In electrical engineering, impedance measures how easily a circuit conducts current when a voltage runs through it. Measured in ohms, impedance is a way of telling you how much of the voltage introduced at one end will really make it to the other end. Impedance depends on other qualities of electricity, such as resistance, reactance, inductance, and capacitance.

The simplest kind of impedance happens with direct current, or DC, electricity. In this case, it's the same thing as resistance. All that gets in the way of electrons making it through the circuit is the circuit's resistance to the flow of current. Resistance usually takes the form of leaked energy that gets diverted from its intended pathway and creates extra heat.

With alternating current, or AC, impedance is much more complicated. The best way to understand how this characteristic works is to use an extended metaphor of freeway traffic. The whole point of a good freeway system is to transport people in vehicles, the way that a circuit carries electrons. The system might become less efficient if there are fewer people on the freeway, too many people, or too few lanes. There could even be bumps or obstacles in the road that cause cars to slow down, or lanes that accidentally route traffic away from its destination. Electrons face similar impediments.

Resistance would be akin to these physical properties of the freeway that obviously get in the way of smooth traffic flow. Wires, conductors, resistors, and other elements of a complex circuit also have these properties. In a standard freeway, adjacent traffic is all moving in the same direction. In direct current, the electrons are also moving in the same direction. However, the situation would get chaotic if cars on the freeway were allowed to move in opposite directions in the same lanes, weaving between each other. This is like alternating current.

Impedance is determined by resistance and another value, reactance. Reactance takes into account opposing forces and frequency; in other words, cars headed in the other direction and how many cars move past the same point at one time. Of course, more people will get to their destination if there are more net cars moving in one direction past a certain point, analogous to the effect frequency has on electron's energy. Two elements of reactance, inductance and capacitance, correspond to frequency. Inductance is in proportion to frequency while capacitance is inversely proportional to frequency.

Once you have determined the inductance and the capacitance, you can determine overall how efficiently the current is making it across the circuit, or the reactance. When you combine, or add, the reactance to resistance, you arrive at impedance. Mathematically, Z=V/I, where Z is impedance, given in ohms; V is voltage, given in volts; and I is current, given in amps.