Conservation of Electric Charge
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AP Physics C: Electricity and Magnetism › Conservation of Electric Charge
A battery drives a steady current $I=2.0,\text{A}$ through a single-loop circuit with one resistor. Charge is not created or destroyed; it redistributes in the conducting wires while the surrounding plastic insulation prevents charge leakage. Based on the described process, how does the current in the wire change as it passes through the resistor?
It stays $2.0,\text{A}$ everywhere in the loop
It becomes $0,\text{A}$ inside the resistor due to insulation
It increases because the resistor creates charge carriers
It decreases because charge is used up in the resistor
Explanation
This question tests understanding of the conservation of electric charge and current continuity in circuits 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, charge flows continuously through the circuit without accumulation or depletion anywhere, demonstrating that current must be the same at all points in a single loop. Choice C is correct because charge conservation requires that the same amount of charge entering the resistor must exit it, maintaining I = 2.0 A throughout the entire loop. Choice A is incorrect because it wrongly assumes charge is consumed in the resistor, but resistors only convert electrical energy to heat - they don't destroy charge carriers. To help students: Emphasize that current is the flow rate of charge, and in steady state, charge cannot accumulate anywhere in the circuit. Use the analogy of water flow in pipes where the flow rate must be constant in a single pipe regardless of obstacles.
A $+3.0,\text{nC}$ metal sphere touches a neutral identical metal sphere on insulating stands, then they separate. Charge redistributes on conductors until equilibrium, conserving total charge. Based on the described process, what is the resulting charge on each sphere after contact?
$-1.5,\text{nC}$ each
$+3.0,\text{nC}$ each
$0,\text{C}$ each
$+1.5,\text{nC}$ each
Explanation
This question tests understanding of the conservation of electric charge and charge distribution in conductors 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 the charged sphere touches the neutral sphere, charge flows between them until they reach the same electric potential, demonstrating how charges redistribute to maintain equilibrium. Choice A is correct because the total charge (+3.0 nC) is conserved and equally distributed between two identical spheres, giving each sphere +1.5 nC. Choice B is incorrect because it wrongly assumes each sphere gets the full original charge, violating conservation by creating charge from nothing. To help students: Emphasize that identical conductors in contact share charge equally, and the total charge before equals the total charge after. Use the analogy of water levels equalizing between connected containers to reinforce the concept of charge equilibrium.
A neutral metal sphere on an insulating stand is brought near a negatively charged rod without touching. Electrons in the conductor shift, but none can leave through the insulating stand, so total charge stays zero. Considering the explained phenomenon, which best explains the charge distribution on the sphere while the rod is nearby?
Charge stays uniformly zero everywhere because metals cannot polarize
Positive near the rod, negative far away; net charge remains zero
Net negative charge appears because induction creates electrons
Negative charge collects inside the metal volume, not the surface
Explanation
This question tests understanding of the conservation of electric charge and electrostatic induction in conductors 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, the negative rod induces charge separation in the neutral conductor - electrons are repelled to the far side, leaving positive charges near the rod, demonstrating charge redistribution without charge creation. Choice B is correct because the conductor's free electrons redistribute with positive charge accumulating near the negative rod and negative charge on the far side, while the total charge remains zero. Choice A is incorrect because it wrongly assumes induction creates new electrons, violating charge conservation - charges only redistribute, they aren't created. To help students: Emphasize that induction causes charge separation, not charge creation, and the net charge on an isolated conductor remains unchanged. Use demonstrations with electroscopes to show how charges redistribute without contact.
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?
$\Phi_E=\dfrac{Q}{\varepsilon_0}=1.13\times10^5,\text{N·m}^2/\text{C}$
$\Phi_E=\dfrac{kQ}{r^2}$, because flux equals field magnitude
$\Phi_E$ increases with shell thickness because more material encloses charge
$\Phi_E=0$, because insulators block electric fields
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.
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.
A hollow conducting spherical shell has net charge $+9.0,\text{nC}$ and no charge inside its cavity. In electrostatic equilibrium, charges redistribute on conductors so the electric field inside the conductor is zero. Considering the explained phenomenon, what is the resulting charge on the inner surface of the shell?
$-9.0,\text{nC}$
$0,\text{C}$
$+9.0,\text{nC}$
Cannot be determined without the shell radius
Explanation
This question tests understanding of the conservation of electric charge and charge distribution in hollow conductors 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, with no charge inside the cavity, the conductor arranges its charges to ensure zero electric field within the conducting material, demonstrating electrostatic shielding. Choice C is correct because with no charge in the cavity, Gauss's Law applied to a surface within the conductor requires zero net charge on the inner surface, so all +9.0 nC resides on the outer surface. Choice A is incorrect because it wrongly assumes the charge distributes between inner and outer surfaces, but this would create a field inside the conductor, violating electrostatic equilibrium. To help students: Emphasize that charges on conductors arrange to make E = 0 inside the conducting material, and use Gauss surfaces within the conductor to prove charge distribution. Practice with nested conductor problems to reinforce how cavity charges affect surface charge distribution.
A charged metal sphere is enclosed by a spherical Gaussian surface of radius $0.10,\text{m}$. The surface is then expanded to radius $0.30,\text{m}$ without crossing any additional charges. Considering the explained phenomenon, how does the electric field at the Gaussian surface change when the radius increases?
It increases proportional to $r^2$
It decreases proportional to $1/r^2$
It stays the same because flux is constant
It becomes zero because the Gaussian surface is larger
Explanation
This question tests understanding of the conservation of electric charge and the application of Gauss's Law with changing Gaussian surfaces 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, expanding the Gaussian surface doesn't change the enclosed charge, but the surface area increases, demonstrating how electric field varies with distance from a spherical charge distribution. Choice C is correct because for a spherical charge distribution, E ∝ 1/r², so when radius increases from 0.10 m to 0.30 m (factor of 3), the field decreases by a factor of 9. Choice A is incorrect because it wrongly assumes constant flux means constant field, confusing the fact that flux = EA remains constant while both E and A change. To help students: Emphasize that while flux through any closed surface around the same charge is constant, the field strength depends on distance. Use the analogy of light intensity decreasing with distance from a bulb to reinforce the inverse square law.
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 stays $0$ because the enclosed net charge remains $0$
Flux increases because the external rod increases the field everywhere
Flux depends on the Gaussian surface radius, so it must change
Flux becomes negative because field lines enter the surface
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.
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?
$+8.0,\mu\text{C}$ and $-2.0,\mu\text{C}$, because charges stay with originals
$+6.0,\mu\text{C}$ each, because contact creates extra charge
$+5.0,\mu\text{C}$ each, because only positive charge moves
$+3.0,\mu\text{C}$ each, by charge conservation and symmetry
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.
A $+6.0,\mu\text{C}$ rod touches a neutral metal sphere on an insulating stand; after separation, what is the sphere’s charge?
$+3.0,\mu\text{C}$, assuming equal sharing by identical conductors
$+6.0,\mu\text{C}$, because all charge transfers to the sphere
$+0,\text{C}$, because charge returns to the rod
$+12,\mu\text{C}$, because contact creates additional charge
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.