Chapter 22
What is the total surface charge qint on the interior surface of the conductor (i.e., on the wall of the cavity)?
-q
The large positive charge inside the shell is roughly 16 times that of the smaller charges shown on the inner and outer surfaces of the spherical shell. Which of the following figures best represents the charge distribution on the inner and outer walls of the shell?
1 (+ signs evenly distributed outside, - signs mostly on the right on the inside)
Consider a point 0.5 m above the midpoint of the two charges. As you can verify by removing one of the positive charges, the electric field due to only one of the positive charges is about 18 V/m. What is the magnitude of the total electric field due to both charges at this location?
25 V/m
What is the voltage 3 m away from the charge?
3 V
The positive charge is equal to roughly 16 of the smaller charges shown on the surfaces of the spherical shell. Which of the pictures best represents the charge distribution on the inner and outer walls of the shell?
4 (+ signs evenly distributed on outside, - signs evenly distributed on inside)
The electric potential (voltage) at a specific location is equal to the potential energy per unit charge a charged object would have if it were at that location. If the zero point of the voltage is at infinity, the numerical value of the voltage is equal to the numerical value of work done to bring in a unit charge from infinity to that location. Select Show numbers and grid in the green menu, and drag one positive charge to the middle of the screen, right on top of two intersecting bold grid lines. Using the voltage meter, you should find that 1 m away from the charge, the voltage is 9 V. What is the voltage 2 m away from the charge?
4.5 V
How does one coulomb of charge compare with the charge of a single electron?
A coulomb of charge is the charge associated with 6.25 × 10^18 electrons
What is a positive ion? What is a negative ion?
A positive ion is a neutral atom that has lost one or more electrons. A negative ion is a neutral atom that has gained one or more electrons.
How does a semiconductor differ from a conductor or an insulator?
A semiconductor is neither a good conductor nor a good insulator - it has a middle range of resistivity
Which molecule is an example of a common electric dipole?
A water molecule
What is meant by saying that charge is quantized?
All charged objects have a charge that is an integer multiple of the charge of an electron
Place several E-Field Sensors at a few points on different equipotential lines, and look at the relationship between the electric field and the equipotential lines. Which statement is true?
At any point, the electric field is perpendicular to the equipotential line at that point, and it is directed toward lines of lower voltages.
If the field strength is E = 9 V/m a distance of 1 m from the charge, what is the field strength E a distance of 3 m from the charge?
E = 1 V/m
What happens to electrons in any charging process?
Electrons transfer from one place to another
What term is used for "electricity at rest"?
Electrostatics
Now, remove the negative charge, and drag two positive charges, placing them 1 m apart, as shown below. What is the voltage at the midpoint of the two charges?
Exactly twice the voltage produced by only one of the charges at the same point
Two point charges, each with 0.6 C of charge, are 0.1 m apart. Given that k=9.0×10^9 N⋅m^2/C^2 (the proportionality constant for Coulomb's law), select the correct equations that show that the force between the charges is 3.24×10^11 N.
F = kq1q2/r^2 = (9.0x10^9 N*m^2/C^2)*(0.6 C)*(0.6 C) / (0.1 m)^2 = 3.2 x 10^11 N (the answer with (0.1 m)^2 as the denominator)
An electron volt, eV , is a unit of energy. Which is larger, a GeV or a MeV?
GeV
Give two examples of common force fields and name the sources of these fields
Gravity fields made by mass, electric fields made by charge
As in the video, we apply a charge +Q to the half-shell that carries the electroscope. This time, we also apply a charge -Q to the other half-shell. When we bring the two halves together, we observe that the electroscope discharges, just as in the video. What does the electroscope needle do when you separate the two half-shells again?
It does not deflect at all
Rank the six combinations of electric charges on the basis of the electric force acting on q1. Define forces pointing to the right as positive and forces pointing to the left as negative. Rank positive forces as larger than negative forces.
Largest +1, -1, -1 & -1, +1, +1 +1, -1, +1 +1, +1, -1 +1, +1, +1 & -1, -1, -1 Smallest
Two glass marbles (1 and 2), each supported by a nylon thread, are rubbed against a piece of silk and then are placed near a third glass marble (3), also supported by a similar thread. Assuming that marble 3 has not been in contact with the piece of fabric, which of the following statements best describes the situation when the three marbles are brought together?
Marbles 1 and 2 repel each other, but no interaction occurs with marble 3
What is the most common net charge of an atom?
Neutral
How is Coulomb's law similar to Newton's law of gravitation? How is it different?
Newton's law of gravitation is attractive, whereas Coulomb's law is attractive or repulsive. Both are proportional to the inverse square of distance
Coulomb's law is most similar to which of these laws?
Newton's law of gravity
Make a long line of positive charges, similar to that shown in the figure below. Try to place all of the charges centered along a horizontal grid line. Feel free to look at the electric field, as it is interesting. Measure the strength of the electric field 1 m directly above the middle as well as 2 m directly above. Does the strength of the electric field decrease as 1 over distance squared (1/r^2)?
No, it decreases less quickly with distance
Make a small dipole by bringing the two charges very close to each other, where they are barely touching. The midpoint of the two charges should still be on one of the grid point intersections. Measure the strength of the electric field 0.5 m directly above the midpoint as well as 1 m directly above. Does the strength of the electric field decrease as 1 over distance squared (1/r^2)?
No, it decreases more quickly with distance
Which particle has exactly one quantum unit of charge?
Proton
How does the flow of current differ in a superconductor compared with the flow in ordinary conductors?
