1 / 22

Chapters 25.4 and 26 to 26.3

Chapters 25.4 and 26 to 26.3. How current flows. x. 1. 2. 3. EMF—Electromotive Force. Any chemical, solar, mechanical, heat method of creating a potential difference Batteries Alternator, dynamo Solarcell, photovoltaic Thermocouple Symbol E Units: volts. A simple circuit. i.

reynold
Download Presentation

Chapters 25.4 and 26 to 26.3

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Chapters 25.4 and 26 to 26.3

  2. How current flows x 1 2 3

  3. EMF—Electromotive Force • Any chemical, solar, mechanical, heat method of creating a potential difference • Batteries • Alternator, dynamo • Solarcell, photovoltaic • Thermocouple • Symbol E • Units: volts

  4. A simple circuit i

  5. EMF Devices • Ideal EMF device– no internal resistance • Real EMF device– some internal resistance

  6. Batteries • Several Different Types of Batteries (called cells) • Wet Cell (left) • Car Batteries • High Current Apps • Dry Cells (Zn-Cu paste) • Dry Cell Ratings • AAA 20 mA • AA 25 mA • C 80 mA • D 150 mA • Akaline Cells • AAA 200 mA • AA 300 mA • C 500 mA • D 600 mA • EMF=1.5 V • Cells are constant voltage sources: 1.5 3 6 9 12 15 24 48 V e-e- PbO2 Pb PbSO4 PbSO4 PbO2 Pb Pb PbO2 SO4 H2

  7. Internal Resistance • The battery itself can have some resistance to current flow • Could be terminals • Could be plates or paste • Could be combination PbO2 Pb PbO2 Pb Pb PbO2 SO4 H2

  8. Internal Resistance • We treat the internal resistance as if an external resistor had been added to the circuit just ahead of the positive terminal

  9. Terminal Voltage (Effective Voltage) EMF R Vab=Va-Vb=EMF-iR va vb

  10. Dr. Womble Loop Rule • The algebraic sum of changes in potential encountered in a complete traversal of the circuit must be zero. • AKA Kirchoff’s Loop Rule • Consider Nashville Panama City Total Elevation Change = 0

  11. From an electrical perspective

  12. Resistance rule • For a move through a resistor in the direction of current, the change in potential is –iR • If the move opposes the current then the change in potential is +iR. move Va-Vb= +iR Va-Vb= -iR Vb Va i

  13. EMF Rule • For a move from the negative terminal to the positive terminal then the change in potential is +EMF • For a move from “+” to “-” then the change in potential is -EMF move -EMF +EMF

  14. Putting these ideas into practice i.e changing a circuit into an equation X • Pick a direction for the current. • Pick a direction of circuit traversal • 3. Sum the potentials as you traverse the circuit My move R=65 W From X, +iR+EMF=0 EMF=-iR 5=-65i i=-0.076 A or -76 mA EMF=5 V i The negative sign means that we guessed the wrong direction for the current.

  15. Resistors in Series • Connected resistances are said to be in series when a potential difference that is applied across their combination is the sum of the resulting potential differences across all the resistances. R3 E-iR1-iR2-iR3=0 E E Req R2 R1 So Req=R1+R2+R3

  16. Reducing Networks, If You Can, then DO SO! • Always try to reduce the total number of variables by using the equivalent resistance. • For N resistors in series, the equivalent resistance is • Req=R1+R2+….+RN

  17. How to find a potential difference • To find the potential difference between any two points • Start at one point • Traverse the circuit following any path • Add algebraically the changes in potential Point A R3 Blue— Va –iR3-iR2=Vb Va-Vb=i(R3+R2) i E R2 Red— Va-E+iR1=Vb Va-Vb=E-iR1 Point B R1

  18. One last word on internal resistance • Recall Power, P=Vi a X E-ir=0 Va+ir-E=Vb Va-Vb=E-ir P=iV <-(Va-Vb) P=i(E-ir) P=Ei-i2r r i E X b Power of EMF Device Thermal dissipation (losses)

  19. Junction Rule • Sum of all currents entering a junction must equal the sum of all currents leaving the junction. i3 i1 i3 i1 i2 i2 i2 i3 i1 i1+i2+i3=0 i1+i2=i3 i1+i3=i2 IN = OUT

  20. Resistors in Parallel • Connected resistances are said to be in parallel when a potential difference that is applied across their combination results in that same potential difference across each resistance. i1 i2 i3 i E E R1 R2 R3 i=i1+i2+i3 i E Req

  21. Resistor Color Codes • Color Codes Color Value Tolerance Black 0 Brown 1 1% Red 2 2% Orange 3 3% Yellow 4 4% Green 5 Blue 6 Violet 7 Gray 8 White 9 Gold 5% Silver 10% No Color 20% 1 2 3 T Band1*10+Band2 x 10^ Band 3 +/- Band T

  22. Guidelines for Problem Solving • Replace network of resistors with their equivalents (if possible) • If you can’t simplify to a single loop, then use the junction rule and the loop rule to set up a series of equations. Be sure to: • Pick a direction of current (sign is a mathematical convention) • If you traverse a resistor against the current then +iR else –iR • If you traverse an EMF source from low potential to high potential then the EMF is positive, else negative. • You have the following arbitrary choices • Directions of currents • Which loops to use • Direction of traversal of each loop • Starting point and ending point • ABOVE ALL, REMEMBER: • TRAVERSE THE LOOP COMPLETELY • ONCE YOU HAVE CHOSEN A DIRECTION OF THE CURRENT YOU MUST STICK WITH THIS DIRECTION UNTIL YOU HAVE FINISHED THE PROBLEM.

More Related