Satellite dishes receive TV signals sent from orbit. (See Figure 2.) (a) What is the intensity of this wave? (b) What is the power received by the antenna? (c) If the orbiting satellite broadcasts uniformly over an area of 1.50 x 10 13 m 2 (a large fraction of North America), how much power does it radiate? Figure 2. This caused identifiable health problems, such as cataracts, for people who worked near them.)Ħ: A 2.50-m-diameter university communications satellite dish receives TV signals that have a maximum electric field strength (for one channel) of 7.50 μV/m. (b) What is the maximum electric field strength at the safe intensity? (Note that early radar units leaked more than modern ones do. (a) If a radar unit leaks 10.0 W of microwaves (other than those sent by its antenna) uniformly in all directions, how far away must you be to be exposed to an intensity considered to be safe? Assume that the power spreads uniformly over the area of a sphere with no complications from absorption or reflection. (a) Assuming all of the radio waves that strike the ground are completely absorbed, and that there is no absorption by the atmosphere or other objects, what is the intensity 30.0 km away? (Hint: Half the power will be spread over the area of a hemisphere.) (b) What is the maximum electric field strength at this distance?ĥ: Suppose the maximum safe intensity of microwaves for human exposure is taken to be 1.00 W/m 2. (c) Find the peak electric field strength.Ĥ: An AM radio transmitter broadcasts 50.0 kW of power uniformly in all directions. (a) If such a laser beam is projected onto a circular spot 1.00 mm in diameter, what is its intensity? (b) Find the peak magnetic field strength. In fact, for a continuous sinusoidal electromagnetic wave, the average intensity I ave is given byġ: What is the intensity of an electromagnetic wave with a peak electric field strength of 125 V/m?Ģ: Find the intensity of an electromagnetic wave having a peak magnetic field strength of 4.00 x 10 -9 T.ģ: Assume the helium-neon lasers commonly used in student physics laboratories have power outputs of 0.250 mW. Thus the energy carried and the intensity I of an electromagnetic wave is proportional to E 2 and B 2. In electromagnetic waves, the amplitude is the maximum field strength of the electric and magnetic fields. This is true for waves on guitar strings, for water waves, and for sound waves, where amplitude is proportional to pressure. Clearly, the larger the strength of the electric and magnetic fields, the more work they can do and the greater the energy the electromagnetic wave carries.Ī wave’s energy is proportional to its amplitude squared E 2 or B 2. If absorbed, the field strengths are diminished and anything left travels on. Once created, the fields carry energy away from a source. With electromagnetic waves, larger E-fields and B-fields exert larger forces and can do more work.īut there is energy in an electromagnetic wave, whether it is absorbed or not. Energy carried by a wave is proportional to its amplitude squared. This simultaneous sharing of wave and particle properties for all submicroscopic entities is one of the great symmetries in nature. ![]() ![]() ![]() These particle characteristics will be used to explain more of the properties of the electromagnetic spectrum and to introduce the formal study of modern physics.Īnother startling discovery of modern physics is that particles, such as electrons and protons, exhibit wave characteristics. ![]() But we shall find in later modules that at high frequencies, electromagnetic radiation also exhibits particle characteristics. The behavior of electromagnetic radiation clearly exhibits wave characteristics.
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