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Questions and Answers of University Physics

An object with charge q = -6.00 × 10-9 C is placed in a region of uniform electric field and is released from rest at point A. After the charge has moved to point B, 0.500 m to the right, it has
If the electric potential at a single point is known, can E̅(vector) at that point be determined? If so, how? If not, why not?
It is easy to produce a potential difference of several thousand volts between your body and the floor by scuffing your shoes across a nylon carpet. When you touch a metal doorknob, you get a mild
The potential difference between the two terminals of an AA battery (used in flashlights and portable stereos) is 1.5 V. If two AA batteries are placed end to end with the positive terminal of one
Two protons are released from rest when they are 0.750 nm apart.(a) What is the maximum speed they will reach? When does this speed occur?(b) What is the maximum acceleration they will achieve? When
If you carry out the integral of the electric field ˆ«EÌ…(vector) ˆ™ dlÌ…(vector) for a closed path like that shown in Fig. Q23.9, the integral will
(a) If the potential (relative to infinity) is zero at a point, is the electric field necessarily zero at that point?(b) If the electric field is zero at a point, is the potential (relative to
Two protons, starting several meters apart, are aimed directly at each other with speeds of 2.00 × 105 m/s, measured relative to the earth. Find the maximum electric force that these protons will
Which way do electric field lines point, from high to low potential or from low to high? Explain.
(See Exercise 21.21.)(a) Calculate the electric potential energy of the adenine€“thymine bond, using the same combinations of molecules (O-H-N and N-H-N) as in Exercise 21.21.(b) Compare
If E̅(vector) is zero throughout a certain region of space, is the potential necessarily also zero in this region? Why or why not? If not, what can be said about the potential?
If E̅(vector) is zero everywhere along a certain path that leads from point A to point B, what is the potential difference between those two points? Does this mean that E̅(vector) is zero
Since potential can have any value you want depending on the choice of the reference level of zero potential, how does a voltmeter know what to read when you connect it between two points?
Is it possible to have an arrangement of two point charges separated by a finite distance such that the electric potential energy of the arrangement is the same as if the two charges were infinitely
The potential (relative to a point at infinity) midway between two charges of equal magnitude and opposite sign is zero. Is it possible to bring a test charge from infinity to this midpoint in such a
A student asked, “Since electrical potential is always proportional to potential energy, why bother with the concept of potential at all?” How would you respond?
Which statement is true about E̅(vector) inside a negatively charged sphere as described here?(a) It points from the center of the sphere to the surface and is largest at the center.(b) It points
What is the direction of E̅(vector) just outside the surface of such a sphere?(a) Tangent to the surface of the sphere;(b) Perpendicular to the surface, pointing toward the sphere;(c) Perpendicular
What is the magnitude of E̅(vector) just outside the surface of such a sphere?(a) 0;(b) 106 N/C;(c) 107 N/C;(d) 108 N/C.One of the hazards facing humans in space is space radiation: high-energy
Suppose that to repel electrons in the radiation from a solar flare, each sphere must produce an electric field E̅(vector) of magnitude 1 × 106 N/C at 25 m from the center of the sphere. What net
The electric field is measured for points at distances r from the center of a uniformly charged insulating sphere that has volume charge density Ï and radius R, where r < R (Fig.
