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lec16_021407_-_Currents_and_Resistors

# lec16_021407_-_Currents_and_Resistors - Currents and...

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Unformatted text preview: Currents and Resistors Q = n q Avd Current: I = t Current Density: Va E Batteries J = nqvd J= I = n q vd A L Vb I e- Resistance: R= dR = V I dL A R= L A Resistivity: E = J Resistivity: Parallel: Series: R = dR Const. V Ceq, parallel = i Ci 1 Const. V Const. I 1 Ceq, series Wednesday, 14 February 2007 Const. Q = i 1 Ci Req, parallel = i 1 Ri Ri Req, series = i 1 Wednesday, 14 February 2007 2 Question The power company bills you based on the number of kW*hr used. This is a unit of A. Power B. Energy Real batteries have an internal resistance which affects their behavior "Load" Load" Real Batteries U energy Power (W ) = t time C. Charge D. Speed energy kW hr = time = energy time r & R are in series: V = I (r + R ) emf & (r & R) are in parallel: = V = I (r + R) Small I Big I VAB = - Ir = IR Big R Small R VAB VAB < 4 Wednesday, 14 February 2007 3 Wednesday, 14 February 2007 1 Question Batteries are rated in Amp*hr. What kind of unit is this? Making Measurements in Circuits Ammeter: measures current Voltmeter: measures potential difference Ohmmeter: measures resistance Applies a know V, measures I, and calculates R A. Power B. Energy Current(Amp) = Amp hr = Q charge t time C. Charge D. Speed charge time = charge time Meters should impact the circuit that they are measuring as little as possible! possible! 5 Wednesday, 14 February 2007 6 Wednesday, 14 February 2007 Question An ideal Ammeter: A. Has infinite resistance B. Has zero resistance Question An ideal Voltmeter: A. Has infinite resistance B. Has zero resistance C. Is placed "in line" as if it is part of the wire line" D. Is placed "across" a circuit element, like a resistor, across" capacitor, or battery E. Both B and C 7 C. Is placed "in line" as if it is part of the wire line" D. Is placed "across" a circuit element, like a resistor, across" capacitor, or battery E. Both A and D 8 Wednesday, 14 February 2007 Wednesday, 14 February 2007 2 Find I and V through each resistor 2 Sample Circuit Find I and V through each resistor -1 Sample Circuit Series: Constant I R27 = 2 + 7 = 9 Parallel: R236 Constant V 1 1 1 1 = + + 2 3 6 = 1 9 Series: Constant I R15 = 1 + 5 = 6 -1 I total = Req = 18V = 2A 9 6 Parallel: R36 = + 3 6 Constant V 1 1 = 2 Wednesday, 14 February 2007 9 Wednesday, 14 February 2007 10 3 ...
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