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**Unformatted text preview: **Physics 122 Sept. 21, 2010 Save the date: Monday, October 4, 6:50 8:50. Review session before Midterm #1. Javits 103. (Of course you are already saving Tuesday, October 5, 8:30 10:00 for Midterm #1.) I will have a practice midterm in your hands one week before the exam. Question 1: Measure voltages and currents. Voltage is between two conductors. (Or 2 parts of 1 conductor) Current passes through a conductor. Ideal voltmeter passes no current (infinite resistance). Ideal ammeter has no voltage drop (zero resistance). e sample which ? Ohms law (empirical). True for many materials. Apply a potential difference V along a conductor, and a current I will flow in proportion. nits of Volt/Amp Volts) Units of R = Volt/Amp = Ohm Frequently written . V (V I (Amperes) Question #2 . A coffee pot operates at 110 Volts and draws a current of 1 Amp. What is its resistance? Question #2. A coffee pot operates at 110 Volts and draws a current of 1 Amp. What is its resistance? R = V / I = 110 Volt / 1 Amp = 110 Resistance = Voltage difference / Current. R = V / I. Unit Ohm ( ). Property of a particular component with two wires, or a ngth of wire. length of wire. Resistivity is a property of the material. Unit Ohm meter ( m). wire of length ross section area resistivity A wire of length , cross section area A , resistivity , will have resistance R = / A . (From SV8) Resistivity is ughly proportional Resistivity of roughly proportional to temperature for most metals. platinum vs. temperature At 293 K (20 C), = 1.1x10-7 m d /dT = 4.3x10-10 m/K 1 d 1 d R = d d 0.00392 /K T R T = = = Define temperature coefficient of resistivity (or resistance) . ( ) ( ) [ 1 ( ) ] T T T T = + has dimensions K-1 . ( ) ( ) [ 1 ( ) ] For a limited range of . R T R T T T T = + Question 3: When it is cold (20 C), the heating element of a toaster (nichrome wire, = 4.0x10-4 /K) has a resistance of 30 , which increases to 37 when it hot How hot is it? is hot. How hot is it? Question 3: When it is cold (20...

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