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# Assign4 - PHY 54! Assignment #4! Summer 2011 Reading: DC...

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Reading : DC Circuits Key concepts: Current and current density, emf, resistance, Joule heating, power to a circuit element, Kirchhoff’s rules. 1. Questions about light bulbs. a. A 100 W light bulb and a 1000 W space heater both operate on 120 V. What is the ratio of the resistance of the bulb to that of the heater? b. Why is the glass enclosure for a 100 W bulb larger than for a 25 W bulb? c. The “drift” speed of electrons in a current is about 1 m/hr. Why does a light bulb begin to glow almost immediately when the switch is closed? 2. More questions about light bulbs. a. Why are lamps in ordinary household wiring connected in parallel? b. Why are the small bulbs for holiday lighting wired in series? c. A bank of batteries connected in series has a total emf of 120 V. When a light bulb rated 25 W at 120 V is connected across this bank it glows with normal brightness. When a bulb rated 250 W at 120 V is connected across the bank, its brightness is much less than normal. Explain. 3. Questions about circuit elements. a. Is the current through a circuit element always proportional to the potential difference across it? If not, give a counter-example. b. The circuit element shown contains a 6 V battery and a 0.2 Ω resistor. The current is 5 A, and at the terminals V a = 5 V, V b = 10 V. What is the power supplied to the element? c. Draw a circuit diagram showing how the battery and resistor might be conFgured in the element shown. 4*. Each bulb in a string of 48 holiday lights (connected in series) has total resistance 10 Ω . It consists of a Flament of resistance R and a 130 Ω shunt resistor, connected in parallel as shown. a. What is the purpose of the shunt resistor? b. What is the Flament resistance R ? c. What power does the string consume if all bulbs are working? d. What power does the string consume if one bulb’s Flament burns out? I a b PHY 54 Assignment #4 Summer 2011

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5. Questions about Ohm’s law. a. Distinguish clearly between the defnition oF resistance and Ohm’s law, since both involve the equation Δ V = IR . b.
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## This note was uploaded on 10/19/2011 for the course PHYSICS 54L taught by Professor Thomas during the Summer '09 term at Duke.

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Assign4 - PHY 54! Assignment #4! Summer 2011 Reading: DC...

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