Lessons · Engineering · Impedance: resistance that depends on frequency
Resistance that depends on frequency
Impedance is opposition to alternating current, and unlike resistance it depends on frequency. A capacitor's opposition falls as frequency rises; an inductor's grows. Resistance does neither. Impedance combines resistance and reactance as arrows at a right angle.
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In beta. This lesson was written for Hone and has not yet been checked by a licensed engineer. Practice material, not professional advice. What that means.
What it is for
It is why a circuit behaves differently at 50 Hz and at 50 kHz with no component changed, and why a capacitor that blocks a battery passes a signal.
How to think about it
Separate what is resistive from what is reactive, then combine them as a right-angled sum rather than adding. Ask what frequency the circuit actually sees before deciding what a capacitor or inductor is doing.
Worked example
Resistance is the same at every frequency.The simple part.
Capacitive reactance falls as frequency rises.Blocks DC, passes high frequency.
Inductive reactance rises as frequency rises.Passes DC, blocks high frequency.
Impedance = root of (R squared + X squared).A right-angled combination.
Your turn
R = 8 ohms, reactance X = 6 ohms. Write the impedance in ohms.
sqrt(8^2 + 6^2) = ohms
Solve one, graded on the server
The trap
Adding resistance and reactance directly. They are at a right angle, so eight and six give ten, not fourteen.