Practice Problems Exam 2

Dear Users - please note students are currently working on these problem sets - please do not edit until their signature tag has been removed --J.M.Pearce (talk) 23:25, 3 April 2008 (UTC)

Students - please put in two questions for the review and work out entire example.

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Make sure to include all equations and units. Make sure to tag your question with --~~~~

Thank you.

Waves

A Sound wave from a tuning fork moves at 340 m/s. It has a wave length of 2.43m. What is it's frequency?

Wave Speed = wavelength * frequency
340m/s/2.43m = 139.91Hz

Name, and describe two types of waves that we discussed in class.

Transverse Wave: motion of the particle is perpendicular to the direction of the wave
Longitudinal Wave: motion of the particle move the same direction

A wave is traveling with a frequency of 70.0 Hz and has a wavelength of 50 m. What is the speed of this wave?

velocity= wavelength X frequency
velocity= 50 m X 70.0 Hz
velocity= 3500 m/s

As a wave moves, intensity _________ with distance, and ________ as it passes through a material.

Intensity DECREASES with distance, and DECREASES as it passes through a materal.

What is the definition of a wave?

Any Pattern with an indentifable periodicity in space and time. No matter is transferred but energy is.

A wave has a wavelength of 70m and a frequency of 80Hz. How fast would this particular wave be traveling?

= Wavelength x frequency
= 70m x 80HZ
= 5600m/s

Illustrate diffraction

Illustrate Hugen's Principle?

At a time(t) consider everypoint on a wave front, a new point source.

What are the names for the highest and lowest points of a wave?

 crest and trough

Sound

1. For sound waves, as the _________ of a material increases, _________ increases

Density and Velocity

2. Dolphins can hear noises at the frequency of 250 kHz. If a dolphin hears a sound with this frequency that is moving with a velocity of 1700 m/s what is it's wavelength?

Use the equation v=λf, for this problem it is v/f=λ. First convert 250kHz to 250000Hz. 
1700m/s divided by 250000Hz = 6.8x10^3m

3. Sound cannot travel through a vacume because it needs what?

     Sound needs a medium to travel through. (solid, liquid, or gas)

4. What is Refraction when dealing with sound waves?

An abrupt change in wave speed at boundary of two mediums.

5. What is the equation for Refraction?

f = v1/wavelength = v2/wavelength2

Electromagnetic Waves

What are the seven type of waves in the electromagnetic spectrum in order of their size (largest to smallest)?

Radio waves, microwave, infrared, visible, ultraviolet, x-ray, and gamma rays.

The wave-like properties of electromagnetic radiation are due to what?

The periodic oscillations in the electric and magnetic components.

If a wave is traveling at a velocity of 3.8m/s and the end of the wave reaches where the fron t of the wave started in 15s.

Wavelength=velocity / Frequency
3.8m/s x 15s=
57m/s(squared)

If an AM radio wave has a length of 300 m , what is it's frequency?

Frequency = speed of light (c) / wave length
Frequncy= (3 x 10^8 m/s)/ 300 m
Frequency= 1 x 10^7 Hz

An electromagnetic wave has a frequency of 1.5x10^7 Hz, what is the lenth of the wave?

  c is constant. c=3x10^8 m/s
legth=c/f=3x10^8/1.5x10^7Hz=20m

Light

What's the difference between a convex mirror and a concave mirror?

A convex mirror curves away from you and a concave mirror curves toward the viewer.

What can the size of an infrared light wave be compared to physically? (e.g. a bread box, an atom, etc).

a pinhead

Reflection

1. A beam of light is sitting in the floor 45 degree and it is shining to a mirror that was sitting at the floor 90 degree, if the mirror turn 45 degree where the light is pointing to and the light remain the same position, what will be the new reflection angle? (Hint: follow the wording use the diagram only as a guide)

No reflection will occour because the mirror and the light become parallel.

Refraction

Empty

Lighting

CFL

A Compact Fluorescent Light bulb (or CFL) uses roughly ____ percentage of the electrical wattage needed for an incandescent light bulb to produce the same amount of light.

Answer: 25%

True or false CFL's are a more economical choice over the standard incandescent bulbs

 Answer: True

If a house has 20 light fixtures and they switch to CFL's, that save $32 per bulb, how much will the household save?

 Answer: 20 x 32
         $640

CFL Bulbs that have the equivalent power of a 60 Watt Incandescent bulb have ____ watts.

