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

mghPE 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.

Crate of “Mad Magazines”

PE increases when raised to h

ho

h

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

2

2mvKE

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?

h

¾ h

½ h

¼ h

KE + PE = E

0 + 100 J = 100 J

25 J + 75 J = 100 J

50 J + 50 J = 100 J

75 J + 25 J = 100 J

100 J + 0 J = 100 J

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

R

VI

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).

PtE

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. Rotsa Ruck!