Aim: How can we account for resistivity and capacitance? HW: Problem Set

Using Kirchoff’s Rules Solve for the current through each resistor! Write a full equation without numbers first! Then solve for the current!

Solution Junction Rule: Single wire with no branches Loop: 0 = -30V + IR1 + 10V + IR2 + IR3 + IR4 I = 0.385A

Defining Resistance The opposition that a device or conductor offers to the flow of electric current. Measured in Ohms (Ω) J.s/C2 Symbol: R + 

What variables determine resistance in a wire or object? Length of wire (L) in meters. Area (A) in meters2. Resistivity of material () in ohmsmeters. Temperature (Increase Temp., Increase R) What is resistivity? How well a certain material “resist” the flow of electrons. Does a Copper cable have the same resistance as an aluminum cable?

Factors that affect resistance Length: Length  Area: Area  Temperature: Temp  Resistance  Graph relationship of resistance vs. cross sectional-area…. Inverse Relationship R vs. L… Direct Resistance  Resistance 

Practice A copper wire has a length of 4.00 m and a diameter of 2 mm. Assume a temperature of 20C. a. What is the wire’s area? (Ω.m) b. What is the resistivity? c. What is the resistance of the wire? d. Why specify 20C? Material Resistivity Aluminum 2.82 x 10-8 Copper 1.72 x 10-8 Gold 2.44 x 10-8 Nichrome 150 x 10-8 Silver 1.59 x 10-8 Tungsten 5.60 x 10-8

Station Activity Determine the resistance of a 6.00 m length of silver wire having a cross-sectional area of 3.14x10-6 m2. Assume a temperature of 20C. Material Resistivity Aluminum 2.82 x 10-8 Copper 1.72 x 10-8 Gold 2.44 x 10-8 Nichrome 150 x 10-8 Silver 1.59 x 10-8 Tungsten 5.60 x 10-8

Summary: How is current, voltage and resistance all related? Symbol Name Units Equation V Potential Difference Volts (V) PE = qV I Current Amps (A) I=q/t R Resistance Ohms (Ω) R = ρL/A What is the graph of Voltage vs. Current? From Lab students know Slope = Resistance

Summary What decides an objects resistivity? How will the resistance of a wire change if I double the area, but cut the length in half? How will current behave in a circuit of parallel resistors? What “rule” does this exemplify?