefore, if an object of charge –2q were placed at the same location where the first charge was, it would feel an electric force of

(A) 

(B) –2FE

(C) –2qFE

(D) 

(E) 

  7. A charge of –3Q is transferred to a solid metal sphere of radius r. Where will this excess charge reside?

(A) –Q at the center, and –2Q on the outer surface

(B) –2Q at the center, and – Q on the outer surface

(C) –3Q at the center

(D) –3Q on the outer surface

(E) –Q at the center, – Q in a ring of radius r, and –Q on the outer surface

  8. Which of the following statements is true?

(A) Electric field vectors point toward a positive source charge, and the resulting electric force on an electron would point in the same direction as the electric field vector.

(B) Electric field vectors point toward a positive source charge, and the resulting electric force on an electron would point in the opposite direction from the electric field vector.

(C) Electric field vectors point toward a negative source charge, and the resulting electric force on an electron would point in the same direction as the electric field vector.

(D) Electric field vectors point toward a negative source charge, and the resulting electric force on an electron would point in the opposite direction from the electric field vector.

(E) None of the above

  9. The figure above shows two point charges, +Q and +Q. If the right-hand charge were absent, the electric field at Point P due to +Q would have a strength of E. With the right-hand charge in place, what is the strength of the total electric field at P, which lies at the midpoint of the line segment joining the charges?

(A) 0

(B) 

(C) 

(D) 2E

(E) 4E

10. The figure above shows four point charges fixed in position at the corners of a square. What charge would have to be present at the bottom right location for the electric field at the center of the square to be zero?

(A) +Q

(B) +Q

(C) +2Q

(D) +3Q

(E) +4Q

Keywords

electromagnetic force

electric charge

charged

ionization

conserved

elementary charge

quantized

coulombs

electric force

Coulomb’s law

Coulomb’s constant

superposition

gravitational field

electric field

electric field vector

electric dipole

conductors

insulators

semiconductors

superconductor

Summary

·        Electric charge is a quality of protons and electrons that gives them an attractive force. Protons are positive, while electrons are negative. Neutrons have no electrical charge.

·        Use Coulomb’s law  to determine the magnitude electric force between two charged particles with charges of q1 and q2 separated by a distance of r2.

·        Superposition refers to the fact that the total electric force acting on a charge can be determined by summing up the individual contributions to the force of each of the other charges. Electric force is a vector quantity.

·        The presence of a charge creates an electric field in the space that surrounds it. The electric field vectors farther from the source charge are shorter than those that are closer because the strength decreases as we get further away from the charge.

·        The electric field points away from positive charges and toward negative charges.

·        Positive charge feel a force in the direction of the electric field and negative charges feel a force opposite to the electric field.

·        Conductors are materials, such as metals, that permit the flow of charge. Electrons are free to flow through metal and redistribute themselves.

·        Insulators are materials, such as wood, glass, rubber and plastic, which inhibit the flow of electrons. Electrons cannot travel through an insulator, so the charge stays put in the material in which it originated.