Physics II Test 2 Conceptual Questions
State three reasons for adding a dielectric material between the plates of a capacitor.
(1) Dielectrics do not break down (allowing electric charge to flow) as readily as air, so a higher voltage can be applied without charge passing across the gap. (2) A dielectric allows the plates to be placed closer together without touching thus allowing an increased capacitance because the plate separation is less. (3) If a dielectric fills the space between the two conductors, it increases the capacitance.
A 1 mF, a 2 mF, and a 3 mF capacitor are connected in parallel, the combination being connected across a 9 volt battery. (a) Which capacitor has the greatest charge?(b) Which capacitor has the greatest voltage?
(a) 3 mF. (b) They all have the same voltage.
A 1 pF capacitor is connected in parallel with a 2 pF capacitor, the parallel combination then being connected in series with a 3 pF capacitor. The resulting combination is then connected across a battery. (a) Which capacitor has the greatest charge? (b) Which capacitor has the greatest voltage?
(a) 3 pF(b) They all have the same voltage.
A capacitor consists of a set of two parallel plates of area A separated by a distance d. The capacitor plates carry charges +Q and -Q. If the separation between the plates is doubled, the electrical energy stored in the capacitor will A) double. B) quadruple. C) be cut in half. D) be cut in fourth. E) not change.
A
A capacitor consists of a set of two parallel plates of area A separated by a distance d. The capacitor plates cary charges +Q and -Q. If the separation between the plates is doubled, the potential difference between the plates will
A
A dielectric is inserted between the plates of an isolated parallel-plate capacitor that carries a charge Q. What happens to the potential energy stored in the capacitor? A) The potential energy of the capacitor decreases. B) The potential energy of the capacitor increases or decreases depending on the value of the dielectric constant of the capacitor. C) The potential energy of the capacitor remains the same. D) The potential energy of the capacitor increases. E) More information is needed to answer the question.
A
A parallel-plate capacitor is charged by a battery and then the battery is removed and a dielectric of constant K is used to fill the gap between the plates. Inserting the dielectric changes the energy stored by a factor of A) 1/K. B) K. C) 2. D) K - 1. E) 1 (no change).
A
Cathode rays are now known to be A) electrons. B) visible light rays. C) protons. D) x-rays. E) short wavelength light rays.
A
Electric dipoles always consist of two charges that are A) equal in magnitude; opposite in sign. B) equal in magnitude; both are negative. C) equal in magnitude; both are positive. D) unequal in magnitude; opposite in sign. E) unequal in magnitude; both are negative.
A
If the voltage on a capacitor is doubled, then the energy stored by the capacitor A) quadruples. B) doubles. 9C) halves. D) stays the same. E) triples.
A
The direction in which the potential changes at the greatest rate is in the direction A) of the gradient of the potential. B) perpendicular to the electric field. C) perpendicular to the gradient of the potential. D) of the partial derivative with respect to x. E) cannot be determined
A
The potential of a uniformly charged sphere is lowest at A) infinity. B) a distance from the sphere equal to its radius. C) a distance from the sphere equal to its diameter. D) the surface of the sphere. E) the center of the sphere.
A
When a parallel-plate capacitor of capacitance C is connected to a battery of voltage V, the energy density in the capacitor is u. If the voltage of battery is doubled, which of the following changes would keep the energy density equal to its previous value u? A) doubling the spacing of the plates B) doubling the size of the plates C) reducing the size of the plates by half D) reducing the spacing of the plates E) The energy density is unaffected by a change in the voltage.
A
When two or more capacitors are connected in parallel to a battery, A) the voltage across each capacitor is the same. B) each capacitor carries the same amount of charge. C) the equivalent capacitance of the combination is less than the capacitance of any one of the capacitors. D) all of the given answers E) none of the given answers
A
Describe the process of storing electrical energy in a capacitor.
A charged capacitor stores electrical energy. The net effect of charging a capacitor is to remove charge from one plate and add it to the other plate. This is what a battery does when it is connected to a capacitor. A capacitor does not become charged instantly. It takes time. Initially, when the capacitor is uncharged, it requires no work to move the first bit of charge over. When some charge is on each plate, it requires work to add more charge of the same sign because of the electric repulsion.
A negative charge, if free, tries to move A) from high potential to low potential. B) from low potential to high potential. C) toward infinity. D) away from infinity. E) in the direction of the electric field.