  Answer: 15 watts

If you have 10 fixtures to replace with CFL light bulbs and the total costs over 8,000 hrs of illumination at Clarion rate of $0.0615 per kW-hr are$18.30 per fixture. How much will your initial costs be to replace the light bulbs with the energy saving bulbs?

  Answer: $18.30*10=$183 as your initial cost for CFL bulbs

How do CFL light bulbs differ from incandescent light bulbs?

Answer: CFL's use mercury vapor and when the mercury vapor is exposed to an
electrical current the electrons begin to move quickly, emitting light.   An
Incandescent light bulb uses a filament which emits mostly heat, which makes it   
much less efficient.

If a CFL light bulb has a pay back time of 5.4 months and you save 1.12 per month and the life time is 3 years, what is your total return on investment?

Answers: 36 months - 5.4 = 31.6 months x $1.12 = $35.40

LEDs

Nothing

Energy

What is the formula for electrical energy?

Answer: Energy=Power(Time)

Name 5 forms of energy

    Light, chemical, electrical, lastic, nuclear, sound, thermal, gravitational.

How much energy is being used while using 3000 watts for 5 hours?

    E=Pt = [w][hrs]
    1000w=1kw
     =3(5)
     =15kw*hr

If you use one electrical space heater at 500w to heat your living room for 8 hrs. How much energy does it use in kw-hr? 1000w= 1kw-hr

    E=Pt
    =(500w)(8hrs)
    = 4,000w-hrs = 4kw-hr
    E= 4kw-hr

What are 3 examples of chemical energy?

  Gasoline, fossil fuels, batteries

Can energy be lost when it is used in a particular task?

     no, energy can only be transfered from on form to another--Natehepler

A 120 watt curling iron plugged in for 1 hour uses how much electrical energy?

.12[kw-hrs]

How many watts are in a kilowatt?

11. A force of 6 N is moving a box over a distance of 2 meters. How much work is being exerted on the object?

      w=6x2; w=12J

Central vs Distributed

Heat

Temperature

When dealing with science, equal temperatures are also known as this term?

Thermodynamic Equilibrium

The boiling point of water in degrees Kelvin is 373. Find the boiling point of water in degrees Celsius.

373-273=100

If you have a temperature of 63 degrees Celcius, what would that same temperature be in Kelvin?

Formula: T Kelvin = T Celcius + 273

T Kelvin = 63 + 273
T = 336

What is Thermal Expansion and what causes it to occur?

Answer: Thermal Expansion is the expansion/contraction of a material due to temperature change. When most materials are heated, the atoms move faster and cause the material to expand; the opposite occurs when it is cooled. Thermal expansion is calculated by using a "coefficient of thermal expansion" specific to that material, since the expansion/contraction of each material varies.

A 10-inch long copper rod is heated from 70°F to 180°F. The coefficient of thermal expansion of copper is 9.3×10^-6 in./°F. What is the length of the rod after it has been heated?

ΔL = α×L×ΔT
Where α=coefficient, L=original length, ΔT=change in Temperature
ΔL=(9.3×10^-6)(10 inches)(180-70°F)
ΔL=(9.3×10^-6)(1100 in./°F)
ΔL=(0.01 inches)
10 inches + 0.01 inches = 10.01 inches

Heat Transfer

How many ways can heat be transfered and what are the specific

ways?

 -3
 -Conduction
 -Convection
 -Radiation

Lets say you have a wall with a window in the middle. The window

is 90cm x 60 cm. The glass in the window is .35 cm thick. The temp outside is 5 c and inside it's 20 c. What is the rate of heat loss through your window? Remember K= .63 w/mk

Icd=Q/t=KAΔT/d

.63w/mk(90x10 raised to the negative 2)(60x10 raised to the -2)(20-5)
divided by .35x 10 raised to the negative 2 m

the answer is 1458 w

Jimmy is replacing the 17ft x 14ft window in his kitchen. The glass he purchased is 0.34cm thick. The current temperature in Jimmy's house is 23 degrees Celsius and the temperature outside is 7 degrees Celsius. K is 0.63w/mK. What is the thermal gradiant?

               Solution: ΔT/d  Change in temp= 23-7 =16.0
               D = 0.34
               ΔT/d = 16/0.34 = 47.06

What is ΔT/d?

It is the thermal gradient.

The Jones are getting new window for in their living room. They are 50cm x 45 cm. The glass is 0.42 cm. The outside temperature is 10°C, the inside is 21.1°C. K is 0.63w/mK. What is the thermal gradient?

