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S E C T I O N 37. 4 • Phasor Addition of Waves 1185 in discussing wave interference. The sinusoidal wave we are discussing can be repre- 0 rotating about the origin counterclock- wise with an angular frequency -, as in Figure 37.8a. Note that the phasor makes an 1 , the magnitude of the wave disturbance at some time t. Hence, as the phasor rotates in a circle about the origin, the projection E 1 oscillates along the verti- cal axis. Now consider a second sinusoidal wave whose electric field component is given by E 2 # E 0 sin(-t ( .) This wave has the same amplitude and frequency as E 1 , but its phase is . with respect to E 1 . The phasor representing E 2 is shown in Figure 37.8b. We can obtain the resultant wave, which is the sum of E 1 and E 2 , graphically by redrawing the phasors as shown in Figure 37.8c, in which the tail of the second phasor is placed E R runs from the tail of the first phasor to the tip of the second. Furthermore, E R rotates along with the two individual phasors at the same angular frequency -. The projection E R along the vertical axis equals the sum of the projections of the two other phasors: E P # E 1 ( E 2 . It is convenient to construct the phasors at t # 0 as in Figure 37.9. From the geom- etry of one of the right triangles, we see that which gives E R # 2E 0 cos 1 Because the sum of the two opposite interior angles equals the exterior angle ., we see Hence, the projection of the phasor E R along the vertical axis at any time t is This is consistent with the result obtained algebraically, Equation 37.10. The resultant 0 cos(./2) and makes an angle ./2 with the first phasor. E P # E R sin " - t ( . 2 # # 2E 0 cos(./2) sin " - t ( . 2 # E R # 2E 0 cos " . 2 # cos 1 # E R
/2 E 0 Figure 37.8 (a) Phasor diagram for the wave disturbance E 1 # E 0 sin - t. The phasor is a vector of length E 0 rotating counterclockwise. (b) Phasor diagram for the wave E 2 # E 0 sin( - t ( . ). (c) The phasor E R represents the combination of the waves in part (a) and (b). Figure 37.9 A reconstruction of the resultant phasor E R . From the geometry, note that 1 # . /2. E 2 E 0 (b) ω t + φ ω φ t ω E 1 E 0
φ (c) E P E 0 E R E 2 E 1 (a) E 0 t ω E 0 φ E 0 E R α α |