HW2 - PHYS 4D Solution to HW 2 January 15, 2011 Problem...

Info iconThis preview shows pages 1–2. Sign up to view the full content.

View Full Document Right Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: PHYS 4D Solution to HW 2 January 15, 2011 Problem Giancoli 31-22 (I) The E field in an EM wave has a peak of 26 . 5 mV/m . What is the average rate at which this wave carries energy across unit area per unit time? Solution: The average rate at which this wave carries energy across unit area per unit time is the average magnitude of the Poynting vector. S = 1 2 cE 2 = 1 2 ( 8 . 85 10- 12 C 2 /N m 2 )( 3 10 8 m/s ) (0 . 0265 V/m ) = 9 . 32 10- 7 W/m 2 . Problem Giancoli 31-24 (II) How much energy is transported across a 1 . 00 cm 2 area per hour by an EM wave whose E field has an rms strength of 32 . 8 mV/m ? Solution: The energy transported across unit area per unit time is the magnitude of the Poynting vector. S = c E 2 rms = U A t U t = c E 2 rms A = ( 3 10 8 m/s )( 8 . 85 10- 12 C 2 /N m 2 ) (0 . 0328 V/m ) 2 ( 1 10- 4 m 2 ) (3600 s/h ) = 1 . 03 10- 6 J/h. Problem Giancoli 31-26 (II) If the amplitude of the B field of an EM wave is 2 . 5 10- 7 T , (a) what is the amplitude of the E field? Solution: E = cB = ( 3 10 8 m/s ) 2 . 5 10- 7 T = 75 V/m (b) What is the average power per unit area of the EM wave? Solution: The average power per unit area is given by the Poynting vector I = E B 2 = 75 V/m 2 . 5 10- 7 T 2 (4 10- 7 Ns 2 /C 2 ) = 7 . 5 W/m 2 . Problem Giancoli 31-28 (II) A 15 . 8- mW laser puts out a narrow beam 2 . 00 mm in diameter. What are the rms values of E and B in the beam? Solution: The power output per unit area is the intensity and also is the magnitude of the Poynting vector. Use Eq. 31-19a with rms values S = P A = c E 2 rms E rms = P Ac = . 0158 W (10- 3 m ) 2 (3 10 8 m/s ) (8 . 85 10- 12...
View Full Document

This note was uploaded on 09/19/2011 for the course PHYS 4d taught by Professor Staff during the Winter '08 term at UCSD.

Page1 / 4

HW2 - PHYS 4D Solution to HW 2 January 15, 2011 Problem...

This preview shows document pages 1 - 2. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online