When thinking about how capacitive reactances, inductive reactances, and resistances combine in a circuit containing resistors, capacitors, and inductors, it’s useful to think in terms of polar coordinates. If you consider that r is the third side of a right triangle made up of the sides r, x, and y, r is equal to the square root of x 2 + y 2. After you’ve computed the net reactance, you plot the resistance on the x-axis and compute the magnitude of the impedance, shown by r in the graph above. The net reactance, X, will be the sum of the two reactances. To figure out the impedance of a circuit, you first plot the inductive reactance on the positive y-axis and the capacitive reactance on the negative y-axis. If the impedance was 50 + j25, then the circuit would have 50 ohms resistance in series with 25 ohm of inductive reactance because +jX represents an inductive reactance. QUESTION: What does the impedance 50-j25 represent? (E5C06) ANSWER: 50 ohms resistance in series with 25 ohms capacitive reactance QUESTION: Which of the following represents capacitive reactance in rectangular notation? (E5C01) ANSWER: -jX When X is negative, the reactance is capacitive. When X is inductive reactance, the reactance is a positive value. In rectangular notation, we’d represent an impedance as R +/- jX, where X is the value of the reactance. QUESTION: Where is the impedance of a pure resistance plotted on rectangular coordinates? (E5C07) ANSWER: On the horizontal axis QUESTION: When using rectangular coordinates to graph the impedance of a circuit, what do the axes represent? (E5C09) ANSWER: The X axis represents the resistive component and the Y axis represents the reactive component QUESTION: What coordinate system is often used to display the resistive, inductive, and/or capacitive reactance components of impedance? (E5C04) ANSWER: Rectangular coordinates When using rectangular coordinates to graph the impedance of a circuit, the X axis, or horizontal axis, represents the resistive component, and the Y axis, or vertical axis, represents the reactive component. Rectangular coordinates is the coordinate system often used to display the resistive, inductive, and/or capacitive reactance components of an impedance. The two numbers that are used to define a point on a graph using rectangular coordinates are the coordinate values along the horizontal and vertical axes. Most often when we plot values on a graph, we use the rectangular, or Cartesian, coordinate system.
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