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Phy122_0921

# Phy122_0921 - Physics 122 Sept 21 2010 Save the date Monday...

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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:

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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 ?
Ohm’s law ( empirical ). True for many materials. A l i l diff V l d d Apply a potential difference along a conductor, and a current I will flow in proportion. Units of R = Volt/Amp Volts) = Ohm Frequently written Ω . V (V I (Amperes) Q ti #2 A ff t t t 110 V lt d Question #2 . A coffee pot operates at 110 Volts and draws a current of 1 Amp. What is its resistance?

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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 Ω
Resistance = Voltage difference / Current. R = Δ V / I . Unit Ohm ( Ω ). Property of a particular component with two wires, or a length of wire. Resistivity is a property of the material. ρ Unit Ohm meter ( Ω m). A wire of length cross section area A resistivity ρ , , resistivity , will have resistance R = ρ / A .

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(From SV8)
Resistivity is roughly proportional Resistivity of to temperature for most metals. platinum vs . temperature At 293 K (20 ° C), ρ = 1.1x10 -7 Ω m d ρ /dT = 4.3x10 -10 Ω m/K 1 d 1 d R ρ α = = d d 0.00392 /K T R T ρ = Define temperature coefficient of resistivity (or resistance) α . h di i K 1 0 0 ( ) ( )[1 ( )] ( ) ( )[1 ( )] T T T T R T R T T T ρ ρ α = + α has dimensions K -1 . 0 0 For a limited range of . T α = +

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Question 3: When it is cold (20 ° C), the heating element of a toaster (nichrome wire, α = 4.0x10 -4 /K) has a resistance of 30 Ω , which increases to 37 Ω when it is hot How hot is it? is hot.
Question 3: When it is cold (20 ° C), the heating element of a toaster (nichrome wire, α = 4.0x10 -4 /K) has a resistance of 30 Ω , which increases to 37 Ω when it is hot How hot is it?

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Phy122_0921 - Physics 122 Sept 21 2010 Save the date Monday...

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