## Mhl

d where e0 is the permittivity of free space (8.9 x 10"12 F/m), d is the separation between the plates and k is the dielectric constant (k = 1, because there is no dielectric, the capacitor being in vacuum). With an area of 10"14 m2, C = 8.9 x 10"26/d farads. If the device can be built with a separation of 10"7 m between the plates, the capacitance will be about 10"18 F. The lower cutoff frequency is fL = 2nC(Rff + RL). (8.12)

Since for maximum power transfer, the load must match the generator, RL = Rh. To have the lowest possible fL, we want the largest possible RH. High-resistivity material should be used. Possibly, some semiconductor would be appropriate. Let us, however, try a metal—gold—as an example. Assume that the resistor is a ribbon of gold 20 atoms thick and 10"7 by 10"7 m in area.

The resistivity of gold at room temperature is 2.44 x 10"8 Qm, and its temperature coefficient^ is 3.4 x 10"11 Qm/K. Thus, at 700 K, the resistivity is 3.8 x 10"8 Qm. The ribbon would have a resistance of 0.4 Q, leading to a lower cutoff frequency of 2 x 1017 Hz, which is much higher than the upper cutoff frequency. The device as planned is patently too small. If the resistor could be made of lightly doped silicon, the lower cutoff frequency would be brought down by a factor of, say, 105, leading to fL = 2 x 1012. Such a cutoff frequency is acceptable because it would result in a bandwidth of 3 x 1015 - 2 x 1012 « 3 x 1015.

References

Yater, Joseph C., Power conversion of energy fluctuations, Phys. Rev. A,

10(4), p. 1361, October 1974. Yater, Joseph C., Rebuttal to "Comments on Power conversion of energy fluctuation," Phys. Rev. A, 20, p. 623, August 1979.

ftTemperature coefficient of resistivity is the ratio of the change in resistance to the corresponding change in temperature.

## Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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