HNRS 227 Fall 2008Chapter 4

Heat and Temperaturepresented by Prof. Geller

Do You Remember These?Units of length, mass and time, and metric PrefixesDensity and its unitsThe Scientific MethodSpeed, velocity, accelerationForceFalling objectsNewton’s Laws of MotionNewton’s Law of Universal GravityWorkPotential Energy and Kinetic EnergyConversion of EnergyTypes/Sources of Energy

iClicker Question

What is the result if you add 1015 to 1015?A 1015

B 1030

C 10150

D 1025

E 2 x 1015

iClicker Question

Which unit of the following is the largest?A KilometerB Astronomical Unit (AU)C Light Year (ly)D Parsec (pc)E Gigameter

iClicker Question

What is acceleration?A The change in distance per change in time.B The change in position per unit of time.C The ratio of the change in

velocity per change in time.D The change of time per unit of

length.

iClicker Question

What happens to the velocity and acceleration of an object in free fall?A The velocity decreases as the acceleration

remains the same.B The velocity increases as the acceleration

remains the same.C The velocity increases and the acceleration

decreases.D The velocity increases and the acceleration

increases.E Both velocity and acceleration decrease.

iClicker Question

Neglecting air resistance, what are the forces acting on a bullet after it has left the barrel of a rifle?

A The force of air acting up and gravity acting down.B Only the force of gravity acting straight

down.C There are no forces acting at this point.D All of the above are true.

Question for Thought

A kWhr isA a unit of workB a unit of energyC a unit of powerD More than one of the above is true.

A kWhr is a unit of work, and since energy is the ability to do work, it is also a unit of energy. In terms of units, a watt is a joule per second, and an hour, as a second, is a unit of time. The time units cancel, leaving a unit of a joule, which can be used to measure either work or energy.

Question for Thought

What happens to the kinetic energy of a falling book when the book hits the floor?

A The kinetic energy is destroyed.B The kinetic energy is converted to heat

only.C The kinetic energy is converted to heat

and sound.

The magnitude of the force that a baseball player exerts with a 5 kilogram baseball bat on a 0.2 kilogram ball is measured to be 50 Newtons. What is the magnitude of the force that the ball exerts on the bat?A. 6 NewtonsB. 10 NewtonsC. 50 NewtonsD. 60 NewtonsE. 1.0 Newtons

In the diagram above, a box is on a frictionless horizontal surface with forces F1 and F2 acting as shown. If the magnitude of F1 is greater than the magnitude of F2, then the box isA. moving at constant speed in the direction of F1B. moving at constant speed in the direction of F2C. accelerating in the direction of F1D. accelerating in the direction of F2E. not moving at all.

Box F1F2

When a satellite is a distance R from the center of Earth, the force due to gravity on the satellite is F. What is the force due to gravity on the satellite when its distance from the center of the Earth is 2R?A. 9 FB. 4 FC. F / 4D. F / 9E. F / 16

Which of the following best describes the law of conservation of energy?A. Energy must not be used up faster than it is created or the supply will run out.B. Energy can be neither created nor destroyed.C. Energy is conserved because it is easily destroyed.D. Conservation is a law describing how to destroy matter.E. Energy conservation is a law recently passed by Congress.

What is the power consumed by a 100 Joules energy source in 4 seconds?A. 400 WattsB. 100 WattsC. 200 WattsD. 50 WattsE. 25 Watts

Main Concepts for Chapter 4

Kinetic Molecular TheoryTemperatureHeatPhases of matterThermodynamics

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 collidingAlternative definition

the average kinetic energy of the molecules in a region

Temperature

UnitsFahrenheitCelsiusKelvin

Where is absolute zero?At what temperature does water freeze? Boil?

Heat

Heat is a form of energythe energy flows from a warmer object to a cooler object

Units of heatcalorie⌧amount 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 energyproportional to massproportional to change in temperature

Q = m*c*ΔT (note: not in textbook)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 componentsheat flows from a warmer body to a cooler bodyentropy increases remains constant or increases in time

Question for Thought

Which is true about temperature and heat.A Temperature is a measure of the average kinetic energy of the molecules of a substance.B Heat is the total internal energy of the molecules involved in an energy transfer.C Both A and B above are true.D Neither A nor B above are true.

Question for ThoughtAs 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.

A This explains why most materials become less dense as their temperature is increased.B This explains why most materials become more dense as their temperature is increased.

Question for Thought

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

A Increase the insulation value.B Decrease the insulation value.

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 ThoughtCooler air is found in low valleys on calm nights.⌧A True⌧B False

True. 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

Is air a good insulator?A YesB No

Air is a good insulator because it 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

Can you 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.

A TrueB False

True. 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

Can you 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

The condensation of water vapor on a bathroom mirror warms the bathroom.

A TrueB False

True. 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?

A One with 10 pounds of ice at 0 degrees C

B One with 10 pounds of ice water at 0 degrees C?

The 10 pounds of ice (A) provide more cooling because as the iceundergoes 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

Consider that 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?

A SummerB Winter

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

A burn from steam at 100 degrees C is more severe than a burn from 100 degrees C water.

A TrueB False

True. 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.

A TrueB False

True. 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 = ΔKE and ΔKE = 12

mv2 ∴ Q = 12

mv2

Q = mcΔT ∴ mcΔT =12

mv2 ∴ ΔT =

12

mv2⎛ ⎝

⎞ ⎠ ×

1.00 cal4.184 J

⎛ ⎝ ⎜ ⎞

⎠ mc

T =

12

1.0kg( ) 5.0ms

⎛ ⎝

⎞ ⎠

2

1.00 cal( )

1.0 kg( ) 0.11kcalkg°C

⎛ ⎝ ⎜ ⎞

⎠ 4.184 J( )

× 1.00 kcal1, 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 = mcΔT

= 250.0 kg( ) 0.028 kcalkg°C

⎛ ⎝ ⎜ ⎞

⎠ 308.0°C( )

= 2,156 kcal

Q2 = mLf

= 250.0 kg( ) 5.5kcalkg

⎛ ⎝ ⎜ ⎞

⎠

= 1,375 kcal

QTotal = Q1 + Q2

= 2,156 kcal( )+ 1,375 kcal( )= 3,531 kcal= 3,500 kcal