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Chapter 18
Electric Currents
Objectives
After studying the material of this chapter, the student should be able to:
Describe AC current.
Explain what is meant by drift velocity.
18.7 Alternating Current
AC vs. DC
Current from a battery
flows steadily in one
direction (direct current,
DC). Current from a
power plant varies
sinusoidally (alternating
current, AC).
18.7 Alternating Current
The voltage varies sinusoidally with time:
as does the current:
(18-7)
18.7 Alternating Current
Multiplying the current and the voltage gives
the power:
Alternating and direct current
• If voltage alternates, so does
current.
• When the voltage is positive,
the current in the circuit is
clockwise.
• When the voltage is negative
the current is the opposite
direction.
• This type of current is called
alternating current, or AC.
Alternating and direct current
• AC current is used for almost all highpower applications because it is easier to
generate and to transmit over long
distances.
• The 120 volt AC (VAC) electricity used in
homes and businesses alternates
between peak values of +170 V and -170
V at a frequency of 60 Hz.
• AC electricity is usually identified by the
average voltage, (120 VAC) not the peak
voltage.
18.7 Alternating Current
Usually we are interested in the average power:
18.7 Alternating Current
The current and voltage both have average
values of zero, so we square them, take the
average, then take the square root, yielding the
root mean square (rms) value.
(18-8a)
(18-8b)
18.8 Microscopic View of Electric Current
Electrons in a conductor have large, random
speeds just due to their temperature. When a
potential difference is applied, the electrons
also acquire an average drift velocity, which is
generally considerably smaller than the
thermal velocity.
What is drift velocity?
• The average velocity that a charged particle
attains due to an electric field.
• In a metal wire, electrons move around
rapidly and randomly. If there is an electric
field through the wire, the vibrating
electrons will drift in one direction—the
speed at which they move is the “drift
velocity.”
What does drift velocity depend on?!
Charge (q)
Current (I)
Cross sectional area (A)
Number of charged particles per unit volume (n)
How is drift velocity calculated?!?
drift velocity
current
cross sectional area
no. of particles per unit volume
charge
Where did that come from?!?!
Watch this informative video for an explanation of
drift velocity, a basic derivation of the equation, and
a sample problem.
The drift velocity of charge in a conductor
by fizzicsorg
http://www.youtube.com/watch?v=KgbqPKZU5I
A
References!
(Look at them if you need more help.)
http://resources.schoolscience.co.uk/cda/16plus/copelech2pg3.html
http://en.wikipedia.org/wiki/Drift_velocity
http://physics.bu.edu/~duffy/semester2/c09_driftvelocity.html
This site has an applet that might interest you!
25-8 Microscopic View of Electric Current:
Current Density and Drift Velocity
We define the current density (current per unit
area) – this is a convenient concept for
relating the microscopic motions of electrons
to the macroscopic current:
If the current density is not uniform:
.
18.8 Microscopic View of Electric Current
This drift speed is related to the current in the
wire, and also to the number of electrons per unit
volume.
(18-10)
Summary of Chapter 18
• A battery is a source of constant potential
difference.
• Electric current is the rate of flow of electric
charge.
• Conventional current is in the direction that
positive charge would flow.
• Resistance is the ratio of voltage to current:
Summary of Chapter 18
• Ohmic materials have constant resistance,
independent of voltage.
• Resistance is determined by shape and
material:
• ρ is the resistivity.
Summary of Chapter 18
• Power in an electric circuit:
• Direct current is constant
• Alternating current varies sinusoidally
Summary of Chapter 18
• The average (rms) current and voltage:
• Relation between drift speed and current:
Which draws more current, a 100-W lightbulb or a 75-W
bulb? Which has the higher resistance?
Which draws more current, a 100-W lightbulb or a 75-W
bulb? Which has the higher resistance?
Electric power is transferred over large distances at very
high voltages. Explain how the high voltage reduces
power losses in the transmission lines.
Electric power is transferred over large distances at very
high voltages. Explain how the high voltage reduces
power losses in the transmission lines.
Driven by ac power, the same electrons pass back and
forth through your reading lamp over and over again.
Explain why the light stays lit instead of going out after
the first pass of electrons.
Driven by ac power, the same electrons pass back and
forth through your reading lamp over and over again.
Explain why the light stays lit instead of going out after
the first pass of electrons.
Homework:
Questions: 12, 13, 16