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PARAMOUNT SCHOOL SYSTEM

Subject: Physics – I

Unit 6: Work and Energy

QI. Multiple Choice Questions

1. The unit of work or energy joule (J) is equal to:

A. horsepower B. watt meter C. watt second D. newton second

2. A car, an elephant, and a cricket ball have the same kinetic energies. Which of these will have a greater speed?

A. Car B. Elephant C. Cricket ball D. All have the same speed

3. A ball weighing 50 N is lifted to a height of 5 meters. The potential energy stored in it is:

A. 10 J B. 25 J C. 45 J D. 250 J

4. What is the power utilized when 100 J of work is done in 5 s?

A. 10 W B. 20 W C. 50 W D. 500 W

5. The SI unit of power is:

A. joule B. watt C. horsepower D. erg

6. A 4 kg body is thrown vertically upward from the ground with a velocity of 5 m/s. If friction is neglected, its kinetic energy just before hitting the ground is:

A. 25 J B. 50 J C. 75 J D. 100 J

7. A ball is thrown downward with an initial velocity, its:

A. E_k increases & E_p decreases B. E_kdecreases & E_pincreases

C. Both E_k & E_p increase D. Both E_k & E_p decrease

8. The type of energy derived from heated groundwater is:

A. tidal energy B. geothermal energy

C. hydroelectric energy D. nuclear energy

9. A weight lifter of power 1960 watt lifts a load of mass ‘M’ from the ground to a height of 2 m in 3 seconds. ‘M’ is:

A. 100 kg B. 200 kg C. 300 kg D. 400 kg

10. Which one is a renewable source of energy?

A. Coal B. Natural gas C. Sunlight D. Uranium

11. One unit of horsepower is equivalent to:

A. 756 watt B. 716 watt C. 736 watt D. 746 watt

12. A practical engine cannot have an efficiency equal to or greater than:

A. 0 B. 0.5 C. 0.8 D. 1

13. A heavy and a lighter object have the same momenta. The object with greater kinetic energy is:

A. lighter B. heavy C. same kinetic energy D. either a or b

14. A force is acting on a body but causes no displacement. The work done on the body is:

A. positive B. negative C. zero D. infinite

15. A box is taken to the second floor of a building by doing some work. This work converts to:

A. kinetic energy B. potential energy C. heat energy D. sound energy

SHORT RESPONSE QUESTIONS

QII. Give a short response to the following questions.

1. A car is moving with a constant speed along a straight road. Is there any work done on the car?

No, there is no work done on the car while it is moving with a constant speed along a straight road. Work is defined as the product of force and displacement in the direction of that force. Since the car is not accelerating or experiencing a net force in the direction of motion, the work done is zero.

2. Does the work done in raising a box up in a building depend upon how fast it is raised up? Through which path? To how much height?

As we know that,

W = E_{P. grav}= mgh

The work done in raising a box up a building does not depend on how fast it is raised or the specific path taken, as long as the height is the same. It solely depends on the weight of the box and the height it is lifted.

3. Work done on the body either speeds it up, slows it down. Keeping it mind, explain how much work is done by centripetal force on an orbiting satellite?

Centripetal force does no work on an orbiting satellite. This is because work is done only when a force causes displacement in its own direction. In the case of a satellite, centripetal force acts perpendicular to its motion (directed toward the center of the orbit), so it does not change the satellite’s speed or perform any work, as there is no displacement in the direction of the force.

4. A car has Kinetic energy E_K. By what factor its kinetic energy would change, if its velocity is doubled?

If the car’s velocity is doubled, its kinetic energy would increase by a factor of four.

When velocity become double:

Thus, the kinetic energy would be four times greater.

5. A bullet is fired from gun, bullet penetrates into sand wall and it stops. Where does its kinetic energy used?

The kinetic energy of the bullet is used in overcoming the resistance of the sand, transforming into other forms of energy such as heat and sound. As the bullet penetrates and comes to a stop, its kinetic energy is dissipated mainly as heat due to friction with the sand particles and as a sound.

6. An LED light bulb has efficiency of 80%. Does it violate conservation of energy principle?

No, an LED light bulb with 80% efficiency does not violate the conservation of energy. It simply converts 80% of the electrical energy into light, with the remaining 20% converted into heat.

7. How does using renewable energy sources contribute to reducing environmental impact compared to non-renewable sources?

