# Is a resistor a capacitor?

by Orthoceras
Tags: capacitor, gauss's law, resistor
 P: 17 Suppose a resistor is connected on both sides to a copper wire, in a simple circuit with a battery. Resistor and wires are cylinders of equal diameter. A DC current is flowing through the resistor and the wires. The current is axial everywhere, and the electric field is also axial. The strength of the electric field is constant in the resistor, E = U/d (U is voltage, d is length of the resistor), and it is close to zero in the copper wires. This E field seems to be similar to the E field of a parallel plate capacitor. The two boundary surfaces S1 and S2 between copper and resistor are the source and the sink of the E field. Is it correct to conclude that the boundary surfaces S1 and S2 carry a surface charge Q = U/C, where C = A εx /d , and that the resistor behaves like a capacitor for AC frequencies ω > 1/RC ?
 HW Helper P: 2,078 That is called parasitic capacitance. Resistors and other circuit components have capacitance. Sometimes (for a given purpose) it is small enough to ignore, other times it must be taken into account. The equations of a circuit are approximations based on assumptions, different approximations are appropriate at different times. What we think of as a resistor, an inductor, or a capacitor is a combination of all of them.
 P: 17 Thanks. I suppose your answer also implies: yes, the boundary surfaces S1 and S2 carry that surface charge Q. Now, I am trying to understand in mechanical terms why there is a surface charge at S1 and S2. DC current through a resistor seems to be mathematically similar to viscous fluid/gas flow through a hydraulic resistor. Is there an equivalent of the electrical surface charge at the entrance of the hydraulic resistor? For example, is the density of the air increased or decreased in a thin layer at the entrance of the air filter in a vacuum cleaner (assuming a constant conduit diameter, to maintain the same particle speed inside and outside the filter)?
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