1. In a circuit, R is defined for any device
Ohm's Law
Using R = rL/A, R can be found using the ____________________________of a metal wire.
physical properties
In a circuit, R is defined for any device
as the ratio of __________________ the device to the
________________________the device:
voltage across
current through
R =
V/I
pos. I
wire
A simple
circuit: I
voltage
source
circuit
element V I

3. Ex. If the potential difference across a resistor is _______
and the current through it is __________, find R.
8.0 V
0.25 A
R = V/I
= 8.0 V / 0.25 A
= 32 V/A
= 32 W
units: [ ] = [ ]/[ ] = R V I
1 ohm
1 volt/ampere
1 W
1 V/A

4. To remember all 3 equations, use: V
V = IR
R = V / I
Solve this for
I = ?
I = V/R
To remember all 3 equations, use: V I R
units: [V] = [I][R] =
[I] = [V]/[R]
1 V
1 A·W
1 A
1 V/W

5. 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

6. 1. Assume the connecting wires have _________________
resistance. (They usually have ________________ R
than the circuit elements.)
little or no
much less
a light or a heater
For simple devices such as _______________________ ,
we often replace the device with the symbol for
__________________ : and assume that
it has all of the ______________________.
a resistor
resistance
through the wires and the R
I = charge flowing _______________________________ .
The charge going __________ any circuit element must
_________ the charge __________ that element. Assume
____ charge flows out of the ____ side of the source.
into
equal
leaving + +
V = potential difference __________________
= ____________________________ available to do work
= energy converted to _______________________ by R
= energy is __________________ by passing through R
= _________________________ across R
= _______ if there is no resistance, e.g. in a __________
across the R
energy per charge
heat or motion
"used up"
potential "dropped"
wire

7. Ex: A simple circuit has 1 _______. All of the __________________ is
loop
Ex: A simple circuit has 1 _______.
All of the __________________ is
dropped across the ___________,
because it is the only element in
the circuit that requires
____________ (voltage). 1 2
voltage
one R V
source R 4
energy 3
positive
Graph the voltage drops as you follow ____________
charge from the _________ potential side of source, through
the _____________ , back to the _______ side of the source.
high
circuit
low 1 2
no V dropped in wire
b/c V = IR = ________ V
I·0
V = IR 3 4
distance around the circuit

8. A simple circuit with ____________:
meters
through
ammeter – measures current passing ____________ R
- Ideally, it has no ____, so no ________ across it R
V drop
across
voltmeter – measures potential difference _________ R
- Ideally, it does not allow any ____ to enter it I A
The voltmeter must be connected
across _______________ sides of R
to measure potential _____________. R V
opposite
difference
Other ____________________ways to hook up the meters:
equivalent A V V V A A

9. Remember:
V ____________ I  So changing V _______________ I.
causes
changes
Ohm’s Law: For __________________ conductors at
_______________ temp., I is ___________ prop. to V.
metallic
constant
directly
Case A: a device obeys Ohm’s Law  _____________
"ohmic" V I
slope = ΔV/ΔI = constant
so the ratio V/I = ____ is ___________ R
constant
non-ohmic
Case B: ________________ devices V I
 slope = V/I = R is ________________
not constant
In the case shown, R _______________
increases
(Traditionally, V is plotted on the ____ axis) y

10. I I R tiny Ex. If R is _____________ , then I is _____________ . huge∞
As R  ____, I  ____ . This is an ___________ circuit.
open V No I =
R = ∞ R I V
tiny
huge
Ex. If R is ___________ , then I is _____________ .
As R  ____, I  ____ . This is a ___________ circuit. ∞
short I I
R = V V = R
extremely dangerous
This situation can be _______________________________ .
Body resistance can be lowered by getting __________ .
wet

11. "It's _______________ that jolts, (shocks you)
But it's ___________ (milliamps of current) that kills."
volts
mills
Currents
and the
harm they
can cause:
AC tends to send heart nerves into_________________, which
can be harder to fix than simply__________________________
defibrillation
restarting a stopped heart.

12. 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.

13. 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.

14. Of course, you can make the case out of plastic,
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…

15. 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.

16. 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.

17. Ground fault detection: With no problem, the currents
into and out of device are equal.
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.

18. Ground Fault Current Interruptors, or GFCIs for short
test button
reset button
This works, independent of the device being properly grounded.

19. A__________, short for 'fusible link', is a type of overcurrent protection device. Its essential component is a __________________________________________________ ____________________ Fuses usually are rated in _______________ . If the current exceeds the rating, the
metal strip melts, and it _________ the circuit. This protects the circuit from __________________ which may damage other circuit parts or ________________ .
A _______________________ is an automatically-operated electrical ______________ . Like a fuse, it is designed to protect an electrical circuit from damage caused by excess_________ Unlike a fuse, which operates once and then must be replaced, a circuit breaker ________________ once the problem that caused the excess current is fixed.
fuse
metal wire or strip that melts when there is
too much current.
amperes
opens
overheating
start fires
circuit breaker
switch
current
can be reset

20. fuses

23. Circuit breakers act like fuses but can be reset.
Two basic types:
solenoid (electromagnet)
thermal (bimetallic strip)

24. A downed power line
can set up a _________
through the ground.
circuit
Since the cables have
_________ R, most voltage
will be dropped along
____________________ .
little
the ground
If the distance between the
downed line and the source
is___________ , there can be a
significant ___________ between
two nearby points along the
ground, ie, between your 2 feet.
_____________ or_______________ !
small
voltage
stay away
stand on 1 foot