Conservation of Electric Charge

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AP Physics C: Electricity and Magnetism › Conservation of Electric Charge

Questions 1 - 10
1

A $+1.0,\mu\text{C}$ charge is placed at center of an insulating spherical shell; considering Gauss’s Law, what is flux through the shell?

$\Phi_E$ increases with shell thickness because more material encloses charge

$\Phi_E=0$, because insulators block electric fields

$\Phi_E=\dfrac{Q}{\varepsilon_0}=1.13\times10^5,\text{N·m}^2/\text{C}$

$\Phi_E=\dfrac{kQ}{r^2}$, because flux equals field magnitude

Explanation

This question tests understanding of the conservation of electric charge and the application of Gauss's Law to insulating materials in AP Physics C. Conservation of charge ensures the charge remains fixed at the center, while Gauss's Law determines the flux through any enclosing surface. In this scenario, the insulating shell doesn't affect the flux calculation since Gauss's Law depends only on enclosed charge, not on the material of the Gaussian surface. Choice B is correct because it correctly applies Gauss's Law: Φ_E = Q_enclosed/ε₀ = (1.0×10⁻⁶)/(8.85×10⁻¹²) = 1.13×10⁵ N·m²/C, independent of whether the shell is conducting or insulating. Choice A is incorrect because it wrongly assumes insulators block electric fields or affect flux calculations, which is a common error when students confuse material properties with field propagation. To help students: Emphasize that Gauss's Law applies universally - the flux depends only on enclosed charge, not on intervening materials. Practice with both conducting and insulating shells to reinforce that material type doesn't affect flux through surfaces.

2

A conducting spherical shell has net charge $+5.0,\mu\text{C}$; considering Gauss’s Law, what is $E$ inside the metal?

$E=\dfrac{kQ}{r^2}$, because enclosed charge is nonzero

$E=0,\text{N/C}$, because charges reside on the surface at equilibrium

$E$ depends on shell thickness, not on charge

$E$ is constant and nonzero throughout the metal

Explanation

This question tests understanding of the conservation of electric charge and the application of Gauss's Law inside conductors in AP Physics C. Conservation of charge combined with electrostatic equilibrium requires that all excess charge on a conductor resides on its outer surface, leaving the interior field-free. In this scenario, the conducting shell's charge distributes entirely on its surface, creating zero field in the metal itself regardless of the charge amount. Choice B is correct because it correctly applies the principle that E = 0 inside any conductor at equilibrium, as free charges would move if any field existed, contradicting the equilibrium condition. Choice A is incorrect because it wrongly assumes Gauss's Law gives a non-zero field inside the conductor, which is a common error when students forget that charges reside only on surfaces. To help students: Emphasize that conductors in electrostatic equilibrium have E = 0 everywhere inside the material itself. Use Gauss's Law with surfaces inside the conductor to show that since no charge is enclosed, the field must be zero.

3

A neutral conductor is placed in a uniform external electric field; which statement best describes its charge distribution at equilibrium?

Charges remain fixed because conductors behave like insulators in fields

Positive and negative charges separate on opposite surfaces; net charge remains zero

Excess charge accumulates uniformly throughout the conductor’s volume

New charge is created on the conductor to cancel the external field

Explanation

This question tests understanding of the conservation of electric charge and charge redistribution in conductors placed in external fields in AP Physics C. Conservation of charge means that the total electric charge in an isolated neutral conductor remains zero, but charges can redistribute internally. In this scenario, the external field causes free electrons to move opposite to the field direction until equilibrium is reached, demonstrating how charges rearrange while maintaining zero net charge. Choice B is correct because it correctly describes that positive and negative charges separate to opposite surfaces (polarization) while the conductor remains neutral overall, creating an internal field that cancels the external field. Choice A is incorrect because it wrongly assumes charge distributes uniformly throughout the volume, which is a common error when students forget that charges in conductors move to surfaces. To help students: Emphasize that conductors in electrostatic equilibrium have zero internal field, achieved by surface charge redistribution. Use diagrams showing induced surface charges creating fields that exactly cancel external fields inside the conductor.

4

A metal sphere has net charge $+2.0,\mu\text{C}$; at $r=0.30,\text{m}$, what is $E$ outside, by Gauss’s Law?

