PHYS 112 EXAM 2 — 250 MULTIPLE-CHOICE
QUESTIONS AND CORRECT ANSWERS WITH
DETAILED RATIONALES
ELECTROSTATICS & COULOMB'S LAW
1. Two-point charges, +3.0 μC and -5.0 μC, are separated by a distance of 0.10
m. What is the magnitude of the electrostatic force between them? (k = 9.0 ×
10⁹ N·m²/C²)
A) 1.35 × 10¹ N
B) 1.35 × 10¹ N
C) 1.35 × 10¹ N
D) 1.35 × 10¹ N
Correct Answer: C
Rationale: Using Coulomb's law F = k|q₁q₂|/r² =
(9.0×10⁹)(3.0×10⁻⁶)(5.0×10⁻⁶)/(0.10)² = 13.5 N.
2. Two electrons are 20.0 mm apart at closest approach. What is the
magnitude of the maximum electric force that they exert on each other?
A) 5.8 × 10⁻²⁵ N
,B) 5.8 × 10⁻²⁵ N
C) 5.8 × 10⁻²⁵ N
D) 5.8 × 10⁻²⁵ N
Correct Answer: A
Rationale: F = kq²/r² = (9.0×10⁹)(1.6×10⁻¹⁹)²/(0.020)² = 5.8×10⁻²⁵ N.
3. Two stationary point charges q₁ and q₂ are shown with field lines. What can
you deduce from the sketch?
A) q₁ is positive, q₂ is negative; |q₁| > |q₂|
B) q₁ is negative, q₂ is positive; |q₁| < |q₂|
C) Both charges are positive
D) Both charges are negative
Correct Answer: B
Rationale: Field lines point toward negative charges and away from positive
charges. The density of field lines indicates the magnitude of charge.
4. A charge of +2.0 C is located at the origin and a charge of -3.0 C is located on
the positive x-axis at (3.0 m, 0). What is the net electric force on a third charge
of +1.0 C placed at (1.0 m, 0)?
A) 1.5 × 10¹⁰ N to the right
B) 1.5 × 10¹⁰ N to the left
C) 3.0 × 10¹⁰ N to the right
,D) Zero
Correct Answer: A
Rationale: Calculate forces from each charge and add vectorially.
5. As charge Q exerts a 1.2 N force on another charge q. If the distance
between the charges is doubled, what is the magnitude of the force exerted on
Q by q?
A) 0.30 N
B) 0.60 N
C) 2.4 N
D) 4.8 N
Correct Answer: A
Rationale: Force is inversely proportional to r². Doubling r reduces force by
factor of 4: 1.2/4 = 0.30 N.
6. A hydrogen atom consists of a proton and an electron. If the orbital radius
of the electron increases, the electric potential energy of the electron due to
the proton:
A) Increases
B) Decreases
C) Remains constant
D) Becomes zero
, Correct Answer: A
Rationale: U = kq₁q₂/r. For opposite charges, U is negative; as r increases, U
becomes less negative (increases).
7. Two objects A and B are electrically attracted to each other. Which of the
following is possible? (Select all that apply)
A) Both objects are neutral
B) Both objects are positively charged
C) One object is positively charged and the other is neutral
D) One object is positively charged and the other is negatively charged
Correct Answer: A and D
Rationale: A charged object can attract a neutral object through polarization.
Opposite charges attract.
8. A plastic rod charged to 40 nC touches a neutral metal sphere. Afterward,
the rod's charge is 15 nC. How much charge is on the metal sphere?
A) 55 nC
B) 25 nC
C) -25 nC
D) -55 nC
Correct Answer: B
Rationale: Charge is conserved. The sphere gains 40 - 15 = 25 nC.
QUESTIONS AND CORRECT ANSWERS WITH
DETAILED RATIONALES
ELECTROSTATICS & COULOMB'S LAW
1. Two-point charges, +3.0 μC and -5.0 μC, are separated by a distance of 0.10
m. What is the magnitude of the electrostatic force between them? (k = 9.0 ×
10⁹ N·m²/C²)
A) 1.35 × 10¹ N
B) 1.35 × 10¹ N
C) 1.35 × 10¹ N
D) 1.35 × 10¹ N
Correct Answer: C
Rationale: Using Coulomb's law F = k|q₁q₂|/r² =
(9.0×10⁹)(3.0×10⁻⁶)(5.0×10⁻⁶)/(0.10)² = 13.5 N.
2. Two electrons are 20.0 mm apart at closest approach. What is the
magnitude of the maximum electric force that they exert on each other?
A) 5.8 × 10⁻²⁵ N
,B) 5.8 × 10⁻²⁵ N
C) 5.8 × 10⁻²⁵ N
D) 5.8 × 10⁻²⁵ N
Correct Answer: A
Rationale: F = kq²/r² = (9.0×10⁹)(1.6×10⁻¹⁹)²/(0.020)² = 5.8×10⁻²⁵ N.
3. Two stationary point charges q₁ and q₂ are shown with field lines. What can
you deduce from the sketch?
A) q₁ is positive, q₂ is negative; |q₁| > |q₂|
B) q₁ is negative, q₂ is positive; |q₁| < |q₂|
C) Both charges are positive
D) Both charges are negative
Correct Answer: B
Rationale: Field lines point toward negative charges and away from positive
charges. The density of field lines indicates the magnitude of charge.
4. A charge of +2.0 C is located at the origin and a charge of -3.0 C is located on
the positive x-axis at (3.0 m, 0). What is the net electric force on a third charge
of +1.0 C placed at (1.0 m, 0)?
A) 1.5 × 10¹⁰ N to the right
B) 1.5 × 10¹⁰ N to the left
C) 3.0 × 10¹⁰ N to the right
,D) Zero
Correct Answer: A
Rationale: Calculate forces from each charge and add vectorially.
5. As charge Q exerts a 1.2 N force on another charge q. If the distance
between the charges is doubled, what is the magnitude of the force exerted on
Q by q?
A) 0.30 N
B) 0.60 N
C) 2.4 N
D) 4.8 N
Correct Answer: A
Rationale: Force is inversely proportional to r². Doubling r reduces force by
factor of 4: 1.2/4 = 0.30 N.
6. A hydrogen atom consists of a proton and an electron. If the orbital radius
of the electron increases, the electric potential energy of the electron due to
the proton:
A) Increases
B) Decreases
C) Remains constant
D) Becomes zero
, Correct Answer: A
Rationale: U = kq₁q₂/r. For opposite charges, U is negative; as r increases, U
becomes less negative (increases).
7. Two objects A and B are electrically attracted to each other. Which of the
following is possible? (Select all that apply)
A) Both objects are neutral
B) Both objects are positively charged
C) One object is positively charged and the other is neutral
D) One object is positively charged and the other is negatively charged
Correct Answer: A and D
Rationale: A charged object can attract a neutral object through polarization.
Opposite charges attract.
8. A plastic rod charged to 40 nC touches a neutral metal sphere. Afterward,
the rod's charge is 15 nC. How much charge is on the metal sphere?
A) 55 nC
B) 25 nC
C) -25 nC
D) -55 nC
Correct Answer: B
Rationale: Charge is conserved. The sphere gains 40 - 15 = 25 nC.
