P7.1 Energy Flashcards
What is law of conservation f energy
What is a closed system..
Energy can not be created or destroyed. Only transferred between different energy stores.
In a closed system energy does not change not effected by anythint
If you in a closed system, this will be true , there is no net change. No external forces act on, nothing goes in or nothing goes out
Remember kWh is just
Energy transferred
So when they say power if 2 kHz used 2 hours
Just power = word down / time
What are the different energy stores
Chemical - coal - thermal- anything hot - kinetic - anything moving Elastic- spring - gravational- anything held up - nuclear hydrogen in sun ?, ATOMIC RADII THAT RELEASE ENERGY IN REACTIONS
- electrostatic : twomoppsote charges held apart
Magnetic - magnet
In energy analysis what must you consider and assume?
You must consider that some of the energy when changing stores actually can get transferred to thermal stiess of surroundings because of FRICTION, SOUND, or AIR RESITSNCE / DRAG
Therefore your final answer may be higher then expected because you assumed no energy was dissipated, but this it was closed.
How to do energy analysis
Like light fillamenr example?
Say you stationary car in race. Your chemical high but kinetic store low
As you start, the Chicago gets lower as kinetic gets higher. By the end the kinetic high and chemical low.
If you add them up, it should be the same in energy is conserved…
2)here chemical energy form battery transferred to thermal energy of light
Another situation?
Car is moving , surroundings relatively cool. Car comes to stop quickly, what happens
Why can’t you calculate the temp of the surroundings when car stops
Kinetic energy high, thermal energy of surroundings low
Then kinetic STORE (NOT ENERGY)zero, but due to friction thermal STORE high. Not completely, because some would have escaped as sound, etc
You can’t calculate the temperature of the surroundings because it changes so quickly?
How to do calculations with different energies? IMPORTSNT AND
Just remember about real situations?
Just put two equations next to each other, cancel out anything on both sides (like mass) or give them a random value, and then re arrange for one thing. Pretty cool . EQUATIONS RELATE TO EACH ORHER
Real situations are not closed systems… thus energy transfer won’t be the same
More realistic
For example
Include other energy stores and what probably is going on, like thermal. And also sound
Sun glasses are falling, kinetic energy is high, and due to resistance thermal store is meh
Then it hits ground and kinetic iz zero but thermal is high
How do you pay for abill
What is KWH anyways
Here what unit of time?
Money= kWh x rate per kWh
Basically they given electrical power rating which tells you how much energy they transfer between stores in a certain time
Power= word done /time so thus 1000=x/3600
And x=360000000 joules
HOURS NOW
What happens to the surroundings of a wire as you heat it up
Also what is light represented by?
Current to wire increases the heat of it, thermal store increase and this energy is wastedi
This always happens In an electrical circuit
2) thermal energy like filament lamp
How do energy get transferred in temp
When a temperature difference happens, a transfer of energy is produced which happens from hot to cold until its equilibrium
What does energy being dissipated mean
IMPORTSNT
Basically once energy is used it transferred to surroundings
Thus
When energy is transferred to the surrounding such that it is not easily accessible again and not in a store that is useful
How to reduce energ dissipated due to friection by and heating by
Lubrication for friction (oil just common sense)
Insulation for heating (like foam)
How do conditions affect ways in which heating is lost in houses
Thickness and thermal conductivity
What is thermal conductivity of a material?
1) thicker wall, lower rate of temp to drop
2) this is TELLS you the rate at which means energy is transferred through a. Wall with 1m2 area, 1m thickness and a temperature difference of 1 °.
The HIGHER THE thermal conductivity, the more energy lost, so greater rate drop of temperature. Therefore lower thermal conductivity Ed are used in house
Efficiency equation
= useful energy/ total energy
Efficiency of 0.5 but 50% efficient
Never fully efficient energy always dissipated
How to draw SANKEY diagram like what to make sure
What does arrow represent
Sanity diagram show where energy goes, width of arrow = AMOUNT IF ENERGY TRANSFERRED
How to increase efficiency
3 ways
Why cant you just say use dissapted energy
- easy
1) insulation
2) lubrications
3) make devices that reduce unwanted energy waste - Develop new technology like LEDs that are just better
More efficient = less power.
You can’t say find something useful with wasted energy because it gets dissapated there is. I thing useful with it as it’s hard to transfer back
When estimating what number do you pick
dismtee of a wheel?
Magnitude ofn10s
remember diameter then need to do circumeference
Remmeber some key speeds
A
Thinking distance, braking and stopping distance
What affects thinks and braking.
What about speed makes it 20mph in areas with house
Thinking distance = time x speed
= reaction time x speed (so like 0.2 but it depends)
- what affects this is your reaction time so sleepy, caffeine , intoxicated, distractions, drugs
- and speed of car
Braking distance = v2 - u2 = 2AD, solve for d
@ affected by mass in car
- brake grip strength
- condition of roads so slippery or what
Because braking distance is very dependent on speed , then built up areas are 20 but not is 30
Stopping = thinking and braking
Reaction time test?
Rule drop test
@) Hand Open, get someone else to drop a ruler. Make sure Finger in line with zero
2) drop ruler, calculate distance
3) find the final velocity using equation of motion
4) Find AVERAGE SPEED, which is final + inital velocity /2,
then do next step
You know s = d/t
And if speed = final velocity and distance = distance dropped, then done
What does 3G mean
If you experience a 3G force, you experience a force that’s 3 x gave its x your mass, or 3 x your weight
Okay so how do you explain why when you multiply speed by 2, the braking distance increases by
4
Also what about relationship between thinking distance and speed
For a car to stop, work must be done by the brakes that is = to all of the energy in the car therefore
0.5 x Mx V2 = FxD
As you can see, if you multiply msss by 2, the braking distance is mutliokied by 2, but if you multiply v byb2, the braking happens by 4
2) since distance = speed x time, multiply speed by 2, distance by 2 they are proportional
Renewable vs non renewable
Non renewable take millions of years to form, renewable fast
Renewable= biofuels, solar, hydro, wind, geothermal
Non renewable, nuclear (diesn’t contributes to global), hyrdocarbins etc
Problem with non renewable affect climate
Problem with renewable expensive ti set up, things like wind turbines ruin the view, and only work when certain conditions met such as solar and windy etc.
