PHYS 012A Lecture Notes - Lecture 14: Drift Velocity, Charge Carrier, Electric Potential Energy
12 Jun 2018
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Chapter 27: Current and Resistance
Electric Current
- Energy can be transferred to a device in an electric current.
- Electric current is the rate of flow of charge through some region of space.
- The SI unit of current is the ampere (A)
o 1 A= 1C/s,
o The symbol for electric current is I.
Average Electric Current
- If ΔQ is the amount of charge that passes through A in time Δt, then
the average current is
Instantaneous Electric Current
- If the rate at which the charge flows varies with time, the instantaneous current, I, is defined as
the differential limit of average current as Δt→0.
Direction of Current
- The charged particles passing through the surface could be positive, negative, or both.
- Current is the flow of positive charges.
- The current goes opposite the flow of electrons.
- Any moving charge is a charge carrier.
Current and Drift Speed
- Charged particles move through a cylindrical conductor
of cross-sectional area A
avg
Q
t
=
I
dQ
dt
I
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Charge Carrier Motion in a Conductor
- When a potential difference is applied across the conductor, an electric
field is set up in the conductor which exerts an electric force on the
electrons. The motion of the electrons is no longer random.
- The zigzag black lines represent the motion of a charge carrier in a
conductor in an electric field. The net drift speed is small.
- The sharp changes are due to collisions.
- Th motion of electrons is opposite the direction of the electric field.
- In the presence of an electric field, in spite of all the collisions, the charge
carriers slowly move along the conductor with a drift velocity,
- The electric field exerts forces on the conduction electrons in the wire.
- These forces cause the electrons to move in the wire and create a
current.
Current Density
- J is the current density of a conductor which is defined as the current per unit area.
o J ≡ I / A = nqvd
o Valid only if the current density is uniform and A is perpendicular to the direction of the
current.
- The current density is in the direction of the positive charge carriers.
Conductivity
- A current density and an electric field are established in a conductor whenever a potential
difference is maintained across the conductor.
- For some materials, the current density is directly proportional to the field.
- The constant of proportionality, σ, is called the conductivity of the conductor.
Oh’s La
- Ohm’s law states that for many materials, the ratio of the current density to the
electric field is a constant σ that is independent of the electric field producing
the current.
o Most etals oe Oh’s la
o Mathematically, J = σ E
o Materials that oe Oh’s la are said to e ohmic
o Not all aterials follo Oh’s la
▪ Materials that do ot oe Oh’s la are said to e nonohmic.
- Oh’s la is not a fundamental law of nature and is only valid for certain
materials.
d
v
IRV
A
IV
V
A
I
V
A
I
EJ
=
=
=
=
=
A
R
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