Transcript Midway High School Science TAKS Review

Midway High School Science
TAKS Review
IPC (6) The student knows the
impact of energy transformations
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 motion and if found from
mv
KE 
2
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 – onethousand 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. Rotsa Ruck!