Maxwell’s equations together with the Lorentz force equation imply the existence of radiation pressure much more generally than this specific example, however.įigure 16.13 Electric and magnetic fields of an electromagnetic wave can combine to produce a force in the direction of propagation, as illustrated for the special case of electrons whose motion is highly damped by the resistance of a metal. ![]() When the E field reverses, the B field does too, and the force is again in the same direction. By applying the right-hand rule, and accounting for the negative charge of the electron, we can see that the force on the electron from the magnetic field is in the direction of the positive x-axis, which is the direction of wave propagation. When the electric field is in the direction of the positive y-axis, electrons move in the negative y-direction, with the magnetic field in the direction of the positive z-axis. This is comparable to an object moving against friction and stopping as soon as the force pushing it stops ( Figure 16.13). To understand the direction of the force for a very specific case, consider a plane electromagnetic wave incident on a metal in which electron motion, as part of a current, is damped by the resistance of the metal, so that the average electron motion is in phase with the force causing it. This force occurs because electromagnetic waves contain and transport momentum. A much less familiar feature of electromagnetic radiation is the extremely weak pressure that electromagnetic radiation produces by exerting a force in the direction of the wave. The energy that sunlight carries is a familiar part of every warm sunny day. These forces do work on the particles of the object, increasing its energy, as discussed in the previous section. An electromagnetic wave incident on the object exerts forces on the charged particles, in accordance with the Lorentz force, Equation 16.11. Material objects consist of charged particles. Explain how the radiation pressure of light, while small, can produce observable astronomical effects. ![]()
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