B
For an electron moving in a direction opposite to the electric field A) its potential energy increases and its electric potential decreases. B) its potential energy decreases and its electric potential increases. C) its potential energy increases and its electric potential increases. 4D) its potential energy decreases and its electric potential decreases. E) both its potential energy and it electric potential remain constant.
B
If the voltage between the plates of a parallel-plate capacitor is doubled, the charge on the capacitor A) is tripled. B) is doubled. C) is halved. D) quadruples. E) remains the same.
B
One electron-volt corresponds to A) 8.0 × 10-20 J. B) 1.6 × 10-19 J. C) 8.0 × 10-20 J. D) 9.5 × 10-17 J. E) 1.9 × 10-16 J.
B
The energy acquired by a particle carrying a charge equal to that on the electron as a result of moving through a potential difference of one volt is referred to as A) a joule. B) an electron-volt. C) a proton-volt. D) a neutron-volt. E) a coulomb.
B
The plates of a parallel-plate capacitor are maintained with constant voltage by a battery as they are pulled apart. During this process, the amount of charge on the plates must A) increase. B) decrease. C) remain constant. D) either increase or decrease. There is no way to tell from the information given.
B
The plates of a parallel-plate capacitor are maintained with constant voltage by a battery as they are pulled apart. What happens to the strength of the electric field during this process? A) It increases. B) It decreases. C) It remains constant. D) cannot be determined from the information given
B
Two parallel-plate capacitors are connected in series to a battery as shown in Fig. 24-4. A dielectric is inserted in capacitor C1. What happens to the charge on capacitor C2? A) The charge on C2 increases or decreases depending on the value of the voltage of the battery. B) The charge on C2 increases. C) The charge on C2 increases or decreases depending on the value of the dielectric constant of the dielectric. D) The charge on C2 decreases. E) The charge on C2 remains the same.
B
A capacitor consists of a set of two parallel plates of area A separated by a distance d. This capacitor is connected to a battery that maintains a constant potential difference between the plates. If the separation between the plates is doubled, the magnitude of the electrical energy stored on the capacitor will A) double. B) quadruple. C) be cut in half. D) be cut in fourth. E) not change.
C
A dielectric material such as paper is placed between the plates of a capacitor holding a fixed charge. What happens to the electric field between the plates? A) no change B) becomes stronger C) becomes weaker D) reduces to zero E) cannot be determined without additional information
C
Fig. 24-1 shows 3 capacitors, of equal capacitance C, connected to a battery of voltage V. What is the equivalent capacitance of this combination? A) 3C/2 B) C/2 C) 2C/3 D) 3 C E) C/3
C
Three capacitors C1, C2, and C3 are connected to a battery as shown in Fig. 24-3. The three capacitors have equal capacitances. Which capacitor stores the most potential energy? A) C3B) C2C) C1D) C2 or C3. They store the same amount of energy E) All three capacitors store the same amount of energy.
C
Three identical capacitors are connected in series to a battery. If a total charge of Q flows from the battery, how much charge does each capacitor carry? 6A) 9Q B) 3Q C) Q D) Q/3 E) Q/9
C
Three negative charges of equal magnitudes are positioned along the x-axis at x = -a, x=0, and x = +a respectively. The charge located at x=0 is moved away along the y-axis to a position (x,y) = (0, +a). How does the potential energy of the system of charges change as a result? A) The potential energy may increase or decrease depending on the magnitude of the charges. B) The potential energy increases. C) The potential energy decreases. D) The potential energy stays the same. E) More information is needed to answer the question.
C
Briefly discuss some of the uses for capacitors in electronic circuits.
Capacitors are devices that can store electric charge. They store charge for later use, such as in a camera flash, and as energy backup in computers if the power fails. Capacitors can also block surges of charge and energy to protect circuits. Very tiny capacitors serve as memory for the "ones" and "zero" of the binary code in the random access memory of computers.
A negative charge is moved from point A to point B along an equipotential surface. Which of the following statements is true for this case? A) The negative charge performs work in moving from point A to point B. B) Work is required to move the negative charge from point A to point B. C) Work is both required and performed in moving the negative charge from point A to point B. D) No work is required to move the negative charge from point A to point B. E) Not enough information is given to make a statement about the work involved.
D
An equipotential surface must be A) parallel to the electric field at every point. B) oriented 30° with respect to the electric field at every point. C) oriented 60° with respect to the electric field at every point. D) perpendicular to the electric field at every point. E) equal to the electric field at every point.
D
Capacitance of capacitors depends upon A) the material between the conductors. B) the geometry of the conductors. C) the space between the conductors. D) all of these answers. E) none of these answers.
