HNRS 227 Lecture 5 and 6 Chapter 4 and Chapter 5 Heat and Temperature Wave Motion and Sound...

HNRS 227 Lecture 5 and 6 Chapter 4 and Chapter 5

Heat and TemperatureWave Motion and Soundpresented by Prof. Geller

Recall from Chapters 1, 2, 3 Units of length, mass and time, and metric Prefixes Density and its units The Scientific Method Speed, velocity, acceleration Force Falling objects Newton’s Laws of Motion Newton’s Law of Universal Gravity Work Potential Energy and Kinetic Energy Conversion of Energy Types/Sources of Energy

Main Concepts for Chapters 4 and 5

Kinetic Molecular TheoryTemperatureHeatPhases of matterThermodynamicsForces, Vibrations and Wave MotionSound Waves and their characteristicsReflection, Refraction, Resonance

1st Law of Thermodynamics

In an isolated system, the total amount of energy, including heat energy, is conserved.

ENERGY IS CONSERVED

Temperature

A relative term reflecting how vigorously atoms in a substance are moving and colliding

Alternative definition the average kinetic energy of the

molecules in a region

Temperature

Units Fahrenheit Celsius Kelvin

Where is absolute zero?At what temperature does water

freeze? Boil?

Heat

Heat is a form of energy the energy flows from a warmer object to

a cooler objectUnits of heat

calorieamount of temperature needed to raise

temperature of 1 gram of water 1 degree Celsius

kilocalorie (kcal or Calorie)amount of temperature needed to raise

temperature of 1 kg of water 1 degree Celsius

Specific Heat

Ability of a material to absorb heat energy proportional to mass proportional to change in temperature

Q = m*c*T c is the SPECIFIC HEAT of the substance

amount of energy needed to increase temperature of 1 gram of substance 1 degree Celsius

Transfer of Heat

Conduction “movement of heat by collisions

between vibrating atoms or molecules”Convection

“transfer of heat by the physical motion of masses”cooler liquids or gasses descend while

warmer liquids or gasses rise

Phases and Phase Diagram (NOT IN TEXT but IMPORTANT)

2nd Law of Thermodynamics

Two key components heat flows from a warmer body to a

cooler body entropy increases remains constant or

increases in time

Question for Thought

Define temperature and heat.

Temperature is a measure of the average kinetic energy of the molecules of a substance. Heat is the total internal energy of the molecules involved in an energy transfer.

Question for Thought

Why do most materials become less dense as their temperature is increased?

As the temperature of a solid increases, the vibrations of the individual molecules become larger. When these vibrations become larger, the average distance between the molecules increases to accommodate these larger oscillations, and the solid expands. In a liquid or a gas, the individual molecules move faster as the temperature increases, and the collisions between individual molecules become more violent. Since the molecules are moving faster, they move farther apart as they travel a larger distance in the time between collisions.

Question for Thought

Would the tight packing of more insulation, such as glass wool, in an enclosed space increase or decrease the insulation value? Explain.

Tight packing would tend to decrease the insulation value of glass wool because it would squeeze the wool together and give the heat more paths to travel. It is the presence of many small pockets of air, with unattached molecules, that gives glass wool and other similar insulation materials their insulating properties.

Question for Thought

A true vacuum bottle has a double-walled, silvered bottle with the air removed from the space between the walls. Describe how this design keeps food hot or cold by dealing with conduction, convection and radiation.

The vacuum between the walls prevents heat transfer by means of convection or conduction, while the silvered walls reflect radiated energy back into (or away from) the food, preventing energy transfer by radiation.

Question for ThoughtWhy is cooler air found in low

valleys on calm nights?

Cooler air is denser than warmer air. This denser air weighs more per volume than the warmer air and pushes the warmer air out of the way as it sinks down to its lowest level. The warmer, less dense air sits on top of the cooler air because it weighs less per volume.

Question for Thought

Why is air a good insulator?

Air is not very dense and conduction is not very efficient at transferring energy because the molecules are much farther apart than they are in solids or liquids.

