EE302Lecture4 - DC Circuits Basic Concepts and Laws Basic...

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Unformatted text preview: DC Circuits Basic Concepts and Laws Basic Concepts Charge and Current Voltage Power Passive Sign Convention Circuit Elements Ohms Law Kirchhoffs Laws Series and parallel circuits Passive Sign Convention In the circuit below: We know that: The battery supplies power (current flows away from +V) The lamp dissipates power (current flows into +V) The current through the battery is the same as the current through the lamp (because they are in series ). The voltage across the battery is the same as the voltage across the lamp. V B = V L (because they are also in parallel ) 1 Passive Sign Convention: Loads Current is flowing from a higher to a lower voltage. (this will be a resistor or a resistance). Downhill in terms of potential (current flows from the + to the - through the resistance). Reversing both the direction of the current and the Voltage polarity means the power still flows in the same direction (show this on the board in class). Passive Sign Convention: Supplies This device is supplying power if actual i<0 and v>0. A current is flowing from the device into a + node voltage (the + node V is > than the reference node V). 2 POWER Later we will write equations in which current away from a node and into the component will be considered as positive. POWER actual i + reference node device Passive Sign Convention: Supplies Or, this device is supplying power if actual i>0 and actual v<0. A current is flowing away from the + node and the actual voltage on that node turns out to be more negative than the reference node. Physical Reality The physical reality of a circuit does not depend on our choice of how we draw the current directions or how we place + and - voltage polarity signs on the nodes. If the physical current moves from a higher to lower voltage, the circuit element is absorbing (or dissipating) power. Power is positive (leaving the circuit) If the physical current moves from lower to higher voltage, the circuit element is supplying (or delivering or providing) power....
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This note was uploaded on 04/04/2008 for the course EE 302 taught by Professor Mccann during the Spring '06 term at University of Texas at Austin.

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EE302Lecture4 - DC Circuits Basic Concepts and Laws Basic...

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