$E=\dfrac{Q}{\varepsilon_0 r}=7.5\times10^5,\text{N/C}$

$E=\dfrac{kQ}{r}=6.0\times10^4,\text{N/C}$

$E=0,\text{N/C}$, because charge is on the surface

$E=\dfrac{kQ}{r^2}=2.0\times10^5,\text{N/C}$

Explanation

This question tests understanding of the conservation of electric charge and applying Gauss's Law to find electric fields outside charged conductors in AP Physics C. Conservation of charge ensures all excess charge resides on the conductor's surface, creating an electric field outside identical to a point charge. In this scenario, the metal sphere's charge distribution creates a radially symmetric field that can be analyzed using a spherical Gaussian surface. Choice B is correct because it correctly applies the principle that outside a spherical conductor, E = kQ/r² = (9×10⁹)(2×10⁻⁶)/(0.30)² = 2.0×10⁵ N/C, treating all charge as if concentrated at the center. Choice C is incorrect because it uses an incorrect formula (missing 4π factor), which is a common error when students mix up different forms of field equations. To help students: Emphasize that conductors with spherical symmetry produce fields identical to point charges when viewed from outside. Practice using both Coulomb's law and Gauss's Law approaches to verify they give the same result for spherical charge distributions.

5

A $-2.0,\mu\text{C}$ metal sphere touches an identical $+8.0,\mu\text{C}$ sphere; after separation, what is each sphere’s charge?

$+5.0,\mu\text{C}$ each, because only positive charge moves

$+3.0,\mu\text{C}$ each, by charge conservation and symmetry

$+6.0,\mu\text{C}$ each, because contact creates extra charge

$+8.0,\mu\text{C}$ and $-2.0,\mu\text{C}$, because charges stay with originals

Explanation

This question tests understanding of the conservation of electric charge when identical conductors make contact in AP Physics C. Conservation of charge means that the total electric charge before contact equals the total after, which is fundamental when analyzing charge redistribution. In this scenario, two identical spheres with different charges touch, allowing charge to flow until they reach the same potential, then separate with equal charges. Choice A is correct because it correctly applies charge conservation: total initial charge is -2.0 + 8.0 = +6.0 μC, which divides equally between identical spheres giving +3.0 μC each, ensuring both conservation and symmetry. Choice B is incorrect because it wrongly assumes charges remain with their original spheres, which is a common error when students don't recognize that contact allows complete charge redistribution. To help students: Emphasize that identical conductors must have equal charge after separation due to symmetry. Practice with various initial charge combinations to reinforce that total charge is conserved while individual charges change.

6

A neutral metal sphere is inside a spherical Gaussian surface; a charged rod outside approaches. Based on Gauss’s Law, how does flux change?

Flux increases because the external rod increases the field everywhere

Flux becomes negative because field lines enter the surface

Flux depends on the Gaussian surface radius, so it must change

Flux stays $0$ because the enclosed net charge remains $0$

Explanation

This question tests understanding of the conservation of electric charge and Gauss's Law for systems with external influences in AP Physics C. Conservation of charge means the neutral sphere remains neutral (zero net charge) regardless of external fields, which is key to applying Gauss's Law correctly. In this scenario, while the external rod induces charge separation on the sphere's surface, the net charge enclosed by the Gaussian surface remains zero. Choice B is correct because it correctly applies Gauss's Law: flux depends only on enclosed charge (Φ = Q_enc/ε₀ = 0), not on external charges or induced polarization, so flux remains zero. Choice A is incorrect because it wrongly assumes external charges affect flux through a surface not enclosing them, which is a common error when students confuse field strength with flux. To help students: Emphasize that Gauss's Law considers only charges inside the Gaussian surface - external charges create fields but don't contribute to flux. Practice with various charge configurations to reinforce the distinction between local field changes and net flux.

7

Considering a charged conducting sphere, based on Gauss’s Law, what is the electric flux through any enclosing spherical surface?

