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We are learning about:
TOPICS
1.4 Work and energy
1.4.1 Work and Power.
1.4.2 Energy.
1.4.3 Energy Conservation.
1.4.4 Impact of Power Generation on the Environment.
Competencies
Competencies
Establish relationships among energy, work, and power to solve related numerical problems in physics.
Describe the effective methods of energy consumption based on physics concepts to save the energy for energy security in future.
Argues the impact of various methods of power generation to make evidence-based decisions to generate power which has minimum negative impact on environment, society, economy and culture in the global and local context.
Scope
Scope
work done, work done against gravity, power.
potential energy, kinetic energy, law of conservation of energy.
efficient ways to use energy, impacts of power generation on environment.
hydroelectricity, solar energy, bioenergy, wind energy, nuclear energy and its impact on environment.
Objectives
Objectives
Examine the concept of work done to calculate power and efficiency of different machines.
Describe transformation of energy between potential and kinetic energy to demonstrate the energy conservation using simulations.
Develop a prototype to demonstrate the concept of energy transformation.
Analyse the change in use of energy due to changing behavior of people and new electrical appliances.
Design an energy efficient building or house integrating efficient ways to use energy (conceptual or physical).
Discuss on various methods of power generation to provide evidence-based ideas to generate power with minimum negative impact on the environment.
Video Lessons
NOTES
NOTES
Work:
Work is defined as the product of the force and the distance covered by the body in the direction of force. Mathematically,
i) When the displacement is in the direction of force.
W= F x d
i) When the displacement and direction of force is different and forms an angle θ
W= F.d cosθ
W=F x d x (adjacent side/hypotenuse)
The S.I unit of the work is joule.
If force is in newton (N) and the displacement in metre (m), then the work done is measured as
W= F.d= newton. metre = Nm or joule (J)
Similarly, if force is in dyne and the displacement is in centimetre (cm), then the work done is measured as
W= F.d= dyne. centimetre = erg
1 J = 107 erg
1 joule work is defined as the amount of work done to cover a distance of 1m in the direction of force when the force of 1N is applied on the body.
It is a scalar quantity.
Work is done against two forces
Work against the gravity.
Work against friction.
Work done against gravity:
When a body is raised to a certain height, work is being done against gravity. From Newton’s law of motion we know that F = ma and here a = g (acceleration due to gravity). Therefore,
F = m g
Work done against gravity,
W = force x displacement
W= m g h
Power:
What is Power?
The rate of doing work is called power.
Power is the measure of work being done in certain interval of time.
Power= (work done)/(time taken)
P=W/t
If the work done is measured in joule and time in second then the unit of power is given by
P = J/s = watt (W).
The power of a machine is the rate at which work is done by it.
Power of a machine = Force (F) Ă— velocity (v)
Efficiency of a machine is the ratio of output work to input work.
Efficiency (η) = (output work)/(input work)
Efficiency (η) = (Energy output)/(Energy input)
Efficiency of a machine is always less than 100 %. A machine having 100 % efficiency is called ideal machine.
Energy
What is Energy?
The capacity for doing work is called Energy. It may exist in kinetic, potential, thermal, chemical, electrical, nuclear, or various other forms. The SI unit of energy is the same as that of work.
The S.I unit of energy is joule.
Potential Energy:
The energy possessed by a body due to its position or state is called potential energy.
Potential energy (P.E.) = m g h
Kinetic energy:
Energy possessed by a body due to its motion is called kinetic energy.
K.E= 1/2 x mv2
m
Law of Conservation of Energy
Energy can neither be created nor be destroyed however, it can be changed from one to another form. The total energy in an isolated system remains the same or energy is conserved over time.
As per the law, energy cannot be created and neither can be destructed but conveniently can be changed to different forms. Chemical energy can be converted into heat energy which could further be used to generate potential energy in a different body. As energy is changed from one to another form, net gain/loss remains the same. Therefore, energy is said to be conserved wherein the total amount remains the same.
