Understanding The Basic Difference Between Resistance and Impedance

Resistance and impedance are both electrical properties that describe how a component or circuit affects the flow of electrical current, but they differ in their response to different types of signals.

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Resistance is a measure of how much a component resists the flow of electrical current. It is defined as the ratio of the voltage across an element to the current flowing through it, and it is measured in ohms. Resistance applies only to direct current (DC) circuits, where the voltage and current are constant over time.

Impedance, on the other hand, is a measure of how much a component resists the flow of alternating current (AC). Impedance takes into account not only the resistance of the component but also its reactance, which is a measure of how much the component resists changes in current flow. Impedance is defined as the ratio of the voltage across a component to the current flowing through it in an AC circuit, and it is also measured in ohms. Unlike resistance, impedance can vary with the frequency of the AC signal. Check also the difference between resistance and impedance formulas as you go down reading this article.

The formula of resistance and impedance

  1. Resistance

The formula for resistance (R) is:

R = V / I

R is resistance in ohms, V is voltage in volts, and I is current in amperes.

  1. Impedance

The formula for impedance (Z) depends on the type of circuit:

For a circuit with only resistance, the impedance is equal to the resistance. So the formula is:

Z = R

Where R is resistance in ohms.

For a circuit with both resistance and reactance, the impedance is the vector sum of the resistance (R) and the reactance (X). So the formula is:

Z = sqrt(R^2 + X^2)

R is resistance in ohms, X is reactance in ohms, and sqrt denotes the square root.

The reactance depends on the component type, and it can be either capacitive or inductive. The reactance of a capacitor is given by:

Xc = -1 / (2 * pi * f * C)

Where Xc is capacitive reactance in ohms, pi is 3.14159 (the mathematical constant), f is the frequency in hertz, and C is capacitance in farads.

The reactance of an inductor is given by:

Xl = 2 * pi * f * L

Where Xl is inductive reactance in ohms, pi is 3.14159 (the mathematical constant), f is the frequency in hertz, and L is inductance in henrys.

So the impedance of a circuit with both resistance and reactance can be calculated using the formula above.

Below are some example circuits about the difference between resistance and impedance.

Example circuit of resistance

There are many types of circuits that can include resistance. Here are a few examples:

  1. Voltage Divider: A voltage divider circuit consists of two resistors in series with an input voltage applied across the pair. The output voltage is taken from the junction of the two resistors. This circuit is used to create a voltage output that is a fraction of the input voltage.
  2. LED Current Limiter: A resistor can be used to limit the current flowing through an LED (Light Emitting Diode). When an LED is connected to a voltage source, it will draw a large amount of current which can damage the LED. A resistor is connected in series with the LED to limit the current and protect the LED.
  3. Temperature Sensor: A thermistor is a type of resistor whose resistance changes with temperature. This property can be used to create a temperature sensor circuit. By measuring the resistance of the thermistor, the temperature of the circuit can be determined.
  4. DC Motor Speed Control: A potentiometer can be used to control the speed of a DC motor. By connecting the potentiometer in series with the motor and a fixed resistor, the resistance of the circuit can be changed, which in turn changes the voltage supplied to the motor and hence its speed.

These are just a few examples of circuits that utilize resistance. In general, resistors are used in circuits to limit current, divide voltage, set gain or bias levels, filter signals, and perform many other functions.

Example circuit of impedance

There are many types of circuits that include impedance, especially AC circuits. Here are some examples:

  1. RC Circuit: An RC circuit consists of a resistor and a capacitor in series or parallel. The impedance of the circuit changes with the frequency of the AC signal due to the reactance of the capacitor. This circuit is used for filtering or timing signals.
  2. RL Circuit: An RL circuit consists of a resistor and an inductor in series or parallel. The impedance of the circuit also changes with the frequency of the AC signal due to the reactance of the inductor. This circuit is used for filtering or timing signals.
  3. RLC Circuit: An RLC circuit consists of a resistor, an inductor, and a capacitor in series or parallel. The impedance of the circuit changes with the frequency of the AC signal due to the reactance of the capacitor and the inductor. This circuit is used for filtering or tuning signals.
  4. Antenna Circuit: The impedance of an antenna circuit is critical for efficient power transfer between the transmitter and the antenna. The impedance of the circuit must match the impedance of the transmitter and the antenna for maximum power transfer.
  5. Speaker Circuit: The impedance of a speaker circuit affects the power and quality of the sound produced. The speaker impedance must match the amplifier output impedance for optimal performance.

These are just a few examples of circuits that utilize impedance. In general, impedance is a critical parameter in AC circuits, and it is used for filtering, tuning, matching, and many other functions.

What is the relation between resistance and impedance?

Although there is a difference between Resistance and impedance take not they are related, but they are not the same thing. Resistance is a measure of how much a material or component resists the flow of direct current (DC) through it, while impedance is a measure of how much a material or component resists the flow of alternating current (AC) through it.

Resistance is represented by the symbol R and is measured in ohms (Ω). Impedance is represented by the symbol Z and is also measured in ohms (Ω).

In a DC circuit, the resistance is the only factor that affects the flow of current. However, in an AC circuit, the impedance of the circuit is a combination of resistance and reactance, which is the opposition to the change in current flow caused by capacitance or inductance.

In a purely resistive circuit, the impedance is equal to the resistance, so the two values are the same. However, in a circuit that contains both resistance and reactance, the impedance is greater than the resistance, and it is calculated using the formula:

Z = sqrt(R^2 + X^2)

R is the resistance in ohms, X is the reactance in ohms, and sqrt denotes the square root.

So, in summary, resistance is a measure of how much a material or component resists the flow of DC through it, while impedance is a measure of how much a material or component resists the flow of AC through it, and impedance is a combination of resistance and reactance. Hope this topic helps you understand the difference between resistance and impedance.

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