Physics 102: Lecture 6, Slide 1 Important Announcements Makeup lecture #5 –Video of lecture available online (see course website) –Homework #3 now due Thursday Feb. 10 –My (and M. Lundsgaard’s) office hours moved to Wednesday –Lectures resume as normal this week (#6 and #7) Discussions & Labs –Discussions & Labs resume as normal this week –Missed discussion or lab? See course website

Physics 102: Lecture 6, Slide 2 Exam I two weeks from today! How do you study for a Phys 102 exam? –Start studying now! (cramming DOES NOT work) –Emphasize understanding concepts & problem solving, NOT memorization –Review lecture notes, problem solver summary –Understand formula sheet (i.e. when to use and when NOT to use an equation) & know what each symbol means –Do practice exam problems (time yourself!) –Go to office hours & review session

Physics 102: Lecture 6, Slide 3 Kirchhoff’s Laws Physics 102: Lecture 06

Physics 102: Lecture 6, Slide 4 Last Time Resistors in series: Resistors in parallel: Current thru is same; Voltage drop across is IR i Voltage drop across is same; Current thru is V/R i Last Lecture Solved Circuits What about this one? Today

Physics 102: Lecture 6, Slide 5 Kirchhoff’s Rules Kirchhoff’s Junction Rule (KJR): –Current going in equals current coming out. Kirchhoff’s Loop Rule (KLR): –Sum of voltage drops around a loop is zero.

Physics 102: Lecture 6, Slide 6 Conceptual basis: conservation of charge At any junction in a circuit, the current that enters the junction equals the current that leaves the junction Example: R1R1 R2R2 R3R3 I1I1 I3I3 I2I2 11 22 Kirchhoff’s Junction Rule (KJR) At junction: I 1 + I 2 = I 3

Physics 102: Lecture 6, Slide 7 Conceptual basis: conservation of energy Going around any complete loop in a circuit, the sum total of all the potential differences is zero Example: R1R1 R2R2 R3R3 I1I1 I3I3 I2I2 11 22 Kirchhoff’s Loop Rule (KLR) Around the right loop:  2 + I 3 R 3 + I 2 R 2 = 0

Physics 102: Lecture 6, Slide 8 Using Kirchhoff’s Rules (1)Label all currents Choose any direction (2)Label +/– for all elements Current goes +  – (for resistors) (3)Choose loop and direction (4)Write down voltage drops Be careful about signs R4R R1R1 ε1ε1 R2R2 R3R3 ε2ε2 ε3ε3 R5R5 A B I1I1 I3I3 I2I2 I4I4 I5I5

Physics 102: Lecture 6, Slide 9 Loop Rule Practice R 1 =5  I +  1 - IR 1 -  2 - IR 2 = I I = 0 I = +2 Amps  1 = 50V R 2 =15   2 = 10V A B Find I: Label currents Label elements +/- Choose loop Write KLR What is the electric potential at V B (assume V A = 0): V A +  1 - IR 1 = V B 0 +  – 2x5 = 40V = V B

Physics 102: Lecture 6, Slide 10 ACT: KLR R 1 =10  E 1 = 10 V IBIB I1I1 E 2 = 5 V R 2 =10  I2I2 Resistors R 1 and R 2 are 1) in parallel 2) in series 3) neither + - Definition of parallel: Two elements are in parallel if (and only if) you can make a loop that contains only those two elements. Upper loop contains R 1 and R 2 but also ε 2.

Physics 102: Lecture 6, Slide 11 Preflight 6.1 R 1 =10  E 1 = 10 V IBIB I1I1 E 2 = 5 V R 2 =10  I2I2 1) I 1 = 0.5 A 2) I 1 = 1.0 A 3) I 1 = 1.5 A E 1 - I 1 R = 0 28% 60% 12% Calculate the current through resistor 1.  I 1 = E 1 /R = 1A

Physics 102: Lecture 6, Slide 12 Preflight 6.1 R 1 =10  E 1 = 10 V IBIB I1I1 R 2 =10  I2I2 1) I 1 = 0.5 A 2) I 1 = 1.0 A 3) I 1 = 1.5 A E 1 - I 1 R = 0  I 1 = E 1 /R = 1A How would I 1 change if the switch was opened? E 2 = 5 V 1) Increase 2) No change 3) Decrease 28% 60% 12% Calculate the current through resistor 1. ACT: Voltage Law

Physics 102: Lecture 6, Slide 13 Preflight 6.2 R 1 =10  E 1 = 10 V IBIB I1I1 E 2 = 5 V R 2 =10  I2I2 1) I 2 = 0.5 A 2) I 2 = 1.0 A 3) I 2 = 1.5 A E 1 - E 2 - I 2 R = 0 57% 30% 13%  I 2 = 0.5A Calculate the current through resistor 2.