Energy Conservation
We need to use energy efficiently and economically for the sustainability of the energy.
The energy efficiency and renewable energy are the twin pillars of sustainable energy policy. Both the strategies are required to slow down the growth in the demands of fossil fuels.
Advantages of energy efficiency and its sustainability.
It reduces the expenditure of the consumers.
It reduces carbon dioxide emission and helps to protect the environment.
It also reduces mining and drilling. So it slows down the rate at which domestic energy resources are depleted.
reduce the level of energy imports from foreign countries.
Efficient ways to use energy
Using energy-efficient appliances
Eg. - LED bulbs are more durable and energy-efficient compared to incandescent and CFL bulbs.
- conventional ovens take hours for baking while the latest microwave ovens can bake within minutes.
Insulation
-use electrical energy which is totally environment-friendly compared to wood and kerosene.
-insulate our homes from cold or heat.
Technology
-automatically turning laptops, PC, etc. off or putting them into low-energy mode when not in use.
-reducing the weight of the vehicle and improved aerodynamics design.
- buildings to be equipped with sensors that monitors and controls all the appliances.
- Advanced architectural designs of buildings also reduce energy use.
Saving and consumption of energy is crucial and the technology makes it all possible.
Potential energy (PE)
1. The energy possessed by a body by virtue of its specific position or changed configuration is called potential energy.
2.Two forms of potential energy are gravitational potential energy and elastic potential energy.
3.Example: A wound up watch spring has potential energy.
4. P.E = mgh
Kinetic energy (KE)
1.The energy possessed by a body by virtue of its state of motion is called the kinetic energy.
2. Forms of kinetic energy are translational, rotational and vibrational kinetic energy.
3. For example: a moving car has kinetic energy.
4. K.E= 1/2 x mv2
Impacts of power generation on environment.
1. Air Pollution
2. Water Pollution
3. Land Degradation
​4. Noise Pollution
Sources of energy and their Impact on Environment:
Renewable energy sources are the sources which can be replenished within a short period of time.
I. Hydroelectricity:
Hydropower generators produce clean electricity, but hydropower does affect the environment.
The most evident advantages of hydropower generation are:
Clean, efficient, and reliable form of energy.
Does not emit any direct pollutants or greenhouse gases.
While the initial cost is high, they are very inexpensive to operate.
IMPACT:
occupy large area which have many environmental impacts.
The natural timing of flow of water, nutrient and sediments may be disturbed.
affect the agriculture, homes, and natural habitats in the dam area
obstruct the migration of fishes and affect their population.
II. Solar energy:
The energy harnessed from the sun is called solar energy.
Advantages of Solar energy generation are:
We cannot run out of solar energy, unlike some of the other sources of energy.
reduction of the emissions of the greenhouse gases
reclamation of degraded land
absence of waste products during their operation.
IMPACT:
Large-scale solar power installations cover very large area.
lead to destruction of ecosystem and habitats
limit movement of animals
effects on the growth of vegetation due to blockage of sunlight.
convert a large amount of radiation into heat, which leads to global or local climate change.
III. Bioenergy:
Bioenergy refers to electricity and gas that is generated from organic matter, known as biomass. This can be anything from plants and timber to agricultural and food waste – and even sewage.
The use of biogas helps in reducing the emission of harmful methane gas into the atmosphere.
Bhutan has almost 65-70% of its area covered by forest. So biomass is one of the largest sources of energy.
Biomass in the form of manure, industrial food waste, agricultural residues and sewage can also be anaerobically digested to produce biogas.
Much of the CO2 emitted from its combustion, is equivalent to CO2 absorbed by the plant during its life cycle to produce biomass.
IV. Wind energy:
The energy harnessed from wind is called wind energy.
Kinetic energy from the wind is converted to mechanical energy in a gearbox.
Most of the land used for wind turbines may still be used for grazing or cultivation.
