# ASSIGN3 - Section Name Section Instructor Assignment 3...

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Section: ______ Name: ______________________________ Section Instructor: ______________________________ Assignment 3: Charges in Motion & Electric Circuits Concepts: electric current current density charge carriers and drift speed resistance and resistivity Ohm’s law energy and power in electric circuits “pumping” charges and emf calculating and measuring voltage and current simple multiloop circuits (series, parallel) RC circuits Reading in text: Chapter 26: Sections 1-5, 7. Chapter 27: Sections 1-9. Assignment: Solutions posted on Friday, February 16. Quiz may be given on or after Tuesday, February 20. 1. In the lower atmosphere of the Earth there are negative and positive ions, created by radioactive elements in the soil and cosmic rays from space. The density of singly charged negative ions is 550 3 / cm , while the density of singly charged positive ions is 620 3 / cm . In a certain region, the strength of the fair weather atmospheric electric field is 120 V/m, directed vertically downward, and the measured conductivity is 27 10 14 . / ( ) × Ω m . (a) What is the direction of drift of each kind of ion? (b) What is the direction of the total electric current? Do the motions of the two kinds of ions produce electric currents that add or subtract? Please explain. (c) What is the electric current density ? (d) What is the ion drift speed , assuming it to be the same for both kinds of ions? (e) If this current density were the same over the entire surface of the Earth, what would be the total electric current carried by these atmospheric ions? (f) Does the charge transfer due to this current tend to enhance or diminish the atmospheric electric field? Please explain. 2. A common flashlight bulb is rated at 0.30 amp and 3.0 volts (the values of the current and voltage under operating conditions). (a) What is the bulb's resistance when lit? (b) What electric power is dissipated into light and thermal energy by the bulb? (c) If the resistance of the bulb's tungsten filament at room temperature (20 ° C) is 1.2 Ω , what is the filament's temperature when the bulb is lit? Please note any assumptions that you make. 3. The “kilowatt-hour” (kWh) is a standard unit for measuring electric energy delivered by your electric power company and commonly appears on household electric bills. 1 kWh is defined as the total energy delivered in 1 hour at a steady rate (power) of 1 kilowatt (kW). Hence, the name. [Note: “kilo” means 10 .] 3 (a) Show that 1 3 . 6 10 6 kWh joules (J) = × . (b) If a jelly donut contains chemical (food) energy equal to about 250 kcal or about 1 million J, how many jelly donuts worth of food energy is equivalent to 1 kWh? (c) If you leave a 100 W light bulb on continuously for 1 day (24 hours), how many kWh of electricity does this bulb use?

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