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physics
university physics
Questions and Answers of
University Physics
A beam of light goes from one material into another. On physical grounds, explain why the wavelength changes but the frequency and period do not.
Sunlight or starlight passing through the earth’s atmosphere is always bent toward the vertical. Why? Does this mean that a star is not really where it appears to be? Explain.
Light requires about 8 minutes to travel from the sun to the earth. Is it delayed appreciably by the earth’s atmosphere? Explain.
Doubling the frequency of a wave in the range of 25 Hz to 3 kHz represents what change in the maximum allowed electromagnetic-wave intensity?(a) A factor of 2;(b) A factor of 1/√2;(c) A factor of
In the United States, household electrical power is provided at a frequency of 60 Hz, so electromagnetic radiation at that frequency is of particular interest. On the basis of the ICNIRP guidelines,
As a physics lab instructor, you conduct an experiment on standing waves of microwaves, similar to the standing waves produced in a microwave oven. A transmitter emits microwaves of frequency f . The
Because the speed of light in vacuum (or air) has such a large value, it is very difficult to measure directly. To measure this speed, you conduct an experiment in which you measure the amplitude of
The energy flow to the earth from sunlight is about 1.4 kW/m2.(a) Find the maximum values of the electric and magnetic fields for a sinusoidal wave of this intensity.(b) The distance from the earth
Does an electromagnetic standing wave have energy? Does it have momentum? Are your answers to these questions the same as for a traveling wave? Why or why not?
A light source radiates a sinusoidal electromagnetic wave uniformly in all directions. This wave exerts an average pressure p on a perfectly reflecting surface a distance R away from it. What average
If a light beam carries momentum, should a person holding a flashlight feel a recoil analogous to the recoil of a rifle when it is fired? Why is this recoil not actually observed?
Most automobiles have vertical antennas for receiving radio broadcasts. Explain what this tells you about the direction of polarization of E̅(vector) in the radio waves used in broadcasting.
The magnetic-field amplitude of the electromagnetic wave from the laser described in Example 32.1 (Section 32.3) is about 100 times greater than the earth’s magnetic field. If you illuminate a
For a certain sinusoidal wave of intensity I, the amplitude of the magnetic field is B. What would be the amplitude (in terms of B) in a similar wave of twice the intensity?
The light beam from a searchlight may have an electricfield magnitude of 1000 V/m, corresponding to a potential difference of 1500 V between the head and feet of a 1.5-m-tall person on whom the light
Suppose that a positive point charge q is initially at rest on the x-axis, in the path of the electromagnetic plane wave described in Section 32.2. Will the charge move after the wave front reaches
Is polarization a property of all electromagnetic waves, or is it unique to visible light? Can sound waves be polarized? What fundamental distinction in wave properties is involved? Explain.
Medical x rays are taken with electromagnetic waves having a wavelength of around 0.10 nm in air. What are the frequency, period, and wave number of such waves?
Sometimes neon signs located near a powerful radio station are seen to glow faintly at night, even though they are not turned on. What is happening?
Give several examples of electromagnetic waves that are encountered in everyday life. How are they all alike? How do they differ?
When driving on the upper level of the Bay Bridge, westbound from Oakland to San Francisco, you can easily pick up a number of radio stations on your car radio. But when driving eastbound on the
By measuring the electric and magnetic fields at a point in space where there is an electromagnetic wave, can you determine the direction from which the wave came? Explain.
If the frequency at which the electrode is oscillated is increased to a very large value, the electrode’s impedance(a) Approaches infinity;(b) Approaches zero;(c) Approaches a constant but nonzero
The signal from the oscillating electrode is fed into an amplifier, which reports the measured voltage as an rms value, 1.5 nV. What is the potential difference between the two extremes?(a) 1.5
If the electrode oscillates between two points 20 mm apart at a frequency of (5000/π)Hz, what is the electrode’s impedance?(a) 0;(b) Infinite;(c) √2 × 104 Ω;(d) √2 × 106 Ω.An individual
What is the dc impedance of the electrode, assuming that it behaves as an ideal capacitor?(a) 0;(b) Infinite;(c) √2 × 104 Ω;(d) √2 × 106 Ω.An individual cell such as an egg cell (an ovum,
An ideal transformer has N1 windings in the primary and N2 windings in its secondary. If you double only the number of secondary windings, by what factor does(a) The voltage amplitude in the
Can a transformer be used with dc? Explain. What happens if a transformer designed for 120-V ac is connected to a 120-V dc line?