IMPACT:
Wind turbine interference with radar and telecommunication facilities
noise produced by the rotor blades
deaths of birds and bats that fly into the rotors.
V. Nuclear energy:
Nuclear energy is the energy in the nucleus, or core, of an atom. Nuclear energy can be used to create electricity, but it must first be released from the atom. In the process of nuclear fission, atoms are split to release that energy.
Uranium is the fuel most widely used to produce nuclear energy.
Produces no polluting gases.
Does not contribute to global warming.
IMPACT:
Waste is radioactive and safe disposal is very difficult and expensive.
Large-scale accidents can be catastrophic.
Local thermal pollution from wastewater affects marine life.
Exercise questions
Exercise questions
I. Fill in the blanks. (Questions will be added as per the lessons carried out in class)
Work is a ______________ quantity.
We need ___________ to perform all kinds of work.
____________ can make something move, change the shape of a body, change the position of a body, and the direction.
Work is said to be done only when a body is ___________ under the application of a force.
The amount of work done by a force on a body depends on __________________, ___________________ and __________________________.
Power is defined as the rate of doing _____________.
Power is a ________quantity.
Efficiency of a machine is the _____________ of output work to input work.
II. State whether following statements are TRUE or FALSE. Rewrite the incorrect statements correctly. (Questions will be added as per the lessons carried out in class)
The rate of doing work varies inversely with the velocity of the body.
Efficiency of machine is always less than 100%.
Work is done when energy is used in doing any activity.
When the displacement is perpendicular to the direction of the force, work done is positive.
Work done against gravity is equal to potential energy.
III. Answer the following questions. (Questions will be added as per the lessons carried out in class)
Q1. Define work. When is work said to be done by a force?
Q2. How is the work done by a force measured when-
(i) force is in direction of displacement,
(ii) force is at an angle to the direction of displacement?
Q3. A force F acts on a body and displaces it by a distance S in a direction at an angle with the direction of force.
(a) Write the expression for the work done by the force.
(b) What should be the angle between force and displacement so that the work done is-
(i) zero,
(ii) maximum?
Q5. A body is acted upon by a force. State two conditions when the work done is zero.
Q6. State the condition when the work done by a force is (i) positive, (ii) negative. Explain with the help of examples.
Q7.
Numerical Problems:
Q1. A weight lifter lifted a load of 200 kg to a height of 2.5 m in 5 s. Calculate: (i) the work done, and (ii) the power developed by him. Take g =10N/kg-1.
Q2. A boy of mass 40kg climbs up the stairs and reaches the roof at a height 8m in 5 s. Calculate: (take g= 10ms-2)
(i) The force of gravity acting on the boy,
(ii)The work done by him against gravity,
(iii)The power spent by boy.
Q3. A machine raises a load of 750N through a height of 16m in 5 s. Calculate:
i. the energy spent by the machine.
ii. the power of the machine if it is 100% efficient.
Q4. A vessel containing 50 kg of water is placed at a height 15m above the ground. Assuming the gravitational potential energy at ground to be zero, what will be the gravitational potential energy of water in the vessel? (g = 10ms-2).
Other notes for reference
Other notes for reference
What is Work, Energy and Power?
What is Work, Energy and Power?
(QUICK REFERENCE NOTES)
Work
Work
Definition: Work is said to be done when a force applied to an object moves that object.
Formula: We can calculate work by multiplying the force by the movement of the object.
W = F Ă— d
Unit: The SI unit of work is the joule (J)
Energy
Energy
Definition: In physics, we can define energy as the capacity to do work.
Formula: For the potential energy the formula is
P.E. = mgh
Unit: The SI unit of energy is joules (J), which is named in honour of James Prescott Joule.
Power
Power
Definition: Power can be defined as the rate at which work is done i.e. energy converted.
Formula: The formula for power is
P = W/t
Unit: The unit of power is watt (W).
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