# HW7_Solutions.pdf - P25.25 Strategize This is a...

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P25.25. Strategize: This is a straightforward application of the fixed relationship between magnetic and electric field amplitudes in electromagnetic waves, given by Equation 25.17. Prepare: The electric and magnetic field amplitudes of an electromagnetic wave are related as Solve: The electric field amplitude of the electromagnetic wave is Assess: Because the magnetic field amplitude is much larger than the earth’s magnetic field, we expected a large electric field amplitude. P25.27. Strategize: This is a straightforward application of the relationship between the intensity of an electromagnetic wave and the magnetic and electric field amplitudes. This is described by Equation 25.18. Note that once we determine the amplitude of one field type, the amplitude of the other is given by Equation 25.17. Prepare: The intensity of the microwaves is related to the amplitude of the oscillating electric field by and the amplitude of the oscillating magnetic field is related to the amplitude of the oscillating electric field by Solve: The amplitude of the oscillating electric field is The amplitude of the oscillating magnetic field is Assess: These are reasonable values for the amplitude of the oscillating electric and magnetic fields of microwaves. P25.32. Strategize: We can use Equation 25.18 to express the intensity in terms of the electric field amplitude. Prepare: In the expression we have all variables except the unknown electric field amplitude. Solve: Rearranging Equation 25.18, we have Assess: This electric field is, of course, always changing direction, and has no fixed orientation. So it does not have a net effect on electric charge. P25.33. Strategize: A radio wave is an electromagnetic wave. The power or energy transported per second by the radio wave is This energy is carried uniformly in all directions. Equation 25.18 connects power of a wave that impinges on an area A (defined as its intensity) with the wave’s electric field amplitude. Prepare: Recall that the intensity of a point source free to spread in three dimensions (over a spherical surface) is We will combine this Equation 25.18. Solve: (a) Using the above expression, the light intensity is (b) Using Equation 25.18, Assess: These values are reasonable, in that they are small enough that they would not upset the normal operation of circuits