SAT Physics Subject Test

Chapter 9 Electric Potential and Capacitance

Chapter 9 Review Questions

See Chapter 17 for solutions.

  1. True statements about the relationship between the electric field and electric potential include which of the following?

    I. If the electric field at a certain point is zero, then the electric potential at the same point is also zero.

   II. If the electric potential at a certain point is zero, then the electric field at the same point is also zero.

  III. The electric potential is inversely proportional to the strength of the electric field.

(A) I only

(B) II only

(C) I and II only

(D) I and III only

(E) None of the above

Questions 2-3

(A) 0

(B) 

(C) 

(D) 

(E) 

  2. Which expresses the value of the electric field at the center of the square?

  3. Which expresses the value of the electric potential at the center of the square?

  4. Negative charges are accelerated by electric fields toward points

(A) at lower electric potential

(B) at higher electric potential

(C) where the electric field is zero

(D) where the electric field is weaker

(E) where the electric field is stronger

  5. Which of the following would be half as great at a distance of 2R from a source charge than it would be at a distance of R from the charge?

    I. Electric force on another charge

   II. Electric field due to the source charge

  III. Electric potential due to the source charge

(A) I only

(B) II only

(C) III only

(D) I and II only

(E) I, II, and III

  6. Which points in the uniform electric field (between the plates of the capacitor) shown above lie on the same equipotential?

(A) 1 and 2 only

(B) 1 and 3 only

(C) 2 and 4 only

(D) 3 and 4 only

(E) 1, 2, 3, and 4 all lie on the same equipotential, since the electric field is uniform.

  7. The potential at point A in an electric field is 10V higher than at point B. If a negative charge, q = –2 C, is moved from point A to point B, then the potential energy of this charge will

(A) decrease by 20 J

(B) decrease by 5 J

(C) increase by 5 J

(D) increase by 20 J

(E) increase by 100 J

  8. A parallel-plate capacitor is charged to a potential difference of ∆V; this results in a charge of +Q on one plate and a charge of –Q on the other. The capacitor is disconnected from the charging source, and a dielectric is then inserted. What happens to the potential difference and the stored electrical potential energy?

(A) The potential difference decreases, and the stored electrical potential energy decreases.

(B) The potential difference decreases, and the stored electrical potential energy increases.

(C) The potential difference increases, and the stored electrical potential energy decreases.

(D) The potential difference increases, and the stored electrical potential energy increases.

(E) The potential difference decreases, and the stored electrical potential energy remains unchanged.

  9. How much work would the electric field (created by the stationary charge Q) perform as a charge q is moved from point B to A along the curved path shown?

(VA = 200 V, VB = 100 V, q = –0.05 C, length of line segment AB = 10 cm, length of curved path = 20 cm.)

(A)   0 J

(B)   5 J

(C) 10 J

(D) 15 J

(E) 20 J

Keyword

electrical potential energy

electric potential

volt

equipotential surfaces

capacitance

parallel-plate capacitors

farad

parallel

equivalent

series

dielectric

dielectric constant

Summary

·        The change in the electrical potential energy of a charge is defined by ∆UE = –WE. Essentially this is the same equation that defines a change in the gravitational potential energy.

·        Electric potential is electric potential energy per unit charge. The units of electric potential are joules per coulomb. The change in electric potential, ∆V, is defined as ∆V = .

·        Capacitance is a measure of the capacity for holding charge. The greater the capacitance, the more charge can be stored on the plates at a given potential difference.

·        Combinations of capacitors are often seen in electric circuits. A collection of capacitors is parallel if the capacitors all share the same potential difference. A collection of capacitors is in a series if they all share the same charge magnitude.

·        Dielectrics are insulators that are placed between the plates of a capacitor so that the capacitor can maintain charge separation and store potential energy. A dielectric always increases the capacitance of a capacitor.