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physics
university physics
Questions and Answers of
University Physics
We have seen that a coulomb is an enormous amount of charge; it is virtually impossible to place a charge of 1 C on an object. Yet, a current of 10 A, 10 C/s, is quite reasonable. Explain this
Batteries are always labeled with their emf; for instance, an AA flashlight battery is labeled “1.5 volts.” Would it also be appropriate to put a label on batteries stating how much current they
A rule of thumb used to determine the internal resistance of a source is that it is the open-circuit voltage divided by the short-circuit current. Is this correct? Why or why not?
Can the potential difference between the terminals of a battery ever be opposite in direction to the emf ? If it can, give an example. If it cannot, explain why not.
When is a 1.5-V AAA battery not actually a 1.5-V battery? That is, when do its terminals provide a potential difference of less than 1.5 V?
Two copper wires with different diameters are joined end to end. If a current flows in the wire combination, what happens to electrons when they move from the larger-diameter wire into the
A cylindrical rod has resistivity ρ. If we triple its length and diameter, what is its resistivity, in terms of ρ?
A cylindrical rod has resistance R. If we triple its length and diameter, what is its resistance, in terms of R?
During lightning strikes from a cloud to the ground, currents as high as 25,000 A can occur and last for about 40 ms. How much charge is transferred from the cloud to the earth during such a strike?
The definition of resistivity (r = E/J) implies that an electric field exists inside a conductor. Yet we saw in Chapter 21 that there can be no electrostatic electric field inside a conductor. Is
What is the minimum amount of work that must be done by the cell to restore Vm to -70 mV?(a) 3 mJ;(b) 3 µJ;(c) 3 nJ;(d) 3 pJ.Upon fertilization, the eggs of many species undergo a rapid change in
Suppose that the change in Vm was caused by the entry of Ca2+ instead of Na+. How many Ca2+ ions would have to enter the cell per unit membrane to produce the change?(a) Half as many as for Na+;(b)
Suppose that the egg has a diameter of 200 mm. What fractional change in the internal Na+ concentration results from the fertilization-induced change in Vm? Assume that Na+ ions are distributed
How many moles of Na+ must m ove per unit area of membrane to change Vm from -70 mV to +30 mV, if we assume that the membrane behaves purely as a capacitor?(a) 10-4 mol/cm2;(b) 10-9 mol/cm2;(c) 10-12
You are conducting experiments with an airfilled parallel-plate capacitor. You connect the capacitor to a battery with voltage 24.0 V. Initially the separation d between the plates is 0.0500 cm. In
You are designing capacitors for various applications. For one application, you want the maximum possible stored energy. For another, you want the maximum stored charge. For a third application, you
Your electronics company has several identical capacitors with capacitance C1 and several others with capacitance C2. You must determine the values of C1 and C2 but don’t have access to
A parallel-plate capacitor has square plates that are 8.00 cm on each side and 3.80 mm apart. The space between the plates is completely filled with two square slabs of dielectric, each 8.00 cm on a
A potential difference Vab= 48.0 V is applied across the capacitor network of Fig. E24.17. If C1= C2= 4.00 µF and C4= 8.00 µF, what must the capacitance C3be if the network is to store
In Fig. E24.20, C1= 3.00 µF and Vab= 150 V. The charge on capacitor C1is 150 µC and the charge on C3is 450 µC. What are the values of the capacitances of C2and C3?Fig.24.20 (C)
Polystyrene has dielectric constant 2.6 and dielectric strength 2.0 × 107 V/m. A piece of polystyrene is used as a dielectric in a parallel-plate capacitor, filling the volume between the plates.(a)
Some cell walls in the human body have a layer of negative charge on the inside surface and a layer of positive charge of equal magnitude on the outside surface. Suppose that the charge density on
The two plates of a capacitor are given charges ±Q. The capacitor is then disconnected from the charging device so that the charges on the plates can’t change, and the capacitor is immersed in a
A conductor is an extreme case of a dielectric, since if an electric field is applied to a conductor, charges are free to move within the conductor to set up “induced charges.” What is the
Liquid dielectrics that have polar molecules (such as water) always have dielectric constants that decrease with increasing temperature. Why?