Physics 102: Lecture 6, Slide 14 Preflight 6.2 R=10  E 1 = 10 V IBIB I1I1 E 2 = 5 V R=10  I2I E 1 - E 2 + I 2 R = 0 Note the sign change from last slide  I 2 = -0.5A Answer has same magnitude as before but opposite sign. That means current goes to the right, as we found before. How do I know the direction of I 2 ? It doesn’t matter. Choose whatever direction you like. Then solve the equations to find I 2. If the result is positive, then your initial guess was correct. If result is negative, then actual direction is opposite to your initial guess. Work through preflight with opposite sign for I 2 :

Physics 102: Lecture 6, Slide 15 Kirchhoff’s Junction Rule Current Entering = Current Leaving I1I1 I2I2 I3I3 I 1 = I 2 + I 3 1) I B = 0.5 A 2) I B = 1.0 A 3) I B = 1.5 A I B = I 1 + I 2 = 1.5 A 12% 32% 57% R=10  E 1 = 10 V IBIB I1I1 E = 5 V R=10  I2I2 + - Preflight 6.3 Calculate the current through battery.

Physics 102: Lecture 6, Slide 16 Kirchhoff’s Laws (1)Label all currents Choose any direction (2)Label +/– for all elements Current goes +  – (for resistors) (3)Choose loop and direction Your choice! (4)Write down voltage drops Follow any loops (5)Write down junction equation I in = I out R4R4 R1R1 ε1ε1 R2R2 R3R3 ε2ε2 ε3ε3 I1I1 I3I3 I2I2 I4I4 R5R5 A B

Physics 102: Lecture 6, Slide 17 You try it! In the circuit below you are given  1,  2, R 1, R 2 and R 3. Find I 1, I 2 and I 3. R1R1 R2R2 R3R3 I1I1 I3I3 I2I2 11 22

Physics 102: Lecture 6, Slide 18 You try it! R1R1 R2R2 R3R3 I1I1 I3I3 I2I Loop 1: +  1 - I 1 R 1 + I 2 R 2 = 0 1.Label all currents (Choose any direction) 2. Label +/- for all elements (Current goes +  - for resistor) 3. Choose loop and direction (Your choice!) 4.Write down voltage drops (Potential increases or decreases?) Loop 2: 11 5. Write down junction equation Node: I 1 + I 2 = I 3 22 3 Equations, 3 unknowns the rest is math! In the circuit below you are given  1,  2, R 1, R 2 and R 3. Find I 1, I 2 and I 3. Loop 1  + -   +  1 - I 1 R 1 - I 3 R 3 -  2 = 0   Loop 2

Physics 102: Lecture 6, Slide 19 ACT: Kirchhoff loop rule What is the correct expression for “Loop 3” in the circuit below? I1I1 I3I3 I2I Loop 3 R2R2 R3R3 11 22 R1R1 1) +  2 – I 3 R 3 – I 2 R 2 = 0 2) +  2 – I 3 R 3 + I 2 R 2 = 0 3) +  2 + I 3 R 3 + I 2 R 2 = 0

Physics 102: Lecture 6, Slide 20 Let’s put in real numbers In the circuit below you are given  1,  2, R 1, R 2 and R 3. Find I 1, I 2 and I 3.  5 10 I1I1 I3I3 I2I  Loop 1: 20 -5I 1 +10I 2 = 0 2. Loop 2: 20 -5I 1 -10I 3 -2=0 3. Junction: I 3 =I 1 +I 2 solution: substitute Eq.3 for I 3 in Eq. 2: I (I 1 +I 2 ) - 2 = 0 rearrange: 15I 1 +10I 2 = 18 rearrange Eq. 1: 5I 1 -10I 2 = 20 Now we have 2 eq., 2 unknowns. Continue on next slide Loop 1 Loop 2

Physics 102: Lecture 6, Slide 21 15I 1 +10I 2 = 18 5I I 2 = 20 Now we have 2 eq., 2 unknowns. Add the equations together: 20I 1 =38 I 1 =1.90 A Plug into bottom equation: 5(1.90)-10I 2 = 20 I 2 =-1.05 A note that this means direction of I 2 is opposite to that shown on the previous slide Use junction equation (eq. 3 from previous page) I 3 =I 1 +I 2 = I 3 = 0.85 A We are done!

Physics 102: Lecture 6, Slide 22 See you next time…