Physics Exam 1 conceptuals
Two identical small charged spheres are a certain distance apart, and each one initially experiences an electrostatic force of magnitude F due to the other. With time, charge gradually leaks off of both spheres. When each of the spheres has lost half its initial charge, the magnitude of the electrostatic force will be
1/4 F
When a potential difference of 10 V is placed across a certain solid cylindrical resistor, the current through it is 2 A. If the diameter of this resistor is now tripled, the current will be
18 A
A wire of resistivity ρ must be replaced in a circuit by a wire of the same material but 4 times as long. If, however, the resistance of the new wire is to be the same as the resistance of the original wire, the diameter of the new wire must be
2 times the diameter of the original wire
An ideal parallel-plate capacitor consists of a set of two parallel plates of area A separated by a very small distance d. When the capacitor plates carry charges +Q and -Q, the capacitor stores energy U0. If the separation between the plates is doubled, how much electrical energy is stored in the capacitor?
2U0
The potential across the right hand capacitor is V2. The potential across the left hand one, is closest to:
2V2
Two point charges Q 1 and Q 2 of equal magnitudes and opposite signs are positioned as shown in the figure. Which of the arrows best represents the net electric field at point P due to these two charges?
A
Three resistors R1=100 Ω, R2=200 Ω, and R3=300 Ω are connected in various ways shown below. For which circuit is the resistance the greatest from end to end?
A (fall 2017)
A constant current flows through a wire shaped like a cone as shown. The surfaces S1 and S2 are equipotential surfaces. How does the magnitude of the electric field E vary with position along the axis moving from S1 to S2?
A) E decreases
The potential difference between the plates of a parallel plate capacitor is set to V0, resulting in a charge of Q0. The distance between the plates is then doubled while maintaining the potential difference at V0. The energy stored in the capacitor is now:
A) Q0V0/4.
As more identical capacitors are added in series across a constant voltage source, the charge on one capacitor:
A) decreases
When two or more capacitors are connected in series across a potential difference
All of the above choices are correct. -each capacitor carries the same amount of charge. the equivalent capacitance of the combination is less than the capacitance of any of the capacitors. the potential difference across the combination is the algebraic sum of the potential differences across the individual capacitors.
A positive point charge Q is fixed on a very large horizontal frictionless tabletop. A second positive point charge q is released from rest near the stationary charge and is free to move. Which statement best describes the motion of q after it is released?
As it moves farther and farther from Q, its speed will keep increasing
The angular direction with respect to the +x axis for the Coulomb force on charge Q1 in question 3 above is closest to:
B) +233 degrees.
The electric field in a region of space is given by: 10 𝑥 𝑖̂+ 30 𝑗̂+ 0 𝑘̂ (the field increases linearly with x). It passes through a cube with edges aligned along the x, y, and z axes. Diagonal corners are at (0,0,0) and (L,L,L). The net electric flux passing through the surface of the cube is:
B) 10𝐿^3
Three equal charges, +Q, are placed on three corners of a square of side L. The field magnitude at the center of the square is:
B) 2𝑘𝑄/𝐿^2
Four equal charges, +Q, are placed at the corners of a square of side L. The potential at the center of the square is:
B) 4√2𝑘𝑄 /L
The resistance of the middle segment is R0. The resistance of the entire conductor from left side to right side is closest to:
B) 9R0.
At which of the horizontal locations (A, B, C) in the sketch is the electric potential lowest?
C
Three equal negative point charges are placed at three of the corners of a square of side d as shown in the figure. Which of the arrows represents the direction of the net electric field at the center of the square?
C
Two identical small charged spheres are a certain distance apart, and each one initially experiences an electrostatic force of magnitude F due to the other. With time, charge gradually leaks off of both spheres. When each of the spheres has lost half its initial charge, the magnitude of the electrostatic force will be:
C) 1/4 F.
Consider two concentric spherical surfaces S1 with radius a and S2 with radius 2a, both centered at the origin. There is a charge +q placed at the origin. There is no other charge or field in the region. The electric flux through S1 is . The flux through S2 is therefore:
C) Electric flux
The conducting rod shown here has no net charge. A negative charge is brought near the right end of the rod. The rod:
C) attracts the charge because it polarizes with positive charge closest to the object.