A parallel-plate capacitor is connected to a power supply that maintains a fixed potential difference between the plates.(a) If a sheet of dielectric is then slid between the plates, what happens
In Fig. 24.8a, let C1= 3.00 µF, C2= 5.00 µF, and Vab= +64.0 V. Calculate(a) The charge on each capacitor and(b) The potential difference across each capacitor.Fig.24.3a Capacitors in
In terms of the dielectric constant K, what happens to the electric flux through the Gaussian surface shown in Fig. 24.22 when the dielectric is inserted into the previously empty space between the
Electrolytic capacitors use as their dielectric an extremely thin layer of nonconducting oxide between a metal plate and a conducting solution. Discuss the advantage of such a capacitor over one
Suppose you bring a slab of dielectric close to the gap between the plates of a charged capacitor, preparing to slide it between the plates. What force will you feel? What does this force tell you
A capacitor made of aluminum foil strips separated by Mylar film was subjected to excessive voltage, and the resulting dielectric breakdown melted holes in the Mylar. After this, the capacitance was
Is dielectric strength the same thing as dielectric constant? Explain any differences between the two quantities. Is there a simple relationship between dielectric strength and dielectric
As shown in Table 24.1, water has a very large dielectric constant K = 80.4. Why do you think water is not commonly used as a dielectric in capacitors?Table 24.1 Values of Dielectric Constant Kat
You have two capacitors and want to connect them across a voltage source (battery) to store the maximum amount of energy. Should they be connected in series or in parallel?
The charged plates of a capacitor attract each other, so to pull the plates farther apart requires work by some external force. What becomes of the energy added by this work? Explain.
Two parallel-plate capacitors, identical except that one has twice the plate separation of the other, are charged by the same voltage source. Which capacitor has a stronger electric field between the
A parallel-plate capacitor is charged by being connected to a battery and is then disconnected from the battery. The separation between the plates is then doubled. How does the electric field change?
A parallel-plate capacitor is charged by being connected to a battery and is kept connected to the battery. The separation between the plates is then doubled. How does the electric field change? The
In the parallel-plate capacitor of Fig. 24.2, suppose the plates are pulled apart so that the separation d is much larger than the size of the plates.(a) Is it still accurate to say that the electric
To store the maximum amount of energy in a parallelplate capacitor with a given battery (voltage source), would it be better to have the plates far apart or close together?
Suppose the two plates of a capacitor have different areas. When the capacitor is charged by connecting it to a battery, do the charges on the two plates have equal magnitude, or may they be
Suppose several different parallel-plate capacitors are charged up by a constant voltage source. Thinking of the actual movement and position of the charges on an atomic level, why does it make sense
Equation (24.2) shows that the capacitance of a parallelplate capacitor becomes larger as the plate separation d decreases. However, there is a practical limit to how small d can be made, which
The maximum voltage at the center of a typical tandem electrostatic accelerator is 6.0 MV. If the distance from one end of the acceleration tube to the midpoint is 12 m, what is the magnitude of the
A helium ion (He++) that comes within about 10 fm of the center of the nucleus of an atom in the sample may induce a nuclear reaction instead of simply scattering. Imagine a helium ion with a kinetic
For a particular experiment, helium ions are to be given a kinetic energy of 3.0 MeV. What should the voltage at the center of the accelerator be, assuming that the ions start essentially at rest?(a)
A small, stationary sphere carries a net charge Q. You perform the following experiment to measure Q: From a large distance you fire a small particle with mass m = 4.00 Ã 10-4kg and
An alpha particle with kinetic energy 9.50 MeV (when far away) collides headon with a lead nucleus at rest. What is the distance of closest approach of the two particles? (Assume that the lead
Charge Q = +4.00 µC is distributed uniformly over the volume of an insulating sphere that has radius R = 5.00 cm. What is the potential difference between the center of the sphere and the surface of
A proton and an alpha particle are released from rest when they are 0.225 nm apart. The alpha particle (a helium nucleus) has essentially four times the mass and two times the charge of a proton.
