Presentation on theme: "Resistance, Ohm’s Law & Circuits"— Presentation transcript:
1 Resistance, Ohm’s Law & Circuits Georg OhmPhysics 1161: Lecture 08Resistance, Ohm’s Law & CircuitsHomework, keep lots of digits!1
2 Today Resistors R = V/I Last Time Capacitors C = Q/V -- definition Physical C = e0A/dSeries /Ceq = 1/C1 + 1/C2Parallel Ceq = C1 + C2Energy U = 1/2 QVTodayResistors R = V/IPhysical R = r L/ASeries Req = R1 + R2Parallel 1/Req = 1/R1 + 1/R2Power P = IV
3 Preflight 8.121Two cylindrical resistors are made from the same material. They are of equal length but one has twice the diameter of the other.R1 > R2R1 = R2R1 < R2
4 Example e Simple Circuit Practice… Calculate I when e=24 Volts and R = 8 WOhm’s Law: V =IRI = V/R= 3 Amps
5 Note: I is the same everywhere in this circuit! Preflight 8.3R1=1We0R10=10WCompare I1 the current through R1, with I10 the current through R10.I1 < I10I1 = I10I1 > I10Note: I is the same everywhere in this circuit!
6 Compare V1 the voltage across R1, with V10 the voltage across R10. V1>V10V1=V10V1< V10R1=1We0R10=10W
7 Compare V1 the voltage across R1, with V10 the voltage across R10. V1>V10V1=V10V1< V10R1=1We0R10=10WV1 = I1 R1 = 1 x IV10 = I10 R10 = 10 x I
8 Practice: Resistors in Series ExamplePractice: Resistors in SeriesR1=1We0R2=10WCalculate the voltage across each resistor if the battery has potential V0= 22 volts.Simplify (R1 and R2 in series):R12 = R1 + R2V12 = V1 + V2I12 = I1 = I2R12e0= 11 W= V0 = 22 Volts= V12/R12 = 2 AmpsExpand:V1 = I1R1V2 = I2R2= 2 x 1 = 2 VoltsR1=1We0R2=10W= 2 x 10 = 20 VoltsCheck: V1 + V2 = V12 ?YES!
9 Preflight 8.5What happens to the current through R2 when the switch is closed?IncreasesRemains SameDecreasesV2 = ε = I2R2
10 What happens to the current through the battery when the switch is closed? IncreasesRemains SameDecreases
11 What happens to the current through the battery when the switch is closed? IncreasesRemains SameDecreases
12 Practice: Resistors in Parallel ExamplePractice: Resistors in ParallelR2R3eDetermine the current through the battery.Let E = 60 Volts, R2 = 20 W and R3=30 W.Simplify: R2 and R3 are in parallelR23e1/R23 = 1/R2 + 1/R3V23 = V2 = V3I23 = I2 + I3R23 = 12 W= 60 Volts= V23 /R23 = 5 Amps
13 Why is it dangerous to use one power strip to plug in and use simultaneously your microwave, coffee pot, toaster, and hair dryer (current through hair dryer is 10 A)?The resistance of the kitchen circuit is too high.The voltage across the kitchen circuit is too high.The current in the kitchen circuit is too high.
14 Why is it dangerous to use one power strip to plug in and use simultaneously your microwave, coffee pot, toaster, and hair dryer (current through hair dryer is 10 A)?The resistance of the kitchen circuit is too high.The voltage across the kitchen circuit is too high.The current in the kitchen circuit is too high.
15 Preflight 8.6, 8.7123R R R/2Which configuration has the smallest resistance?123Which configuration has the largest resistance?2
16 Parallel + Series Tests Resistors R1 and R2 are in series if and only if every loop that contains R1 also contains R2Resistors R1 and R2 are in parallel if and only if you can make a loop that has ONLY R1 and R2Same rules apply to capacitors!!
17 Summary Each resistor on the same wire. SeriesParallelR1R1R2R2Each resistor on the same wire.Each resistor on a different wire.WiringDifferent for each resistor.Vtotal = V1 + V2Same for each resistor.Vtotal = V1 = V2VoltageSame for each resistorItotal = I1 = I2Different for each resistorItotal = I1 + I2CurrentIncreasesReq = R1 + R2Decreases1/Req = 1/R1 + 1/R2Resistance
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