Physics
Electromagnetic waves
Light has Momentum
A
ca
We will now analyze a "chunk" of this light energy in motion, exactly like
we did for the spacecraft in motion in the previous Rocket Science
Challenge (Session 5). But now, instead of matter energy we will be
dealing with field energy-the Physics is the same (remember relativity
unifies Physics, making it simpler!)
B
Consider a small cube of length ? and cross-sectional area A (volume
V = {A) centered at the origin of the coordinate system in the diagram at
the top of the previous page. This cube will contain electric and magnetic
fields, and thus energy. Suppose the amount of energy in the cube is E..
1. How much time will it take for this cube of energy to move a distance l to the right?
At =
2. What is the associated power (rate of energy transfer)? Express your answer in terms of E., c, and l.
P=
3. What is the energy flux (power per unit area, SI units Watts per square meter) involved in this energy
transfer? Express your answer in terms of E., c, and V.
S =
4. In Session 5 we learned that, just like energy is equivalent to mass (E = mc2), energy flux is equivalent to
momentum density: S = Pc2. This is the fundamental meaning of “momentum” in Physics! Whenever
there is energy in motion there is momentum, and vice versa. If we let E = E./V denote energy density
(energy per unit volume in the cube of light), use S = Pc2 and your result from #3 to find an expression for
P, the magnitude of the momentum density. Express your answer in terms of E and c:
=
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