Chapter 18
Two plastic spheres with uniform charge repel each other with a force of 10. N. If you remove the charge from one sphere, what will be the force between the spheres?
0.0 N
What is the charge on a proton?
1.6*10^-19
If you double the distance between two point charges, by which factor does the force between the particles change?
1/4
Assume you make the following changes to a parallel-plate capacitor: 1.Double the area 2.Reduce the distance by a factor of four 3.Add a dielectric with kappa=3 How is the capacitance affected? The capacitance will increase by a factor of
24
Electrostatic forces are enormous compared to gravitational force. Why do you NOT notice electrostatic forces in everyday life, whereas you do notice the force due to gravity?
Because opposite charges cancel each other, while gravity does not cancel out
Why is Coulomb's law called an inverse-square law?
Because the force is proportional to the inverse of the distance squared between charges.
Why should capacitance be inversely proportional to the separation between the plates of a capacitor?
Capacitance is directly proportional to the electric field, which is inversely proportional to the distance between the capacitor plates.
You have four neutral conducting spheres and a charging device that allows you to place charge q on any neutral object. You want to charge one sphere with a charge q/2 and the other three with a charge q/6. How do you proceed?
Charge one sphere with charge q. Touch it to one other sphere to produce two spheres with charge q/2. Touch one of these spheres simultaneously to the two remaining neutral spheres.
Given the potential difference between two points and the distance between the points, how can you obtain the electric field between the points?
Divide the electric potential by the distance to obtain the electric field.
An arbitrary electric field passes through a box-shaped volume. There are no charges in the box. If eight electric-field lines enter the box, how many electric-field lines must exit the box?
Eight
How does a conductor differ from an insulator?
Electric charges move easily in a conductor but not in an insulator.
Gravitational potential energy is the potential for two masses to do work by virtue of their positions with respect to each other. What is the analogous definition of electric potential energy?
Electric potential energy is the potential for two charges to do work by virtue of their positions with respect to each other.
How is electric potential related to electric potential energy?
Electric potential is the electric potential energy per unit charge at a given position in space.
You insert a dielectric into a charged air-filled capacitor. How does this affect the energy stored in the capacitor?
Energy stored in the capacitor will decrease.
The students in your class count off consecutively so each student has a number. The odd-numbered students are told to act as negative charge, and the even-numbered students are told to act as positive charge. How would you organize the students to represent a polarized material?
Even-numbered students will be asked to come to the front, whereas odd-numbered students will be asked to go to the back of the class.
An electron moves in an electric field. Does it move toward regions of higher potential or lower potential? Explain.
It moves toward regions of higher potential because its charge is negative.
Imagine that the magnitude of the charge on the electron differed very slightly from that of the proton. How would this affect life on Earth and physics in general?
Many macroscopic objects would be charged, so we would experience the enormous force of electricity on a daily basis.
There are very large numbers of charged particles in most objects. Why do most objects NOT exhibit static electric effects?
Most objects have equal numbers of protons and electrons.
A charge distribution has electric field lines pointing into it. What sign is the net charge?
Negative
In a science-fiction movie, a villain emits a radial electric field to repulse the hero. Knowing that the hero is electrically neutral, is this possible?
No, because an electrically neutral body can be attracted but not repelled
If you dive into a pool of seawater through which an equal amount of positively and negatively charged particles are moving, will you receive an electric shock?
No, because equal amounts of positively and negatively charged particles are moving
How many electric field lines can exist at a given point in space? Why?
One; since electric field lines point in the direction that a positive charge would be forced, and there is a single direction of net force, there can only be one field line at any point in space.
How does the mass of the electron compare with the mass of the proton? The mass of the is about times that of the
Proton, 2000, electron
How can you modify two charged objects to quadruple the force between them without transferring any charge?
Reduce the distance between the charged objects to half the original distance.
A negative lightning strike occurs when a negatively charged cloud discharges its excess electrons to the positively charged ground. If you observe a cloud-to-cloud lightning strike, what can you say about the charge on the area of the cloud struck by lightning?
The area of the cloud that was struck by lightning had a positive charge.
If the area of a parallel-plate capacitor doubles, how is the capacitance affected?
The capacitance will double.
The combined charge of all the electrons in a dime is hundreds of thousands of coulombs. Because like charges repel, what keeps the dime from exploding?
The dime has an equal number of protons, with positive charge.
When electric field lines get closer together, what does that tell you about the electric field?
The electric field is directly proportional to the density of electric field lines.
How are electrostatic force and charge related?
The force is proportional to the product of two charges.
How are potential difference and electric-field strength related for a constant electric field?
The magnitude of electric-field strength is equivalent to the potential divided by the distance.
How is the charge of the proton related to the charge of the electron?
The magnitudes of charge of the proton and the electron are equal, but the charge of the proton is positive, whereas the charge of the electron is negative.