Question for Thought

Explain the meaning of the mechanical equivalent of heat.

Mechanical energy can be converted to heat as it changes from external mechanical energy to the internal kinetic energy of the molecules. A given quantity of mechanical energy always yields a known amount of heat.

Question for Thought

What do people really mean when they say that a certain food “has a lot of Calories?

When people refer to the “Calorie content of food,” they are referring to the amount of chemical energy available from the food. One way to measure the chemical energy of foodstuff is to find out how much heat is released by complete oxidation. A Calorie (kcal) is a measure of the heat release and thus is a measure of the chemical energy released.

Question for Thought

A piece of metal feels cooler than a piece of wood at the same temperature. Explain why.

The metal is more efficient at conducting heat away from your hand than wood, so it feels cooler because your hand senses heat leaving your body.

Question for Thought

Explain how latent heat of fusion and latent heat of vaporization are “hidden.”

The latent heats of fusion and vaporization go into or are released from internal energy during a phase transition. There is no temperature change associated with these heat transfers, so they are “hidden.”

Question for Thought

What is condensation? Explain on a molecular level, how the condensation of water vapor on a bathroom mirror warms the bathroom.

Condensation occurs when more vapor molecules are returning to the liquid state than are leaving the liquid state. When a water vapor molecule joins a group of liquid water molecules, it has to give up its latent heat of vaporization. This heat is transferred to the surrounding air molecules such as the air in the bathroom.

Question for Thought

Which provides more cooling for a styrofoam cooler, one with 10 pounds of ice at 0 degrees C or one with 10 pounds of ice water at 0 degrees C?

The 10 pounds of ice provide more cooling because as the ice undergoes the phase change into water, it absorbs heat. Ten pounds of ice water simply absorbs heat according to the value of its specific heat until it reaches room temperature and therefore absorbs less heat.

Question for Thought

Explain why a glass filled with a cold beverage seems to “sweat.” Would you expect more sweating inside a house during the summer or during the winter? Explain.

Water condenses out of the air onto the cooler surface of a glass because the air near the glass is cooled, lowering its temperature to the dew point. Since the warmer air can hold more water vapor in the summer, it would have more water vapor to condense. Therefore, you would expect more condensation in the summer.

Question for Thought

Why is a burn from steam at 100 degrees C more severe than a burn from 100 degrees C water?

One hundred degree Celsius steam contains more energy (540 cal/g) than 100°C water, so the steam burn would be more severe.

Question for Thought

Relative humidity typically increases after sunset. Explain how this is possible when no additional water vapor is added or removed from the air.

Cooling of air reduces the capacity of air to hold water vapor. Relative humidity is a ratio of water in the air to how much water it can hold. Thus a decrease of capacity increases the relative humidity, even when the amount of water vapor in the air is constant.

Question

A 1.0 kg metal head of a geology hammer strikes a solid rock with a velocity of 5.0 meters per second. Assuming that all of the energy is retained by the hammer head, how much will the temperature increase? (specific heat of hammer head is 0.11 kcal/kgC)

Answer

4.

Q KE and KE 12mv2 Q 1

2mv2

Q mcT mcT 1

2mv2 T

1

2mv2

1.00 cal

4.184 J

mc

T

1

21.0kg 5.0

m

s

2

1.00 cal

1.0 kg 0.11kcal

kgC

4.184 J

1.00 kcal

1, 000.0 cal

0.03 C

Question

Lead is a soft, dense metal with a specific heat of 0.028 kcal/kgC, a melting point of 328.0o C and a heat of fusion of 5.5 kcal/kg. How much heat must be provided to melt a 250.0 kg sample of lead with a temperature of 20.0o C?

Answer13.