T=21.1-10=11.1  D=0.42     ΔT/d.... 11.1/.42= 26.4

Name the 3 types of heat transfer.

Conduction, Convection, and Radiation

We are getting new windows in the classroom.From the example we had in class 20cm x 15cm,

the glass is .32cm outside temp. is -15°C inside temp. is 22°C and K is 0.63w/mK. 
Using this information what is the Thermal gradient?

ΔT/d 22-(-15)= 37 d=.32 37/.32= 115.62

All of the following are ways to improve windows except:

A. Having multiple panes B. Use an inert gas--vacuum C. Low conductivity spacers, either plastic or wood D. Having a single pane E. Low e coating

D is the correct answer.

What is the rate of heat loss thru a 30 cm by 25 cm single pane window, with a thickness of 0.45 cm thick, when it is -10 degrees C outside and 25 degree C inside. If given K=0.63 W/mK, what is the rate of loss?

Icd=KA(change in temp)/d
Plug in the numbers and the answer is 368 W.

Describe the 3 different types of heat transfer.

Conduction-a collision of molecules
Convection- energy transfer by movement of material (forced air)
Radiation-energy transfer with electromagnetic waves

A baseball player has a surface area of 1.2m² with a skin temperature of 34 degrees celcius. He is standing in center field. The air temperature is 28 degrees celcius and the wind velocity is 4m/s. The coefficient of convection is 32. I_cv=?

I_cv=(h)(A)(change in temperature)
I_cv=(32)(1.2m²)(6 degrees celcius)
I_cv=230.4 W

A shipwrecked sailor is lying on a beach. The sailors skin area is 1.8m², the air temp is 25°c,

  the windspeed is 3m/s, and the sailors skin temp is 30°c.  What is the Icv? ( h=26)                                                             -

   use the formula Icv = hAΔT = 26(1.8m²)(30 - 25) = 234w

A hockey player has a surface area of 1.6 m2 with a skin temperature of 35 degrees celcius while he is warming up for a big game. The temperature in the arena is 45 degrees celcius and there is a slight breeze from the top being open and it is 2 m/s. What is the Icv? (h=37)

-Use the formula Icv=(h)(a)(change in temperature) Plug in the numbers in the correct places. Icv= (37)(1.6 m2) (45-35) iCV= (37)(1.6 m2) (10) Icv=592 w

Which form of heat transfer involves electromagnetic waves?

Radiation

Temperature moves from a___________object to a____________object until the temperatures of both objects are equal.

Hot to cold

Thermodynamics

What is the second law of Thermodynamics?

  No refrigerator or heat pump can have an inefficiency of zero.

What is the third law of Thermodynamics?

 It is impossible to cool a system to absolute 0.

Heat Engines

A heat engine is rated as 85% efficient. If the heat input is 500W what is the work output to make the efficiency true?

  E=work output/Q input
  .85=output/500W
  .85x500=425W
  output=425W

Refrigerators

The fridge in your kitchen has a heat output of 60 and work input of 250. What is the inefficiency of the refrigerator?

    Answer: The answer is 4.2
               The inefficienct of the fridge is e_in = work input / heat output
                                                                = 250 / 60
                                                                =  4.2

The 2nd Law of Thermodynamics states what about refrigerators or heat pumps?

   Answer: The 2nd Law of Thermodynamics states that No refrigerator or heat pump can have an efficiency of 0 (zero).

The 3rd law of thermodynamics states that it is impossible to cool a system to ________.

Absolute zero (or, 0 degrees Kelvin)

Heat pumps

. The 2nd law of thermodynamics is no refrigerator or heat pump can have a(n) underline text of zero.

inefficiency

. You have an air conditioner which has a heat output of 100 and an inefficiency of 6.5. What is the work input?

e_in= work input/heat output
work input= 100 x 6.5
work input= 650

Microturbines

What is a Microturbine?

A Small Electric generation systems based on turbine technology.

If you put 100 units in how many units will it produce out? 150 units?

85 units, it is 85% efficient 150 x .85= 85% ... 150 x .85= 127.5 units

What does the 1st law of thermodynamics deal with?

Energy conservation

What is 2 of the 5 ways one can improve windows?

    Multiple layers of glass, no frames of metal, insulate frames, fill glass with inert      gas,Low emissivity coating

Introduction to Physical Science/A.2

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