Using renewable energy sources reduces environmental impact compared to non-renewable sources by minimizing greenhouse gas emissions, promoting sustainability, and decreasing pollution. Renewables like solar and wind generate clean energy without depleting resources, while non-renewable sources lead to environmental degradation and contribute to climate change.

8. Will we eventually rely entirely on renewable energy sources? Why or why not?

Yes, it is likely that we will eventually rely entirely on renewable energy sources. Using renewable energy sources reduces environmental impact compared to non-renewable sources by minimizing greenhouse gas emissions, promoting sustainability, and decreasing pollution. Renewables like solar and wind generate clean energy without depleting resources, while non-renewable sources lead to environmental degradation and contribute to climate change.

9. How can increasing the power of a machine impact its energy consumption?

Increasing the power of a machine typically results in higher energy consumption, as power is the rate at which energy is used. A more powerful machine may perform tasks more quickly or effectively, but this often requires additional energy input.

10. A perpetual engine has an efficiency equal to 1. Why it will not work?

A perpetual engine with an efficiency of 1 means it would produce as much energy as it uses, with no losses. However, in real machines, some energy is always lost as heat and sound. For the engine to operate continuously, it would have to generate energy from nothing, which goes against the law of conservation of energy. As a result, a perpetual engine cannot function because it cannot compensate for these unavoidable energy losses.

LONG RESPONSE QUESTIONS

QIII. Give an extended response to the following

1. Define work and its unit. Describe the conditions for maximum and minimum work.

Work:

Work is said to be done when a force displaces a body in its own direction.

When an object moves distance S in the direction of applied force F, then work done W is given mathematically as:

Quantity:

Work is a scalar quantity.

Unit:

SI unit of work is joule (J). It is defined as

The amount of work is one joule when a force of one newton displaces a body through one metre in the direction of force.

Thus,

Condition for Maximum Work:

Work is maximized when the force is applied in the exact direction of the displacement (),

Condition for Minimum Work:

Work is minimized, or zero, when the force is perpendicular to the displacement (),

2. What is kinetic energy? Derive its expression by using graphical analysis.

Kinetic Energy:

The energy possessed by a body due to its motion is called Kinetic energy.

Example:

A car in motion has kinetic energy.
Water flowing in a river

Derivation:

Consider a constant force ‘F’ is acting on an object of mass ‘m’ and as a result the object moves on a frictionless surface. The kinetic energy of an object will be equal to work done.

The area under force displacement graph is the area of rectangle, thus:

As the speed is increasing its velocity from to therefore the average speed is therefore,

And acceleration can also be written as:

Putting equation 2 and equation 3 in equation 1, we get

3. What is potential energy? What are its different types? Show that gravitational potential energy is equal to the product of mass ‘m’, gravitational field strength ‘g’ and height ‘h’.

Potential Energy:

Potential energy is the energy a body possesses due to its position or configuration in a force field.

Example:

A rock held above the ground has potential energy due to its height.
Water stored in a dam holds gravitational potential energy.

Types:

Gravitational Potential Energy:

The energy stored in an object as a result of its position in a gravitational field.

Example:

A book on a shelf has gravitational potential energy.
A skier at the top of a hill has gravitational potential energy.

Elastic Potential Energy:

The energy stored in an elastic object (like a spring or rubber band) when it is stretched or compressed.

A stretched rubber band holds elastic potential energy.
A compressed spring stores elastic potential energy.

Chemical Potential Energy:

The energy stored in chemical bonds, such as in food, batteries, or fuels.

Food stores chemical potential energy.
Gasoline in a car holds chemical potential energy.

Electrical Potential Energy:

The energy due to the position of charges.

A charged battery has electrical potential energy.
Charged particles in a thundercloud hold electrical potential energy.

Derivation:

Consider an object of mass ‘m’ being lifted vertically by a force ‘F’ to ‘h’ as shown in figure. The work done by the force F is given by equation.

Since the force in this case is equal to its weight

Here the distance moved is the height ‘h’

Putting equation 2 and 3 in equation 1, we get

4. What is meant by energy conversion and energy conservation?

Energy Conversion:

Energy conversion is the process of changing energy from one form to another. This transformation occurs in various systems and devices.

Examples:

Hydroelectric Power:

Potential energy of water stored in a dam is converted into kinetic energy as it flows down, which then turn’s turbines to generate electrical energy.

Electric Bulb:

Electrical energy is converted into light and heat energy when current flows through the filament of a bulb.

Energy Conservation:

The principle of energy conservation states that energy cannot be created or destroyed; it can only be transformed from one form to another, and the total amount of energy in an isolated system remains constant.

