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

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

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

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

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

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

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

Phys 133 – Chapter 30

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

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

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

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

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

Phys 133 – Chapter 30

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

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

Simulation Phys 133 – Chapter 30

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

Potential and current in a wire Phys 133 – Chapter 30

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