"Impedance" is a broader term than "resistance," and reflects the opposition to current flow that can be imposed by resistance, capacitance, or inductance.

The impedance of an ideal resistor (in practice there is no such thing, as all resistors will also have some amount of inductance and capacitance) is the same as its resistance and is the same at all frequencies. Resistance is usually denoted as R and is measured in ohms.

The impedance of an ideal capacitor (again, there is no such thing) decreases as frequency increases, and is equal to:

Xc = 1/(2 x pi x f x C)

where Xc denotes what is referred to as capacitive reactance (one form of impedance), and is also measured in ohms;

f = frequency in Hertz

C = capacitance in Farads

The impedance of an ideal inductor (again, there is no such thing) increases as frequency increases, and is equal to:

Xl (that's a small "L", not the number "1" or a capital "i") = 2 x pi x f x L

where Xl denotes what is referred to as inductive reactance (one form of impedance), and is also measured in ohms;

f = frequency in Hertz

L = inductance in Henries

Depending on the frequency and the circuit application, it is often (but certainly not always) possible to treat a practical resistor as being a close enough approximation to an ideal resistor to neglect its stray capacitance and inductance, as well as other non-ideal effects it may have. Likewise for practical capacitors and inductors.

The combined impedance of some amount of inductance, capacitance, and resistance is NOT calculated by directly combining the number of ohms of each, because the three parameters have different effects on the phase relation between voltage and current.

In the case of a pure resistance, voltage and current are in phase with each other. In the case of a pure inductance, voltage leads current by 90 degrees (1/4 cycle of the signal frequency). In the case of a pure capacitance, voltage lags current by 90 degrees.

Given that, the magnitude of the impedance of a circuit element combining all three types of impedance is calculated, for a specific frequency, by subtracting Xc from Xl, and then taking the square root of the sum of the squares of that difference and R. The phase angle between voltage and current at a specific frequency, corresponding to that combined impedance, is equal to the (arctangent of ((Xl-Xc)/R)), among other ways that it can be calculated.

Best regards,

-- Al

The impedance of an ideal resistor (in practice there is no such thing, as all resistors will also have some amount of inductance and capacitance) is the same as its resistance and is the same at all frequencies. Resistance is usually denoted as R and is measured in ohms.

The impedance of an ideal capacitor (again, there is no such thing) decreases as frequency increases, and is equal to:

Xc = 1/(2 x pi x f x C)

where Xc denotes what is referred to as capacitive reactance (one form of impedance), and is also measured in ohms;

f = frequency in Hertz

C = capacitance in Farads

The impedance of an ideal inductor (again, there is no such thing) increases as frequency increases, and is equal to:

Xl (that's a small "L", not the number "1" or a capital "i") = 2 x pi x f x L

where Xl denotes what is referred to as inductive reactance (one form of impedance), and is also measured in ohms;

f = frequency in Hertz

L = inductance in Henries

Depending on the frequency and the circuit application, it is often (but certainly not always) possible to treat a practical resistor as being a close enough approximation to an ideal resistor to neglect its stray capacitance and inductance, as well as other non-ideal effects it may have. Likewise for practical capacitors and inductors.

The combined impedance of some amount of inductance, capacitance, and resistance is NOT calculated by directly combining the number of ohms of each, because the three parameters have different effects on the phase relation between voltage and current.

In the case of a pure resistance, voltage and current are in phase with each other. In the case of a pure inductance, voltage leads current by 90 degrees (1/4 cycle of the signal frequency). In the case of a pure capacitance, voltage lags current by 90 degrees.

Given that, the magnitude of the impedance of a circuit element combining all three types of impedance is calculated, for a specific frequency, by subtracting Xc from Xl, and then taking the square root of the sum of the squares of that difference and R. The phase angle between voltage and current at a specific frequency, corresponding to that combined impedance, is equal to the (arctangent of ((Xl-Xc)/R)), among other ways that it can be calculated.

Best regards,

-- Al