D
For a proton moving in the direction of the electric field A) its potential energy increases and its electric potential decreases. B) its potential energy decreases and its electric potential increases. C) its potential energy increases and its electric potential increases. D) its potential energy decreases and its electric potential decreases. E) both its potential energy and it electric potential remain constant.
D
The work done in moving a positive charge against an electric field does not depend on the path chosen in moving the charge in that field. Based on the statement, what kind of force field is the electrostatic field? A) discrete B) quantized C) polarized D) conservative E) nonconservative
D
Three identical capacitors are connected in parallel to a battery. If a total charge of Q flows from the battery, how much charge does each capacitor carry? A) 9Q B) 3Q C) Q D) Q/3 E) Q/9
D
When two or more capacitors are connected in series to a battery, A) the total voltage across the combination is the algebraic sum of the voltages across the individual capacitors. B) each capacitor carries the same amount of charge. C) the equivalent capacitance of the combination is less than the capacitance of any of the capacitors. D) all of the given answers E) none of the given answers
D
shows equipotentials surrounding a pair of charges QA and QB. The value of the potential half-way between the charges is indicated. Which of the statements below applies to the charges? A) The two charges have the same sign and equal magnitudes. B) Nothing can be said about the charges. C) The two charges have the same sign but different magnitudes. D) The two charges have opposite signs and equal magnitudes. E) The two charges have opposite signs and different magnitudes.
D
A parallel-plate capacitor of capacitance C is connected to a battery of voltage V until it is fully charged. The energy density in the capacitor is then equal to u. If the same capacitor is then connected to a battery of voltage 2V its energy density becomes equal to A) u. B) u/4. C) u/2. D) 2u. E) 4u.
E
Computer monitors are often referred to as CRT's. CRT stands for A) computer registration tube. B) color relay transmission. C) computer resource transmission. D) nothing; it's computer jargon. E) cathode ray tube.
E
Fig. 24-2 shows three capacitors connected to a battery. The capacitances are such that C1 = 2 C2 and C1= 3C3. Which capacitor stores the smallest amount of charge? A) C2B) C3C) C1D) C2 or C3E) All three capacitors store the same amount of charge.
E
If the voltage applied to a parallel-plate capacitor is doubled, the electric field between the plates A) is halved. B) quadruples. C) is tripled. D) remains the same. E) is doubled.
E
If the voltage between the plates of a parallel-plate capacitor is doubled, the capacitance of the capacitor A) is tripled. B) is halved. C) is doubled. D) quadruples. E) remains the same.
E
As more and more capacitors are connected in series, the equivalent capacitance of the combination increases.
FALSE
The change in electric potential energy, Ub - Ua, is the work done on a charge by the electric force as it moves from point a to point b.
FALSE
When a proton moves in a direction of the electric field, its potential increases but its potential energy decreases.
FALSE
Explain why equipotential surfaces are always perpendicular to the electric field vectors.
If the electric field is not perpendicular to an equipotential, the field would do work on a charge that moves along the equipotential. In this case, the potential energy of the charge would change, and the surface would not in fact be an equipotential.
If you were a parallel plate capacitor manufacturer, state three ways you might make larger valued capacitors.
Increase plate area, decrease separation, and increase the dielectric constant.
All points on the perpendicular bisector of the line joining two equal but opposite charges have a potential of zero.
TRUE
As more and more capacitors are connected in parallel, the equivalent capacitance of the combination increases.
TRUE
Capacitors in series share the same charge and capacitors in parallel share the same voltage.
TRUE
Equipotential lines and electric field lines meet perpendicular to one another.
TRUE
Every point on an equipotential surface is at the same potential.
TRUE
The component of the electric field in any direction is equal to the negative of the rate of change of the electric potential with distance in that direction.
TRUE
The direction of an electric field is from higher to lower potential.
TRUE
The energy stored in a capacitor will be equal to the work done to charge it.
TRUE
The potential outside a uniformly charged sphere is the same as if all the charge were concentrated at its center.
TRUE
How much work is required to move a charge from one location on an equipotential to another point on the same equipotential? Explain.
Zero. The electric field is perpendicular to an equipotential, therefore the work done in moving along an equipotential is zero.
A 1 mF, a 2 mF, and a 3 mF capacitor are connected in series, the combination being connected across a 9 volt battery. (a) Which capacitor has the greates charge?(b) Which capacitor has the greatest voltage?
a) They all have the same charge. (b) 1 mF
