Physics 115 Final Exam Review

Renewable Energy

-naturally replenishing but flow limited
-virtually inexhaustible in duration but limited in amount of energy per unit of time
-Ex. biomass, hydropower, wind, solar geothermal

Nonrenewable Energy

sources do not form or replenish over a short period of time
ex. fossil fuels, nuclear fission fuels, nuclear fusion fuels

Direct conversion of solar energy

-electricity using photovoltaic devices (solar panels)
-solar thermal power stations
-passive and active solar heating
-hot water generation

indirect conversion of solar energy

-wind power
-hydropower
-biomass

What month is Earth closest to the sun?

January

The \___ of Earth's axis of rotation determines amount of daylight

tilt

Equinoxes have \_______ day and night, while solstices have \_______ day and night

equal, longest/shortest

Solar Constant

power per unit of sunlight that shines on the day side of the Earth and is independent of season

Insolation

how much sunlight reaches the surface
-unit: power per area (ex W/m^2)
-cannot be larger than the solar constant
-varies with weather

Insolation data available for both \____ and \____ tilted surfaces

horizontal and tilted

World leader in using solar energy

Germany

Building block device manufactured from silicon wafers in solar panels

cell

A semiconductor photovoltaic device converts sunlight's \____ energy into \___

-electromagnetic wave energy into electrecity

how are photovoltaics and semiconductors made

-by sandwiching a n-type semiconductor with a p-type semiconductor
-semiconductors are made by doping silicon with bits of elements adjacent to silicon on the perioid table

p-n junction

-energy barrier associated with electric force
- analogous to tossing a ball over a wall
- if ball overthrown efficiency smaller
barrier is 1.1 eV
-electrons absorb the photons energy and kick them over the junction energy barrier

Solar conversion efficiency is maximum __%

33%

T/F: Some utilities will buy excess solar power

T; called net metering

invertor

converts DC to AC (oscillating current)

Duck Curve

graph of power production over the course of a day that shows the timing imbalance between peak demand and renewable energy production

Heating water accounts for \___% of home energy use

20

Passive solar space heating

homes and buildings can be designed with favorable views towards sun or Trombe wall

photovoltaic cells are mostly made of what material?

silicon
-no CO2 emissions, but manufacturing is not carbon-free

Direct solar cost is the \____-cost electricity many places in the world

lowest

Wind

directed kinetic energy of atmospheric
- moves from high atmospheric pressure to low atmospheric pressure

wind power growing \_____

exponentially

Annual Average Wind Speed

100 m

Offshore wind resource will be important why?

population density high in costal areas

Wind turbine extracts energy in the air's \____ \____ and converts to electricity using a generator

bulk motion

Anatomy of ana electricity-generating windmill

-rotation of blades connected directly to an electric generator
-efficiency of mechanical to electrical energy 80-90%

Max possible efficiency extracting wind's kinetic energy

59%

T/F: It is possible to extract 100% wind's kinetic energy

F

Betz Limit

the theoretical maximum fraction of the wind's energy that a wind turbine can extract; equal to 16/27, or about 59%

GE Haliade-X

highest power windmill
-60-64% capacity factor
-1 rotation provides daily energy for two homes

In the US, which source provides the largest fraction of renewable energy?

Biomass

Biomass derived from

recently living matter

hydroelectric is _% of renewable energy

83%

Earliest energy source used by humans

Biomass
-used for food, fuel, and electricity generation

Biomass is carbon neutral

Burning biomass releases CO2 , but this "same" CO2is reused as the food for the
next generation of living organisms

Chemical reaction for photosynthesis

CO2 + H20 + photon energy --\> 02 + carbohydrates

T/F: Biomass most energy dense indirect solar energy

F

Primary Productivity

rate of energy storage by photosynthesis

Net primary productivity

portion left after plant's consumption of about 1/2 energy, measured in power/area

Dominant biofuel produced today

ethanol

ethanol produced by

fermenting corn and other plants such as sugarcane

Biodiesel made by

converting oils and fats in plants

Recycled cooking oil made into

biodiesel

T/F: Biodiesel sold in mixture that is B5 (5% biodiesel) and B20 (20% biodiesel)

T

Power grid

-efficiently carried energy from electric generators to consumers
-ex. LED bulb: energy produced by nuclear steam plant, electric grid transports electricity to home, electric current in LED bulb creates photons
-electric grid in US is 60 Hz AC, meaning 60 cycles per second
-uses high voltage for long-distance transmission lines to
minimize the energy loss from electrical resistance

T/F: electricity is an energy source

F: energy carrier

Electric motors vs electric generators

Generators convert mechanical energy to electric
Motors convert electricity to mechanical energy

Electricity

current (flow) of electric charge in a circut

Circut

Closed loop

Electric charge is a \____ quantity

conserved

SI Unit for electric charge

Coulomb, C

Open and closed circuts

-Closed: electric field created and current flowing
-Open: A gap in this circuit, so current = 0 (a switch does this on purpose)