$\Phi_E = Q/\varepsilon_0$, independent of radius

$\Phi_E = kQ/r^2$, because flux equals field

$\Phi_E = 0$, because conductors block electric fields

$\Phi_E$ increases with surface area $4\pi r^2$

Explanation

This question tests understanding of the conservation of electric charge and the application of Gauss's Law to conducting spheres in AP Physics C. Conservation of charge combined with Gauss's Law states that electric flux through any closed surface depends only on the enclosed charge, not on the surface size or shape. In this scenario, a charged conducting sphere has all its charge on the surface, but any Gaussian surface enclosing it will have the same flux. Choice A is correct because it correctly applies Gauss's Law: Φ_E = Q_enclosed/ε₀, which is independent of the Gaussian surface radius as long as it encloses all the charge. Choice D is incorrect because it wrongly assumes flux increases with surface area, which is a common error when students confuse flux (scalar) with field strength (which does decrease with distance). To help students: Emphasize that Gauss's Law relates flux to enclosed charge only - the size of the Gaussian surface doesn't matter. Practice calculating flux for various surface sizes to reinforce that while E-field decreases with r², the product E·A remains constant.

8

Based on induction, a $-4.0,\mu\text{C}$ rod nears a grounded metal sphere; after removing ground, what is sphere’s net charge?

$0,\text{C}$, because polarization cannot leave net charge

$+4.0,\mu\text{C}$, because electrons flowed to ground

$-4.0,\mu\text{C}$, because contact transfers charge

$-8.0,\mu\text{C}$, because induction doubles charge

Explanation

This question tests understanding of the conservation of electric charge and electrostatic induction in grounded conductors in AP Physics C. Conservation of charge means that the total electric charge in an isolated system remains constant, but grounding allows charge to flow to/from Earth. In this scenario, the negative rod induces positive charges on the near side of the sphere by repelling electrons through the ground connection, demonstrating how charges rearrange to maintain equilibrium. Choice C is correct because it correctly applies the principle that electrons flow to ground when repelled by the negative rod, leaving the sphere with a net positive charge of +4.0 μC after the ground is removed. Choice B is incorrect because it wrongly assumes polarization alone cannot leave net charge, which is a common error when students overlook the role of grounding in allowing charge transfer. To help students: Emphasize the sequence of events in grounding - first induction occurs, then charge flows through ground, finally removing ground traps the remaining charge. Use diagrams showing electron flow direction to reinforce that grounding provides a path for charge movement.

9

A charged insulating rod is brought near a metal sphere on an insulating stand; based on induction, how does the sphere’s net charge change?

It becomes oppositely charged because electrons jump across the air gap

It gains the same sign of charge as the rod without any grounding

Its net charge increases because polarization adds charge to the system

It stays neutral overall, but charges redistribute on its surface

Explanation

This question tests understanding of the conservation of electric charge and electrostatic induction without grounding in AP Physics C. Conservation of charge means that in an isolated system (sphere on insulating stand), the total charge cannot change without physical contact or grounding. In this scenario, the charged rod induces charge separation on the metal sphere through electrostatic forces, but no charge transfer occurs across the air gap. Choice B is correct because it correctly applies the principle that induction only redistributes existing charges - the sphere remains neutral overall but develops opposite charges on different regions of its surface. Choice A is incorrect because it wrongly assumes electrons can jump across air gaps under normal conditions, which is a common error when students confuse induction with conduction. To help students: Emphasize the distinction between induction (charge redistribution) and conduction (charge transfer through contact). Practice identifying when systems are isolated (no grounding) versus when charge can flow, reinforcing that air is an excellent insulator.

10

A $+6.0,\mu\text{C}$ rod touches a neutral metal sphere on an insulating stand; after separation, what is the sphere’s charge?

$+12,\mu\text{C}$, because contact creates additional charge

$+0,\text{C}$, because charge returns to the rod

$+6.0,\mu\text{C}$, because all charge transfers to the sphere

$+3.0,\mu\text{C}$, assuming equal sharing by identical conductors

Explanation

This question tests understanding of the conservation of electric charge and charge distribution when conductors make contact in AP Physics C. Conservation of charge means that the total electric charge in an isolated system remains constant, which is foundational in analyzing electric phenomena. In this scenario, when a charged rod touches a neutral conductor, charge redistributes between them until they reach the same electric potential. Choice B is correct because it correctly applies the principle that identical conductors will share charge equally, resulting in each having half the original charge (+3.0 μC each), ensuring charge is conserved and correctly distributed. Choice C is incorrect because it wrongly assumes all charge transfers to the sphere, which is a common error when students overlook that both objects are conductors that must reach equilibrium. To help students: Emphasize that identical conductors share charge equally when in contact, and use the principle that total charge before equals total charge after. Practice with scenarios involving different conductor sizes to reinforce that charge sharing depends on capacitance ratios.

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