1. Electric Circuits

2. What is Electric Current? Current in a river –The movement of water molecules Higher current = more water passing a given point each second. Current in a pipe (flow rate) –How fast water is flowing in the pipe. What is needed for water to flow? –A path for flow –Pressure to move the water

3. Electric Current –The movement of charge past a given point. Higher current = more charge passing a given point each second. What is needed for electric current in a wire? –A closed path for flow called a circuit –“Pressure”, called a potential difference (V) What is Electric Current?

4. How does a power source work? –Creates a potential difference Current flows from high potential to low potential Electrons flow from negative to positive Battery –Symbol = –Potential Difference measured in Volts (V) 

5. Conductivity (Solids) The ability of a material to conduct electricity depends on the following: –T–The number of free charges per unit volume. –T–The mobility of charges Conductor, insulator, and semiconductor –C–Conductor:Many free electrons (Metals) –I–Insulator:Few free electrons (Non-metals) –S–Semiconductor: In between a conductor and insulator.

6. Conductivity (Liquids) Electrolytes –Separate into positive and negative ions Ions are free to move Salt (NaCl) Na + and Cl 

7. Conductivity (Gases) How do gases conduct electricity? –Ionized to form a plasma –Caused by high energy radiation, electric fields, or collisions.

8. Remember –A path for flow called a circuit –“Pressure”, called a potential difference (V) Conditions necessary for current flow 

9. What supplies potential difference in a circuit? –Battery or Generator How can we measure it? –Voltmeter –Measured across device Voltage  V

10. Ex) 6.0 joules of work are done to move 2.0 coulombs of charge from point A to point B. Determine the potential difference between points A and B. 1.6.0 v 2.2.0 v 3.3.0 v 4.12 v Voltage

11. Current What is current? –The rate of electron flow What units do we use? –Ampere (A) or Amps How is it measured? –In Series (ammeter) –Current through a device  A

12. Current Ex) Find the current in a wire, if 14c of charge pass by in 7s? 1.2 A 2.7 A 3.14 A 4.21 A

13. Resistance

14. What is Resistance? The opposition that a device or conductor offers to the flow of electric current. –Measured in Ohms (Ω) 

15. What variables determine resistance in a wire? –Length of wire (L) in meters. –Area (A) in meters 2. –Resistivity of material (  ) in ohms  meters. What is resistivity? –How well a certain material “resists” the flow of electrons.

16. The Laws of Resistance Length: Length  Area: Area  Temperature: Temp  Resistance  Resistance  Resistance 

17. 3.14x10 -6 m 2 R = 2.19 x 10 -2 Ω Sample Problem Determine the resistance of a 4.00 m length of copper wire having a diameter of 2.00 mm. Assume a temperature of 20  c. KnownUnknown L = 4.00 m A =  = = 1.72 x 10 -8 Ω  m R = ? MaterialResistivity Aluminum2.82 x 10 -8 Copper1.72 x 10 -8 Gold2.44 x 10 -8 Nichrome150 x 10 -8 Silver1.59 x 10 -8 Tungsten5.60 x 10 -8

18. 3.14 x10 -6 m 2 R = 3.04 x 10 -2 Ω Sample Problem Determine the resistance of a 6.00 m length of silver wire having a cross-sectional area of 3.14 x10 -6 m 2. Assume a temperature of 20  c. KnownUnknown L = 6.00 m A =  = = 1.59 x 10 -8 Ω  m R = ? MaterialResistivity Aluminum2.82 x 10 -8 Copper1.72 x 10 -8 Gold2.44 x 10 -8 Nichrome150 x 10 -8 Silver1.59 x 10 -8 Tungsten5.60 x 10 -8

19. Superconductivity Cool a conductor below a critical temperature –Resistance decreases to zero. –Magnetic field lines will not penetrate.