Figure 2 shows the circuit symbol for a resistor a

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Figure 2 shows the circuit symbol for a resistor (a zigzag line) along with the reference polarity of voltage across and the reference direction of current through the resistor. Electric resistance is always positive for resistors. In other words, electric current always enters the more positive terminal of a resistor; if the polarity of voltage is swapped, the direction of current gets reversed (and vice versa ). Figure 2. The circuit symbol of a resistor is shown along with the reference polarity of voltage across and the reference direction of current through this resistor. The current-voltage (or I-V) characteristic of a resistor is linear according to Ohm’s law. Figure 2 also includes the plot that relates the current through and the voltage across a circuit component. This is the volt-amp, or I-V characteristic – one of the favorite tools of Electrical Engineers. According to Ohm’s law, this plot is linear. Book Page 32
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EE for the 21 st century Review the basics 1-1-3 Ohm’s law – and beyond © 2015 Alexander Ganago Page 4 of 11 Last printed 2015-07-24 6:18 PM File: 2015 1-1-3 Ohm's.docx <Sidebar> Electrical Engineers often use the volt-amp, or I-V characteristics – the plots that relate the current through a circuit component and the voltage across this component. However, Georg Simon Ohm discovered his law as an empirical rule; now we understand that it is not a universal law of nature. The circuit components, to which Ohm’s law applies, are called resistors. But Ohm’s law may fail even for resistors, if they are used outside of the recommended operating conditions. For example, if the power absorbed by the resistor (Unit 1-1-4) exceeds the manufacturer’s specifications, it may simply burn out (go in flames) and quit obeying Ohm’s law. <Sidebar> Ohm’s law is not a universal law of nature: it applies to certain types of materials but fails in many other important cases. E LECTRIC CONDUCTANCE RECIPROCAL OF ELECTRIC RESISTANCE Reciprocal of resistance is the electric conductance ࠵? , with which Ohm’s law can be expressed as the following: ࠵? = ࠵? ࠵? ; ࠵? = ࠵? ࠵? ; ࠵? = ࠵? ࠵? = 1 ࠵? [Equation 3] The SI unit of conductance is siemens (S), named after German inventor and industrialist Ernst Werner von Siemens (Figure 3): 1 ࠵? = 1 ࠵? 1 ࠵? [Equation 4] Book Page 33
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EE for the 21 st century Review the basics 1-1-3 Ohm’s law – and beyond © 2015 Alexander Ganago Page 5 of 11 Last printed 2015-07-24 6:18 PM File: 2015 1-1-3 Ohm's.docx Figure 3. Ernst Werner von Siemens (1816 – 1892) was a German inventor and industrialist, founder of one of the largest electro- technological companies in the world, which is known today as Siemens AG. The fourth (out of fourteen) son of a tenant farmer, he left school without finishing education to become a soldier. When he become inventor and founded his own company, his younger brothers became its representatives in England (Carl Wilhelm Siemens was knighted and became Sir William) and in Russia (Carl Heinrich was ennobled by Tsar Nicholas II).
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  • Fall '07
  • Ganago
  • Electric charge, Alexander Ganago

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