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essential university physics
Essential University Physics 3rd Edition Volume 2 Richard Wolfsonby - Solutions
In some cities, streetlights are wired in such a way that when one burns out, they all go out. Are the lights in series or parallel?
When the switch in Fig. 25.25 is open, what’s the voltage across the resistor? Across the switch? FIGURE 25.25 For Thought and Discussion 6
Two identical resistors in series dissipate equal power. How can this be, when electric charge loses energy in flowing through the first resistor?
When a large electric load such as a washing machine or oven comes on, lights throughout a house often dim. Why?
How would you connect a pair of equal resistors across an ideal battery in order to get the greatest power dissipation?
You have a battery whose voltage and internal resistance are unknown. Using an ideal voltmeter and an ideal ammeter, how would you determine each of these characteristics?
A student who’s confused about voltage and current hooks a nearly ideal ammeter across a car battery. What happens?
A student who’s confused about voltage and current tries to measure the voltage across a lighted lightbulb by inserting a voltmeter in series with the bulb. What happens to the bulb?
An unmagnetized piece of iron has no net magnetic dipole moment, yet it’s attracted to either pole of a bar magnet. Why?
Would there be a magnetic force on a piece of iron deep inside a long solenoid? Explain.
Why is a piece of iron attracted into a solenoid?
Figure 26.38 shows some magnetic field lines associated with two parallel wires carrying equal currents perpendicular to the page. Are the currents in the same or opposite directions? How can you tell? The only currents in Fig. 26.38 are those in the two wires. FIGURE 26.38 For Thought and
If a current is passed through an unstretched spring, will the spring contract or expand? Explain.
Do currents in the same direction attract or repel? Explain.
The Biot–Savart law shows that the magnetic field of a current element decreases as 1/r2 . Could you put together a complete circuit whose field exhibits this decrease? Why or why not?
In what two senses does a current loop behave like a magnetic dipole?
Two identical particles carrying equal charge are moving in opposite directions, perpendicular to a uniform magnetic field, when they collide elastically head-on. Describe their subsequent motion.
An electron and a proton moving at the same speed enter a region containing a uniform magnetic field. Which is deflected more from its original path?
Do particles in a cyclotron gain energy from the electric field, the magnetic field, or both? Explain.
A magnetic field points out of this page. Will a positively charged particle moving in the plane of the page circle clockwise or counterclockwise as viewed from above?
An electron moving with velocity v(vector)through a magnetic field B(vector) experiences a magnetic force F(vector) . Which of the vectors F(vector), v(vector) , and B(vector) must be at right angles?
A charged particle moves through a region containing only a magnetic field. Under what condition will it experience no force?
In Fig. 27.35, a bar magnet moves toward a conducting ring. What’s the direction of the induced current in the ring? FIGURE 27.35 For Thought and Discussion 1
Figure 27.36 shows two concentric conducting loops, the outer connected to a battery and a switch. The switch is initially open. It’s then closed, left closed for a while, and then reopened. Describe the currents in the inner loop during the entire procedure. FIGURE 27.36 For Thought and
Fluctuations in Earth’s magnetic field due to changing solar activity can wreak havoc with communications, even those using underground cables. How is this possible?
A static magnetic field cannot change the energy of a charged particle. Is this true of a changing magnetic field? Discuss.
Can an induced electric field exist in the absence of a conductor?
A car battery has a 12-V emf, yet energy from the battery provides the 30,000-V spark that ignites the gasoline. How is this possible?
You have a fixed length of wire to wind into an inductor. Will you get more inductance if you wind a short coil with large diameter, or a long coil with small diameter?
In a popular demonstration of induced emf, a lightbulb is connected across a large inductor in an RL circuit, as shown in Fig. 27.37. When the switch is opened, the bulb flashes brightly and may even burn out. Why? Lightbulb FIGURE 27.37 For Thought and Discussion 8
List some similarities and differences between inductors and capacitors.
A 1-H inductor carries 10 A, and a 10-H inductor carries 1A. Which contains more stored energy?
It takes work to push two bar magnets together with like poles facing. Where does this energy go?
A small magnet is dropped into each of two hollow vertical tubes of equal length, one made of copper and one of aluminum. Does it take longer for the magnet to fall through the aluminum tube or the copper tube, or does it take the same amount of time for each?
Figures 27.1b and 27.2 actually describe the same situation, just from the viewpoints of two different inertial reference frames. In Fig. 27.2, in the reference frame of the magnet, you can think of the induced current as arising from the magnetic force on the electrons in the coil (motional emf).
Two AC signals have the same amplitude but different frequencies. Are their rms amplitudes the same?
What’s meant by the statement, “A capacitor acts like a DC open circuit”?
There’s an insulating gap between capacitor plates, so how can current flow in an AC circuit containing a capacitor?
Why does it make sense that inductive reactance increases with frequency?
The same AC voltage appears across a capacitor and a resistor, and the same rms current flows in each. Is the power dissipation the same in each?
When a particular inductor and capacitor are connected across the same AC voltage, the current in the inductor is higher than in the capacitor. Is this true at all frequencies?
An inductor and capacitor are connected in series across an AC generator, and the voltage across the inductor is higher than across the capacitor. Is the generator frequency above or below resonance?
When the capacitor voltage in an undriven LC circuit reaches zero, why don’t the oscillations stop?
Why is Equation 28.5 not a full description of the relation between voltage and current in a capacitor? Vp (28.5) Xc 1/@C
The applied voltage in a series RLC circuit lags the current. Is the frequency above or below resonance?
The voltage across two components in series is zero. Is it possible that the voltages across the individual components aren’t zero? Give an example.
