How much energy is transported by an EM wave with a given E field strength?

AI Thread Summary
The discussion focuses on calculating the energy transported by an electromagnetic (EM) wave with a given electric field strength. The rms strength of the E field is 36.5 mV/m, and the area in question is 1.15 cm². Participants clarify the use of the formula for energy density, ε₀E², and the conversion of units to ensure correct calculations. The correct approach involves multiplying the energy density by the area and the speed of light, followed by converting the result to joules per hour. The final calculations aim to resolve any discrepancies in the expected answer.
Queue
Messages
34
Reaction score
0

Homework Statement


How much energy is transported across a 1.15 cm^2 area per hour by an EM wave whose E field has an rms strength of 36.5 mV/m?

Homework Equations



E=hv?
The big problem I'm having is that I don't know what formulae to use.

The Attempt at a Solution


I know what V/m = J/C = J/(A*s) and I know I need to get to J/h. But I don't know what I have that would include Amps or Coloumbs to get rid of that part (I've resorted to trying to sort out units in the face of the lack of equations.

Thanks!
 
Physics news on Phys.org
Queue said:

Homework Statement


How much energy is transported across a 1.15 cm^2 area per hour by an EM wave whose E field has an rms strength of 36.5 mV/m?

Homework Equations



E=hv?
The big problem I'm having is that I don't know what formulae to use.

The Attempt at a Solution


I know what V/m = J/C = J/(A*s) and I know I need to get to J/h. But I don't know what I have that would include Amps or Coloumbs to get rid of that part (I've resorted to trying to sort out units in the face of the lack of equations.

Thanks!

Ok, I have from my book (which was just returned to me) that energy in a wave is given by \epsilon_0 E2 with units of J/m3. So I have u = 8.85*10-12*(36.5*10-3)2. From there I divided by the area I am given (1.15 cm2) and then multiplied by the speed of light (in hours) to give me the right units but my answer was incorrect according to mastering physics. Perhaps I made a math error or perhaps I'm having conceptual issue. Any help would be appreciated.
 
I think you did it correctly, but I'll just ask if you converted the area to m2 units
 
I did convert to square meters.

I multiplied by area, didn't divide like I thought I was supposed to which actually makes my units work out.

This gives me u = \epsilon_0 E^2 = 8.85*10-12*(3.65*103)2 J/(m3). From there I have u*A = 8.85*10-12*(3.65*103)2*0.000115 J/m. Then I multiplied by c giving u*A*c = 8.85*10-12*(3.65*103)2*0.000115*3*109 = whatever it is J/s. Then I just multiplied by 3600 (seconds per hour) to get J/h.

Thanks!
 
Last edited:
I multiplied the values first without the error limit. Got 19.38. rounded it off to 2 significant figures since the given data has 2 significant figures. So = 19. For error I used the above formula. It comes out about 1.48. Now my question is. Should I write the answer as 19±1.5 (rounding 1.48 to 2 significant figures) OR should I write it as 19±1. So in short, should the error have same number of significant figures as the mean value or should it have the same number of decimal places as...
Thread 'A cylinder connected to a hanging mass'
Let's declare that for the cylinder, mass = M = 10 kg Radius = R = 4 m For the wall and the floor, Friction coeff = ##\mu## = 0.5 For the hanging mass, mass = m = 11 kg First, we divide the force according to their respective plane (x and y thing, correct me if I'm wrong) and according to which, cylinder or the hanging mass, they're working on. Force on the hanging mass $$mg - T = ma$$ Force(Cylinder) on y $$N_f + f_w - Mg = 0$$ Force(Cylinder) on x $$T + f_f - N_w = Ma$$ There's also...
Back
Top