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Current. A charge in an electric field is subject to a force. Acceleration Positive charges move with the field. Negative charges move against the field. -. +. Moving Charge. E. Moving water in a pipe forms a current. Steady flow at constant velocity

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Moving charge

A charge in an electric field is subject to a force.

Acceleration

Positive charges move with the field.

Negative charges move against the field.

-

+

Moving Charge

E


Water flow

Moving water in a pipe forms a current.

Steady flow at constant velocity

The mass flow is the density times velocity times area.

Pressure provides the force.

Water Flow

v

r

A


Charge flow

Moving charge forms a current.

Charge density ne instead of mass density

The charge flow is the density times velocity times area.

Defined for positive charge

Potential provides the force.

+

Charge Flow

v

ne

A


Current direction

In a conductors electrons are free to move.

Collide with other charges

Random motion

A potential along a wire causes a net velocity for electrons.

Negative charge moves opposite to current

Current Direction


Ampere
Ampere

  • Current is the average amount of charge flowing through an area in a unit of time.

  • The unit of electric current is the ampere (A).

    • 1 A = 1 C/s

  • The ampere is a fundamental unit of the SI.

    • Coulomb is derived from it

    • 1 C = 1 A s


Electron beam

The current comes from the definition.

I = Dq/Dt

(3.20 x 10-6 C)/(200 x 10-3 s)

I = -16.0 mA (negative beam)

The charge of one electron is used to get the number rate.

(-1.60 x 10-5)/(-1.60 x 10-19)

De/Dt = 1.00 x 1014 electrons/s

In an electron microscope a constant electron beam transports 3.20 mC of electrons in 200 ms across a vacuum.

Find the current and number of electrons per second.

Electron Beam

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