1 / 29

Electric Current & Circuits: Fundamentals, Laws, & Applications

Dive into the world of electric current and circuits! Learn about Ohm's Law, resistors, resistivity, power in circuits, and more. Explore the concept of resistance, temperature effects, and energy conversion. Test your knowledge with practical examples and circuit analyses.

wshell
Download Presentation

Electric Current & Circuits: Fundamentals, Laws, & Applications

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. Electric current Physics 114 Lecture V

  2. Up to this point • Static situation – charges are not moving • Coulombs force = charge * electric field • Deeper look in the properties of the electric field - Gauss’s law • Potential energy= charge * electric potential • Electric potential – integral of electric field • Electric field = gradient of the electric potential • Next – dymanics = moving charges = electric current Lecture V

  3. Concepts • Primary concepts: • Electric current • Resistor and resistivity • Electric circuit Lecture V

  4. Laws • Ohm’s law • Power in electric circuits Lecture V

  5. Electric current - - - - - - - + + + + + + + • A flow of charge is called an electric current Note: net charge =0 • It is measured in ampere (A=C/s) • Need free charge to have electric current. Use conductors. Lecture V

  6. Skiing  electric circuit High PE High PE Low PE Skiers Charges go from points with high PE to low PE To complete the circuit need a device that brings you back to high PE: Ski lift Battery Low PE Lecture V

  7. Electric circuit • Need free charge  electric circuit must consist of conductive material (wires). • Electric circuit must be closed. • Battery supplies constant potential difference – voltage. e - • Battery converts • chemical energy into • electric energy. Symbol for battery Lecture V

  8. Electric circuit b). Will not work, Circuit is at the same potential (+), no potential difference - voltage. c). Will work. a). Will not work, Circuit is not closed Lecture V

  9. Ohm’s law • Electric current is proportional to voltage. • Coefficient in this dependence is called resistance R • Resistance is measured in Ohm (W = V/A) I R V Lecture V

  10. Resistors • First digit • Second digit • Multiplier • Tolerance • 2.5 x103W +- 5%. Lecture V

  11. Resistivity • traffic Electric current • Long narrow street  high resistance • Condition of the road  material property called resistivityr. r is measured in W m L – length of the conductor A – its area. Lecture V

  12. Resistance and Temperature • When electrons move through the conductor they collide with atoms: • Resistivity grows with temperature ( more collisions) r0 – resistivity measured at some reference temperature T0 a – temperature coefficient of resistivity Lecture V

  13. Resistance and Temperature • When electrons move through the conductor they collide with atoms: • Temperature of the conductor increases because of the current (through collisions) • Electrical energy is transformed into thermal energy • Resistors dissipate energy • Power – energy per unit of time- (in W=J/s) dissipated by a resistor Lecture V

  14. Electric power • Electric energy can be converted into other kinds of energy: • Thermal ( toaster) • Light (bulbs) • Mechanical (washer) • Chemical • Electric power (energy per unit of time): Lecture V

  15. Test problem • You have an open working refrigerator in your room. It makes your room • A hotter • B colder Lecture V

  16. Test problem • A light bulb is connected to a battery. It is then cooled and its resistance decreased. Brightness is proportional to consumed power. The light bulb burns • A Brighter • B dimmer P=IV P=I2R P=V2/R Lecture V

  17. Alternating current (AC) • Voltage changes sign  current changes the direction I Req ~ Lecture V

  18. Electric circuits: resistors • Current in=current out I1=I2 • No electrons are lost inside • Resistors dissipate power (energy/time) • P=I2R • Drop of voltage over a resistor DV=-IR: • V2=V1-IR R I2,V2 I1,V1 Lecture V

  19. Electric circuits: wires I2,V2 I1,V1 • We assume that wire have very small resistance (R=0) • Current in=current out I1=I2 • Power dissipated in wires • P=I2R=0 • Drop of voltage over a resistor DV=-IR=0 • V2=V1 • From the point of electric circuit wires can be • stretched, • Bended • Straightened • Collapsed to a point without changing the electrical properties of the circuit I2,V2 I1,V1 I2,V2 I1,V1 Lecture V

  20. Electric circuit: battery R1 R2 R3 Energy conservation • Drop of voltage in electric circuit is always equal to voltage supplied by an external source (e.g. battery). • Current (the effective flow of positive charge) goes from + to – • Electrons (negative charge!) go from – to + I V Lecture V

  21. Electric circuits: branches • Charge is conserved • Current – what goes in, goes out I1 I I2 I I3 V Lecture V

  22. Symbols • Circuits can be rearranged: • Wires with negligible resistance can be • Stretched • Bended • Collapsed to a point Lecture V

  23. Skiing  electric circuit a Cannot stop at b, must get to c – ski lift: V=V1+V2- Net voltage drop in a circuit is always equal to the supplied voltage (e.g. battery) b Ski lift c Battery Lecture V

  24. Series connection • Charge conservation: • I=I1=I2=I3 • Ohm’s law • V1=IR1; V2=IR2; V3=IR3 • Energy conservation: • qV=qV1+qV2+qV3 • V=V1+V2+V3 • IReq=IR1+IR2+IR3 • Req=R1+R2+R3 Lecture V

  25. Parallel connection • Charge conservation:I=I1+I2+I3 • Energy conservation: V=V1=V2=V3 • Ohm’s law: I1=V/R1; I2=V/R2; I3=V/R3 Lecture V

  26. DC circuits • Series connection • I=I1=I2=I3 • V=V1+V2+V3 • Req=R1+R2+R3 • Parallel connection • I=I1+I2+I3 • V=V1=V2=V3 Lecture V

  27. R1=R2=R3=R Req=3R I=V/(3R) I1=I2=I3=I=V/(3R) R1=R2=R3=R Req=R/3 I=3V/R I1=I2=I3=I/3=V/R Series vs parallel - I < < Total current and individual currents are smaller in series connection. Lecture V

  28. R1=R2=R3=R Req=3R R1=R2=R3=R Req=R/3 Series vs parallel - Req > Equivalent resistance is larger in series connection. Lecture V

  29. R1=R2=R3=R Req=3R I=V/3R  Pnet=V2/3R I1=V/3R P1=V2/9R R1=R2=R3=R Req=R/3 I=3V/R  Pnet=3V2/R I1=V/R  P1=V2/R Series vs parallel - P P1=I2R Pnet=IV Brightness proportional to power < < Total and individual power consumptions are smaller in series connection. Light bulbs are brighter in parallel connection. Lecture V

More Related