PrepGo

AP Physics C: Electricity and Magnetism Practice Quiz: Conservation of Electric Charge and the Process of Charging

Written by AP Content Team, Verified for 2026 AP Exams, Last updated: June 2026

Test your understanding with short quizzes. This quiz has 10 questions to check your progress.

Question 1 of 10

An isolated system consists of two objects, A and B, both initially electrically neutral. After the objects are rubbed together, object A has a net charge of -2.0 µC. According to the principle of conservation of charge, what is the net charge on object B?

All Questions (10)

An isolated system consists of two objects, A and B, both initially electrically neutral. After the objects are rubbed together, object A has a net charge of -2.0 µC. According to the principle of conservation of charge, what is the net charge on object B?

A) 0 µC

B) +2.0 µC

C) -2.0 µC

D) The charge cannot be determined without knowing the materials.

Correct Answer: B

The principle of conservation of charge states that the total charge of an isolated system remains constant. The initial total charge of the system (A + B) was 0. For the final total charge to also be 0, if object A gained a charge of -2.0 µC, object B must have acquired an equal and opposite charge of +2.0 µC. The net charge change is due to the transfer of electrons from B to A.

A positively charged rod is brought near, but does not touch, an isolated, neutral conducting sphere. Which of the following correctly describes the state of the sphere?

A) The net charge of the sphere becomes negative.

B) The net charge of the sphere remains zero, but electrons move to the side of the sphere closer to the rod.

C) The net charge of the sphere becomes positive as electrons are repelled by the rod.

D) The net charge of the sphere remains zero, and its charge distribution is unaffected.

Correct Answer: B

The presence of the external positive charge causes the charge distribution of the neutral sphere to change. Mobile electrons within the conductor are attracted to the nearby positive rod, creating an accumulation of negative charge on the near side and leaving a deficit of electrons (a net positive charge) on the far side. Since the sphere is isolated and no charge is transferred, its overall net charge remains zero.

A metal plate initially has a net charge of +Q. The plate is then briefly touched by a student, and its net charge becomes 0. This process demonstrates that:

A) The net charge of a system can change due to a transfer of charge with its surroundings.

B) The charge distribution of a system can change in response to another system.

C) The net charge of an isolated system is always conserved.

D) Grounding can only be accomplished by connecting an object to the Earth.

Correct Answer: A

The plate's net charge changed from +Q to 0. This change occurred because charge was transferred between the plate (the system) and the student (the surroundings). The student acts as a large, approximately neutral system, similar to the Earth in grounding, allowing electrons to flow to the plate and neutralize it. This is a direct example of a system's net charge changing due to charge transfer.

An object with a large excess of electrons is connected by a copper wire to the Earth. What is the immediate result of this connection?

A) Protons flow from the Earth to the object to neutralize the charge.

B) The excess electrons on the object are redistributed to be on the side closest to the wire.

C) The excess electrons flow from the object through the wire to the Earth.

D) The net charge of the object remains negative but decreases slightly.

Correct Answer: C

This process is known as grounding. The Earth is a very large, approximately neutral system that can act as a source or sink for electrons. Since the object has an excess of electrons (a net negative charge), these mobile charges will flow from the area of high concentration (the object) to the Earth, effectively neutralizing the object.

A student wants to give a neutral conducting sphere a net positive charge using a negatively charged rod. Which set of steps will accomplish this?

A) Touch the negatively charged rod to the sphere.

B) Bring the rod near the sphere, ground the opposite side of the sphere, remove the rod, and then remove the ground.

C) Bring the rod near the sphere, ground the opposite side of thesphere, remove the ground, and then remove the rod.

D) Ground the sphere, then touch the negatively charged rod to it.

Correct Answer: C

This is the process of charging by induction. 1) Bringing the negative rod near repels the sphere's electrons to the far side. 2) Grounding provides a path for these repelled electrons to leave the sphere and flow to the Earth. 3) The ground connection must be removed *before* the rod is taken away to trap the net charge (which is now positive due to a deficit of electrons). 4) Removing the rod allows the net positive charge to spread evenly over the sphere.

Two isolated metal spheres are charged such that the total charge of the system is +10 nC. If the spheres are moved so that the charge distribution on each sphere changes, but no charge is transferred between the spheres or with the surroundings, what is the total charge of the system?

A) Less than +10 nC

B) Greater than +10 nC

C) +10 nC

D) Zero

Correct Answer: C

The principle of conservation of charge applies to the isolated system of the two spheres. A change in charge distribution (polarization) within the objects does not change the net charge of the system. Since no charge was transferred to or from the system, the total net charge must remain constant at +10 nC.

A system's net charge can only be altered by which of the following mechanisms?

A) Bringing a charged object nearby, causing polarization.

B) Connecting the system to an insulator.

C) The transfer of charge between the system and its surroundings.

D) The creation or destruction of charge within the system.

Correct Answer: C

Based on the provided content, any change to a system's net charge is explicitly due to a transfer of charge between the system and its surroundings. Polarization (A) only redistributes existing charge. Connecting to an insulator (B) prevents charge transfer. The creation/destruction of net charge (D) violates the conservation of charge.

A positively charged metal plate is connected to ground. Which statement best describes the charge flow?

A) Positive charges (protons) flow from the plate to the ground.

B) Positive charges (protons) flow from the ground to the plate.

C) Negative charges (electrons) flow from the plate to the ground.

D) Negative charges (electrons) flow from the ground to the plate.

Correct Answer: D

A net positive charge on a metal indicates a deficit of mobile electrons. The ground acts as a vast reservoir of electrons. When the connection is made, electrons are attracted to the positively charged plate and flow from the ground to the plate until the plate is neutralized. Protons are fixed in the metal's atomic nuclei and do not flow.

Consider a system defined as a single, initially neutral, conducting sphere on an insulating stand. A positively charged rod is brought near the sphere, and the sphere is then grounded. How does the net charge of the defined system (the sphere) change during this process?

A) The system's net charge remains zero because charge is conserved.

B) The system's net charge becomes positive because protons are transferred from the ground.

C) The system's net charge becomes negative because charge is transferred from the surroundings (ground) into the system.

D) The system's net charge first becomes positive, then negative.

Correct Answer: C

The system is just the sphere. The positive rod attracts electrons within the sphere. When grounded, the strong attraction from the nearby positive rod pulls additional electrons from the surroundings (the ground) onto the sphere. This transfer of charge from the surroundings changes the net charge of the system from zero to negative.

The behavior of a system where its total electric charge remains constant over time, even if charges are transferred between different parts of the system, is best described by:

A) The process of grounding.

B) The law of charge redistribution.

C) The principle of conservation of charge.

D) The process of charging by induction.

Correct Answer: C

This is the definition of the conservation of charge. For an isolated system, the net electric charge is a conserved quantity. While charges can move around within the system or be transferred between its components, the total sum of all charges does not change.