Voltmeter has two terminals probe and reference for

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Voltmeter has two terminals – Probe and Reference – for voltage measurements. The connections of cables determine the reference polarity of the measured voltage. The two choices of the reference polarity in Figure 4 are shown in Figure 6. Figure 6. Measurements of the unknown voltage (Y or Z) involve opposite reference polarities for the same actual polarity and magnitude (compare with Figure 4). In general, we can talk about ü The reference marks for the direction of the current and the polarity of the voltage vs. ü The actual direction of the current and the actual polarity of the voltage. Book Page 25
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EE for the 21 st century Review the basics 1-1-2 Kirchhoff’s law for electric voltages © 2015 Alexander Ganago Page 8 of 11 Last printed 2015-07-24 6:16 PM File: 2015 1-1-2 KVL.docx It is especially valuable to distinguish them when you work with alternating (sinusoidal, etc.) currents and voltages: ü The reference marks remain fixed, while ü The actual direction of current (and/or polarity of voltage) varies many times per second. On the other hand, reference marks are like the scaffolding used in construction: ü Everyone knows the Statue of Liberty but only few historians care to learn what scaffolding was used for its construction ü Similarly, we most often need to know are the actual currents and voltages in the circuit, and very seldom look into what reference marks were used for their calculations or measurements. C ONNECTION IN PARALLEL Parallel connection of circuit elements, sketched in Figure 7, is very important in practice, because it can be found in many real-world circuits. Figure 7. Parallel connection of three circuit elements. If two or more circuit elements are connected between the same two nodes in the circuit, we call it parallel connection. According to KVL, the voltages across circuit elements, which are connected in parallel, all equal each other. <Sidebar> Two or more circuit elements, which are connected between the same two nodes in the circuit, are in parallel to each other. The voltages across all elements, which are connected in parallel, are all the same. Indeed, for the left loop of the circuit in Figure 21, the KVL equation is: ࠵? ! + ࠵? ! = 0 For the right loop: ࠵? ! + ࠵? ! = 0 For the large loop: Book Page 26
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EE for the 21 st century Review the basics 1-1-2 Kirchhoff’s law for electric voltages © 2015 Alexander Ganago Page 9 of 11 Last printed 2015-07-24 6:16 PM File: 2015 1-1-2 KVL.docx ࠵? ! + ࠵? ! = 0 Thus, ࠵? ! = ࠵? ! = ࠵? ! Parallel connection is present as a building block in many circuits; thus the ability to recognize it and to apply its consequences is a very valuable skill for circuit analysis. E XAMPLE Figure 8. Calculate the unknown voltages V 1 and V 2 in this circuit. Let us calculate the unknown voltage ࠵?
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  • Fall '07
  • Ganago
  • Electric charge, Alexander Ganago

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