As more resistors are added in series across a constant voltage source, the power supplied by the source:
C) decreases.
A very long solid cylinder is made from a material in which the net charge density is constant. The electric field within the cylinder is proportional to:
C) r
The figure shows a graph of the resistance of a wire as a function of its length. What physical quantities does the slope of this graph represent?
C) the resistivity of the material divided by the cross-sectional area of the wire
A conducting sphere of radius R carries an excess positive charge and is very far from any other charges. Which one of the following graphs best illustrates the potential (relative to infinity) produced by this sphere as a function of the distance r from the center of the sphere?
C.) flat the negative slope graph
Which of the following changes to an ideal parallel plate capacitor will result in an increase in charge stored on the capacitor?
D) Inserting a dielectric with k>1.
How does the resistance of an Ohmic substance depend on magnitude E of the applied electric field?
D) R is independent of E
At a distance D from a very long (essentially infinite) uniform line of charge, the electric field strength is 1000 N/C. At what distance from the line will the field strength to be 2000 N/C?
D/2
The electric field between square the plates of a parallel-plate capacitor has magnitude E. The potential across the plates is maintained with constant voltage by a battery as they are pulled apart to twice their original separation, which is small compared to the dimensions of the plates. The magnitude of the electric field between the plates is now equal to
E/2.
A narrow copper wire of length L and radius b is attached to a wide copper wire of length L and radius 2b, forming one long wire of length 2L. This long wire is attached to a battery, and a current is flowing through it. If the electric field in the narrow wire is E, the electric field in the wide wire is
E/4.
The electric field at location B is E0. The field at location C is closest to:
E0/4.
A point charge Q is located a short distance from a point charge 3Q, and no other charges are present. If the electrical force on Q is F, what is the electrical force on 3Q
F
If the electric field is zero everywhere inside a region of space, the potential must also be zero in that region
False
If the electric flux through a closed surface is zero, the electric field at points on that surface must be zero.
False
A negative charge is moved from point A to point B along an equipotential surface. Which of the following statements must be true for this case?
No work is required to move the negative charge from point A to point B
Positive charge is uniformly distributed around the circular loop as shown to the right. At which of the points on the x-axis is the electric field the greatest? (Point A is at the center of the ring. Point B is about one radius out along the axis. Point C is 3 radii out along the axis. Point D is 5 radii out.)
Point B
The four identical capacitors in the circuit shown in the figure are initially uncharged. Let the charges on the capacitors be Q 1, Q 2, Q 3, and Q 4 and the potential differences across them be V 1, V 2, V 3, and V 4. The switch is thrown first to position A and kept there for a long time. It is then thrown to position B. Which of the following conditions is true with the switch in position B?
Q 1 = 3 Q 2
The figure shows three electric charges labeled Q 1, Q 2, Q 3, and some electric field lines in the region surrounding the charges. What are the signs of the three charges?
Q 1 is positive, Q 2 is negative, Q 3 is positive.
The charge on the square plates of a parallel-plate capacitor is Q. The potential across the plates is maintained with constant voltage by a battery as they are pulled apart to twice their original separation, which is small compared to the dimensions of the plates. The amount of charge on the plates is now equal to
Q/2
The figure above shows three identical lightbulbs connected to a battery having a constant voltage across its terminals. What happens to the brightness of lightbulb 2 when the switch S is closed? Assume the resistance of the bulbs does not change with brightness.
The brightness decreases.
The figure shows three identical lightbulbs connected to a battery having a constant voltage across its terminals. What happens to the brightness of lightbulb 1 when the switch S is closed
The brightness increases permanently
Consider a spherical Gaussian surface of radius R centered at the origin. A charge Q is placed inside the sphere. To maximize the magnitude of the flux of the electric field through the Gaussian surface, the charge should be located
The charge can be located anywhere, since flux does not depend on the position of the charge as long as it is inside the sphere.
The figure shows a steady electric current passing through a wire with a narrow region. What happens to the drift velocity of the moving charges as they go from region A to region B and then to region C?
