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University Physics Volume 2

Conceptual Questions

University Physics Volume 2Conceptual Questions

Conceptual Questions

16.1 Maxwell’s Equations and Electromagnetic Waves


Explain how the displacement current maintains the continuity of current in a circuit containing a capacitor.


Describe the field lines of the induced magnetic field along the edge of the imaginary horizontal cylinder shown below if the cylinder is in a spatially uniform electric field that is horizontal, pointing to the right, and increasing in magnitude.

Figure shows a cylinder placed horizontally. There are three columns of arrows labeled vector E across the cylinder. The arrows point right. The column to the left has the shortest arrows and that to the right has the longest.

Why is it much easier to demonstrate in a student lab that a changing magnetic field induces an electric field than it is to demonstrate that a changing electric field produces a magnetic field?

16.2 Plane Electromagnetic Waves


If the electric field of an electromagnetic wave is oscillating along the z-axis and the magnetic field is oscillating along the x-axis, in what possible direction is the wave traveling?


In which situation shown below will the electromagnetic wave be more successful in inducing a current in the wire? Explain.

Figures a and b show electromagnetic waves with both electic and magnetic components. In figure a, the electric field is parallel to the wire and the magnetic field is perpendicular. In figure b, the magnetic field is parallel to the wire and the electric field is perpendicular.

In which situation shown below will the electromagnetic wave be more successful in inducing a current in the loop? Explain.

Figures a and b show electromagnetic waves with both electic and magnetic components going through a loop connected to a tuner. In figure a, the electric field is parallel to the loop and the magnetic field is perpendicular. In figure b, the magnetic field is parallel to the loop and the electric field is perpendicular.

Under what conditions might wires in a circuit where the current flows in only one direction emit electromagnetic waves?


Shown below is the interference pattern of two radio antennas broadcasting the same signal. Explain how this is analogous to the interference pattern for sound produced by two speakers. Could this be used to make a directional antenna system that broadcasts preferentially in certain directions? Explain.

Figure shows waves as circles radiating from two points lying side by side. The points where the circles intersect are highlighted and labeled constructive interference. Arrows connecting the points of constructive interference radiate outwards. These are labeled direction of constructive interference.

16.3 Energy Carried by Electromagnetic Waves


When you stand outdoors in the sunlight, why can you feel the energy that the sunlight carries, but not the momentum it carries?


How does the intensity of an electromagnetic wave depend on its electric field? How does it depend on its magnetic field?


What is the physical significance of the Poynting vector?


A 2.0-mW helium-neon laser transmits a continuous beam of red light of cross-sectional area 0.25cm20.25cm2. If the beam does not diverge appreciably, how would its rms electric field vary with distance from the laser? Explain.

16.4 Momentum and Radiation Pressure


Why is the radiation pressure of an electromagnetic wave on a perfectly reflecting surface twice as large as the pressure on a perfectly absorbing surface?


Why did the early Hubble Telescope photos of Comet Ison approaching Earth show it to have merely a fuzzy coma around it, and not the pronounced double tail that developed later (see below)?

A Hubble Telescope photo of a comet. It appears as a bright dot with fuzzy light around it.
Figure 16.21 (credit: modification of work by NASA, ESA, J.-Y. Li (Planetary Science Institute), and the Hubble Comet ISON Imaging Science Team)

(a) If the electric field and magnetic field in a sinusoidal plane wave were interchanged, in which direction relative to before would the energy propagate?
(b) What if the electric and the magnetic fields were both changed to their negatives?

16.5 The Electromagnetic Spectrum


Compare the speed, wavelength, and frequency of radio waves and X-rays traveling in a vacuum.


Accelerating electric charge emits electromagnetic radiation. How does this apply in each case: (a) radio waves, (b) infrared radiation.


Compare and contrast the meaning of the prefix “micro” in the names of SI units in the term microwaves.


Part of the light passing through the air is scattered in all directions by the molecules comprising the atmosphere. The wavelengths of visible light are larger than molecular sizes, and the scattering is strongest for wavelengths of light closest to sizes of molecules.
(a) Which of the main colors of light is scattered the most? (b) Explain why this would give the sky its familiar background color at midday.


When a bowl of soup is removed from a microwave oven, the soup is found to be steaming hot, whereas the bowl is only warm to the touch. Discuss the temperature changes that have occurred in terms of energy transfer.


Certain orientations of a broadcast television antenna give better reception than others for a particular station. Explain.


What property of light corresponds to loudness in sound?


Is the visible region a major portion of the electromagnetic spectrum?


Can the human body detect electromagnetic radiation that is outside the visible region of the spectrum?


Radio waves normally have their E and B fields in specific directions, whereas visible light usually has its E and B fields in random and rapidly changing directions that are perpendicular to each other and to the propagation direction. Can you explain why?


Give an example of resonance in the reception of electromagnetic waves.


Illustrate that the size of details of an object that can be detected with electromagnetic waves is related to their wavelength, by comparing details observable with two different types (for example, radar and visible light).


In which part of the electromagnetic spectrum are each of these waves:
(a) f = 10.0 kHz, (b) f=λ=750nmf=λ=750nm,
(c) f=1.25×108Hzf=1.25×108Hz, (d) 0.30 nm


In what range of electromagnetic radiation are the electromagnetic waves emitted by power lines in a country that uses 50-Hz ac current?


If a microwave oven could be modified to merely tune the waves generated to be in the infrared range instead of using microwaves, how would this affect the uneven heating of the oven?


A leaky microwave oven in a home can sometimes cause interference with the homeowner’s WiFi system. Why?


When a television news anchor in a studio speaks to a reporter in a distant country, there is sometimes a noticeable lag between when the anchor speaks in the studio and when the remote reporter hears it and replies. Explain what causes this delay.

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