# Resistance - TeacherWeb

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15 Νοε 2013 (πριν από 4 χρόνια και 7 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

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
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
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