# Group Work - PowerPoint PPT Presentation

1 / 31

Group Work The 2004 Tour de France’s Alpe d’Huez time trial was a climb with its finish 1200 m higher than the start. The winner, Lance Armstrong, and his gear had a combined mass of 84 kg . The work Lance had to do was ( 84 kg )( 9.8 N/kg )( 1200 m ) = about 10 6 J .

## Related searches for Group Work

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.

Group Work

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

### Group Work

• The 2004 Tour de France’s Alpe d’Huez time trial was a climb with its finish 1200 m higher than the start. The winner, Lance Armstrong, and his gear had a combined mass of 84 kg.

The work Lance had to do was

(84 kg)(9.8 N/kg)(1200 m)= about106 J.

Muscle is about 20% efficient, so Lance had to deplete 5 times as much potential energy as the work he produced. How much energy did he consume?

How much of the energy he consumed was not converted to work?

## Heat and Phase Changes

Our interaction with the world of the small

### Announcements

• Questions on drill?

### What’s the Point?

• What happens to all our energy?

• How does energy convert in freezing, melting, evaporation, and condensation?

### Objectives

• Define and differentiate heat and temperature.

• Explain the role of heat in conservation of energy.

• Describe the energy transfer of phase changes.

### CPS Question

• Transfer of force.

• Transfer of momentum.

• Transfer of mass.

• Transfer of energy.

What is work?

### Mechanical Equivalent of Heat

James Joule’s life-long obsession

Identical effects of adding heat to a system and doing work on it.

Source: Griffith, The Physics of Everyday Phenomena

### Heat and work

• Doing work on an object changes its energy.

• Heat transfer is another way to change an object’s energy!

• Work heat: easy

• Heat  work: more difficult

### CPS Question

• Its speed increases.

• Its height increases.

• Its temperature increases.

• Its energy does not change.

When heat is added to an object, how does the object’s energy change?

### Temperature and Energy

• Averagetranslational molecular kinetic energy is

1/2 kBT

per mode of motion.

• kB = 1.3806610–23 J/K (Boltzmann constant)

• Individual molecules can have higher or lower kinetic energies than average.

### Terms

• Temperature is proportional to average molecular translational kinetic energy.

• Internal energy (U) is total molecular kinetic + potential energy.

• Heat is transferred molecular energy.

When a red-hot piece of iron is dropped into a bucket of water, the water

### CPS Question

becomes hotter.

increases its temperature.

increases its internal energy.

all of the above.

c =

q

mDT

### Specific Heat (Capacity)

• Heat needed to change the temperature of a unit amount of a substance.

• q= heat input

• m = mass of sample

• Dt = temperature change

• Units: J/(kg K) or cal/(kg K)

• Intensive

• ### Heat Capacity

• Heat needed to change the temperature of a particular sample.

C = q/DT = mc

• Units: J/K

• Extensive

### Group Work

• Show that the temperature change DT of an object to which an amount of heat q is added is given by the formula

DT = q/(mc)

where m is the object’s mass and c is its specific heat.

Hint: What is the formula for specific heat?

### Group Work

• The specific heat of Lance Armstrong is about the same as the specific heat of water: 4184 J/(kg °C).

If all the non-work energy he converted in the Alpe d’Huez climb stayed in his 75-kg body, how much would his body temperature have risen?

### Heat Units

• Joule

• Calorie (cal): heat needed to raise 1 gram of water 1 degree C (or K) = 4.184 J.

• British Thermal Unit (BTU): heat needed to raise 1 pound of water 1 degree F = 1054.35 J

### Another Heat Unit

• U.S. Food Calorie: Cal = 1000 cal

• Food energy values are often presented in kJ in other countries

### Quick Group Work

• How many Calories (food calories) did Lance burn in the Alpe d’Huez climb?

## Phase Changes

### Phase Changes

• Melting, boiling, freezing, condensing…

• Added or removed heat changes the substance’s potential rather than kinetic energy

• Water freezes at 0 °C, boils at 100 °C (well, at 92 °C in Laramie)

• Not all heat transfer is expressed as a temperature change.

### Molecular simulations

• Water phase changes

### Evaporation of a Liquid

• More energetic jostling = higher temperature

• An especially fast molecule at the surface may detach!

### Evaporation of a Liquid

• More energetic jostling = higher temperature

• An especially fast molecule at the surface may detach!

• KEPE

### Evaporation

• Evaporating molecules carry away energy

• Remaining liquid cools (KE decreases)

### CPS Question

When are the molecules’ potential energies highest?

• When they are together in the liquid.

• When they are separated in the gas.

### Phase Changes

• Potential energies:

Solid < Liquid <Gas

• During a phase change, potential energy, not kinetic energy (temperature) changes.

• Heating or cooling a changing phase does not change its temperature!

steam

Water boils

Liquid water

Ice melts

ice

### Latent heat

• Potential energy of phase change (energy required to change the phase of 1 kg of substance)

• Water’s latent heat of fusion (melting):

335,000 J/kg

• Water’s latent heat of vaporization:

2,255,000 J/kg

### Group Work

• During the Alpe d’Huez climb, how much sweat would Lance have needed to evaporate to keep his body temperature constant?

The heat q needed to vaporize a mass m of water is

q = m (2.255 106 J/kg).

Solve for mass m and substitute in the values.