Aim: How does resistivity affect resistance? Do Now: The current flowing through a wire is 4 amperes. How many electrons pass a point inside the wire in 2 seconds?
Electrical Stopping Power Introduction to Resistance
Resistance Resistance: is a measurement of how strongly an object will oppose current An object’s resistance depends on FOUR factors: Resistivity Length Is specific to a material – the higher it is, the more natural resistance the material has. How long is the object? The longer it is, the more it will oppose current flow. Cross-sectional Area Temperature How big is the object across? The wider it is, the more current it will allow to pass. If the object is warm, the molecules inside will be bouncing around more – opposing current.
Equation Unit for Resistance is the: OHM (Ω) R = resistance J.s/C2 Symbol: R R = resistance +
Resistance in Conductors We would like to have conductors that allow CURRENT to flow from one place to another EASILY “IDEAL” conductor minimum RESISTANCE WIDE or NARROW? LONG or SHORT? HOT or COLD? The “IDEAL” conductor has a LOW RESISTIVITY and is SHORT, WIDE, and COLD COLDER conductors have lower resistance because: atoms are not as energetic – don’t interfere with electrons WIDER conductors have lower resistance because: electrons have more room to pass! SHORTER conductors have lower resistance because: electrons do not have to travel as far!
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 2x10-3 m. Assume a temperature of 20C. (Ω.m) a. What is the wire’s area? b. What is the resistivity? c. What is the resistance of the wire? d. Why specify 20C? 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
Envelope Activity Answer the following two questions regarding resistivity with a partner.
Example #1 Calculate the electrical resistance provided by a piece of copper wire with a length of 1 meter and a cross sectional area of 0.01 m2.
Example #2 Determine the composition of a wire with a resistance of 31.8 Ohms if it is 5 x 107 meters long and has a cross-sectional area of 0.025 m2. SILVER
Graph relationships Predict & SKETCH what he following graphs would look like. 1. R vs. L 2. R vs. A Students sketch on smartboard
SUMMARY Which factors affect resistance? How does each factor affect resistance? How can you calculate resistance? What are the units for resistance? Describe an ideal wire with the LEAST amount of resistance