Chapter P.6, Problem 19P

Wireless Power Transmission

Your laptop has wireless communications connectivity, and you might even have a wireless keyboard or mouse. But there’s one wire you haven’t been able to get rid of yet—the power cord.

Researchers are working on ways to circumvent the need for a direct electrical connection for power, and they are experiencing some success. Recently, investigators were able to use current lowing through a primary coil to power a 60 W lightbulb connected to a secondary coil 2.0 m away, with approximately 15% efficiency. The coils were large and the efficiency low, but it's a start.

Figure VI.2

The wireless power transfer system is outlined in Figure VI.2. An AC supply generates a current through the primary coil, creating a varying magnetic Held. This field induces a current in the secondary coil, which is connected to a resistance (the lightbulb) and a capacitor that sets the resonance frequency of the secondary circuit to match the frequency of the primary circuit.

What are the rms and peak currents for a 60 W bulb? (The rms voltage is the usual 120 V.)

1. A. 0.71 A, 0.71 A
2. B. 0.71 A, 0.50 A
3. C. 0.50 A, 0.71 A
4. D. 0.50 A, 0.50 A