Electricity,
Power,
and
Circuits
The
juice
that
drives
us
Power
and
Energy
•  The
rate
at
which
energy
is
converted
into
another
form
is
power
•  Measured
in
Joules/s
or
Wa@s
(W)
•  MathemaCcally,
Wa@s
(P)
=
Volts
(V)
x
Amps
(I)
–  Current
=
charge/Cme
–  Wa@
=
(N‐m)/s
V = IR, I =
V
, P = VI
R
I
P =  I = I 2 R
 R
V  V 2
P = V  =
 R R
€
Electrical
Energy
•  Measures
power
over
a
period
of
Cme
•  Energy
=
Power
x
Cme
•  Example:
A
dryer
has
a
power
raCng
of
2000
wa@s
and
is
used
for
2
hours.
–  The
energy
consumed
by
the
refrigerator
would
be
4
kWh
Energy
units
Voltage
•  The
electrical
force
of
“pressure”
that
causes
current
to
flow
through
a
circuit
•  Measured
in
volts
•  Measuring
voltage
is
done
outside
the
circuit,
in
parallel
Current
•  Current
is
the
movement
of
electrical
charge
through
a
circuit
•  Flows
in
a
circuit
as
electrons
travel
from
high
voltage
to
low
voltage.
•  Measuring
current
in
a
circuit
must
be
done
in
series
Resistors
•  A
resistor
allows
current
to
flow,
but
not
as
freely
as
a
wire.
–  An
insulator
completely
restricts
the
flow
of
current
•  Composed
of
carbon
or
metal
films
–  Metal??
Ohm’s
Law,
V
=
IR
•  States
that
there
is
a
direct
relaConship
between
voltage
and
current
•  Current
is
inversely
related
to
resistance
–  As
flow
rate
increases,
reisstance
must
decrease
for
the
same
amount
of
“push”
(voltage)
Who
Likes
a
Good
Analogy?
h@p://hyperphysics.phy‐astr.gsu.edu/hbase/electric/watcir2.html
• Increase
pump
size
=
increase
voltage
• Increase
pipe
size
=
increase
current
• Decrease
“R”
=
increase
resistance
Wa@s
up
with
Units
•
•
•
•
•
Energy
=
Joule
=
kg∙m2/s2
Wa@
=
Joule/s
=
kg∙m2/s3
=
N∙m/s
100
Wa@s
=
0.134
HP
Ampere
=
coulomb/s
→
A
=
C/s
Volt
=
wa@/ampere
→
V
=
W/A
–  Or,
V
=
kg∙m2/C∙s2
•  Resistance
=
Voltage/current
=
kg∙m2/C2∙s
•  Wa@
=
Voltage
x
current
(W
=
VA
or
W
=
VI)
Energy
Loss
in
Power
Lines
•
•
•
•
All
wires
have
some
finite
resistance
Each
wire
has
resistance
Energy
loss
in
one
second
is
I2R
Make
current
(I)
small
to
minimize
–  Power
lines
carry
high
voltage,
low
current
•  Transformers
convert
AC
Voltages
up
or
down
Measuring
Current,
Voltage,
and
Resistance
Voltage
•  Voltage
is
measured
outside
a
circuit,
in
parallel
•  A
voltmeter
is
considered
to
have
infinite
resistance,
so
no
current
passes
•  The
PotenCal
difference
is
measured
between
two
points.
Current
•  Current
is
measured
within
the
circuit
•  A
measure
of
the
actual
number
of
electrons
flowing
past
a
certain
point
per
Cme.
•  Since
electrons
are
flowing,
current
is
hard
to
measure
–  Circuit
must
be
broken
for
tesCng
–  OpConal
“clamp
on”
meters
that
use
electromagneCsm
Measuring
Current,
Voltage,
and
Resistance
•  Small
voltage
is
placed
across
a
resistor
•  The
current
that
flows
between
probes
calculates
resistance
•  V
=
IR
Our
MulCmeters
Capacitors
•  Hold
charge
like
ba@eries
–  Do
not
funcCon
by
redox
chemical
reacCons
–  Store
staCc‐like
charge
•
•
•
•
•
•
Quickly
dissipates
charge
Cannot
overcharge
Long
lifeCme
No
heat
discharge
Unlimited
recharge
cycles
Measured
in
Farads
(A∙s/V)
Motors
and
Turbines
•  Current
induces
a
temporary
magneCc
field
in
a
wire
•  Spins
around
a
permanent
magnet(s)
•  As
magneCc
domains
in
the
electromagnet
equal
polarity
of
the
permanent
magnet,
forces
oppose
resulCng
in
moCon