ELECTRIC OSCILLATIONS AND ELECTRIC WAVES. 273 



ergs per second, using the ordinary formula P= EL (b) 

 The electric field intensity between the points pp in Fig. 

 203 is 33^3 volts per centimeter and the magnetic field between 

 the strips is uniform and perpendicular to the plane of the paper. 

 Let the intensity of the magnetic field be H. Express the 

 intensity of the energy stream across pp in ergs per square 

 centimeter per second in terms of H .and the electric field, 

 intensity using the proportionality factor found in problem 147. 

 Multiply this intensity of the energy stream by the sectional area 

 across which it flows at pp and place this result equal to El 

 (expressed in c.g.s. units of course) and thus find the intensity of 

 the uniform magnetic field between the two ribbons. Ans. (a) 

 io 10 ergs per second. () 47T/5O gauss. 



146. A water wave travels along a canal in which the normal 

 depth of water is 6 feet, the width of the canal being 12 feet. 

 The wave is 30 feet long and the water in the wave has a uniform 

 velocity of 0.3 foot per second. Find the total energy of the 

 wave counting both potential energy and kinetic energy. Ans. 

 379.7 foot-pounds. 



147. A rectangular electromagnetic wave-pulse is bounded by 

 two broad sheets of metal as shown in Fig. 195. The width of 

 the sheets is 50 centimeters, their distance apart is 3 centimeters 

 and the length of the wave pulse is 100 centimeters. The inten- 

 sity of the uniform magnetic field in the region of the wave is io 

 gausses. Find the total energy of the wave including electric 

 and magnetic energy. Ans. 3,ooo,ooo/87r ergs. 



148. A battery of which the electromotive force is i^QoeTvolts 

 is connected at a given instant to the two wires of a transmission 

 line. Treating the transmission line as though it consisted of two 

 flat ribbons, make a diagram somewhat similar to Figs. 195 and 

 196 showing the distribution of current in the bounding metal 

 sheets, the distribution of charge on the bounding metal sheets, 

 and the distribution of electric and magnetic fields in the region 

 between the sheets at an instant t seconds after the battery is 

 connected, Vt being less than the length of the line, where V 



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