When radium-226 decays radioactively, it emits an alpha particle (the nucleus of helium), and the end product is radon-222. We can model this decay by thinking of the radium-226 as consisting of an
A small sphere with mass 5.00 × 10-7 kg and charge +7.00 µC is released from rest a distance of 0.400 m above a large horizontal insulating sheet of charge that has uniform surface charge density
A gold nucleus has a radius of 7.3 × 10-15 m and a charge of +79e. Through what voltage must an alpha particle, with charge +2e, be accelerated so that it has just enough energy to reach a distance
A positive point charge q1 = +5.00 × 10-4 C is held at a fixed position. A small object with mass 4.00 × 10-3 kg and charge q2 = -3.00 × 10-4 C is projected directly at q1. Ignore gravity. When q2
A point charge q1 = +5.00 µC is held fixed in space. From a horizontal distance of 6.00 cm, a small sphere with mass 4.00 × 10-3 kg and charge q2 = +2.00 µC is fired toward the fixed charge with
In a certain region of space the electric potential is given by V = +Ax2y - Bxy2, where A = 5.00 V/m3 and B = 8.00 V/m3. Calculate the magnitude and direction of the electric field at the point
Certain sharks can detect an electric field as weak as 1.0 µ/m. To grasp how weak this field is, if you wanted to produce it between two parallel metal plates by connecting an ordinary 1.5V AA
A very small sphere with positive charge q = +8.00 µC is released from rest at a point 1.50 cm from a very long line of uniform linear charge density l = +3.00 µC/m. What is the kinetic energy of
Charge Q = 5.00 µC is distributed uniformly over the volume of an insulating sphere that has radius R = 12.0 cm. A small sphere with charge q = +3.00 µC and mass 6.00 × 10-5 kg is projected toward
A thin spherical shell with radius R1 = 3.00 cm is concentric with a larger thin spherical shell with radius R2 = 5.00 cm. Both shells are made of insulating material. The smaller shell has charge q1
A positive point charge is placed near a very large conducting plane. A professor of physics asserted that the field caused by this configuration is the same as would be obtained by removing the
When a thunderstorm is approaching, sailors at sea sometimes observe a phenomenon called “St. Elmo’s fire,” a bluish flickering light at the tips of masts. What causes this? Why does it occur
A highvoltage dc power line falls on a car, so the entire metal body of the car is at a potential of 10,000 V with respect to the ground. What happens to the occupants (a) When they are sitting
A conductor that carries a net charge Q has a hollow, empty cavity in its interior. Does the potential vary from point to point within the material of the conductor? What about within the cavity? How
A conducting sphere is placed between two charged parallel plates such as those shown in Fig. 23.2. Does the electric field inside the sphere depend on precisely where between the plates the sphere
Point charges q1 = +2.00 µC and q2 = -2.00 µC are placed at adjacent corners of a square for which the length of each side is 3.00 cm. Point a is at the center of the square, and point b is at the
In electronics it is customary to define the potential of ground (thinking of the earth as a large conductor) as zero. Is this consistent with the fact that the earth has a net electric charge that
A conducting sphere is to be charged by bringing in positive charge a little at a time until the total charge is Q. The total work required for this process is alleged to be proportional to Q2. Is
We often say that if point A is at a higher potential than point B, A is at positive potential and B is at negative potential. Does it necessarily follow that a point at positive potential is
Because electric field lines and equipotential surfaces are always perpendicular, two equipotential surfaces can never cross; if they did, the direction of EÌ (vector) would be ambiguous
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 «EÌ (vector) 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 adeninethymine 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
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