Giancoli 7th Edition textbook cover
Giancoli's Physics: Principles with Applications, 7th Edition
22
Electromagnetic Waves
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22-1: B Produced by Changing E
22-2: EM Waves
22-3: Electromagnetic Spectrum
22-4: Measuring the Speed of Light
22-5: Energy in EM Wave
22-6: Radiation Pressure
22-7: Radio, TV

Question by Giancoli, Douglas C., Physics: Principles with Applications, 7th Ed., ©2014, Reprinted by permission of Pearson Education Inc., New York.
Problem 31
Q

Estimate the radiation pressure due to a bulb that emits 25 W of EM radiation at a distance of 9.5 cm from the center of the bulb. Estimate the force exerted on your fingertip if you place it at this point.

A
  1. 7.3×107 N/m27.3 \times 10^{-7} \textrm{ N/m}^2
  2. 3×1010 N3 \times 10^{-10} \textrm{ N}
Giancoli 7th Edition, Chapter 22, Problem 31 solution video poster
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COMMENTS
By roynunez273 on Mon, 4/4/2016 - 3:00 AM

I think part B is wrong. We are given the area of the finger. All we have to do is just convert the area from cm^2 to m^2

By Mr. Dychko on Mon, 4/4/2016 - 5:25 AM

Hi roynunez273, thanks for the comment. In part B we're calculating force by multiplying pressure by the area of the finger tip. The finger tip area is πr2\pi r^2, and we're estimating the radius to be 1×102 m1 \times 10^{-2} \textrm{ m}, which is already in meters so that no conversion is necessary. When we multiply the pressure by that area we get the force shown in the solution, so please just let me know if I'm missing something, and please be specific.

All the best,
Mr. Dychko

By jaclynrgile on Sun, 4/10/2016 - 1:13 PM

roynunez273 is correct about B, however I took a different approach to the answer and ended up getting it right. Need to remove pi from your calculation:

So, F=PA= (7.3479x10^-7 N/m^2) (1.0x10^-4) should give you the correct answer.

By Mr. Dychko on Mon, 4/11/2016 - 3:44 AM

Hi jaclynrgile, thanks for jumping in. I love the discussion this solution is making. In order to duplicate the answer in the back of the text, both you and roynunez273 are correct that you need to multiply the radiation pressure by a figure tip area approximated as 1.0 cm21.0 \textrm{ cm}^2. Since part B is an estimate, and the figure tip area isn't specified, this creates some room for interpretation. While it's fine to estimate the finger tip area as 1.0 cm21.0 \textrm{ cm}^2, as the text does, it's also OK to estimate a finger tip radius as 1.0 cm21.0 \textrm{ cm}^2 and then calculate the area using πr2\pi r^2, because who's finger tip is it? Each student, looking at their own finger tips, should get slightly different answers, and the only important point is that the estimate really is something close to the area of a finger tip.

Best wishes,
Mr. Dychko

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