Question

19. The Pointing S vector indicates the direction in which the energy of an electromagnetic wave is transported per unit area per unit time. Radiation intensity I is defined as the average magnitude of the Pointing vector, Savg. Which is equal to Savg = I = Erms2 / c µo, where c is the speed of light. What will be the Erms value on the Moon's surface of solar radiation if the power per unit area is 1,372 Watts / m2?

a) 0 b) 23V / m c) 224V / m d) 374V / m e) 720V / m

20. The intensity I = Power of the source / Area = PS / A. In the case of the Earth, the PS / A that reaches us from the Sun is 1.4 Watts / m2. To know the total power of the Sun you just have to clear for PS. The area is given by a sphere with a radius equal to the Earth-Sun distance, R = 1UA. (1UA = distance from Earth to the Sun.) With this intensity of radiation, a planet with mass similar to Earth must have liquid water. For the Brown Dwarf star Next to Alpha Centaury, with hardly an intensity I = 0.0017

that of the Sun, how far must Earth orbital from that star to have an intensity equal to that we have with the Sun?

a) 1UA b) 0.5UA c) 0.04UA d) 0.0017UA e) On the surface of the star.

17. Calculate the magnetic field in region 2 due to that displacement current 5cm (= 0.05m) from the center of the plates at t = 2sec. Use the Ampere-Maxwell Law. Leave the result expressed in terms of ε0 and µo. Remember that the area that appears in the expression of the electric flow ФE = AE (t), is now the one enclosed by the 5cm radius.

a) 9.5ε0µo. b) 0.7 ε0µo c) 0.15 ε0µo d) 0.075 ε0µo d) 0

Answer 1