An insulating hollow sphere has inner radius a and outer radius b. Within the insulating material the volume charge density is given by ρ(r) = α/r, where a is a positive constant.(a) In terms of a
In a region of space there is an electric field E̅(vector) that is in the z-direction and that has magnitude E = [3964 N/ (C ∙ m)]x. Find the flux for this field through a square in the xy-plane
An infinitely long cylindrical conductor has radius R and uniform surface charge density σ.(a) In terms of σ and λ, what is the charge per unit length l for the cylinder?(b) In terms of σ, what
A very large, horizontal, nonconducting sheet of charge has uniform charge per unit area s = 5.00 × 10-6 C/m2.(a) A small sphere of mass m = 8.00 × 10-6 kg and charge q is placed 3.00 cm above the
Charge Q is distributed uniformly throughout the volume of an insulating sphere of radius R = 4.00 cm. At a distance of r = 8.00 cm from the center of the sphere, the electric field due to the charge
Two very long uniform lines of charge are parallel and are separated by 0.300 m. Each line of charge has charge per unit length +5.20 µC/m. What magnitude of force does one line of charge exert on a
Explain this statement: “In a static situation, the electric field at the surface of a conductor can have no component parallel to the surface because this would violate the condition that the
A solid conductor has a cavity in its interior. Would the presence of a point charge inside the cavity affect the electric field outside the conductor? Why or why not? Would the presence of a point
A lightning rod is a rounded copper rod mounted on top of a building and welded to a heavy copper cable running down into the ground. Lightning rods are used to protect houses and barns from
You charge up the Van de Graaff generator shown in Fig. 22.26, and then bring an identical but uncharged hollow conducting sphere near it, without letting the two spheres touch. Sketch the
In a conductor, one or more electrons from each atom are free to roam throughout the volume of the conductor. Does this contradict the statement that any excess charge on a solid conductor must
If the electric field of a point charge were proportional to 1/r3 instead of 1/r2, would Gauss’s law still be valid? Explain your reasoning. Consider a spherical Gaussian surface centered on a
A solid copper sphere has a net positive charge. The charge is distributed uniformly over the surface of the sphere, and the electric field inside the sphere is zero. Then a negative point charge
You find a sealed box on your doorstep. You suspect that the box contains several charged metal spheres packed in insulating material. How can you determine the total net charge inside the box
A spherical Gaussian surface encloses a point charge q. If the point charge is moved from the center of the sphere to a point away from the center, does the electric field at a point on the surface
A certain region of space bounded by an imaginary closed surface contains no charge. Is the electric field always zero everywhere on the surface? If not, under what circumstances is it zero on the
In Fig. 22.15, suppose a third point charge were placed outside the purple Gaussian surface C. Would this affect the electric flux through any of the surfaces A, B, C, or D in the figure? Why or why
Suppose that in Fig. 22.15 both charges were positive. What would be the fluxes through each of the four surfaces in the example?Fig. 22.15 B.
A rubber balloon has a single point charge in its interior. Does the electric flux through the balloon depend on whether or not it is fully inflated? Explain your reasoning.
In a follow-up experiment, a charge of +40 pC was placed at the center of an artificial flower at the end of a 30-cm long stem. Bees were observed to approach no closer than 15 cm from the center of
After one bee left a flower with a positive charge, that bee flew away and another bee with the same amount of positive charge flew close to the plant. Which diagram in Fig. P21.100 best represents
What is the best explanation for the observation that the electric charge on the stem became positive as the charged bee approached (before it landed)?(a) Because air is a good conductor, the
Consider a bee with the mean electric charge found in the experiment. This charge represents roughly how many missing electrons?(a) 1.9 × 108;(b) 3.0 × 108;(c) 1.9 × 1018;(d) 3.0 × 1018.Flying
Positive charge Q is distributed uniformly around a very thin conducting ring of radius a, as in Fig. 21.23. You measure the electric field E at points on the ring axis, at a distance x from the
Two small spheres, each carrying a net positive charge, are separated by 0.400 m. You have been asked to perform measurements that will allow you to determine the charge on each sphere. You set up a
In a region where there is a uniform electric field that is upward and has magnitude 3.60 × 104 N/C, a small object is projected upward with an initial speed of 1.92 m/s. The object travels upward a
The earth has a downward-directed electric field near its surface of about 150 N/C. If a raindrop with a diameter of 0.020 mm is suspended, motionless, in this field, how many excess electrons must
A nerve signal is transmitted through a neuron when an excess of Na+ ions suddenly enters the axon, a long cylindrical part of the neuron. Axons are approximately 10.0 mm in diameter, and
Two point charges Q and +q (where q is positive) produce the net electric field shown at point P in Fig. E21.36. The field points parallel to the line connecting the two charges.(a) What can you
The air temperature and the velocity of the air have different values at different places in the earth’s atmosphere. Is the air velocity a vector field? Why or why not? Is the air temperature a
The electric fields at point P due to the positive charges q1and q2are shown in Fig. Q21.22. Does the fact that they cross each other violate the statement in Section 21.6 that electric field lines
Refer to Exercise 21.21. Figure E21.22 shows the bonding of cytosine and guanine. The O€”H and H€”N distances are each 0.110 nm. In this case, assume that the bonding is due only
Sufficiently strong electric fields can cause atoms to become positively ionized that is, to lose one or more electrons. Explain how this can happen. What determines how strong the field must be to
The two sides of the DNA double helix are connected by pairs of bases (adenine, thymine, cytosine, and guanine). Because of the geometric shape of these molecules, adenine bonds with thymine and
Atomic nuclei are made of protons and neutrons. This shows that there must be another kind of interaction in addition to gravitational and electric forces. Explain.
