What is the maximum electric field strength in an electromagnetic wave that has a maximum magnetic field strength of 5.00×10−4 T (about 10 times the Earth’s)?
Answer: 1.5 x 105 V/m (Click and drag over the answer to highlight it)
7.2
Distances in space are often quoted in units of light years, the distance light travels in one year.
(a) How many meters is a light year?
(b) How many meters is it to Andromeda, the nearest large galaxy, given that it is 2.00×106 light years away?
(c) The most distant galaxy yet discovered is 12.0×109 light years away. How far is this in meters?
Answer: (a) 9.46 x 1015 m (b)1.89 x 1022 m (c) 1.14 x 1026 m
7.3
(a) Calculate the ratio of the highest to lowest frequencies of electromagnetic waves the eye can see, given the wavelength range of visible light is from 380 to 760 nm.
(b) Compare this with the ratio of highest to lowest frequencies the ear can hear.
Answer: (a) 2 (b) 1000
7.4
(a) What is the frequency of the 193-nm ultraviolet radiation used in laser eye surgery?
(b) Assuming the accuracy with which this EM radiation can ablate the cornea is directly proportional to wavelength, how much more accurate can this UV be than the shortest visible wavelength of light?
Answer: (a) 1.55 x 1015 Hz (b) about 2 times
7.5
During normal beating, the heart creates a maximum 4.00-mV potential across 0.300 m of a person’s chest, creating a 1.00-Hz electromagnetic wave.
(a) What is the maximum electric field strength created?
(b) What is the corresponding maximum magnetic field strength in the electromagnetic wave?
(c) What is the wavelength of the electromagnetic wave?
Answer: (a) 0.013 V/m (b) 4.4 x 10-11 T (c) 3 x 108 m
7.6
If you wish to detect details of the size of atoms (about 1×10−10 m) with electromagnetic radiation, it must have a wavelength of about this size.
(a) What is its frequency?
(b) What type of electromagnetic radiation might this be?
Answer: (a) 3 x 1018 Hz
7.7
Combing your hair leads to excess electrons on the comb. How fast would you have to move the comb up and down to produce red light?
Answer: about 4 x 1014 times per second
7.8
Assume the helium-neon lasers commonly used in student physics laboratories have power outputs of 0.250 mW.
(a) If such a laser beam is projected onto a circular spot 1.00 mm in diameter, what is its intensity?
(b) Find the peak magnetic field strength.
(c) Find the peak electric field strength.
Answer: (a) 318 W/m2 (b) 1.63 x 10-6 T (c) 490 V/m
7.9
Electromagnetic radiation from a 5.00-mW laser is concentrated on a 1.00-mm2 area.
(a) What is the intensity in W/m2?
(b) Suppose a 2.00-nC static charge is in the beam. What is the maximum electric force it experiences?
(c) If the static charge moves at 400 m/s, what maximum magnetic force can it feel?
Answer: (a) 5000 W/m2 (b) 3.88 µN (c) 5.18 pN
7.10
If electric and magnetic field strengths vary sinusoidally in time, being zero at 𝑡 = 0, then 𝐸 = 𝐸0sin(2π𝑓𝑡) and 𝐵 = 𝐵0sin(2π𝑓𝑡). Let 𝑓 = 1.00 GHz here.
(a) When are the field strengths first zero?
(b) When do they reach their most negative value?
(c) How much time is needed for them to complete one cycle?
Answer: (a) at t = 0 s and t = 0.5 ns (b) at t = 0.75 ns (c) 1 ns
Physics Education Research at UNG 