A circuit consists of a light bulb, a capacitor, and an inductor connected in series to an ac source. Is it possible for both the capacitor and the inductor to be removed and the brightness of the
A coil of wire wrapped on a hollow tube and a light bulb are connected in series to an ac source. What happens to the brightness of the bulb when an iron rod is inserted in the tube?
A light bulb and a parallel-plate capacitor with air between the plates are connected in series to an ac source. What happens to the brightness of the bulb when a dielectric is inserted between the
In Example 31.6 (Section 31.4), a hair dryer is treated as a pure resistor. But because there are coils in the heating element and in the motor that drives the blower fan, a hair dryer also has
When an L-R-C series circuit is connected across a 120-V ac line, the voltage rating of the capacitor may be exceeded even if it is rated at 200 or 400 V. How can this be?
In an L-R-C series circuit, what are the phase angle ϕ and power factor cos ϕ when the resistance is much smaller than the inductive or capacitive reactance and the circuit is operated far from
In an L-R-C series circuit, can the instantaneous voltage across the capacitor exceed the source voltage at that same instant? Can this be true for the instantaneous voltage across the inductor?
Is it possible for the power factor of an L-R-C series ac circuit to be zero? Justify your answer on physical grounds.
Equation (31.9) says that vab= L di/dt (see Fig. 31.8a). Using Faradays law, explain why point a is at higher potential than point b when i is in the direction shown in Fig. 31.8a and is
Fluorescent lights often use an inductor, called a ballast, to limit the current through the tubes. Why is it better to use an inductor rather than a resistor for this purpose?
Equation (31.14) was derived by using the relationship i = dq/dt between the current and the charge on the capacitor. In Fig. 31.9a the positive counterclockwise current increases the charge on the
In an ac circuit, why is the average power for an inductor and a capacitor zero, but not for a resistor?
The current in an ac power line changes direction 120 times per second, and its average value is zero. Explain how it is possible for power to be transmitted in such a system.
Household electric power in most of western Europe is supplied at 240 V, rather than the 120 V that is standard in the United States and Canada. What are the advantages and disadvantages of each
If all of the magnetic energy stored in this MRI magnet is converted to thermal energy, how much liquid helium will boil off? (a) 27 kg; (b) 38 kg; (c) 60 kg; (d) 110 kg.
If part of the magnet develops resistance and liquid helium boils away, rendering more and more of the magnet nonsuperconducting, how will this quench affect the time for the current to drop to half
If a small part of this magnet loses its superconducting properties and the resistance of the magnet wire suddenly rises from 0 to a constant 0.005 Ω, how much time will it take for the current to
How many turns does this typical MRI magnet have?(a) 1100;(b) 3000;(c) 4000;(d) 22,000.Magnets carrying very large currents are used to produce the uniform, large-magnitude magnetic fields that are
Consider the circuit in Exercise 30.23.(a) Just after the circuit is completed, at what rate is the battery supplying electrical energy to the circuit?(b) When the current has reached its final
In an L-R-C series circuit, what criteria could be used to decide whether the system is overdamped or underdamped? For example, could we compare the maximum energy stored during one cycle to the
Suppose there is a steady current in an inductor. If you attempt to reduce the current to zero instantaneously by quickly opening a switch, an arc can appear at the switch contacts. Why? Is it
In the R-L circuit shown in Fig. 30.11, is the current in the resistor always the same as the current in the inductor? How do you know?Fig.30.11 Closing switch S, connects the R-L combination in
A toroidal solenoid has mean radius 12.0 cm and crosssectional area 0.600 cm2.(a) How many turns does the solenoid have if its inductance is 0.100 mH?(b) What is the resistance of the solenoid if the
In the R-L circuit shown in Fig. 30.11, when switch S1 is closed, the potential vac changes suddenly and discontinuously, but the current does not. Explain why the voltage can change suddenly
In Section 30.5 the relationship i = dq/dt is used in deriving Eq. (30.20). But a flow of current corresponds to a decrease in the charge on the capacitor. Explain, therefore, why this is the correct
In Section 30.5 Kirchhoff’s loop rule is applied to an L-C circuit where the capacitor is initially fully charged and the equation -L (di/dt) - (q/C) = 0 is derived. But as the capacitor starts to
In an R-C circuit, a resistor, an uncharged capacitor, a dc battery, and an open switch are in series. In an R-L circuit, a resistor, an inductor, a dc battery, and an open switch are in series.
