phys 133 – chapter 301 chapter 30 current and conductivity

Phys 133 – Chapter 30 1

Chapter 30

Current and Conductivity

Phys 133 – Chapter 30 2

CurrentWhen there is current, the bulb glows and the compass needle deflects.

3

Electron Current (i)i = electrons/sec

n = electrons/m3

vd 10-4 m/s

Phys 133 – Chapter 30

4

QuestionThe 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)

Do Workbook 28.7


5

Conservation of electron currentElectrons 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.


6

How to create currentneed an electric field

(not static equilibrium)

static dynamic


7

What creates Electric Field?Surface charges make E field - creates current


8

Do Workbook 28.8 & 9


9

BatteryBattery: charge escalator

“Pump”, no charge created

Move charge against

electric field


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


11

Kirchoff’s junction rule(conservation of charge/current)


12Phys 133 – Chapter 30

13

Collisions/energy transfer

No electric field With electric field

Electrons move randomly Electrons tend to move against electric field


14

Conduction/resitivity-Fields cause current (add energy)

-Collisions (take away energy)

-material dependent

-environment (temperature, magnetic field, …)

-Current density (J) linear in electric field

-Depends on conductivity (resistivity)


15

Simulation


16

The electron beam inside a TV tube is 0.4 mm in diameter with current 50 A. Electrons strike screen.

a) How many electrons strike screen each second?

b) What is the current density in the beam?

c) 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?

d) Assume each electron gives its kinetic energy to the screen. What power is delivered to the screen?

Problem 28.33


17

a) How many electrons strike screen each second?

b) What is the current density in the beam?

c) 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?

d) Assume each electron gives its kinetic energy to the screen. What power is delivered to the screen?

Problem 28.33 ans


18

Potential in a circuit/wire

Apply to wire


19

Potential and current in a wire


20

“Ohmic” vs “non-ohmic” materials


phys 133 – chapter 301 chapter 30 current and conductivity

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