2. Part 1 Current & Energy Transfer Electric Current

3. Objectives Define an electric current, and the ampere Describe conditions that create current in an electric circuit Define power in electric circuits Draw circuits and recognize that they are closed loops Define resistance and state Ohm’s Law Use equations to solve problems

4. Electric Circuit o Closed path along which charged particles move

5. Current Flow of charged particles Unit is ampere (A) or “amp” Rate at which charge passes a point in a circuit 1 ampere = 1 coulomb per second Symbol is “I”

6. Electric Current Conducting path Closed circuit Supply of energy Potential difference (voltage) Conventional current Flows from positive to negative Electrons actually move from negative to positive

7. Conductivity Ability of a material to conduct electricity Depends on number of free charges per unit volume Charges move under the influence of an electric field Potential difference created between two points Pure metals are good conductors Many electrons are not bound or are loosely bound to a particular atom

8. Resistivity Reciprocal of conductivity Characteristic of a material that depends on its electronic structure & temperature Increases with temperature Conductors Low resistivity Insulators High resistivity Electrons are tightly bound; few are free to move

9. Energy Transfer Power Rate at which energy (work) is transferred Units are watts (W) Joules per second Multiply the potential difference by the current

10. Power

11. Example Textbook, Page 509 Current through the motor is 3.0 coulombs per second Potential difference is 120 volts 1 volt = 1 joule per coulomb What is the power? 1 watt = 1 joule per sec

12. Practice Problem A 6.0 V battery delivers a 0.50 A current to an electric motor that is connected across its terminals What power is consumed by the motor? If the motor runs for 5.0 minutes, how much electric energy is used? If the motor does 600 J of work, what is the efficiency of the motor?

13. Practice Problem Draw a diagram Show the conventional current List the knowns and unknowns

14. Practice Problem Do Textbook, Pg. 511 #1 – 4

15. Part 2 Resistance & Ohm’s Law Electric Current

16. Resistance Property that determines how much current will flow between two points at a given potential difference Measured by placing a potential difference across two points on a conductor, and measuring the current Resistance (R) is the ratio between the voltage (V) and the current (I)

17. Resistance Opposition to the flow of electric current Ratio of potential difference to current flow Unit is “ohm” (Ω) Ohm’s law Device which obeys this relationship is “ohmic” Not a general physical “law”

18. Resistance Unit of resistance is the ohm (  ) One ohm is the resistance that will allow one ampere of current to flow when the potential difference is one volt Ohm’s law Describes the behavior of a “pure” resistor Devices that behave this way are called, “ohmic”

19. Ohm’s Law Current R = slope of the VI curve

20. Ohm’s Law

21. Application Circuit diagrams Pg. 512 Potentiometer (variable resistor) Pg. 513 See Pg. 514, Table 22-1

22. Example A 30.0 V battery is connected to a 10.0 ohm resistor What is the current in the circuit? Draw a diagram List the knowns and unknowns Calculate your answer Check your answer

23. Example

24. Resistance of a Wire Depends on its physical properties Length Cross-sectional area (thickness) Resistivity of material (ρ)

25. Units of Resistivity What are the units of resistivity? Ohm·meter (Ω·m)

26. Example What is the resistance of 500 yards of 12AWG copper wire? If one amp of current is flowing in the wire, what is the potential difference (voltage)?

27. Solution

28. Check

29. Temperature Coefficient of Resistance The temperature coefficient of resistance is the change in resistance per unit resistance per degree change in temperature. Where:  = temperature coefficient of resistance  R = change in resistance R 0 = initial resistance  t = change in time Where:  = temperature coefficient of resistance  R = change in resistance R 0 = initial resistance  t = change in time

30. Superconductivity Superconductivity is a condition of zero resistance encountered at relatively low temperatures for certain materials. The transition temperature is the temperature at which the resistivity of a superconductor material rapidly approaches zero.

31. Extra Credit What animal is capable of producing nearly 1 amp of current? Genus & species Where found

32. Practice Problems Drawing schematic diagrams Pg. 516 Textbook, Pg. 515 #5 – 10 Textbook, Pg. 517 #11 – 13