Calculating Energy in a Box with a Flashlight

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SUMMARY

The discussion focuses on calculating the energy associated with the electromagnetic field in a box illuminated by a flashlight. The key takeaway is that to determine the energy, one must multiply the power output of the flashlight (in Watts) by the duration it is turned on, assuming 100% absorption of light energy within the box. The absence of reflections and the indefinite duration of illumination simplifies the calculation to a straightforward multiplication of power and time.

PREREQUISITES
  • Understanding of basic physics concepts, specifically energy and power.
  • Familiarity with the units of measurement: Watts, Joules, and seconds.
  • Knowledge of electromagnetic fields and their properties.
  • Basic integration concepts in relation to power density (optional for deeper understanding).
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  • Explore the concept of electromagnetic fields and their energy density.
  • Learn about the principles of light absorption and reflection in materials.
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Students studying physics, educators teaching energy concepts, and anyone interested in understanding the principles of energy calculations in illuminated environments.

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Homework Statement



A flashlight is shining into a box. There are no reflections. We know the power of the flashlight in Watts. We know the volume of the box.

Question: How much energy (associated with the EM field) is in the box?

Homework Equations



The Attempt at a Solution



Integrate the power density (J s^-1 m^-3) over the volume of the box. But what is the power density?

Hints would be preferred over full solutions :)
 
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You don't need the power "density" if you already know the power! A Watt is one Joule per second. Assuming all the light energy of the is absorbed by the box, which is implied in your question, just multiply the given wattage of theflashlight by the length of time the flashlight is shined on the box.
 
The flashlight is left on indefinitely, and the light is transmitted (100%) out of the box. I should have been clear that there is no absorption or reflection.
 

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