Building these require a lot of energy too
Why energy sources change dived time?
Problem of energy supply
like what happen to renwable prices
what governemnt consider when picking renwable
also for renwable
Population expand from 1 billion to 7 billion today so more energy
More devices using fuels used
Non renewable finite and run out , and as basic rule of supply and demand as supply lower deman increase and expense too
Burning fossil fuels produce co2 contribute to climate change , leading to more sea level rising and weather pattern and food disruption
Govemnemt tryha. Increase renewable as a streust but consider price, effect in environment to set them up (energy used), contribution to climate change and how worth it is it compared to the effect it brings
also like geothermal cant bare be used everywhere
Ac / a v uk
Frequency and voltage
230 v with 50 hz = mains
DC comes from batteries and the marinas, AC comes when alternating of happens creating ac, but in the national grid
National grid thing
Transformers
National grid is just grid of wires network from everywhere
higher currents means p= i2r somire power is lost, meaning more fuels needed and energy overall- eneegu transferred to surroundings and wires heat up.
- wasted energy costs more, much cheaper to step up and thus NATIONAL GRIDE EFFICIENT WAY TO TRASNPORT CURRENT
thus step UP used from geberator to grid network so the plwer lost is minimal ans xosn step down to homes, from like 400000 to 230
this is possible using ratios of cables and idea that power= power2
Transformers change the old of wires to reduce the heating effect and therefore MORE EFFICENT way to distribute electricity .
When current flows, then heat of wire increase- inefficient as resistance increase too,
You transfer a power = vI. High powers must be transferred, so either high I or high V. But high I will increase to high temp and is inefficient . Power dissipated = I2 R that’s why.
- so transformer used to step up / down pd to make it effiecnet.
Okay so transformers equations and more understanding
High current used means more power lost, so more power will need to be generated to accumulate- resulting in more fuels being used er. Therefore high voltage used , then low voltage.
Power coil1= power coil 2, where none is lost and temp doesn’t increase
VC= V2C2
Remember when an alternating current is inputted into the primary coil, an alternating magnetic current is formed around it, inducing the iron core with an alternating magnetic field. This carries over to the second coil, and an alternating current is induced in the conductor , base don the coils ratio. More in 1 then 2, step down, more in 2 then 1, step up.
Assume they are almost 100% efficient
Wires
What do each do
What colours
What happens in case something bad, how does earth pattern it
what is double insulate
Earth wire = green and yellow
Live wire= BROWN
Natural wire = blue
Live to neutral and earth = 230
Neutral to earth = 0
1) life carries to voltage alternating between - and positive 230. Neutral completes the circuit, out of the wire so 0v. Earth for safety
If live wire becomes loose inside and touches metal casing the casing would become live and is now dangerous, as body is 9v so pd is huge snd current will be big killing you
Therefore earth wire connected to casing so if that happens, current will go to earth wire, and this resistance is low increasing current because v=is, and this melts the fuse. Breaking the circuit, prevent harm. Earth goes to ground connecting circuit there
If it has non conductive casing like plastic don’t need earth wire, as it has casing and non conductive like plastic it’s double insulated
What is energy and what is it relation to work done
Energy is the ability of an object to do work
When word is done by an object , energy transfers from the object to something
When work is done by the object energy is transferred to the object
How is energy transferred between sources4ways
Mechanically, due to a force acting on it
Electrically with an electric current
Heating with particles where energy go from hot to cold
Heating by radiation , energy transferred by light or sound waves so like sun.
How and where is energy lost in house
What about thermal conductivity of ice
Walls floor roof and window
Low so igloos can be warm
What is convection
Conduction + how to reduce ? So
Radiation
These are the three ways in which energy is transferred thermal wise
Conduction happens in solids
- hot object has higher kinetic
- this vibrates and collided with other particles adjacent, making them vibrate too. Increase their temp and this happens until temp across object is same
Convection happens in gas and liquid
- idea that particles with more kinetic energy move from hotter to cooler region and take kinetic energy with them
- this means cool air takes its place and circulation of convection currents occur heating evenly
- reduce convection stop movement of air, so like clothes traps air pockets , and so energy conducts slowly and you warm .
3) radiation is where energy is transferred via em wave of infra red which don’t need particles
- hot objects emit more infra cold emit less
- cooler object absorb this infra and temp increase
- black absorb more white less
How to reduce nervy loss in house
Loft insulation of FIBREGLASS WOOL
- hot water tank
- double glazing reduces conduction
- thick curtains
- cavity wall insulation- where gaps between wall stops conduction and insulation foam stops convection
THICKER WALLS AND LOWER THERMAL CONDUCTIVITY
- pipes insulate paint white etc
Basic energy equations
also force?
P=e/t
GPE= m X G X H
Elastic?= 1/2 spring constant X extension ^2
Kinetic =1/2 mv^2
force = m xa
whynis thinking distance bigger in highway code
because used exagerated reaction time
now how do engineers reduce firce exerted by crash
what is this force equal to
force = m x a
a revilves time , increase time reduce a and reduce force
the time is collision time. to increase this, you need CRUMPLE ZONES AIR BAGS SEATBELTS TOO, which all deform rsther thsn shatter allowing collison time ti increase.
finally motorway railings also have. the max speed motorway should be 70