For the same magnetic field strength B, is the energy density greater in vacuum or in a magnetic material? Explain. Does Eq. (30.11) imply that for a long solenoid in which the current is I the
You are to make a resistor by winding a wire around a cylindrical form. To make the inductance as small as possible, it is proposed that you wind half the wire in one direction and the other half in
Two closely wound circular coils have the same number of turns, but one has twice the radius of the other. How are the self-inductances of the two coils related? Explain your reasoning.
Two identical, closely wound, circular coils, each having self-inductance L, are placed next to each other, so that they are coaxial and almost touching. If they are connected in series, what is the
The tightly wound toroidal solenoid is one of the few configurations for which it is easy to calculate self-inductance. What features of the toroidal solenoid give it this simplicity?
In Fig. 30.1, if coil 2 is turned 90° so that its axis is vertical, does the mutual inductance increase or decrease? Explain.Fig.30.1 Coil 1 Nj turns Coil 2 N2 turns Фв2 it
In an electric trolley or bus system, the vehicle’s motor draws current from an overhead wire by means of a long arm with an attachment at the end that slides along the overhead wire. A brilliant
Which graph in Fig. P29.74 best represents the time t dependence of the current i induced in the brain tissue, assuming that this tissue can be modeled as a resistive circuit? (The units of i are
It may be desirable to increase the maximum induced current in the brain tissue. In Fig. P29.73, which time-dependent graph of the magnetic field B in the coil achieves that goal? Assume that
Consider the brain tissue at the level of the dashed line to be a series of concentric circles, each behaving independently of the others. Where will the induced emf be the greatest?(a) At the center
In part (a) of the figure, a current pulse increases to a peak and then decreases to zero in the direction shown in the stimulating coil. What will be the direction of the induced current (dashed
Blood contains positive and negative ions and thus is a conductor. A blood vessel, therefore, can be viewed as an electrical wire. We can even picture the flowing blood as a series of parallel
A 0.250-m-long bar moves on parallel rails that are connected through a 6.00-Ω resistor, as shown in Fig. E29.33, so the apparatus makes a complete circuit. You can ignore the resistance
Consider the circuit shown in Fig. E29.31, but with the bar moving to the right with speed v. As in Exercise 29.31, the bar has length 0.360 m, R = 45.0 Ω, and B = 0.650 T.(a) Is the
A rectangular loop of wire with dimensions 1.50 cm by 8.00 cm and resistance R = 0.600 Ω is being pulled to the right out of a region of uniform magnetic field. The magnetic field has
If magnetic monopoles existed, the right-hand side of Eq. (29.20) would include a term proportional to the current of magnetic monopoles. Suppose a steady monopole current is moving in a long
Match the mathematical statements of Maxwell’s equations as given in Section 29.7 to these verbal statements.(a) Closed electric field lines are evidently produced only by changing magnetic
Your physics study partner asks you to consider a parallel plate capacitor that has a dielectric completely filling the volume between the plates. He then claims that Eqs. (29.13) and (29.14) show
Two closed loops A and C are close to a long wire carrying a current I (Fig. E29.17).(a) Find the direction (clockwise or counterclockwise) of the current induced in each loop if I is steadily
Can one have a displacement current as well as a conduction current within a conductor? Explain.