Superconductors have infinite conductivity (current flows forever), whereas ordinary conductors have a small resistance to the flow of electric charge
Remove the positive charge by dragging it back to the basket, and drag a negative charge (blue) toward the middle of the screen. Determine how the electric field is different from that of the positive charge. Which statement best describes the differences in the electric field due to a negative charge as compared to a positive charge?
The electric field changes direction (now points radially inward), but the electric field strength does not change.
Now, remove the negative charge, and drag two positive charges, placing them 1 m apart, as shown below. Let's look at the resulting electric field due to both charges. Recall that the electric field is a vector, so the net electric field is the vector sum of the electric fields due to each of the two charges. Where is the magnitude of the electric field roughly equal to zero (other than very far away from the charges)?
The electric field is roughly zero near the midpoint of the two charges.
Try to have the equipotential lines equally spaced in voltage. Then, use an E-Field Sensor to measure the electric field at a few points while looking at the relationship between the electric field and the equipotential lines. Which of the following statements is true?
The electric field strength is greatest where the equipotential lines are very close to each other
Where is the energy stored in a capacitor?
The energy is stored in the electric field between the plates
Equipotential lines are usually shown in a manner similar to topographical contour lines, in which the difference in the value of consecutive lines is constant. Clear the equipotential lines using the Clear button on the voltage tool. Place the first equipotential line 1 m away from the charge. It should have a value of roughly 9 V. Now, produce several additional equipotential lines, increasing and decreasing by an interval of 3 V (e.g., one with 12 V, one with 15 V, and one with 6 V). Don't worry about getting these exact values. You can be off by a few tenths of a volt.Which statement best describes the distribution of the equipotential lines?
The equipotential lines are closer together in regions where the electric field is stronger.
A glass marble is rubbed against a piece of silk. As a result the piece of fabric acquires extra electrons. What happens to the glass marble? Check all that apply
The marble has lost the same number of electrons acquired by the piece of silk The marble acquires a positive charge and attracts the piece of silk
Now, remove the positive charge by dragging it back to the basket, and drag one negative charge toward the middle of the screen. Determine how the voltage is different from that of the positive charge.How does the voltage differ from that of the positive charge?
The voltages become negative instead of positive and keep the same magnitudes
The direction of an electric field is the direction of the force exerted on
a proton
An electric field is basically
a vector quantity
The operation of a microwave oven makes use of
all of the above (the polar nature of water molecules, an oscillating electric field, flip flopping of polarized molecules)
A semiconductor can be
both of these (an insulator, a conductor)
Electric charge can be transferred from one conducting body to another
by both contact and closeness
It is said that electric charge is conserved, which means that electric charge
can be neither created nor destroyed
The electrical force between charges is strongest when the charges are
close together
A highly-charged party balloon is not dangerous when it
contains little overall energy
Make an electric dipole by replacing one of the positive charges with a negative charge, so the final configuration looks like the figure shown below. The electric field at the midpoint is
directed to the right
The primary purpose of a lightning rod is to
discharge the structure to which it is attached
Two charged particles held close to each other are released. As they move, their speeds increase. Therefore, their charges have
either of these
The force that binds atoms together to form molecules is
electrical
The fundamental force underlying all chemical reactions is
electrical
Just as "gravity" is the study of a wide range of gravitational interactions, "electricity" is the study of a wide range of
electrical interactions
What is the magnitude of the electric field 1 m away from the positive charge compared to the magnitude of the electric field 2 m away?
four times
Select Show E-field and grid in the green menu. Drag one positive charge and place it near the middle of the screen, right on top of two intersecting bold grid lines. You should see something similar to the figure below. The electric field produced by the positive charge
is directed radially away from the charge at all locations near the charge
What is the magnitude Eint of the electric field inside the cavity as a function of the distance r from the point charge? Let k, as usual, denote 1/4πϵ0
kq/r^2
Superconductors most often require
low operating temperatures
Much of the charge on a conducting cube is
mutually repelled toward its corners
Two charged particles repel each other with a force F. If the charge of one of the particles is doubled and the distance between them is halved, then the force will be
none of the above (none: F, F/2, 2F, F/4)
Before a thunder storm, clouds in the sky likely become
polarized
What is the total surface charge qext on the exterior surface of the conductor?
q
Place the E-Field Sensor 1 m away from the positive charge (1 m is two bold grid lines away if going in a horizontal or vertical direction), and look at the resulting field strength. Consider the locations to the right, left, above, and below the positive charge, all 1 m away. For these four locations, the magnitude of the electric field is
the same
What is the electric field Eext outside the conductor?
the same as the field produced by a point charge q located at the center of the sphere
When we say charge is conserved, we mainly mean that charge can be
transferred without loss like money in a bank
Electric potential, unlike electric potential energy, is measured in units of
volts
The electric field outside the conductor, Eext
would change
Now a second charge, q2, is brought near the outside of the conductor. Which of the following quantities would change?
would not change
The electric field within the cavity, Ecav
would not change
The total surface charge on the exterior of the conductor, qext
would not change
Most atoms normally have a net charge that is
zero
Now, make an electric dipole by replacing one of the positive charges with a negative charge, so the final configuration looks like the figure shown below. What is the voltage at the midpoint of the dipole?
zero
The electric field inside the dome of a highly-charged Van de Graaff generator is
zero
The net charge on a polarized molecule is normally
zero
The net charge on an energized capacitor is normally
zero
When a car is struck by lightning, the resulting electric field inside the car is
zero
