1. cards, on the Metro, contain a radio frequency identification (RFID) chip, operating at a frequency of 13.56 MHz. The radio waves that are used to read SmarTrip cards are electromagnetic waves, traveling at c, the speed of light. What is the wavelength of these waves?

2. An electric quadrupole is 4 charges with the same magnitude arranged at the corners of a square, with 2 positive charges in opposite corners, and 2 negative charges in the other opposite corners. You have a quadrupole where each charge has magnitude q, and the square has sides of length d. You want to find the electric field at a distance r away from the quadrupole (where r is much larger than d). Which of these expressions could be correct?

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