Q1 mcT

250.0kg 0.028kcal

kgC

308.0C

2,156kcal

Q2 mLf

250.0kg 5.5kcal

kg

1,375kcal

QTotalQ1Q22,156kcal 1,375kcal 3,531kcal

3,500kcal

Forces and VibrationsVibration

back and forth motionAmplitude

extent of displacement from the equilibrium position

Cycle one complete vibration

Period time required to complete one cycle

Frequency number of cycles per second

Relationship between period and frequency T = 1 / f f = 1 / T

Waves

Longitudinal disturbance that causes particles to move

closer together or farther apart IN THE SAME DIRECTION the wave is moving

Transverse disturbance that causes motion

PERPENDICULAR to the direction that the wave is moving

In general, liquids carry longitudinal waves but not transverse waves

Same terms as vibrationsv =* f

Sound Waves

Sound does not travel in a vacuumSound moves through solids faster

than any gasVelocity of sound is effected by

composition and temperature of gas

Reflection, Refraction and Interference

Reflection waves bouncing back off of a boundary

Refraction change in direction of wave crossing a

boundaryInterference

interaction of wavesdestructive interferenceconstructive interference

Resonance

Natural frequency frequency of vibration determined by

the object’s composition and shapeResonance

when frequency of external force matches natural frequency

Doppler Effect and Sonic Boom

Doppler Effect Apparent change in frequency of a

wave caused by the relative motion of the source or observerpitch of train approaching, departing

Sonic boom shock wave caused by object

moving at speed of sound or faster

Question for Thought

What is a wave?

A wave is a disturbance that moves through a medium such as a solid or the air.

Question for Thought

Is it possible for a transverse wave to move through air? Explain.

The book view is “No,” because there is no force acting on the air to return it to its original position after the wave has dislocated it. However, what about EM.

Question for Thought

A piano tuner hears three beats per second when a tuning fork and a note are sounded together and six beats per second after the string is tightened. What should the tuner do next, tighten or loosen the string? Explain.

Loosen. Since the beat frequency depends upon the difference between the two frequencies, you wish to go in the direction of fewer beats per second.

Question for Thought

Why do astronauts on the moon have to communicate by radio even when close to one another?

There is no medium such as air to transmit sound on the moon.

Question for Thought

What is resonance?

The condition where the frequency of an external force matches the frequency of an object is resonance.

Question for Thought

Explain why sound travels faster in warm air than in cool air.

Gas molecules have a greater kinetic energy and move faster in warm air than in cold air. These molecules are able to transfer an impulse from one molecule to the next faster.

Question for Thought

Do all frequencies of sound travel with the same velocity? Explain using the wave equation.

Longer wavelengths have lower frequencies. Since the velocity of sound is equal to the product of the frequency times the wavelength, the velocity is a constant.

Question for Thought

What eventually happens to a sound wave traveling through the air?

The energy of the sound wave is eventually dissipated into heat.

Question for Thought

What gives a musical note its characteristic quality?

The presence and strength of various overtones determine the characteristic sound of a musical note.

Question for Thought

Does a supersonic aircraft make a sonic boom only when it cracks the sound barrier? Explain.

The sonic boom is from the building up of a pressure wave in front of the moving aircraft. Since this pressure wave is present as long as the plane is moving faster than the speed of sound, the aircraft continually makes a sonic boom.

Question for Thought

What is an echo?

An echo is the return of a sound wave to its source after the wave has been reflected.

Question for Thought

Why are fundamental frequencies and overtones also called resonant frequencies?

They all produce standing waves or resonance in whatever is oscillating.

Question

The distance between the center of a condensation and the center of an adjacent rarefaction is 65.23 cm. If the frequency is 256.0 Hz, how fast are these waves moving?

Answer

2.vff 256.0Hz 265.23cm

v256.01s

265.23cm

3.340104cm/s (or 343.0 m/s)

Question

A warning buoy is observed to rise every 5.0 seconds as crests of waves pass by it. What is the period of these waves? What is the frequency?

Answer

3.aT5.0s

bf1T

15.0s

0.2Hz

Question

Sound from the siren of an emergency vehicle has a frequency of 750 Hz and moves with a velocity of 343 meters per second. What is the distance from one condensation to the next?

Answer

4.

vfvf

v343.0m

sf750.0Hz

343.0

m

s

750.01

s

343.0

750.0

m

ss

1

0.457m