If you measure the rms voltages across the resistor, capacitor, and inductor in a series RLC circuit, will they add to the rms generator voltage?
A step-up transformer increases voltage, or energy per unit charge. Why doesn’t this violate energy conservation?
Why is Maxwell’s modification of Ampere's law essential to the existence of electromagnetic waves?
The presence of magnetic monopoles would require a modification of Gauss’s law for magnetism. Which other Maxwell equation would need modification?
Is there displacement current in an electromagnetic wave? Is there ordinary conduction current?
List some similarities and differences between electromagnetic waves and sound waves.
The speed of an electromagnetic wave is given by c = λf. How does the speed depend on frequency? On wavelength?
When astronomers observe a supernova explosion in a distant galaxy, they see a sudden, simultaneous rise in visible light and other forms of electromagnetic radiation. How is this evidence that the speed of light is independent of frequency?
Turning a TV antenna so its rods point vertically may change the quality of your TV reception. Why?
The Sun emits about half of its electromagnetic-wave energy in the visible region of the spectrum. Where do you think it emits most of the remainder?
An LC circuit is made entirely from superconducting materials, yet its oscillations eventually damp out. Why?
If you double the field strength in an electromagnetic wave, what happens to the intensity?
The intensity of light drops as the inverse square of the distance from the source. Does this mean that electromagnetic energy is lost? Explain.
Electromagnetic waves don’t readily penetrate metals. Why not?
Why is it usually inappropriate to consider low-frequency sound waves as traveling in rays? Why is the ray approximation more appropriate for high-frequency sound and for light?
Why does a spoon appear bent when it’s in a glass of water?
Why do a diamond and an identically shaped piece of glass sparkle differently?
White light goes from air through a glass slab with parallel surfaces. Will its colors be dispersed when it emerges from the glass?
You send white light through two identical glass prisms, oriented as shown in Fig. 30.17. Describe the beam that emerges from the right-hand prism. White light FIGURE 30.17 For Thought and Discussion 5
In glass, which end of the visible spectrum has the smallest critical angle for total internal reflection?
Why can’t you walk to the end of the rainbow?
An attempt to rework Example 30.4 with an isosceles prism results in imaginary numbers for the two angles of refraction. What could this mean?
Why are polarizing sunglasses better than glasses that simply reduce the total amount of light?
Under what conditions will the polarizing angle be smaller than 45°?
Give at least three reasons why reflecting telescopes are superior to refractors.
Do you want a long or short focal length for a telescope’s objective lens? What about a microscope’s?
The refractive index of the human cornea is about 1.4. If you can see clearly in air, why can’t you see clearly underwater? Why do goggles help?
A block of ice contains a hollow, air-filled space in the shape of a double-convex lens. Describe the optical behavior of this space.
Does a fish in a spherical bowl appear larger or smaller than it actually is?
Is the image on a movie screen real or virtual? How do you know?
If you look into the bowl of a metal spoon, you see yourself upside down. Flip the spoon so you’re looking at the back side, and now you’re right-side up. Explain.
If you placed a screen at the location of a virtual image, would the image appear on the screen? Why or why not?
Can a concave mirror make a reduced real image? A reduced virtual image? An enlarged real image? An enlarged virtual image? Specify conditions for each possible image.
Is there any limit to the temperature you can achieve by focusing sunlight? (Think about the second law of thermodynamics.)
A diverging lens always makes a reduced image. Could you use such a lens to start a fire by focusing sunlight? Explain.
If you’re handed a converging lens, what can you do to estimate its focal length quickly?
Under what circumstances will the image in a concave mirror be the same size as the object?
How can you see a virtual image, when it’s not “really there”?
Why should the test charge used to measure an electric field be small?
A free neutron is unstable and soon decays to other particles, one of them a proton. Must there be others? If so, what electric properties must it or they have?
If a charged particle were released from rest on a curved field line, would its subsequent motion follow the field line? Explain.
Eight field lines emerge from a closed surface surrounding an isolated point charge. Would the number of field lines change if a second identical charge were brought to a point just outside the surface? If not, would anything change? Explain.
Under what conditions can the electric flux through a surface be written as EA,where A is the surface area?
If the flux of the gravitational field through a closed surface is zero, what can you conclude about the region interior to the surface?
The electric flux through a closed surface is zero. Must the electric field be zero on that surface? If not, give an example.
Can electric field lines ever cross? Why or why not?
Dipoles A and B are both located in the field of a point charge Q, as shown in Fig. 20.27. Does either experience a net torque? A net force? If each dipole is released from rest, describe qualitatively its subsequent motion. +q +9 +Q FIGURE 20.27 For Thought and Discussion 12
Why should there be a force between two dipoles, which each have zero net charge?
Under what circumstances is the path of a charged particle a parabola? A circle?
A spherical balloon is initially uncharged. If you spread positive charge uniformly over the balloon’s surface, would it expand or contract? What would happen if you spread negative charge instead?
The ring in Example 20.6 carries total charge Q, and the point P is the same distance r = √x2 + a2 from all parts of the ring. So why isn’t the electric field of the ring just kQ/r2?
Why does a dipole, which has no net charge, produce an electric field?
Equation 20.3 gives the electric field of a point charge. Does the direction of(a). r̂ nor(b). E(vector) depend on whether the charge is positive or negative? kq (field of a point charge) 9test
Where in Fig. 20.5 could you put a third charge so it would experience no net force? Would it be in stable or unstable equilibrium? 9. = 1.0µC 92 =-1.5MC -x(cm) The charges have opposite signs, so q192 is negative, and F is opposite the direction of î. The unit vector is in the direction from q,
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