Knowt
Note
Studied by 8 people
Energy
7.1
Work:
An object’s motion changes due to force and how long the force acts.
“How long” can correspond to time or distance.
Impulse → force x time
Work → force x distance
Work is the effort exerted on something that will change its energy.
Work being done involves:
The application of a force
The movement of something caused by that force
Simplest case: Force is constant, motion is in a straight line in the direction of the force.
Work = Force x Distance
Eg:
Consider lifting a load of gravel up a story.
Work is being done against the force of gravity.
Consider lifting two loads of gravel.
Twice the work is being done.
Consider lifting the load of gravel up two stories.
Twice the work is being done.
Heavier the load → More force required, more the work done
Longer the distance over which the load travels → More the work done
Work always has to involve both force and distance.
Consider a weightlifter holding a barbell above her head.
If she holds it for a long time, she gets tired.
But, as long as she isn’t moving the barbell, no work is being done (no distance covered).
If we considered the act of her lifting the barbell off the floor, work would be done then, because a distance is being covered.
There are two categories of work:
Work being done against another force.
Eg:
Archer stretching a bowstring → work done against elastic forces of bow
Doing pushups → work done against your own weight
Work being done to change the speed of an object.
Eg:
Speeding up an automobile
Club hitting a stationary golf ball
In both these categories, there is a transfer of energy.
Unit of measurement of work: Joule (J)
1 J = 1 N x 1 m
J → Joule, N → Newton (unit of force), m → meter (unit of distance)
One Joule of work is done when a force of one Newton is exerted over a distance of one meter.
Other units:
kilojoules (kJ) → thousands of joules
megajoules (MJ) → millions of joules
Power:
Power talks about how long the work is being done.
Power = Work done/Time interval
Higher power → work is being done faster
“Twice the power” means:
Engine does twice the work in the same time interval, or
Engine does the same work in half the time interval
Unit of power: Joule per second (J/s)
J/s = Watt
One Watt is the power expended when 1 Joule of work is done in 1 second.
Other units:
kilowatt (kW) → a thousand watts
megawatt (MW) → a million watts
One horsepower = 746 Watts
Mechanical Energy:
This is the energy an object has due to its position and movement.
It can be in the form of:
Kinetic energy
Potential energy
Both
Eg: When you wind a spring, the spring acquires enough energy to run a clock.
7.2
Potential Energy:
This is the energy an object has due to its position.
The energy is called potential energy because the energy is stored in the object and gives it the potential to do work.
Eg: A stretched rubber band being part of a slingshot.
The rubber band stores energy when it gets stretched out.
Chemical energy in fuels = potential energy at a submicroscopic level.
Energy becomes available when positions of electric charges change within and between molecules that get altered.
Gravitational Potential Energy → Potential energy an elevated object has
It is equal to the work done against gravity while lifting the object.
Gravitational Potential Energy = weight x height
PE = mgh
m → mass of object
g → acceleration due to gravity
h → height to which the object is lifted
Potential energy is significant only when there is change.
When work is done
When there is a transformation of energy from one form to another
7.3
Kinetic Energy:
This is the energy an object has as a result of its motion.
Kinetic energy depends on the mass and speed of the object.
Kinetic energy = (mass x speed^2)/2
Kinetic energy → work done to bring an object from rest to speed
Or,
Kinetic energy → work done by the object while its being brought to rest
KE = Fd
KE → kinetic energy
F → net force required to change object’s state of motion
d → distance over which object moves
Fd = (mv^2)/2
m → mass of the object
v → speed of the object
7.4
Work-Energy Theorem:
Work = change in kinetic energy
Car speeding up → gain in KE due to work done on it
Car slowing down → work done to reduce KE
Work = ΔKE
Here, work is net work (work based on net force)
Net work → work based on net force
Eg: pushing an object on which friction also acts. Change in KE = push - friction
Here, only some of the work done changes the object’s kinetic energy.
If friction = push, net force = 0, so work done = 0.
Work-Energy theorem also applies to objects with decreasing speed.
Eg: applying the breaks on a skidding car → work done by road on the car = frictional force x distance over which frictional force acts
The road supplies the same maximum friction to the car however quickly or slowly the car moves.
For two cars, if one moves at twice the speed of the other, the faster one skids four times as far before stopping.
The frictional force on both the cars is the same.
Kinetic energy depends on speed squared.
This is the logic behind antilock brakes.
For a braked automobile:
When the tires don’t skid → kinetic energy is converted to heat energy by the brake drums.
When the tires skid → brakes don’t get heated, treads and road get heated.
Vehicles can also be stopped by shifting them to low gear and letting the engine brake.
Hybrid cars use electric generators to convert KE to electrical energy that gets stored in the battery.
When work done by an outside force, work = ΔE
E = energy (all kinds)
Work is not a form of energy.