Two irregular objects A and B carry charges of opposite sign. Figure Q21.19 shows the electric field lines near each of these objects.(a) Which object is positive, A or B? How do you know?(b) Where
What similarities do electric forces have with gravitational forces? What are the most significant differences?
In Example 21.1 (Section 21.3) we saw that the electric force between two a particles is of the order of 1035 times as strong as the gravitational force. So why do we readily feel the gravity of the
A proton is placed in a uniform electric field and then released. Then an electron is placed at this same point and released. Do these two particles experience the same force? The same acceleration?
A point charge of mass m and charge Q and another point charge of mass m but charge 2Q are released on a frictionless table. If the charge Q has an initial acceleration a0, what will be the
When two point charges of equal mass and charge are released on a frictionless table, each has an initial acceleration (magnitude) a0. If instead you keep one fixed and release the other one, what
You have a negatively charged object. How can you use it to place a net negative charge on an insulated metal sphere? To place a net positive charge on the sphere?
If you walk across a nylon rug and then touch a large metal object such as a doorknob, you may get a spark and a shock. Why does this tend to happen more on dry days than on humid days? Why are you
Two identical metal objects are mounted on insulating stands. Describe how you could place charges of opposite sign but exactly equal magnitude on the two objects.
Good conductors of electricity, such as metals, are typically good conductors of heat; insulators, such as wood, are typically poor conductors of heat. Explain why there is a relationship between
Your clothing tends to cling together after going through the dryer. Why? Would you expect more or less clinging if all your clothing were made of the same material (say, cotton) than if you dried
You have a pure (24-karat) gold ring of mass 10.8 g. Gold has an atomic mass of 197 g/mol and an atomic number of 79.(a) How many protons are in the ring, and what is their total positive charge?(b)
The electric force between two charged particles becomes weaker with increasing distance. Suppose instead that the electric force were independent of distance. In this case, would a charged comb
If a proton and an electron are released when they are 2.0 × 10-10 m apart (a typical atomic distance), find the initial acceleration of each particle.
