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GE1305 - Everyday Physics

Chapter 1. Energy and Power

Michel A. Van Hove

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Chapter contents Gaining a feeling for energy Energy vs. power Comparing amounts of energy The "man-day" of energy Other forms of energy Conversion of energy Making a scientific discovery Conservation of energy Power The "man-power" Electric and hybrid cars Renewable and non-renewable energy

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Surprising comparisons What requires more energy?

walking up 100 floors chemical food energy

transformed to potential energy (if 20% efficient)

or heating a cup of tea from 0 to 100 C thermal energy

about the same!

ICC Hong Kong: 118 floors observation deck on 100th floor

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Comparing amounts of energy Intake of energy by human body

energy is taken in mainly as food (and much less as oxygen)

it is used to do work and keep warm Daily intake

in this course, we call the daily intake of energy by the human body: one "man-day" of energy it is also called "human-equivalent" (H-e)

we will compare other forms of energy to the "man-day" example: an AA battery contains

~1/1000 "man-days" of energy so 1000 AA batteries contain about

as much energy as a "man-day"

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Diet versus exercise: in numbers Food intake

daily intake (“man-day”): ~2000 Cal = 2000 kcal ~ 0.3 kg fat 1 can of Coke: ~150 Cal (more for milk and juice)

~ 1/13 “man-day” ~ 0.02 kg of fat = 20 g of fat Reducing food intake by 150 Cal per day

reduce by 1/13 “man-day" (~8% of daily intake): reduces body weight by ~20 g per day ~ 150 g per week [to lose ~0.5 kg per week: skip ~¼ “man-day” each day]

Burning off 150 Cal per day vigorous exercise for ~½ hour each day light exercise for ~1 hour each day

advantage of exercise: also produces 3x same energy as heat

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Other forms of energy: mechanical energy Two types:

potential energy: see next kinetic energy: energy of motion,

felt in collisions; used to store energy in flywheels (can power a bus)

Comparison of kinetic energy with “man-day” of energy 1 "man-day" (2000 kcal) ~ man moving at ~2000 km/h

(at ~2 x speed of a jet or sound) man walking at 4 km/h ~ 1 m/s:

1/300,000 “man-days” so 1 “man-day” ~ motion energy of army of

300,000 men walking

1 high-speed train ~ 250 "man-days"

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Other forms of energy: thermal energy

Comparison with “man-day” of energy 1 "man-day" (2000 kcal) ~ rise from 0° to 100°C

of 20 kg (20 liters) of water

How fast do the molecules and atoms move? at room temperature, molecules in air and atoms in solids

move at about the speed of sound in air: ~ 1000 km/h so thermal energy in 1 kg of air (~ 1 m3) or 1 liter of water

~ kinetic energy of object of 1 kg flying at speed of sound or jet plane ~ energy needed to lift 1 kg by 5000 m

this is a lot of energy!

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A most important discovery of the 17th and 18th centuries: conservation of energy The questions above were raised by the technical

progress of the 18th and 19th centuries development of engines, especially steam engines, to

replace human and animal labor (“horsepower”) to answer these questions required a better

understanding of “work”, “heat”, “energy”, “power” it also required measuring (quantifying) these concepts,

to make more precise comparisons All this produced the First Law of Thermodynamics,

“conservation of energy”, which helped start the Industrial Revolution; let’s see how!

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Making a scientific discovery: a daily example Will the Sun rise tomorrow? (Ignore clouds.) What do you think? no maybe probably certainly

How much would you bet on the Sun rising tomorrow? $1,000,000,000 $1,000,000 $1,000 $1

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Energy conservation This makes perpetual

motion machines impossible because there is

always some friction patent offices

frequently receive patent applications that violate physical laws, like conservation of energy

drawing by M.C. Escher

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Who contributed? James Watt (1736–1819)

Scottish inventor and mechanical engineer, started as maker of astronomical instruments at the University of Glasgow, then made musical instruments and toys

vastly improved the steam engine, requiring understanding heat (and the manufacture of a large cylinder with a tightly fitting piston)

the Watt engine started the Industrial Revolution

also invented an early copying machine (transfer of ink through thin paper) ~1779

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"Power" considers speed of energy conversion What has more power?

1 Hiroshima bomb: much higher delivers its energy in ~1/10,000,000 second think of firecracker: quick small explosion

or petroleum in a B747 jet plane: medium burns for hours

or water in Three Gorges Dam reservoir: much lower takes years to empty (at river's refill speed)

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Power vs energy

Daily examples climbing fast vs slowly

same energy is needed (converting fat to potential energy) climbing fast is more powerful than climbing slowly

falling on concrete vs mattress concrete converts energy of motion to heat very fast: painful a mattress converts the energy of motion more slowly: comfortable

car collision vs braking a collision stops a vehicle or person very quickly: dangerous braking stops a vehicle more slowly: safe

car acceleration a powerful car engine reaches 100 km/h quickly a weak engine needs more time to reach the same speed,

producing the same energy of motion but more gradually

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How big is a "man-power"? "Man-power"

1 "man-power" ~ 1 "man-day" / day ~ 2000 kcal / day ~ 8,000,000 J / 86,400 sec ~ 100 W ~ power of a 100 W light bulb (incl. heat) ~ 1/10 horsepower (hp)

maximum human power ~ 1 horsepower this power comes from:

chemical energy contained in fat

this chemical energy is converted to: mostly heat (~80%), incl. when descending and slowing down kinetic energy (motion) potential energy (climbing) other muscular work (heart, breathing, lifting, pushing, squeezing, …)

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Electric cars Oil vs auto batteries (both chemical energy)

oil contains much energy: ~42,000,000 J/kg (excl. air) but only ~20% efficient (rest goes to heat)

lead-acid batteries contain less energy: ~125,000 J/kg but ~85% efficient!

so oil is ~300x more energetic than batteries ~80x after counting efficiencies we need batteries weighing ~80x weight of oil to drive the same

distance, for example 80 x 40 = 3200 kg of batteries (a typical oil-driven car weighs ~1,000 kg!)

but oil cost ~ lead-acid battery cost + electricity cost oil cost ~ 1/5x (lithium-ion battery cost) + electricity cost lithium-ion battery weight ~ half lead-acid

What is environmental impact of electric cars?

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From non-renewable to renewable energy Replacing non-renewable by renewable energy

1 major power plant (~1 GW, coal, oil, nuclear, …) powers ~1,000,000 Americans or ~5,000,000 Chinese

~ 500 large wind turbines (~ 500 km2) if wind blows non-stop (but typical capacity factor ~ 25%)

~ 100 km of waves (~ 100 km of coastline) if ~ 20% efficient and uses large waves non-stop

~ 15 km2 of photovoltaics if ~ 15% efficient and Sun shines non-stop in daytime

~ 1/20x Three Gorges Dam

Gansu wind farm wave power Florida Power & Light plant

Yes, but … … not so fast!