Work is a way of transferring energy from one place to another, and from one form to another.
7.5
Conservation of Energy:
There are multiple different types of energy.
All these types can be transformed into every other type.
Energy being transformed into other forms or energy being transferred from one location to another is how processes happen in nature.
Consider a pile driver.
Work is done to raise the ram.
The ram acquires potential energy.
When the ram is released, this becomes kinetic energy.
The energy gets transferred to the piling below.
Initial potential energy of the ram = distance piling penetrates ground x average force of impact
This applies in an ideal scenario.
There is no gain or loss of energy during this process.
Law of conservation of energy:
Energy cannot be created or destroyed, it can only be transformed from one form to another. The total amount of energy never changes.
In any system, the energy is never created or destroyed.
The energy score stays the same.
Matter is made of atoms. These atoms are bundles of energy.
When the atoms’ nuclei are rearranged, energy is released.
The Sun shines because this nuclear energy gets converted into radiant energy.
In the interior of the sun, there is compression due to gravity and high temperatures.
This causes fusion of hydrogen nuclei to form helium nuclei.
This is thermonuclear fusion.
Thermonuclear fusion releases a large amount of energy. Some of this reaches the Earth.
The energy that reaches the Earth follows different paths.
Some falls on plants.
Some is stored as coal.
Some is stored in oil.
Some goes into evaporation of ocean water.
Some of this returns as rain that’s trapped in a dam.
Water behind a dam has potential energy since it’s in an elevated position. This energy is converted to electric energy in a generating plant. This energy then goes to homes.
Recycled Energy:
This is energy that is reused.
It would be wasted otherwise.
A fossil fuel fired plant wastes about 2/3rds of the energy inputted.
This energy is dissipated as thermal energy.
Only 1/3rd of the energy is used.
Edison’s power plants in the 1880s utilized energy better than today’s plants.
He used the cast off heat to warm nearby homes and factories.
Most homes in Copenhagen, Denmark are warmed this way.
Half the energy is Denmark is recycled energy.
Less than 10% of energy in the US is recycled.
Reason → Most power plants are built far away from residential areas.
7.6
Machines:
A machine is a device that multiplies or changes the direction of force.
Principle behind machines → principle of conservation of energy.
A lever is the simplest machine we have.
When we apply force on one end of the lever, it does work on the load at the other end.
Change in direction of force → we push down, load is lifted up.
When work done by friction and unbalanced weight of the level are ignored:
Work input = Work output.
Work = Force x distance
(Force x distance) for input = (Force into distance) for output
The point on which a lever rotates is the fulcrum.
When the fulcrum is close to the load, a small input force produces a large output force.
Reason → force is exerted over a large distance, load is moved over a short distance.
A lever can be a force multiplier.
Archimedes understood the principle of the lever in the 3rd century BC.
No machine can multiply work or energy.
A pulley is another simple machine.
Its mechanism is very similar to a lever.
A pulley can be used to change the direction of force or to double the force output.
When it’s used to double the force output, force is increased and distance moved is decreased.
In any machine, forces can change while work input and output don’t.
A block and tackle is a system of pulleys that are capable of multiplying force more than a single pulley.
Consider a man pulling a rope that goes through a pulley. At the other end of the rope is the load.
Energy is transferred from the man to the load.
Machines multiply force at the expense of distance and distance at the expense of force.
No machine can put out more energy than what is put into it.
Machines can only transfer energy and transform it from one form to another.
7.7
Efficiency:
Ideal machine → 100% of the work input is converted to work output.
This is 100% efficiency.
No machine in reality operates at 100% efficiency.
During energy transformations, some energy is always converted to thermal energy.
This is dissipated to the surroundings.
When a lever rocks on its fulcrum, some energy is converted to thermal energy.
Consider a lever with 98% efficiency.
If 100J is the work input, 98J will be the work output.
2J of energy will be converted to thermal energy.
In a pulley system, a larger amount of input energy is converted to thermal energy.
Some input energy also gets wasted by converting into potential energy of the pulley system when it is raised.
The lower the efficiency of a machine, the greater the amount of energy converted to thermal energy.
Inefficiency exists whenever energy transformations happen.
Efficiency = useful energy output / total energy input
An automobile is a machine that converts the chemical energy of fuel into mechanical energy.
When the fuel burns, bonds between petroleum molecules break.
Carbon atoms in the fuel combine with oxygen atoms of the air and form CO2.
Hydrogen atoms combine with oxygen atoms and form water.
Energy is released.
Much of the energy is transformed into thermal energy.
Some of this is used to warm passengers in the winter.
Some goes out in the hot exhaust gases.
Some is dissipated to the air through the cooling system or directly from hot engine parts.
In all energy transformations, there is a dilution of useful energy.
