Transcript Slide 1

Ohm's Law
Using R = rL/A, R can be found using the
physical properties
____________________________of
a metal wire.
In a circuit, R is defined for any device
voltage across the device to the
as the ratio of __________________
current through
________________________the
device:
R = V/I
A simple
circuit:
pos. I
wire
I
voltage
source
circuit
element
I
V
8.0 V
Ex. If the potential difference across a resistor is _______
0.25 A
and the current through it is __________,
find R.
R = V/I
= 8.0 V / 0.25 A
= 32 V/A
units:
[R ]
= 32 W
=
1 ohm =
1 W =
[V ]/[ I ]
1 volt/ampere
1 V/A
R = V / I Solve this for
V=?
V = IR
I=?
I = V/R
To remember all 3 equations, use:
V
I
units:
[V] = [I][R]
1 V = 1 A·W
[I] =
1A =
[V]/[R]
1 V/W
R
Ex: What is the potential difference across a
25-W resistor when it carries a current of 3.0 A?
V = IR
= (3.0 A) (25 W)
= (75 A·W) =
75 V
Ex: How much energy is required to make each coulomb
of charge pass through the above resistor?
V = W/q

W = qV = (1.0 C) (75 V)
= (75 J)
Ex: What is the potential difference across a wire that
has no resistance?
V = IR = I (0 W)
=0
little or no
1. Assume the connecting wires have _________________
much less
resistance. (They usually have ________________
R
than the circuit elements.)
a light or a heater
2. For simple devices such as _______________________
,
we often replace the device with the symbol for
a resistor
__________________
:
and assume that
resistance
it has all of the ______________________.
through the wires and the R .
3. I = charge flowing _______________________________
The charge going __________
any circuit element must
into
equal
leaving that element. Assume
_________
the charge __________
+ side of the source.
____
+ charge flows out of the ____
across the R
4. V = potential difference __________________
energy per charge
= ____________________________
available to do work
heat or motion
= energy converted to _______________________
by R
= energy is __________________
by passing through R
"used up"
potential "dropped"
= _________________________
across R
0
wire
= _______
if there is no resistance, e.g. in a __________
Ex: A simple circuit has 1 loop
_______.
1
All of the __________________
is
voltage
one R
dropped across the ___________,
V
source
because it is the only element in
the circuit that requires
4
energy
____________ (voltage).
2
R
3
positive
Graph the voltage drops as you follow ____________
high
charge from the _________
potential side of source, through
circuit
low side of the source.
the _____________
, back to the _______
1
V
2
no V dropped in wire
I·0
b/c V = IR = ________
V = IR
3
distance around the circuit
4
meters
A simple circuit with ____________:
through R
ammeter – measures current passing ____________
R so no ________
V drop across it
- Ideally, it has no ____,
across R
voltmeter – measures potential difference _________
I to enter it
- Ideally, it does not allow any ____
A
R
V
The voltmeter must be connected
opposite
across _______________
sides of R
difference
to measure potential _____________.
Other ____________________ways
equivalent
to hook up the meters:
V
A
V
A
V
A
Remember:
causes I  So changing V _______________
changes
V ____________
I.
metallic
Ohm’s Law: For __________________
conductors at
constant temp., I is ___________
_______________
directly prop. to V.
"ohmic"
Case A: a device obeys Ohm’s Law  _____________
V
 slope = ΔV/ΔI = constant
R
so the ratio V/I = ____
is constant
___________
I
non-ohmic
Case B: ________________
devices
V
not constant
 slope = V/I = R is ________________
increases
In the case shown, R _______________
I
(Traditionally, V is plotted on the ____
y axis)
tiny
Ex. If R is _____________
, then I is _____________
.
huge
0 . This is an ___________
open
∞ I  ____
As R  ____,
circuit.
V
R
=
I
V
No
I
R= ∞
tiny
huge
Ex. If R is ___________
, then I is _____________
.
