FCC Exam Question: 8-45F4

In the circuit contained in Fig. 8F12, there are 5 volts present at points B and C, and there are zero volts present at points A and D. What is the voltage at point E?

A. -1.5 Volts.
B. 3.75 Volts.
C. 23.75 Volts.
D. 4.5 Volts.2009 FCC Commercial Element 8 Question Pool (approved 25 June 2009) PAGE 54
Correct Answer: A

Explanation: In the circuit (Fig. 8F12), points A and D are at 0 volts (ground), and points B and C are at +5 volts. In a simple resistive network powered only by positive voltage and ground, all intermediate nodal voltages must fall between 0V and +5V. For "the voltage at point E" to be -1.5 Volts, E must represent a *differential voltage* across two points within the circuit, typical of a Wheatstone bridge configuration. If E represents the voltage difference between two nodes, say Vp1 and Vp2, such that E = Vp1 - Vp2, then a negative value is possible if Vp2 is greater than Vp1. For example, if Vp1 = +2.0V and Vp2 = +3.5V, then E = 2.0V - 3.5V = -1.5V. This indicates an unbalanced bridge where one potential is lower than the other. Options B, C, and D are incorrect because they represent positive voltages, which would be expected for a single node's potential referenced to ground in a simple voltage divider, but not for the specific condition that yields a negative result from a differential measurement.

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Includes Elements 1, 3, 6, 7R, 8, and 9.