In every transformation, thermal energy is released.
Eventually, all that’s left will be thermal energy at ordinary temperature.
Thermal energy is only useful when it can be transformed to lower temperatures.
Once it reaches the lowest practical temperature, it is useless.
7.8
Sources of Energy:
The Sun is the source of practically all our energy.
Sunlight evaporates water.
This comes back down as rain.
Rainwater flows into rivers and reservoirs.
It is used to run turbines.
It returns to the sea. The cycle continues from there.
The Sun is not a source of nuclear power.
Energy from coal, petroleum, wood, natural gas is originally from the Sun.
Reason → These fuels are created by photosynthesis.
Photosynthesis → process by which plants trap solar energy and store it.
A square mile of sunlight at midday can produce one gigawatt of electric power.
Photovoltaic Cells:
These are used for building materials, roofing, tiles, and transparent windows.
They were initially crystal wafers that were produced the same way semiconductors for computers are.
Light, flexible “power sheets” are produced now.
They are made my machines similar to printing presses.
They can be mounted almost anywhere without sturdy surfaces.
Volkswagen Chattanooga Solar Plant, Tennessee → contributes power for the production of Volkswagen automobiles.
Boilers:
Mirrors reflect sunlight into water-filled boilers on towers.
When sunlight is concentrated by thousands of mirrors, water is heated to upto four times the boiling point.
It then becomes superhot steam that drives electricity generating turbines.
Sometimes, parabolic mirrors are used.
Mostly, inexpensive, easy to use, flat mirrors are used.
The mirrors are kept focused at optimal angles by computerized tracking.
The turbines are run by natural gas when the Sun isn’t shining.
Solar hybridization → combining solar power with fossil fuel power plants to increase their efficiencies.
Wind Energy:
Wind comes from unequal warming of the Earth’s surface.
This is a form of solar power.
Wind energy is used to turn generator turbines.
Specially equipped windmills are used.
The turbines are placed where the wind blows steady and strong.
The objections of nearby residents are taken into consideration.
When the water isn’t too deep, the turbines are anchored to the ocean floor.
When the water is too deep, floating platforms are used.
There’s no carbon footprint when power is produced by wind.
Hydrogen:
It is the least polluting of all fuels.
In the US, most hydrogen comes from natural gas.
High temperatures and pressures separate hydrogen from hydrocarbon molecules.
Disadvantage → production of carbon dioxide (greenhouse gas)
Simpler, cleaner method → electrolysis.
Water is electrically split into its constituents.
Method:
Two platinum wires are connected to the terminals of a battery. They shouldn’t touch each other.
The wires are placed in a glass of water.
An electrolyte like salt dissolved in water is present for conductivity.
Bubbles of hydrogen form on one wire. Bubbles of oxygen form on the other.
Fuel cells are similar, but they run in reverse.
Hydrogen and oxygen gas are compressed at the electrodes.
Electric current and water are produced.
The International Space Station uses fuel cells to meet electrical needs while producing drinking water for astronauts.
China already has a hydrogen economy.
Railroad trains are powered by fuel cells.
Hydrogen for fuel cells is produced by conventional means or with solar cells.
Solar energy can extract hydrogen from water.
Hydrogen is not a source of energy.
It is a means of storing and transporting energy.
Energy from the Ocean:
Energy from the ocean waves is tapped into.
Generators on the ocean floor are turned by bubbling pontoons on the surface.
Ocean tides are also a clean source of energy.
The rise and fall of ocean tides turns electric power producing turbines in plants positioned across an inlet or estuary.
The River of Rance in France has been producing electric power this way for over 40 years.
This energy is not nuclear or solar.
This energy is due to the rotational energy of the Earth.
Nuclear Energy:
Energy from nuclear fuels is the most concentrated source of usable energy.
Nuclear fuels → uranium, plutonium, eventually deuterium
Nuclear reactions produce a million times more energy than chemical and food reactions for the same weight of fuel.
The Earth’s interior is kept hot because of nuclear power.
By-product of nuclear power in Earth’s interior → geothermal energy.
Geothermal Energy:
It is contained in underground reserves of hot water and hot rock.
Geothermal energy close to the surface is present in areas of volcanic activity.
Here, heated water is tapped to provide steam for powering electric generators.
Places → Iceland, New Zealand, Japan, Hawaii
Using dry-rock geothermal power:
Water is pumped into hot fractured rock far below the surface.
When the water turns to steam, it is piped to a turbine at the surface.
After the turbine is turned, it is pumped back into the ground to be reused.
The demand for energy increases with an increase in the world’s population.
Junk Science:
Energy has contributed to “junk science” more than any other scientific concept.
Many practitioners have promised machines that give out more energy than what is inputted.
This doesn’t align with any scientific law.
Yet, many gullible speculators invest in these schemes.