Resistance - TeacherWeb

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15 Νοε 2013 (πριν από 3 χρόνια και 8 μήνες)

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19.2 Resistance

pp. 700
-

707

Mr. Richter

Agenda


Review HW


Chapter 19 Quiz Monday (second half)


Notes:


Resistance


Ohm’s Law


Human Resistance


Resistors and Superconductors


Objectives: We Will Be Able To…


Understand the relationship between resistance, current and
voltage.


Calculate using Ohm’s Law.


Know what factors affect resistance.


Define resistors and superconductors.

Warm
-
Up:


If current is like water flowing in a river, what do you think
resistance is?

Resistance

Resistance


What does resistance mean?


Resistance

is the measure of
how strongly an object inhibits
the flow of electric current.


High resistance = low current,
few electrons flow


Low resistance = high current,
many electrons flow.

Resistance


Every
device that uses electrical energy adds
to the total
resistance
to a circuit.


The more
total resistance
the circuit has, the less the current.


Resistance


Remember, electron
flow is bumpy and
indirect.


The resistance of a
material largely
depends
on (p. 701):


length


cross
-
sectional area


type of material


temperature


Ohm’s Law

Ohm’s Law


The
resistance (R)
of a material is the ratio of the applied
voltage to the current that flows through.


This ratio is known as
Ohm’s Law:





Resistance is measured in Ohms [
Ω
]

A.K.A.:


V = IR

Ohm’s Law: Relationships


Voltage and current are directly proportional.


With constant resistance


Resistance and current are inversely proportional.


With constant voltage.


A.K.A.:


V = IR

Human Resistance

Human Resistance


Why will a 9
-
volt battery not shock you
on your arm, but shock your tongue?


Human skin usually has a resistance of
approximately 500,000
Ω
.


A 9
-
volt battery creates a current of
about 9/500,000 or ~1.8 x 10
-
5
A
(0.000018 A)


Humans only feel current at about 0.01 A

Human Resistance


The resistance of wet skin can decrease to
as little as 1000
Ω

or even 100
Ω
, or about
1000 times less resistance.


Water, and especially salt water, creates a
better conductor.


Lots of free electrons!


This creates as much as 0.09 amps of
current.


This is enough for human nerves to sense.
And hurt.

Resistors and Superconductors

Resistors


Resistors

can be used to
control the flow of current in a
circuit.


Current can also be controlled
by adjusting voltage, but
voltage is usually constant (like
a 120
-
volt outlet)


A resistor is an element that
provides a specified resistance.

Superconductors


Superconductors

have no resistance
below a critical temperature
, usually near
absolute zero.


Weird stuff: once current is established, it
can continue even when the potential
difference is removed.


This can create semi
-
permanent
electromagnets, and super
-
efficient
electric circuits.


Wrap
-
Up: Did we meet our objectives?


Understand the relationship between resistance, current and
voltage.


Calculate using Ohm’s Law.


Know what factors affect resistance.


Define resistors and superconductors.


Homework


p707 #1
-
5, 7