1. Current and Conductivity
Chapter 30
Current and Conductivity
Phys 133 – Chapter 30

2. Current
When there is current, the bulb glows and the compass needle deflects.
Phys 133 – Chapter 30

3. Electron Current (i) i = electrons/sec n = electrons/m3 vd  10-4 m/s
Phys 133 – Chapter 30

4. Question
The light switch is located approximately 2 m from the light. How long will it take an electron to travel this distance? (vD=10-4 m/s)
Discuss: The electron drift velocity is very slow. Yet when you turn on a light switch, a light bulb several meters away seems to come on instantly. Explain how to resolve this apparent paradox.
Phys 133 – Chapter 30

5. Conservation of electron current
Electrons cannot be created or destroyed
(conservation of charge)
The electron current is the same at all points in a current-carrying wire.
The electron current into a junction is the same as the electron current leaving a junction.
Phys 133 – Chapter 30

6. How to create current need an electric field static dynamic
(not static equilibrium)
Phys 133 – Chapter 30

7. What creates Electric Field?
Surface charges make E field - creates current
Phys 133 – Chapter 30

8. Phys 133 – Chapter 30

9. Battery Battery: charge escalator “Pump”, no charge created
Move charge against
electric field
Phys 133 – Chapter 30

10. Current (I) (as opposed to electron current)
Current - amount of charge passing per unit time
Current Density -charge passing per unit time per unit area
Phys 133 – Chapter 30

11. Problem 28.33
The electron beam inside a TV tube is 0.4 mm in diameter with current 50 A. Electrons strike screen.
How many electrons strike screen each second?
What is the current density in the beam?
The electrons move at 4.0x107m/s. What electric field is necessary to accelerate the electrons to this speed over a distance of 5.0 mm?
Assume each electron gives its kinetic energy to the screen. What power is delivered to the screen?
Phys 133 – Chapter 30

12. Problem 28.33 ans How many electrons strike screen each second?
What is the current density in the beam?
The electrons move at 4.0x107m/s. What electric field is necessary to accelerate the electrons to this speed over a distance of 5.0 mm?
Assume each electron gives its kinetic energy to the screen. What power is delivered to the screen?
Phys 133 – Chapter 30

13. Kirchoff’s junction rule
(conservation of charge/current)
Phys 133 – Chapter 30

14. Phys 133 – Chapter 30

15. Collisions/energy transfer
No electric field
With electric field
Electrons move randomly
Electrons tend to move against electric field
Phys 133 – Chapter 30

16. Conduction/resitivity
-Fields cause current (add energy)
-Collisions (take away energy)
-Current density (J) linear in electric field
-Depends on conductivity (resistivity)
-material dependent
-environment (temperature, magnetic field, …)
Phys 133 – Chapter 30

17. Simulation
Phys 133 – Chapter 30

18. Potential in a circuit/wire
Apply to wire
Phys 133 – Chapter 30

19. Potential and current in a wire
Phys 133 – Chapter 30

20. “Ohmic” vs “non-ohmic” materials
Phys 133 – Chapter 30