Direct current (DC)

charge flows in one direction (ex. battery)

Alternating current (AC)

the direction of current oscillates in time

Energy is carried (transferred) by the motion of \____, which happens in both DC and AC circuits

electric charge

T/F: Thermal power plants, windmills, hydropower dams all have
generators that convert rotational mechanical energy into electricity

T

hydropower

-indriect solar energy
-extracts directed kinetic energy of flowing water
-largest fraction of renewable energy in the world

Lines of force

lines that point in the direction

Voltage

-electric potential energy per charge
-measures energy related to the electric force
= voltage = Ep/Q

Ohm's law

dissipation of electricity due to friction

Electrical Resistance (R)

friction that impedes flow of electric current
-portion of work done by the electric field to overcome the electrons' collisions with atoms
-Resistance, R, to the flow of electricity is described by Ohm's law, Vr=IR

Power dissipated in the resistance

Pr=I * VR = I^2R

Transformers

Used to "step-up" and "step-down" voltage to minimize resistive heating (energy loss) during transmission

Internal combustion engine

dominates market, burns petroleum or biofuel, crankshaft rotation is connected to vehicle's drivetrain

Gas-electric hybrid vehicles

Parallel or series type Gas-electric hybrid vehicles

Vehicle's drivetrain is connected to a combustion engine and/or electric motor(s) with a rechargeable battery for energy storage

Plug-in type vehicle

larger battery that can be pre-charged from the grid

Battery-electric vehicle (BEV)

-no combustion engine and very large battery
-must be recharged from the grid
-does not have catalytic converters

Regenerative breaking

-greatly increases the efficiency of all types of electric vehicles
-The electric motor operates in reverse as a generator, (re)charging the battery
-The car slows down with much less frictional heating

Are electric vehicles carbon free?

Yes if:
-charged using electricity from renewable sources, like a rooftop solar array on a house
No if:
-the grid's electricity comes from a fossil fuel power plant, since the CO2emission is inherited

battery

device that separates opposite charges usng a chemical reaction

Vehicle to grid energy storage

grid-connected electric vehicle batteries could provide large energy storage
-battery charged when electricity demand is low
-When demand is high, a portion of each battery's energy is drawn back to the grid with modest inconvenience to the car owner

Hydrogen

energy carrier that can be used as fuel
-not abundant naturally, but is abundant in chemical compounds
-produced by electrolysis and steam reforming

T/F: Hydrogen is an energy source

F: not an energy source

Electrolysis

way of producing hydrogen gas; apply electricity to water to separate the H2 and O

Steam reforming

way of producing hydrogen gas; reacting natural gas with water

Hydrogen fuel cell

-A fuel cell produces electricity from a flow of H1 gas which reacts with O1 from air (but no flame!)
-The output is water and waste heat

Hydrogen must be produced by consuming \_____

energy

Hydrogen fuel cell electric vehicles

hydrogen stored in a tank as compressed gas (13,000 psi)

atmosphere is \____ km thick, but \___ lies within 16km of Earth's surface

10,000; 80%

Solar constant

power per unit area of the sunlight that shines on the day side of Earth

Earth's temperature

-determined by balance of radiation

Sun radiates energy from its photosphere at Tsun = ???

5,700K

Bare rock

expectation of Earth's temperature

Power absorbed by Earth =

(piR^2)S

Earth's thermal radiation =

(4piR^2)e(boltzmann)T^$

Greenhouse effect

-caused by insulating atmosphere
-Earth warmed by solar radiation
-gases in atmosphere absorb then re-radiate outgoing IF radiation
-Half of the re-radiated energy is directed back down toward Earth's surface

Average temp of Earth \___ larger than if there was no atmosphere

33K

Which gas has the smallest concentration in Earth's atmospere

CO2

"Warmer below, cooler above"

Earth's upper atmosphere colder than surface, consistent with the greenhouse energy balance

Concentration of CO2 increased \___% since beginning of industrial revolution: anthropogenic

45

T/F: Earth's energy flow is balanced

F: Earth's energy flow is not quite balanced, instead absorbing 0.6 W/m^2 more than it is emitting as a result of global warming drivers

Dry greenhouse gases __% of atmosphere, and play a __ role in regulating Earth's energy balance

Two-step radiation model for Earth's energy balance

outgoing infrared energy must balance the incoming solar energy at the top of the atmosphere
-remaining portion leaks into space (1-e)

Three planet climate experiment

-Venus: bare rock predictions, 96% CO2 and 100 times denser than Earth's atmosphere (864 F)
-Earth: 78% N2, 21% O2, 1% Ar, < 1% CO2 and other gases (59F)
-Mars: mostly CO2but its density is only 1% of Earth's atmosphere (-81F)

Can renewable energy sources be used in a heat engine to make elelctricity

No

transformer

-A transformer is used to change the transmission voltage and current in electrical transmission power lines.
-This is done to reduce energy losses, which decrease as the current decreases.