Does Faraday’s law say that a large magnetic flux induces a large emf in a coil? Explain.
Does Lenz’s law say that the induced current in a metal loop always flows to oppose the magnetic flux through that loop? Explain.
Small one-cylinder gasoline engines sometimes use a device called a magneto to supply current to the spark plug. A permanent magnet is attached to the flywheel, and a stationary coil is mounted
A metal ring is oriented with the plane of its area perpendicular to a spatially uniform magnetic field that increases at a steady rate. If the radius of the ring is doubled, by what factor do(a) The
In many magnetic resonance imaging (MRI) systems, the magnetic field is produced by a superconducting magnet that must be kept cooled below the superconducting transition temperature. If the
In the situation shown in Fig. 29.18, would it be appropriate to ask how much energy an electron gains during a complete trip around the wire loop with current I'? Would it be appropriate to ask what
Discusses the external force that must be applied to the slidewire to move it at constant speed. If there were a break in the left-hand end of the U-shaped conductor, how much force would be needed
A square conducting loop is in a region of uniform, constant magnetic field. Can the loop be rotated about an axis along one side and no emf be induced in the loop? Discuss, in terms of the
A metal rectangle is close to a long, straight, current-carrying wire, with two of its sides parallel to the wire. If the current in the long wire is decreasing, is the rectangle repelled by or
Consider the situation in Exercise 29.21. In part (a), find the direction of the force that the large circuit exerts on the small one. Explain how this result is consistent with Lenzs
The current in the long, straight wire AB shown in Fig. E29.7 is upward and is increasing steadily at a rate di/dt.(a) At an instant when the current is i, what are the magnitude and direction of the
An airplane is in level flight over Antarctica, where the magnetic field of the earth is mostly directed upward away from the ground. As viewed by a passenger facing toward the front of the plane, is
A student asserted that if a permanent magnet is dropped down a vertical copper pipe, it eventually reaches a terminal velocity even if there is no air resistance. Why should this be? Or should it?
A long, straight conductor passes through the center of a metal ring, perpendicular to its plane. If the current in the conductor increases, is a current induced in the ring? Explain.
Two circular loops lie side by side in the same plane. One is connected to a source that supplies an increasing current; the other is a simple closed ring. Is the induced current in the ring in the
In Fig. 29.8, if the angular speed v of the loop is doubled, then the frequency with which the induced current changes direction doubles, and the maximum emf also doubles. Why? Does the torque
A sheet of copper is placed between the poles of an electromagnet with the magnetic field perpendicular to the sheet. When the sheet is pulled out, a considerable force is required, and the force
The solenoid is removed from the enclosure and then used in a location where the earth’s magnetic field is 50 µT and points horizontally. A sample of bacteria is placed in the center of the
To use a larger sample, the experimenters construct a solenoid that has the same length, type of wire, and loop spacing but twice the diameter of the original. How does the maximum possible magnetic
What current is needed in the wire so that the magnetic field experienced by the bacteria has a magnitude of 150 µT?(a) 0.095 A;(b) 0.12 A;(c) 0.30 A;(d) 14 A.Some types of bacteria contain chains
An electric bus operates by drawing direct current from two parallel overhead cables, at a potential difference of 600 V, and spaced 55 cm apart. When the power input to the bus’s motor is at its
Calculate the magnitude and direction of the magnetic field at point P due to the current in the semicircular section of wire shown in Fig. E28.34.Figure e28.34 R P
The magnetic field around the head has been measured to be approximately 3.0 × 10-8 G. Although the currents that cause this field are quite complicated, we can get a rough estimate of their size by
A cylinder of iron is placed so that it is free to rotate around its axis. Initially the cylinder is at rest, and a magnetic field is applied to the cylinder so that it is magnetized in a direction
The magnetic susceptibility of paramagnetic materials is quite strongly temperature dependent, but that of diamagnetic materials is nearly independent of temperature. Why the difference?
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