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Lecture 27b

# Lecture 27b - Circuits(contd What is the current CT 29.18b...

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Circuits (contd.)

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©University of Colorado, Boulder (2008) CT 29.18b What is the current flowing through R3? A: 1.5 A B: 2 A C: 3 A D: 4 A E: 12 A V=IR
A Light bulb is a resistor We buy a light bulb rated 120V 100W. What is its’ resistance? 𝑃 = 𝑉 2 𝑅 𝑅 100 = V 2 𝑃 = (120)(120) 100 = 144 Ω We buy a light bulb rated 120V 60W. What is its’ resistance? 𝑅 60 = V 2 𝑃 = (120)(120) 60 = 240 Ω The resistance of a bulb is a constant!

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Worksheet 27a…
5 In the circuit below, the bulbs are identical. What happens to the brightness of bulb A, when bulb C burns out? (When a bulb burns out, its resistance becomes infinite.) A) Bulb A gets brighter B) Bulb A gets dimmer. C) It's brightness remains the same. (Hint: What happens to the current from the battery? Do demo

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6 It is good to be able to think through (without lots of equations) how the components of these circuits work. However, we will need to use our equations to work out detailed values for current and electric potential in more complicated circuits. We now outline a systematic procedure for analyzing circuits.
7 Example 1: Choose R1 = R2 = R3 = 10 W V=12V 1 2 3 1. Longer line on battery is higher voltage and the shorter line is lower voltage, by convention. + - 2. We label the current in each section as shown. Note that we can guess the direction at this point. The answer will be independent of our guess. i b i b i b i b i 2 i 3 i 1

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8 V=12V 1 2 3 + - i b i b i b i b i 2 i 3 i 1 3. Apply the Voltage Loop Rule around each possible loop in the circuit.
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Lecture 27b - Circuits(contd What is the current CT 29.18b...

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