db_lec07_post

# db_lec07_post - Lecture 7 Last lecture Current Electrical...

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Lecture 7 1/31/2012 1 q I t Δ = Δ L R A ρ = VI R = • Last lecture: • Current • Electrical Resistance • Ohm’s Law • This lecture: • Kirchhoff's Rules • Resistor in series • Resistors in parallel D. Bortoletto PHYS221

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1/31/2012 2 Predicting Current in Circuits How could one predict currents running in a complicated circuit? Answer: Kirchhoff’s Rules D. Bortoletto PHYS221
1/31/2012 3 How much current will flow in a circuit? Want to predict I Analogy: If water height difference (potential energy) is maintained, the flow rate of water will not change The flow rate of water will depend on the water height difference Potential difference Δ V defines amount of the current that flows in a circuit D. Bortoletto PHYS221

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1/31/2012 4 Kirchhoff’s Rules 2. Kirchhoff’s Junction Rule: 0 = I This is the consequence of conservation of charge: (charge cannot be created or destroyed). I 1 + I 2 = I 3 so: I 1 + I 2 - I 3 = 0 1.Kirchhoff’s Loop Rule: 0 = Δ V Change in PE of a charge as it travels around a complete circuit loop must be zero D. Bortoletto PHYS221
1/31/2012 5 Kirchhoff’s First Rule (“Loop Rule” or “Kirchhoff’s Voltage Law”) The algebraic sum of the changes in potential encountered in a complete traversal of any loop of circuit must be zero. Move around circuit: ε 1 R 1 R 2 I 2 D. Bortoletto PHYS221 -IR 1 -IR 2 + 1 - 2 =0

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6 Rules ε 1 R 1 R 2 I 2 Voltage Gains enter with a + sign . Voltage Drops enter with a - sign . Note: - + a b - + b a I a b I b a Moving from a to b Opposite arrow then - Moving from a to b in the direction of arrow then + Moving from a to b In the direction opposite to the current Δ V=+IR Moving from a to b
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db_lec07_post - Lecture 7 Last lecture Current Electrical...

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