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**Kirchhoff’s voltage law (KVL)**

Voltage (potential difference): total work per unit charge associated with the motion of charge between two points. 1 volt (V) = 1 joule (J) / coulomb (C) Potential energy: qV, produced by electric field mgh, produced by gravitation field

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**Kirchhoff’s voltage law (KVL): Cont.**

The sum of voltages around a closed loop is zero. -v1 + v2 = 0 + - Potential different (V) vs. potential (V) Potential: at each node, va, vb Potential difference: v2=va-vb Ground: true ground (earth) chassis ground (enclosure)

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**Circuit Elements and Their i-v Characterisitcs**

Resistance and Ohm’s Law: Resistance (R) depends on materials and geometry, and resistivity (r) only depends on materials.

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**Electric Power Electric power: moving charge doing work (energy)**

Power (P) = Work / time = work / charge x charge / time = V I P = Joules / sec = Watts (W) What are paying for electricity bill ? kW H = energy

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More on Resistance Home electric appliances: what does a 1000 W hair dryer tell you? Always assume 120V if the voltage is not specified. Never exceed the rated power. Can you use these appliances in 220 V lines. You destroy the appliance! You can use appliances rated with 220V in 110 V, but with much lower power than rated. (Exercise)

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**Open and Short Circuits**

Short Circuit: R=0, v=0 for any i. Particularly bad for voltage source. Open Circuit: R ∞, i=0 for any v. Particularly bad for current source.

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**Series Resistors and the Voltage Divider Rule**

Series Circuit: One branch same current through each resistor. Voltage Divider:

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**Example: voltage divider**

If you want to use your car battery (12 V) to power your laptop computer which requires 3V power supplier, how would you do it.

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**Parallel Resistors and the Current Divider Rule**

Parallel Circuit: resistors share the same terminal same voltage on each resistor. Current Divider: Large current though smaller R Advantage of parallel circuit: a broken branch will not affect other branches

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