Using R = L/A, R can be found using the ____________________________of a metal wire.

In a circuit, R is defined for any device as the ratio of __________________ the device to the________________________the device:

R =

A simple circuit:

V

I

physical properties

voltage across current through

V/I

voltagesource

wire

I

pos. I

circuitelement

III. Ohm's Law

Ex. If the potential difference across a resistor is _______

and the current through it is __________, find R.

R = V/I

= 8.0 V / 0.25 A

= 32 V/A = 32

units: [ ] = [ ]/[ ]

=

=

8.0 V0.25 A

R

1 ohmV I

1 volt/ampere

1 V/A

1

R = V / I Solve this forV = IR

I = V/R

To remember all 3 equations, use:V

I R

V = ?

I = ?

units: [V] = [I][R] =

[I] = [V]/[R]

=

1 V

1 A

1 A·

1 V/

Ex: What is the potential difference across a 25- resistor when it carries a current of 3.0 A?

Ex: What is the potential difference across a wire that has no resistance?

Ex: How much energy is required to make each coulombof charge pass through the above resistor?

V = W/q W = qV =

V = IR

= (3.0 A) (25 )

= (75 A·) = 75 V

(1.0 C) (75 V) = (75 J)

V = IR = I (0 ) = 0

3. I = charge flowing _______________________________ . The charge going __________ any circuit element must_________ the charge __________ that element. Assume ____ charge flows out of the ____ side of the source.

4. 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 __________

1. Assume the connecting wires have _________________ resistance. (They usually have ________________ R than the circuit elements.)

2. For simple devices such as _______________________ ,we often replace the device with the symbol for__________________ : and assume that it has all of the ______________________.

little or no

a light or a heater

a resistorresistance

much less

through the wires and the R

across the Renergy per charge

heat or motion"used up"

potential "dropped"

into

+leavingequal

+

0 wire

distance around the circuit

V

1 2

V = IR

3 4

Vsource

R

1 2

34

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).

Graph the voltage drops as you follow ____________ charge from the _________ potential side of source, throughthe _____________ , back to the _______ side of the source.

loopvoltage

one R

energy

positivehigh

low

no V dropped in wireb/c V = IR = ________

circuit

I·0

Other ____________________ways to hook up the meters:

ammeter – measures current passing ____________ R - Ideally, it has no ____, so no ________ across it

voltmeter – measures potential difference _________ R - Ideally, it does not allow any ____ to enter it

R

A simple circuit with ____________:

R

equivalent

through

acrossI

The voltmeter must be connected across _______________ sides of Rto measure potential _____________.

A

V

V drop

oppositedifference

A

V

A

VA

V

meters

Ohm’s Law: For __________________ conductors at _______________ temp., I is ___________ prop. to V.

V

I

V

I

Case A: a device obeys Ohm’s Law _____________

Case B: ________________ devices

slope = ΔV/ΔI = constantso the ratio V/I = ____ is ___________

slope = V/I = R is ________________

Remember:V ____________ I So changing V _______________ I.

(Traditionally, V is plotted on the ____ axis)

In the case shown, R _______________

causes changes

metallicconstant directly

"ohmic"

R constant

non-ohmic

not constantincreases

y

Ex. If R is _____________ , then I is _____________ .

V

Ex. If R is ___________ , then I is _____________ .

= V

As R ____, I ____ . This is an ___________ circuit.

= V R =

V R =

This situation can be _______________________________ .Body resistance can be lowered by getting __________ .

extremely dangerous

huge tiny

∞ 0 open

∞

RI

tiny huge

As R ____, I ____ . This is a ___________ circuit.∞0 short

I R

0

No I

Iwet

Currents and the harm they can cause:

"It's _______________ that jolts, (shocks you)

But it's ___________ (milliamps of current) that kills."

volts

mills

AC tends to send heart nerves into_________________, whichcan be harder to fix than simply__________________________

defibrillationrestarting a stopped heart.

Ground fault detection:

With no problem, the currentsinto and out of device are equal.

If there is currentthrough the ground,it is a "fault."Then the currentin and the currentout will NOT be equal. A differenceas small as 5 ma can be detected.

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

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.

opens

overheatingstart fires

amperes

circuit breaker

switch

current

can be reset

fuses

Circuit breakers act like fuses but can be reset.Two basic types:

1. solenoid (electromagnet)2. thermal (bimetallic strip)

A downed power linecan set up a _________through the ground.

Since the cables have _________ R, most voltage will be dropped along____________________ .

If the distance between thedowned line and the sourceis___________ , there can be asignificant ___________ betweentwo nearby points along theground, ie, between your 2 feet._____________ or_______________ !

circuit

little

the ground

smallvoltage

stay away stand on 1 foot