Practice important
Insulator Charging
MCQ questions and answers.
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MCQ Questions
1. A neutral insulating rod is rubbed with a dry cloth. The rod becomes negatively charged, while the cloth becomes positively charged. Which statement best explains this observation?
A.
Electrons are transferred from the rod to the cloth, and protons move into the rod
B.
Electrons are transferred from the cloth to the rod, while protons remain bound in their nuclei
C.
Equal numbers of positive and negative ions are created inside the rod
D.
The rod develops charge only because its electric field attracts positive charge from the surroundings
Correct Answer:
Electrons are transferred from the cloth to the rod, while protons remain bound in their nuclei
Explanation:
Charging an insulator by friction involves transfer of electrons between the two materials. The nuclei and their protons remain essentially fixed, so the material gaining electrons becomes negative and the material losing electrons becomes positive.
2. A positively charged glass rod is brought near, but does not touch, a neutral insulating sphere. After the rod is removed, the sphere is again neutral. During the approach, what occurs within the sphere?
A.
Free electrons migrate completely to the far side of the sphere
B.
Positive charge is transferred from the rod to the near surface
C.
Bound charges undergo a small displacement, producing polarization without net charging
D.
The sphere becomes permanently negatively charged by electrostatic induction
Correct Answer:
Bound charges undergo a small displacement, producing polarization without net charging
Explanation:
In an insulator, charges are not free to move throughout the material. An external electric field slightly displaces bound positive and negative charges, creating polarization. Since no charge is transferred, removing the rod restores the original neutral state.
3. Two identical neutral insulating spheres are rubbed together, and sphere X becomes positively charged. If the system is isolated from the surroundings, what can be concluded about sphere Y?
A.
Sphere Y must become negatively charged with charge equal in magnitude to that of X
B.
Sphere Y must remain neutral because insulators cannot acquire charge
C.
Sphere Y must become positively charged with twice the charge of X
D.
Sphere Y may acquire any charge independently because charge conservation does not apply to insulators
Correct Answer:
Sphere Y must become negatively charged with charge equal in magnitude to that of X
Explanation:
Initially, the total charge of the isolated pair is zero. If sphere X loses electrons and becomes positive, those electrons must have been transferred to sphere Y, giving it an equal-magnitude negative charge.
4. A charged insulating plastic rod is held near small neutral paper pieces, which are attracted to the rod. What is the primary reason for this attraction?
A.
The paper pieces acquire a net charge of the same sign as the rod through permanent electron transfer
B.
The electric field polarizes the molecules of the paper, making the nearer opposite charge slightly closer
C.
The paper pieces become charged by nuclear fission caused by the rod
D.
The rod repels all charges in the paper equally, producing a net force toward the rod
Correct Answer:
The electric field polarizes the molecules of the paper, making the nearer opposite charge slightly closer
Explanation:
The charged rod induces polarization in the initially neutral paper. The opposite bound charge is slightly nearer to the rod than the like bound charge, so the stronger attractive force produces a net attraction without requiring a net charge on the paper.
5. A negatively charged insulating rod is rubbed with wool. The rod gains a charge of -4.8 microcoulombs. Approximately how many electrons were transferred to the rod?
A.
3.0 x 10^13 electrons
B.
7.5 x 10^12 electrons
C.
1.6 x 10^19 electrons
D.
4.8 x 10^6 electrons
Correct Answer:
3.0 x 10^13 electrons
Explanation:
The number of transferred electrons is n = |Q|/e = (4.8 x 10^-6 C)/(1.6 x 10^-19 C) = 3.0 x 10^13 electrons. Since the rod is negatively charged, these electrons were added to it.
6. A positively charged insulating sphere is produced by removing electrons from its surface. Which change would most directly increase its total positive charge without changing the sphere's material?
A.
Reducing the sphere's temperature while keeping its charge distribution unchanged
B.
Removing additional electrons from the sphere
C.
Moving the existing positive charges closer to the center without removing electrons
D.
Bringing a neutral insulator near the sphere without contact
Correct Answer:
Removing additional electrons from the sphere
Explanation:
An insulator becomes positively charged when electrons are removed. Removing more electrons increases the net positive charge. Redistribution or polarization can change charge density or dipole behavior but cannot by itself change the total charge.
7. A charged insulating surface is touched at one point by a small neutral insulating object. Compared with charging by friction, which statement is most appropriate?
A.
Charge transfer is impossible because both objects are insulators
B.
Charge may be transferred locally, but it generally does not spread rapidly over the entire object
C.
All charge instantly moves to the opposite side of the object
D.
The object necessarily acquires a charge opposite to that of the surface
Correct Answer:
Charge may be transferred locally, but it generally does not spread rapidly over the entire object
Explanation:
Insulators can acquire charge through contact when electrons are transferred across the contact region. Because charge carriers are not freely mobile, the acquired charge tends to remain localized rather than distributing quickly over the entire object.
8. A neutral insulating slab is placed in a uniform electric field. Its molecules become slightly polarized. If the field is doubled while the material remains in its linear polarization range, what happens to the induced dipole moment of each molecule?
A.
It becomes half as large
B.
It remains exactly zero because the slab is neutral
C.
It approximately doubles in magnitude and remains aligned with the field
D.
It reverses direction and becomes twice as large
Correct Answer:
It approximately doubles in magnitude and remains aligned with the field
Explanation:
For a linearly polarizable insulator, the induced dipole moment is proportional to the applied electric field. Therefore, doubling the field approximately doubles the molecular dipole moment, with its direction aligned with the field.
9. A neutral insulating object is charged by friction and then cut into two equal pieces. Ignoring charge leakage, what is the most accurate statement about the charges on the pieces?
A.
Each piece necessarily has exactly half the original charge
B.
One piece must be positive and the other negative
C.
The total charge of the two pieces equals the original charge, but the division depends on where the cut passes through the charge distribution
D.
Both pieces become neutral because cutting destroys the original surface charge
Correct Answer:
The total charge of the two pieces equals the original charge, but the division depends on where the cut passes through the charge distribution
Explanation:
Charge is conserved when an insulating object is cut. However, charge on an insulator may be nonuniform and localized, especially after frictional charging, so the two pieces need not receive equal amounts of charge.
10. A charged insulating rod is brought near a neutral insulating sphere and held fixed. The sphere is then placed in a region where the external electric field is zero, without touching the rod. What happens to the sphere's induced polarization?
A.
It becomes stronger because the sphere retains all induced charges permanently
B.
It disappears or greatly decreases as the bound charges return toward their original arrangement
C.
It reverses direction because the sphere's net charge changes sign
D.
It becomes a permanent net charge equal to the rod's charge
Correct Answer:
It disappears or greatly decreases as the bound charges return toward their original arrangement
Explanation:
Polarization of an insulator is caused by displacement of bound charges in an external electric field. When the field is removed without charge transfer, the bound charges tend to return toward their original positions, so the induced polarization decreases.
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