0 I  ____
∞ . This is a ___________
short
As R  ____,
circuit.
V
R
=
I
V
I
R=0
extremely dangerous
This situation can be _______________________________
.
wet
Body resistance can be lowered by getting __________
.
volts
"It's _______________
that jolts, (shocks you)
mills
But it's ___________
(milliamps of current) that kills."
Currents
and the
harm they
can cause:
defibrillation which
AC tends to send heart nerves into_________________,
can be harder to fix than simply__________________________
restarting a stopped heart.
Understanding safe circuits:
"Grounded" side of
circuit is safe to touch
pipe driven
into ground
No shock hazard as long as wires inside
toaster do not touch the outside metal case.
If the hot wire in the
toaster touches the
metal case, and you
touch the case,
current can flow
through you to
ground.
If the neutral wire
in the toaster
touches the metal
case, and you
touch the case,
no current flows
through you to
ground.
Polarized plugs can only be
plugged in one way because one
side of the plug is bigger than the
other. Then the case can be
connected to the neutral wire, and
you will not be shocked if you
touch the case.
Of course, you can make the case out of plastic,
so it doesn't conduct…..or…
The wide prong on the plug links the threaded base of light bulbs to the neutral
terminal (the wider slot) in the receptacle. If the wires are reversed, the hot side of the
outlet (the side that can deliver a shock) is wired to the threaded socket. Normally
there’s a cardboard insulator isolating the base from the socket. However, if the
cardboard insulator is worn out (common on old lamps), the metal parts of the lamp
also could become “hot” (charged). You can then get a dangerous shock if you touch
the metal, which is easy to do when you’re “grounded” and changing a dead bulb. For
example, if you’re standing on damp concrete or in contact with a radiator or other
plumbing pipe while you’re touching a miswired lamp, you could get a dangerous jolt
of electricity.
Buy a “polarized”
replacement plug, that
is, one that has a normal
prong and a wide one.
The neutral line on the
lamp cord is the one
that’s odd; it’ll have
ribbing, a sharp ridge or
printing on it. If the cord
is translucent, the
neutral is silver. The
wire that goes to the
narrower prong has a
smooth, plain surface or
is gold in color.
The third prong on the power cord
provides a direct electrical connection
from the appliance case to earth
ground, making the two points
electrically "common" with each other.
If they're electrically common, then
there cannot be any voltage dropped
between them.
Ground fault
detection:
If there is current
through the ground,
it is a "fault."
Then the current
in and the current
out will NOT be
equal. A difference
as small as 5 ma can
be detected.
With no problem, the currents
into and out of device are equal.
Ground Fault Current Interruptors, or GFCIs for short
test button
reset button
This works, independent of the device being properly grounded.
A__________,
short for 'fusible link', is a type of
fuse
overcurrent protection device. Its essential component
metal wire or strip that melts when there is
is a __________________________________________________
too much current..
____________________
Fuses usually are rated in
amperes
_______________
. If the current exceeds the rating, the
opens
metal strip melts, and it _________
the circuit. This
overheating
protects the circuit from __________________
which may
start fires
damage other circuit parts or ________________
.
circuit breaker
A _______________________
is an automaticallyswitch
operated electrical ______________
. Like a fuse, it is
designed to protect an electrical circuit from damage
current
caused by excess_________.
Unlike a fuse, which
operates once and then must be replaced, a circuit
breaker ________________
once the problem that caused
can be reset
the excess current is fixed.
fuses
Circuit breakers act like fuses but can be reset.
Two basic types:
1. solenoid (electromagnet)
2. thermal (bimetallic strip)
A downed power line
circuit
can set up a _________
through the ground.
Since the cables have
little R, most voltage
_________
will be dropped along
the ground
____________________
.
If the distance between the
downed line and the source
small
is___________
, there can be a
voltage between
significant ___________
two nearby points along the
ground, ie, between your 2 feet.
stay away or_______________
stand on 1 foot !
_____________