Conduction and Current

1. Conduction and Current • Polarization vs. Conduction • Batteries, Current, Resistance • Ohm’s Law and Examples • Resistivity and Examples • Power and Examples

2. Electrical Properties of Materials Materials can do 2 things: • Store charge • Initial alignment of charge with applied voltage • Charge proportional to voltage • Temporary short-range alignment • Conduct charge • Continuous flow of charge with applied voltage • Current proportional to voltage • Continuous long-range movement

3. Charge Storage vs Conduction • Storage • Q = CV • Charge in Coulombs • Energy stored in Joules • Conduction • V= IR (I=GV G=1/R) • Charge flow in Coulombs/second (amps) • Power created or expended in Watts

4. Batteries • Battery • Electrochemical • Source of voltage • Positive and negative • 1.5 volt, 3 volt, 9 volt, 12 volt • Circuit symbol

5. Current • Current • Coulombs/second = amps • I = ΔQ / Δt • Example 18-1 • Requires complete circuit • Circuit diagram • Positive vs. negative flow

6. Resistance • Resist flow of current (regulate) • Atomic scale collisions dissipate energy • Energy appears in other forms (heat, light) • Applications • Characterize appliance behavior • Heater (collisions cause heat) • Regulate current/voltage on circuit board • Resistors and color code

7. Resistance and Ohm’s Law • Storage vs. Conduction • Q = CV (storage) • I = GV (conduction) • Current proportional to voltage • Proportionality is conductance • Use inverse relation • V = IR • Resistance • Units volts/amps = ohms • Ohm’s law • If V proportional to I, ohmic • Otherwise non-ohmic • Example 18-3

8. Ohm’s Law Examples

9. Ohm’s Law Examples OK, but do not touch the other wire! (heat of vaporization of squirrel)

10. Resistivity vs. Resistance • Property of material vs. property of device • Similar to dielectric constant vs. capacitance • Becomes resistance vs. resistivity • We use reciprocals, resistance and resistivity • ρ published for materials, like K. • High ρ poor conductor, σ good conductor (similar to K for storage)

11. Resistivity of materials

12. Resistivity Example • Area of wire from resistivity and length • Radius of wire • Voltage Drop along wire

13. Resistivity examples

14. Resistivity example • Along x • Along y • Along z

15. Power • Work done/ loss of PE for (+) going with field • No ½ because voltage is constant • Alternate forms P=IV = I2R = V2/R V + 0

16. Power example • Calculate current • Calculate Resistance • In one step

17. Power example • Power • Electric Company charges for energy not power • But instead of using Joules, they use kW-hours

18. Power examples • Problems 31, 32, 33, 38,

19. Power transmission • All customer cares about to run his home or factory: • Can do low voltage at high current • High voltage at low current • Transformers can switch back and forth

20. Power transmission – 12,000 V • At 12,000 V current must be • Voltage drop along wire will be • Power wasted in wire will be

21. Power transmission – 50,000 V • At 50,000 V current must be • Voltage drop along wire will be • Power wasted in wire will be

22. Power transmission – 2,000 V • At 2,000 V current must be • Voltage drop along wire will be • Power wasted in wire will be << Half the voltage and power are wasted!