Transcript h - Granbury ISD

PHYSICS REVIEW II
TAKS REVIEW
IPC (6) The student knows the impact of energy transformations
Content found from link from Midway ISD…..
IPC (6) (A) – The student
describes the law of
conservation of energy
One form of energy we will
consider is gravitational
potential energy
PE  mgh
Which is energy an object
has relative to some zero
reference point. Like
work, energy is measured
in Joules.
In the formula, m is mass (in
kg), g is acceleration due to
gravity (a constant, 9.81
m/s2) and h is the height
above the floor, the ground or
any other reference we choose.
If we move an object to a
higher position, we increase
its gravitational potential
energy.
h
ho
PE increases when
raised to h
Crate of “Mad
Magazines”
Ex. A 2.0 kg book is raised
from the floor to a table that
is 1.2 m high. What is the
increase in gravitational
potential energy?
Another form of energy we
will consider is kinetic
energy. This is the energy an
object has because of its
2 from
motion and if found
mv
KE 
2
where m is the mass and v
is the speed of the object.
Kinetic energy is measured
in Joules as are all forms
of energy.
The boy has kinetic
energy because he
is moving
Ex. A velociraptor with a
mass of 15 kg runs with a
speed of 8 m/s. What is the
kinetic energy of the
velociraptor?
Now, back to conservation of
energy. Suppose a ball is
raised to an unknown height
above the floor. What type of
energy does the ball have?
Let’s arbitrarily say the
ball has 100 J of energy. If
the ball is dropped, when it
falls to ¾ of its initial
height, it now has two forms
of energy. What are they?
¼ of the ball’s initial
gravitational potential
energy has been converted to
kinetic energy, so if we
ignore air resistance, and
the total energy of the ball
is conserved, what is the
value of each?
What is the PE, KE and total
E when the ball falls to a) ½
its initial height? b) ¼ its
initial height c) a point
just before the ball hits the
floor?
KE
+
PE
0
+
100 J
=
100 J
¾h
25 J
+
75 J
=
100 J
½h
50 J
+
50 J
=
100 J
75 J
+
25 J
=
100 J
100 J
+
0J
=
100 J
h
¼h
=
E
Ex. Tarzan (mass 85 kg) swings
from a limb to a second limb
that is 10.0 m below the
first. a) What type of energy
does Tarzan have initially? b)
What type of energy does
Tarzan have at the bottom of
the swing?
IPC (6) (B) – The student
investigates and demonstrates
the movement of heat through
solids liquids and gasses by
convection, conduction and
radiation.
If you place an iron skillet
over a an electric burner and
leave it for a little while,
when you return, you will
find the handle (if it is
uninsulated) of the skillet
is very hot, even though it
was not in direct contact
with the burner. Why?
The burner increases the
kinetic energy of the iron
atoms on the bottom of the
skillet. As they move faster,
they transfer energy to
neighboring atoms.
This transfer of energy is
repeated along the entire
surface area of the skillet,
so eventually the handle
becomes hot also.
This transfer of energy
between two materials of
different temperatures (in
this case the burner element
and the iron skillet) is
called conduction.
Some materials transfer heat
energy at a higher rate. These
materials are thermal
conductors. Name some common
thermal conductors.
Other materials transfer
energy at lower rates. These
are called thermal insulators.
Name some thermal insulators.
A cardboard container and an
aluminum ice tray in a freezer
have the same temperature.
However, when you touch each
with your hands, the ice tray
feels colder. Why?
A metal cake pan and the
surrounding air in an oven
have the same temperature. If
you touch the pan with an
unprotected hand, you will
certainly get burned. Yet you
can place your unprotected
hand inside the oven briefly
and experience no pain. Why?
Air, along with most gasses,
is an excellent thermal
insulator. How does air serve
as an insulator in
 Fiberglass insulation blown into the attic of a home
 A down comforter
 Layering your clothing on a cold day
Another activity to try at
home without adult
supervision. Place a metal pot
of cold water under the
broiler (top element) of your
oven.
When the water begins to boil,
place your hand in the pot and
feel the water near the bottom
of the pan. You will find the
water near the bottom is still
cold although your wrist is
scalded by the boiling water
at the surface. Why?
When boiling water, we place
the pan on a burner. Heat is
transferred to the pan and
then water molecules in
contact with the pan (What
type of heat transfer is
this?).
The water becomes less dense
at the bottom of the pan
because of the faster moving
water molecules. This water
rises to the surface and
replace the colder, more
dense water molecules which
then come into contact with
the warmer pan.
This replacement of colder
matter with warmer matter is
called convection.
Use convection to explain
why
 You can hold your fingers around the side of a
candle flame with being burned, but not
above the flame.
 In warmer climates (like Central Texas) the
ac/heating vents are placed in the ceiling, not
in the floor (as they are in colder climates)
What do the sun, an armadillo
and a high school sophomore
all have in common?
Answer – they all give off
heat. Specifically, the heat
given off is in the form of
electromagnetic radiation in
the infrared region of the
electromagnetic spectrum.
Infrared radiation has long
wavelengths we can’t see that
warm us (and if we are sitting
close enough to someone, warms
them). A fire gives off light
but also infrared radiation,
which makes us feel toasty.
Examples of heat transfer by
radiation
 Heat lamps
 Fires
 Mr. Sun
 Space heaters
IPC (6) (F) – Students
investigate and compare
series and parallel circuits
If you have old fashioned
Christmas lights at your
house, you may have noticed
one particularly annoying
feature about them. If one
bulb burns out . . .
Electrical devices are
connected by conducting paths
called circuits. A circuit is
a complete path connecting a
battery or generator to a
load, such as a light bulb.
A series circuit has only
one path for the electrical
charges to travel.
If any part of that circuit is
interrupted (for instance, the
filament of one bulb in a
string of bulbs breaks) the
circuit is open and none of
the devices will receive
current. So, the entire string
of lights goes out.
Three important aspects of
any circuit are
1. Current – the rate of flow of charge in the
circuit
2. Voltage – the electrical potential energy per
charge or “boost” the charges receive from a
battery or generator
3. Resistance – the opposition of the flow of
charge from the devices in the circuit (like
light bulbs, or hair dryers, etc.)
Current, voltage and
resistance are related by a
formula called Ohm’s Law.
Ohm’s Law
V
I
R
Where I is current measured
in amperes (A), V is voltage
measured in volts (V) and R
is resistance measured in
Ohms (Ω).
Ex. What is the current in a
circuit connected to a 12 V
battery and connected to a
total resistance of 6.0 Ω?
In a series circuit, the
total resistance is the sum
of the resistors so the total
resistance in the example
could have come from two 3.0
Ω resistors, three 2.0 Ω
resistors or six 1.0 Ω
resistors.
Now, it wouldn’t be too cool
if in your room you turned
off your CD player and the
fan and lights also went off.
Most of the circuits we
encounter are parallel
circuits, which have more than
one conducting path. So if one
path is open, current still
flows in the other paths.
We discussed power earlier by
describing the rate at which
work is done. In general, the
product of power and time
gives us energy used. (in this
case electrical energy).
E  Pt
As always, energy (E) is
measured in Joules, power (P)
is measured in Watts (or for
electricity, kilowatts, kW –
one-thousand Watts), and time
(t) is measured in seconds.
Ex. How much energy does a
100 W light bulb use in 5
minutes?
That’s all folks! It was a
whirlwind review and there are
many other concepts we could
have discussed. The physics
part of the science TAKS is
the easiest part – no kidding.
Use the formula chart. You
will do great.