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physics
university physics

Questions and Answers of University Physics

In Fig. E26.11 the battery has emf 35.0 V and negligible internal resistance. R1= 5.00 Î©. The current through R1is 1.50 A, and the current through R3= 4.50 A. What are the resistances
Consider the circuit shown in Fig. Q26.12. What happens to the brightnesses of the bulbs when the switch S is closed if the battery(a) Has no internal resistance and(b) Has nonnegligible internal
In Fig. E26.11, R1= 3.00 Î©, R2= 6.00 Î©, and R3= 5.00 Î©. The battery has negligible internal resistance. The current I2through R2is 4.00 A.(a) What are the currents
A real battery, having nonnegligible internal resistance, is connected across a light bulb as shown in Fig. Q26.10. When the switch S is closed, what happens to the brightness of the bulb? Why?Figure
A light bulb is connected in the circuit shown in Fig. Q26.9. If we close the switch S, does the bulb€™s brightness increase, decrease, or remain the same? Explain why.Figure Q26.9 ww
A resistor consists of three identical metal strips connected as shown in Fig. Q26.8. If one of the strips is cut out, does the ammeter reading increase, decrease, or stay the same? Why?Figure Q26.8
For the circuit shown in Fig. E26.7 find the reading of the idealized ammeter if the battery has an internal resistance of 3.26 Î©.Figure E26.7 45.0 0 25.0 V 18.0 N -w 15.0 N
A battery with no internal resistance is connected across identical light bulbs as shown in Fig. Q26.7. When you close the switch S, will the brightness of bulbs B1 and B2change? If so, how will
If two resistors R1and R2(R2> R1) are connected in parallel as shown in Fig. Q26.6, which of the following must be true? In each case justify your answer.(a) I1 = I2.(b) I3 = I4.(c) The current is
If two resistors R1and R2(R2> R1) are connected in series as shown in Fig. Q26.5, which of the following must be true? In each case justify your answer.(a) I1 = I2 = I3.(b) The current is greater
In the circuit shown in Fig. Q26.4, three identical light bulbs are connected to a flashlight battery. How do the brightnesses of the bulbs compare? Which light bulb has the greatest current passing
You connect a number of identical light bulbs to a flashlight battery.(a) What happens to the brightness of each bulb as more and more bulbs are added to the circuit if you connect them(i) In series
Two 120-V light bulbs, one 25-W and one 200-W, were connected in series across a 240-V line. It seemed like a good idea at the time, but one bulb burned out almost immediately. Which one burned out,
The text states that good thermal conductors are also good electrical conductors. If so, why don’t the cords used to connect toasters, irons, and similar heat producing appliances get hot by
High-voltage power supplies are sometimes designed intentionally to have rather large internal resistance as a safety precaution. Why is such a power supply with a large internal resistance safer
A fuse is a device designed to break a circuit, usually by melting when the current exceeds a certain value. What characteristics should the material of the fuse have?
Ordinary household electric lines in North America usually operate at 120 V. Why is this a desirable voltage, rather than a value considerably larger or smaller? On the other hand, automobiles
Long-distance, electric-power, transmission lines always operate at very high voltage, sometimes as much as 750 kV. What are the advantages of such high voltages? What are the disadvantages?
Small aircraft often have 24-V electrical systems rather than the 12-V systems in automobiles, even though the electrical power requirements are roughly the same in both applications. The explanation
Eight flashlight batteries in series have an emf of about 12 V, similar to that of a car battery. Could they be used to start a car with a dead battery? Why or why not?
The energy that can be extracted from a storage battery is always less than the energy that goes into it while it is being charged. Why?
A ductile metal wire has resistance R. What will be the resistance of this wire in terms of R if it is stretched to three times its original length, assuming that the density and resistivity of the
(See Discussion Question Q25.14.) An ideal ammeter A is placed in a circuit with a battery and a light bulb as shown in Fig. Q25.15a, and the ammeter reading is noted. The circuit is then reconnected
A light bulb glows because it has resistance. The brightness of a light bulb increases with the electrical power dissipated in the bulb.(a) In the circuit shown in Fig. Q25.14a, the two bulbs A and B
A 14-gauge copper wire of diameter 1.628 mm carries a current of 12.5 mA.(a) What is the potential difference across a 2.00-m length of the wire?(b) What would the potential difference in part (a) be
Why does an electric light bulb nearly always burn out just as you turn on the light, almost never while the light is shining?
Which of the graphs in Fig. Q25.12 best illustrates the current I in a real resistor as a function of the potential difference V across it? Explain.Figure Q25.12 (a) (b) (c) (d)
Temperature coefficients of resistivity are given in Table 25.2.(a) If a copper heating element is connected to a source of constant voltage, does the electrical power consumed by the heating element
Electrons in an electric circuit pass through a resistor. The wire on either side of the resistor has the same diameter.(a) How does the drift speed of the electrons before entering the resistor
Nerve cells transmit electric signals through their long tubular axons. These signals propagate due to a sudden rush of Na+ ions, each with charge +e, into the axon. Measurements have revealed that
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

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