Two metal spheres are hanging from nylon threads. When you bring the spheres close to each other, they tend to attract. Based on this information alone, discuss all the possible ways that the spheres
If you peel two strips of transparent tape off the same roll and immediately let them hang near each other, they will repel each other. If you then stick the sticky side of one to the shiny side of
Ocean thermal energy conversion is a process that uses the temperature difference between the warm surface water of tropical oceans and the cold deep ocean water to run a heat engine. The graph shows
Compare the entropy change of the warmer water to that of the colder water during one cycle of the heat engine, assuming an ideal Carnot cycle.(a) The entropy does not change during one cycle in
What is the change in entropy of the ammonia vaporized per second in the 10-MW power plant, assuming an ideal Carnot efficiency of 6.5%?(a) +6 Ã— 106 J/K per second;(b) +5 Ã—
If the power plant uses a Carnot cycle and the desired theoretical efficiency is 6.5%, from what depth must cold water be brought?(a) 100 m;(b) 400 m;(c) 800 m;(d) deeper than 1000 m.Ocean thermal
You are conducting experiments to study prototype heat engines. In one test, 4.00 mol of argon gas are taken around the cycle shown in Fig. P20.57. The pressure is low enough for the gas to be
For a refrigerator or air conditioner, the coefficient of performance K (often denoted as COP) is, as in Eq. (20.9), the ratio of cooling output |QC| to the required electrical energy input |W| ,
In your summer job with a venture capital firm, you are given funding requests from four inventors of heat engines. The inventors claim the following data for their operating prototypes:(a) Based on
The pV-diagram in Fig. P20.51 shows the cycle for a refrigerator operating on 0.850 mol of H2. Assume that the gas can be treated as ideal. Process ab is isothermal. Find the coefficient of
Premium gasoline produces 1.23 × 108 J of heat per gallon when it is burned at approximately 400°C (although the amount can vary with the fuel mixture). If a car’s engine is 25% efficient,
A growing plant creates a highly complex and organized structure out of simple materials such as air, water, and trace minerals. Does this violate the second law of thermodynamics? Why or why not?
If you run a movie film backward, it is as if the direction of time were reversed. In the time-reversed movie, would you see processes that violate conservation of energy? Conservation of linear
Suppose that you put a hot object in thermal contact with a cold object and observe (much to your surprise) that heat flows from the cold object to the hot object, making the cold one colder and the
Are the earth and sun in thermal equilibrium? Are there entropy changes associated with the transmission of energy from the sun to the earth? Does radiation differ from other modes of heat transfer
The free expansion of an ideal gas is an adiabatic process and so no heat is transferred. No work is done, so the internal energy does not change. Thus, Q/T = 0, yet the randomness of the system and
Explain why each of the following processes is an example of increasing randomness: mixing hot and cold water; free expansion of a gas; irreversible heat flow; developing heat by mechanical friction.
How can the thermal conduction of heat from a hot object to a cold object increase entropy when the same amount of heat that flows out of the hot object flows into the cold one?
In Example 20.4, a Carnot refrigerator requires a work input of only 230 J to extract 346 J of heat from the cold reservoir. Doesn’t this discrepancy imply a violation of the law of conservation of
Does a refrigerator full of food consume more power if the room temperature is 20°C than if it is 15°C? Or is the power consumption the same? Explain your reasoning.
Real heat engines, like the gasoline engine in a car, always have some friction between their moving parts, although lubricants keep the friction to a minimum. Would a heat engine with completely
What would be the efficiency of a Carnot engine operating with TH = TC? What would be the efficiency if TC = 0 K and TH were any temperature above 0 K? Interpret your answers.
The efficiency of heat engines is high when the temperature difference between the hot and cold reservoirs is large. Refrigerators, on the other hand, work better when the temperature difference is
Compare the pV-diagram for the Otto cycle in Fig. 20.6 with the diagram for the Carnot heat engine in Fig. 20.13. Explain some of the important differences between the two cycles.Fig. 20.6 Otto cycle
When a wet cloth is hung up in a hot wind in the desert, it is cooled by evaporation to a temperature that may be 20 C° or so below that of the air. Discuss this process in light of the second law
Imagine a special air filter placed in a window of a house. The tiny holes in the filter allow only air molecules moving faster than a certain speed to exit the house, and allow only air molecules
Is it a violation of the second law of thermodynamics to convert mechanical energy completely into heat? To convert heat completely into work? Explain your answers.
Why must a room air conditioner be placed in a window rather than just set on the floor and plugged in? Why can a refrigerator be set on the floor and plugged in?
Suppose you try to cool the kitchen of your house by leaving the refrigerator door open. What happens? Why? Would the result be the same if you left open a picnic cooler full of ice? Explain the
Household refrigerators have arrays or coils of tubing on the outside, usually at the back or bottom. When the refrigerator is running, the tubing becomes quite hot. Where does the heat come from?

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