Preliminary Physics - World communicates 1

World
Communicates
1
Waves
Preliminary
Physics
8.2.1
By
S.
Choi

Energy
(recap)
• Energy
is
what
causes
things
to
change
or
happen.
• Types
of
energy:
– Kine?c
(moving)
– Poten?al
(stored)
• Gravita?onal
• Elas?c
• Chemical
• Nuclear
– Heat
– Sound
– Electrical

Energy
transforma?on
in
a
mobile
phone
(a)
• Energy
is
stored
as
chemical
energy
in
the
phone’s
baMery
• Chemical
energy
is
transformed
to
electrical
energy
to
operate
the
phone
• The
microphone
converts
sound
energy
to
electrical
energy
• Antenna
converts
electrical
to
electromagne?c
energy
(microwaves)
and
incoming
electromagne?c
to
electrical
energy
• The
earphone/speaker
converts
electrical
energy
to
sound
energy
• The
LCD
colour
screen
converts
electrical
energy
to
light
energy

Waves
(b)
• Waves
are
carriers
of
energy
–
they
transfer
energy
from
one
place
to
another.
• Wave
mo?on
is
the
result
of
a
periodic
disturbance
of
a
medium,
or
of
space,
by
some
form
of
vibra=on
(or
oscilla=on)
which
transmits
energy
away
from
the
oscilla?ng
source
of
the
wave
• Some
waves
need
a
medium,
a
substance
that
vibrates
to
allow
the
energy
to
pass
through
(note
only
the
energy
moves
NOT
the
medium).
• Some
waves
do
NOT
need
a
medium.
These
waves
can
travel
through
a
vacuum.

How
do
we
know
waves
carry
energy?
(b)
• Microwaves
cook
food
• X-‐rays
can
damage
DNA
molecules
in
living
cells
• Earthquake
waves
can
knock
down
buildings
• Ultrasound
waves
can
warm
human
flesh
• Sound
waves
can
make
small
objects
move
• Water
waves
can
move
even
massive
ships

Propaga?on
dimension
of
waves
(b)
Waves
may
travel
in…
• One
dimension
e.g.
– Waves
in
a
slinky
– Waves
in
a
stretched
rope
or
string
(guitar)
– A
sound
wave
confined
to
a
long,
narrow
tube
(didgeridoo,
flute)
• Two
dimensions
e.g.
– Waves
on
the
surface
of
the
water
– The
skin
on
a
drum
– Human
eardrum
– Some
loud
speakers
• Three
dimensions
– Light
from
a
lamp
or
a
candle
– Sound
waves
– Microwaves
from
mobile
phones
– Light
from
the
sun
The
dimensions
of
waves
depend
on
the
MEDIUM!

There
are
two
major
categories
of
waves
(c)
Mechanical
waves
• Require
a
physical
substance
or
medium
in
which
to
propagate/travel
through
• E.g.
sound
waves,
water
waves
Electromagne=c
waves
• Do
NOT
require
a
physical
medium
in
which
to
propagate/travel
through
• E.g.
light,
radio
waves,
x-‐
rays

The
wave
model
(d)
Longitudinal
waves
Transverse
waves
The
par?cles
in
a
longitudinal
wave
vibrate
back
and
forth,
parallel
to
the
direc?on
in
which
the
wave
is
travelling
The
par?cles
in
a
transverse
wave
vibrate
up
and
down,
perpendicular
to
the
direc?on
in
which
the
wave
is
travelling

Proper?es
of
waves
(d)
• All
waves
have
a
source
of
energy,
which
involves
vibra?on
of
some
sort
– The
rate
of
the
vibra?on
is
called
the
frequency
(f)
and
is
measured
in
hertz
(Hz).
1
Hz
=
one
cycle
OR
vibra?on
per
second.
– The
maximum
displacement
of
the
par?cle
from
its
original
posi?on
is
called
the
amplitude
(A),
measured
in
meters
(m)
• All
waves
have
a
means
by
which
the
energy
can
propagate
outwards
away
from
the
source
as
a
vibra?on
– The
distance
between
adjacent
crests
or
troughs
of
the
wave
is
called
the
wavelength
(λ),
measured
in
meters
(m)
– The
rate
at
which
the
wave
travels
away
from
the
source
is
called
the
speed
or
velocity
(v)
of
the
wave
and
is
measured
in
metres
per
second
(ms-‐1)
Period
(T)
Period
(T)
Period
(T)
Wavelength
(λ)
Wavelength
(λ)
Amplitude
(A)
Amplitude
(A)
Wavelength
(λ)
Amplitude
(A)
Amplitude
(A)
Period
(T)
is
the
?me
it
takes
for
one
cycle
of
wave,
measured
in
seconds
(s).
T
=
1/f
Trough
Crests
Trough
Crests

The
wave
equa?on
(f)
• The
wave
equa?on
relates
three
proper?es
of
waves:
– Frequency
(f)
– Velocity
(v)
– Wavelength
(λ)
v = fλ
v
f λ

Prac?ce
ques?ons
(VI)
1. What
is
the
frequency
of
your
favourite
FM
radio
sta?on?
(Hint:
frequency
for
FM
is
given
as
MHz)
2. Calculate
the
wavelength
of
the
the
radio
wave
from
the
radio
sta?on.
(Hint:
use
3
x
108
ms-‐1
as
speed
of
light/EM
waves)

Preliminary Physics - World communicates 1

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