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

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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

Phys 133 – Chapter 30

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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.

Phys 133 – Chapter 30

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How to create currentneed an electric field

(not static equilibrium)

static dynamic

Phys 133 – Chapter 30

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

Phys 133 – Chapter 30

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Do Workbook 28.8 & 9

Phys 133 – Chapter 30

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BatteryBattery: charge escalator

“Pump”, no charge created

Move charge against

electric field

Phys 133 – Chapter 30

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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

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

Phys 133 – Chapter 30

12Phys 133 – Chapter 30

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Collisions/energy transfer

No electric field With electric field

Electrons move randomly Electrons tend to move against electric field

Phys 133 – Chapter 30

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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)

Phys 133 – Chapter 30

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Simulation

Phys 133 – Chapter 30

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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

Phys 133 – Chapter 30

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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

Phys 133 – Chapter 30

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Potential in a circuit/wire

Apply to wire

Phys 133 – Chapter 30

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Potential and current in a wire

Phys 133 – Chapter 30

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“Ohmic” vs “non-ohmic” materials

Phys 133 – Chapter 30