I need to build an amp circuit for an electromagnetic signal, but I don't know much about amp cicuitry. I am goin to amplify a 7800 Hz siganl from an mp3 player through a coil. I'm making one of those cart locker things. Any ideas?

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Nope. Impedance is a more general concept than resistance. For DC, the two are equivalent.

For AC, the two are only identical when voltage and current are in phase with each other. If your circuit contains capacitors or inductors, the voltage and current will typically be out of phase with each other. For example, for an inductor, the current will lag behind the voltage (because it takes time to switch the current flowing through an inductor), whereas for a capacitor, the voltage lags behind the current (because it takes time for a capacitor to charge).

If the voltage and current are out of phase, the impedance takes on a complex value, where the magnitude (aka absolute value, or modulus) of the impedance is the ratio of the voltage amplitude divided by the current amplitude, and the angle of the impedance is equal to the difference in phase between them.

Both the magnitude and angle of the impedance typically change depending on the frequency.

As usual, Wikipedia has a pretty decent explanation of impedance...

. heehee Welcome to the club. . Although the math can get complicated, the basic principle is fairly simple. As capacitors and inductors (dis)charge, they change how much the circuit resists current flow. The effect is more of an aversion to change. . Since DC voltage doesn't vary, once the capacitors and inductors are charged, they have no effect.