What creates a greater magnitude of force, two charges +q a distance r apart or two charges −q the same distance apart?
The magnitudes of forces are equal.
Which factors determine the capacitance of a device? Select all that apply. The materials used to construct the capacitor The electric field surrounding the capacitor The geometric properties of the capacitor The electric potential across the capacitor The charge stored in the capacitor
The materials used to construct the capacitor The geometric properties of the capacitor
A negative charge is 10 m from a positive charge. Where would you have to move the negative charge to increase the potential energy of the system?
The negative charge should be moved farther away from the positive charge.
True or false: Although wood is an insulator, lightning can travel through a tree to reach Earth.
True
Three parallel plates are stacked above each other, with a separation between each plate. If the potential difference between the first two plates is ΔV1 and the potential between the second two plates is ΔV2 what is the potential difference between the first and the third plates?
V2+V1
What is the relationship between voltage and energy?
Voltage is the energy per unit charge at some point in space.
Can an insulating material be used to charge a conductor?
Yes, a charged insulator can charge a conductor upon contact.
whtat are you going to do to make sure you dont mess up
check to see if it asks for absolute value or magnitude and check sig figs
By considering the molecules of an insulator, how can an insulator be overall neutral but carry a surface charge when polarized? The atoms or molecules in the insulator can become polarized. The distribution in all the molecules can shift in a given direction, leaving an excess of charge on the opposite ends of each molecule. Inside the insulator, the oppositely charged ends of the molecules each other. On the surface of the material, however, a net surface charge appears due to the polarized molecules.
electron, positive, cancel, non-zero
Why does dust stick to the computer screen?
its polarized
what is the sign in the answer for the following problem: If it takes 62.9 mJ to move a charge q from xi=42.6 cm to xf=−42.6 cm in an electric field of (−44.5 NC)x^, what is the charge q?
negative (If it takes neg)
The positive terminal of a battery is connected to one connection of a lightbulb, and the other connection of the lightbulb is connected to the negative terminal of the battery. The battery pushes charge through the circuit but does not become charged itself. Does this violate the law of conservation of charge? , because this circuit is . To function, a battery must be connected to a circuit that makes a loop, with the battery being part of the loop. Thus, the charge that the battery pushes out of one terminal is , so the net charge of the battery .
no, closed, recuperated at the other terminal, does not change
A long straight wire has a uniform positive charge distribution. The electric field lines would be drawn the wire.
perpendicular to
A neutral hydrogen atom has one proton and one electron. If you remove the electron, what will be the leftover sign of the charge?
positive
What is the sign in the answer for the following problem: An electron accelerates from rest to 7.55×104 m/s in an electric field. Through what potential difference did the electron travel?
positive (an electron accelerates pos)
what is the sign in the answer for the following problem: How much work is required from an outside agent to move an electron from xi=0 to xf=38.7 cm in an electric field (33.2 NC)x^
positive (outside agent pos)
What is the sign in the answer for the following problem: The energy required to bring a charge q=−2.82 nC from far away to 2.67 cm away from a point charge Q is 10.8 mJ. What is the absolute value of the potential at the final position of q?
positive (the energy reqired pos)
A closed, isolated system contains two objects with initial charges �1 and �2. If object 1 loses half of its charge, what is the final charge on object 2?
q2 + q1/2
Which equation describes conservation of charge for a closed, isolated system?
qi = qf = constant
The electric-field lines from an isolated positive point charge spread out and point .
radially, outward
Describe the force between two positive point charges that interact. The force is and acts the line joining the two point charges.
repulsive, along
If you double the voltage across the plates of a capacitor, how is the stored energy affected?
stored energy will increases to four times its previous value
In Newton's law of universal gravitation, the force between two masses is proportional to the product of the two masses. What is analogous to mass in Coulomb's law?
the electric charge
A positive charge is located at x=0. When a negative charge is placed at x=10 cm, what happens to the electric-field lines near x=5 cm?
the electric field lines become denser
A constant electric field is acting in the positive y direction. In which direction is the force on a negatively charged object that is inside this field?
the negative y directiom
The fact that Coulomb's constant (k=8.99×109 N⋅m2C2) is much greater than Newton's gravitational constant (G=6.67×10−11 N⋅m2kg2) tells you that the relative strength of the electrostatic force is stronger than that of gravity.
true
True or false: A liquid can be an insulating material.
true
True or false: Placing a dielectric between the plates of a capacitor increases the maximum amount of energy that can be stored in the capacitor.
true
True or false: The potential from a group of charges is the sum of the potentials from each individual charge.
true
If five electric-field lines come out of a +5 nC charge, how many electric-field lines should come out of a +20 nC charge?
twenty field lines
How many types of electric charge exist?
two
Why are water molecules attracted by positive and negative charges?
water molecules are polar