The drift velocity increases from A to B and decreases from B to C
The potential as a function of position xxx is shown in the graph in the figure. Which statement about the electric field is true?
The electric field is zero at x = 0, its magnitude is at a maximum at x = 5 cm, and the field is directed to the right there.
An uncharged conductor has a hollow cavity inside of it. Within this cavity there is a charge of +10 µC that does not touch the conductor. There are no other charges in the vicinity. Which statement about this conductor is true?
The outer surface of the conductor contains +10 µC of charge and the inner surface contains -10 µC.
Equal but opposite charges Q are placed on the square plates of an air-filled parallel-plate capacitor. The plates are then pulled apart to twice their original separation, which is small compared to the dimensions of the plates. Which of the following statements about this capacitor are true? (There may be more than one correct choice.)
The potential difference across the plates has doubled. The energy stored in the capacitor has doubled
A nonconducting sphere contains positive charge distributed uniformly throughout its volume. Which statements about the potential due to this sphere are true? All potentials are measured relative to infinity. (There may be more than one correct choice.)
The potential is highest at the center of the sphere.
A series of equipotential lines separated by 10 V are drawn in the figure. The innermost circle has potential of +20 V and the outermost has potential of 0 V. In which region, if any, does the electric field have the greatest magnitude?
The region to the right of the smallest circle.
If the electrical potential in a region is constant, the electric field must be zero everywhere in that region.
True
X and Y are two uncharged metal spheres on insulating stands, and are in contact with each other. A positively charged rod R is brought close to X as shown in Figure (a). Sphere Y is now moved away from X, as in Figure (b). What are the final charge states of X and Y?
X is negative and Y is positive.
Two long straight parallel lines, #1 and #2, carry uniform positive linear charge densities. The charge density on line #2 is twice as great as the charge density on line #1. The locus of points where the electric field due to these lines is zero is
along a line between the lines closer to line #1 than line #2.
The figure shows four Gaussian surfaces surrounding a distribtuion of charges. (a) Which Gaussian surfaces have an electric flux of +q/ϵ0+q/ϵ0 through them?
b
If a negative charged rod is held near a neutral metal ball, the ball is attracted to the rod. This happens
because the rod polarizes the metal
In the circuit shown in the figure, four identical resistors labeled A to D are connected to a battery as shown. S 1 and S 2 are switches. Which of the following actions would result in the GREATEST amount of current through resistor A?
closing both switches
The graph in the figure shows the variation of the electric potential V (measured in volts) as a function of the radial direction r (measured in meters). For which range or value of r is the magnitude of the electric field the largest?
from r = 3 m to r = 4 m
As more resistors are added in parallel across a constant voltage source, the power supplied by the source
increases.
Suppose you have two point charges of opposite sign. As you move them farther and farther apart, the potential energy of this system relative to infinity
increases.
Under electrostatic conditions, the electric field just outside the surface of any charged conductor
is always perpendicular to the surface of the conductor.
The figure shows two unequal point charges, q and Q, of opposite sign. Charge Q has greater magnitude than charge q. In which of the regions X, Y, Z will there be a point at which the net electric field due to these two charges is zero?
only region X
An air-filled parallel-plate capacitor is connected to a battery and allowed to charge up. Now a slab of dielectric material is placed between the plates of the capacitor while the capacitor is still connected to the battery. After this is done, we find that
the charge on the capacitor had increased.
When two or more capacitors are connected in parallel across a potential difference
the potential difference across each capacitor is the same.
An electron is initially moving to the right when it enters a uniform electric field directed upwards. Which trajectory shown below will the electron follow?
trajectory Z (Downwards)
An ideal air-filled parallel-plate capacitor has round plates and carries a fixed amount of equal but opposite charge on its plates. All the geometric parameters of the capacitor (plate diameter and plate separation) are now DOUBLED. If the original energy density between the plates was u0, what is the new energy density?
u0/16
Charge Q is uniformly distributed throughout a spherical insulator of radius R. The electric field at a point a distance R/4 from the center of the shell is closest to:
𝑘𝑄/4𝑅^2
