Electromagnetic Waves

Transcription

1 Welcome Back to Physics 1308 Electromagnetic Waves James Clerk Maxwell 13 June November 1879

2 Announcements Assignments for Tuesday, November 13th: - Reading: Chapter Watch Videos: - Lecture 21 - Reflection and Refraction Homework 11 Assigned - due before class on Tuesday, November 13th. Midterm Exam 3 will be in class on Thursday, November 15th. It will cover material corresponding to that covered through chapter 29 in your textbook, and all associated lecture, homework and in class material.

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4 Review Question 1 Circuit A contains a battery, a switch, and a resistor connected in series. Circuit B contains a battery, a switch, an inductor, and a resistor connected in series. Initially, the switch is closed in both circuits. How does the behavior of the current in circuit B compare with that in circuit A as both switches are opened at the same time? A) The current in both circuits decreases at the same rate because inductors do not affect the current in a circuit. B) The current in circuit B decreases more slowly than that for circuit A since the inductor acts to maintain the current in the circuit. C) The current in circuit B decreases more quickly than that for circuit A since the inductor increases the current in the circuit as its stored energy is released. D) The behavior of the current in the circuit depends on the inductance of the inductor. If the inductance is small, the current will decrease rapidly; and if the inductance is large, the current will increase for a short time before decreasing.

5 Review Key Concepts Electric charge exists in two kinds, positive and negative. These exert influence on each other via a field of force. The electric field is a conservative force field, with associated concepts of potential energy and the new concept of electric potential. Fields accelerate charges, and this is the basis of circuits. There are relationships between fields and the energy stored and release in circuits (e.g. in capacitors and resistors). Magnetic fields, fields of force exerted by certain materials such as magnetite and iron, also affect electric charges. However, they do so at right angles to the field lines and the direction of motion of the charges. Moving electric charge creates magnetic field. Changing magnetic field penetration in a conductor causes voltage, and thus electric current. A conductor that encloses its OWN magnetic field possesses selfinduction. Magnetic fields store energy, just like electric fields. Light is an electromagnetic wave.

6 Key Concepts

7 Key Concepts Electromagnetic waves are always perpendicular to the direction they are traveling (transverse waves). The electric field is always perpendicular to the magnetic field. The cross product E x B gives the direction in which the wave travels The fields always vary sinusoidally. The fields vary with the same frequency and in phase with each other. By Lookang many thanks to Fu-Kwun Hwang and author of Easy Java Simulation = Francisco Esquembre - Own work, CC BY-SA 3.0, curid=

8 Question 1 Consider the region of space that you are occupying. Which of the following types of electromagnetic waves are present around you? A. visible light waves B. radio waves C. microwaves D. infrared waves E. all of the above

9 Question 2 For which one of the following properties do visible light and ultraviolet waves have the same value? A. wavelength B. frequency C. speed D. energy E. period

10 Student Problem: Galileo Speed of Light In 1667, Galileo attempted to measure the speed of light by having two people hold covered lanterns on hills that were about 1.5 km apart. One person would measure time. One of the people with a lantern would uncover it. The other person would then uncover his lantern when he saw the light from the first lantern. Repeated attempts failed. To see why, determine the approximate time it takes light to travel the 1.5 km distance.

11 To solve this problem, we need only the definition of speed. c = x t! t = x c = m m/s t = s =5.0 µs This time is way too short for any clock of the period to measure. Also, it is worth noting that human reaction time, on average, is only 0.25 seconds. The time which was being measured is even faster than most humans could react.

12 Question 3 What is the correct order, beginning with longest wavelength and extending to the shortest wavelength, of the following colors in the visible light spectrum: blue, green, red, violet, and yellow? A. red, yellow, green, blue, violet B. violet, blue, green, yellow, red C. red, blue, violet, green, yellow D. red, yellow, blue, green, violet E. violet, blue, yellow, red, green

13 Student Problem: Romer s Method The diameter of Earth s orbit is 3.00 x m. If light takes 16.6 minutes to travel this distance, what is the speed of light?

14 To solve this problem, we need only the definition of speed. c = x t = m 16.6 min 1 min 60 s = m/s Note: Römer obtained a considerably smaller value for c because he used 22 min for Δt.

15 Student Problem: Fizeau s Method Minute: 4:00 In Fizeau s experiment, the wheel had 720 teeth, and light was observed when the wheel roared at 25.2 revolutions per second. If the distance from the wheel to the distant mirror was 8.63 km, what was Fizeau s value for the speed of light?

16 First find the total distance traveled by the light. This is the distance to the mirror and back. x = km = 17.3 km The light was reflected through the next gap in the wheel. Thus, the wheel made it through 1/720 revolution in the time the light traveled. Since the wheel made 25.3 revolutions in 1 second, the time it took to make 1/720 revolution was t = 1 s 25.3 rev rev = s The value of the speed of light from this experiment is then c = x t = m s = m/s

17 Other Techniques Cavity Resonance - Measure frequency (f) and wavelength (λ) in a vacuum and take advantage of the relationship c = fλ. Interferometry - A coherent beam of light (e.g. from a laser), with a known frequency (f), is split to follow two paths and then recombined. By adjusting the path length while observing the interference pattern and carefully measuring the change in path length, the wavelength of the light (λ) can be determined. The speed of light is then calculated using the equation c = λf. - This technique (interferometry) was used to discover gravitational waves (whose discovery won the 2017 Nobel Prize in Physics).

18 The End for Today