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PH 105
Dr. Cecilia Vogel
Lecture 21
OUTLINE
Electricity
Magnetism
E & M
Electronic circuits
Circuit elements
Electricity
Electricity is the force that effects
______________:
Makes electrons flow as current in wires.
Makes clothes cling together in laundry
Makes strands of hair push away from each
other
so hair stands on end.
etc
Like charges _________
Opposite charges ________
Magnetism
Magnetism is the force that effects
__________
Makes compasses point North.
Makes magnets cling to fridge.
Tells certain worms which way is down, so
they can burrow in muck.
etc.
Like poles ________
Opposite poles ________
E&M
Magnetism also affects moving charges:
Moving charges act like magnets.
Current flowing around in a coil of wire
acts just like a
Right hand rule:
Wrap your right-hand fingers around
the coil,
pointing in the direction the current is
flowing.
then your thumb points out the north
magnetic pole
E&M
Moving magnets also affect charges:
If a magnet moves near a coil
it will induce current to flow in the coil.
If magnetism changes,
current will resist or oppose change.
So if you try to change the current
flowing thru a coil,
you will meet with
Current Events
If current is to flow, we need 3 things

such as a battery or electric outlet.
voltage provides the force that makes
electrons move.

such as a wire
electrons in a conductor are free to move.
and
Electric Circuit
In a circuit,
there is a complete loop for electrons
to flow around.
maybe more than one
Current never flows away from a
battery and disappears.
Current never shows up out of
nowhere at the battery.
Whatever flows out of one terminal of
battery, flows back in the other.
AC/DC
Direct Current (DC)
is current that flows always in the
__________
like from the + to the - terminal of a
battery.
Alternating Current (AC)
flows in one direction, then flows in
the opposite direction, then ….
Like current from the wall outlet
(60Hz)
Circuit Elements
Battery or other voltage source
causes
Wires
allow the current to .
Resistors,
allow current to flow,
but resist the flow, so
The more current flows or the bigger
the resistance, the faster energy is lost:
P = I2R
+
Circuit Elements
Capacitor
does not allow charge to
charge flows into one side,
and out of the other,
so one side becomes __, the other __.
Energy is stored in separated charges.
Inductor
allows the current to flow in a loop,
so it acts like a
Energy is stored in magnetism
Impedance
Impedance is voltage/current,
Z = V/I
How much ______ flows in response to
______.
Resistors resist flow
independent of
don’t care
Z = R
Impedance of Capacitor
Capacitors
can resist flow because of their ____
flow of charge of the same sign is
resisted
flow of charge of opposite sign is
enhanced
impedance is less for AC, especially hi
freq
Z = 1/2p f C
Impedance of Inductor
Inductors
resist flow because they have
magnetism
that opposes change (change not charge!)
impedance is greater for AC, especially
hi freq
Z = 2p f L
Summary
Electricity and Magnetism
exert forces on charges and magnets.
Electricity and magnetism affect each other:
current in coil acts like magnet
changing magnetic field induces current
Current flow occurs in circuits with voltage
source
Impedance of current flow comes from
resistors, capacitors, inductors
Exam Friday