2015+1-1,2,3+all.pdf

# German physicist georg simon ohm 1789 1854 formulated

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German physicist Georg Simon Ohm (1789 – 1854) formulated the relationship between electric voltage and current based on his experiments with circuits done when he taught physics at a prestigious high school in 1820s. (Portrait from Wikipedia) Ohm’s law, which we discuss in a later Unit, is important because it bridges the gap between currents (charged particles) and voltages (forces that acts on charged particles). Together with Kirchhoff’s laws, Ohm’s law is our key tool for circuit analysis. Figure 6. German physicist Gustav Robert Kirchhoff (1824-1887) developed two basic laws of electric circuits when he was 21-23 years old. He is also remembered for co-invention of spectral analysis, for his study of the chemical composition of the sun, and for his contribution to thermodynamics, which helped lead to the development of quantum mechanics. (Portrait from Wikipedia) The two universal laws discovered by Gustav Kirchhoff apply to all electric circuits, regardless of the nature of charge carriers and circuit elements. In today’s terms, we can say: Book Page 8

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EE for the 21 st century Review the basics 1-1-1 Kirchhoff’s law for electric currents © 2015 Alexander Ganago Page 8 of 17 Last printed 2015-07-24 6:15 PM File: 2015 1-1-1 KCL.docx ü Kirchhoff’s Current Law (KCL) is a consequence of the universal principle for the conservation of electric charges ü Kirchhoff’s Voltage Law (KVL), which will be discussed in the next Unit, is a consequence of the universal principle for conservation of energy. Gustav Kirchhoff formulated both his laws about 50 years before the electrons were identified as charged particles. T HE REFERENCE DIRECTION AND THE ACTUAL DIRECTION OF ELECTRIC CURRENT Figures 3 and 4 above show the definition and the conventional direction of electric current. Figure 7 presents two ways of describing the same current, which is very useful for circuit analysis. Figure 7. Mathematically, the same current can be expressed in two different ways. This flexibility is useful for writing equations for circuit analysis. Working with algebraic equations for electric currents (such as Node Voltage equations, which we will explain and apply in further Units), it is convenient to proceed in two steps: 1) Assign a reference direction for each unknown current, then 2) Solve the equations, in order to obtain the actual direction and magnitude of each current. If the actual direction of a particular current is opposite of the reference direction for this current, we simply swap the algebraic sign (see Figure 7). For example, assume that you are writing an equation for the unknown current in Figure 7. You can introduce it as ࠵? ! that flows from left to right; alternatively, you can introduce the unknown current as ࠵? ! that flows from right to left. Suppose that, after solving the equation, you found that the actual current flows from left to right. This result can be expressed in two ways; both mean the same: ࠵?
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• Fall '07
• Ganago
• Electric charge, Alexander Ganago

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