Examples:

Falling Object:

A ball dropped from a height converts its gravitational potential energy into kinetic energy as it falls, but the total mechanical energy remains the same if air resistance is negligible.

Pendulum Swing:

As a pendulum swings, its energy alternates between potential energy at the highest points and kinetic energy at the lowest point. The total energy remains constant, assuming no air resistance.

5. Describe how useful energy may be obtained from natural resources.

Useful energy can be obtained from natural resources through the process of energy conversion, where different forms of raw energy are transformed into a usable form, such as electricity, heat, or mechanical power. These resources are categorized into two main types: non-renewable and renewable energy sources.

Non-renewable Energy Resources:

Non-renewable energy sources are those that do not replenish within a human lifetime. They are limited in supply and once consumed, cannot be replaced naturally on a short timescale. These sources are extracted from the Earth and are usually rich in hydrocarbons. They include fossil fuels (coal, oil, and natural gas) and nuclear fuels.

Fossil Fuels:

Fossil fuels are derived from the remains of ancient plants and animals.
The energy is extracted by burning these fuels in the presence of oxygen, producing heat that can be used to generate steam. This steam drives turbines connected to generators, producing electricity.

Example: Burning coal in power plants to generate electricity.

Nuclear Fuel:

Nuclear power plants use uranium or plutonium as fuel. These atoms undergo nuclear fission, where their nuclei split to release a significant amount of heat energy.
The heat generated is used to produce steam, which drives turbines connected to generators for electricity production.

Renewable Energy Resources:

Renewable energy sources are those that are naturally replenished in a short period of time. They capture energy from ongoing natural processes such as sunlight, wind, water flow, and biological processes. These sources are sustainable and do not run out over time, provided they are harnessed efficiently. They include solar energy, wind, hydroelectric power, geothermal energy, biofuels, and wave energy.

Solar Energy:

Solar panels use photovoltaic cells to convert sunlight directly into electricity.
Solar thermal systems use mirrors or lenses to concentrate sunlight, generating heat for producing steam, which can drive turbines.

Wind Energy:

Wind turbines convert the kinetic energy of the wind into mechanical energy, which is then transformed into electrical energy by a generator.

Hydroelectric Power:

Water from rivers or reservoirs is directed to flow over turbines, converting the gravitational potential energy of water into mechanical energy, which is then converted into electricity.

Geothermal Energy:

Geothermal power plants use heat from beneath the Earth’s surface to produce steam. This steam is used to drive turbines and generate electricity.

Biomass Energy:

Biomass includes organic materials like wood, agricultural residues, and animal waste. It can be burned directly for heat or converted into biofuels like ethanol and biodiesel, which can be used for heating and electricity generation.

Wave and Tidal Energy:

Devices like Wave Energy Converters (WECs) capture the kinetic energy of ocean waves, converting it into mechanical or electrical energy.

6. Differentiate between renewable and non-renewable energy sources with examples. Write down advantages and disadvantages of each in reference to their availability and environmental impact.

Non-renewable Energy Resources:

Fossil Fuels:

Fossil fuels are derived from the remains of ancient plants and animals.
The energy is extracted by burning these fuels in the presence of oxygen, producing heat that can be used to generate steam. This steam drives turbines connected to generators, producing electricity.

Example: Burning coal in power plants to generate electricity.

Advantages:

Widely available and cost-effective to extract.
High energy density, providing significant energy output from a small amount of fuel.
Easily stored and transported via pipelines or shipping.

Disadvantages:

Limited reserves, making them non-renewable and likely to run out.
Burning fossil fuels emits carbon dioxide (CO₂) and other harmful substances, contributing to air pollution and climate change.
Extraction and combustion can lead to environmental damage, including water contamination and land degradation.

Nuclear Fuel:

Nuclear power plants use uranium or plutonium as fuel. These atoms undergo nuclear fission, where their nuclei split to release a significant amount of heat energy.
The heat generated is used to produce steam, which drives turbines connected to generators for electricity production.

Advantages:

Produces a lot of energy from a small amount of fuel.
Does not release greenhouse gases when generating electricity.
Can be built in different places without damaging the environment.

Disadvantages:

Creates highly dangerous radioactive waste that can harm people’s health.
Building and running nuclear power plants needs advanced technology and skilled workers, which many areas might not have.
Accidents can cause serious damage to the environment and people’s health.

Renewable Energy Resources:

Solar Energy:

Solar panels use photovoltaic cells to convert sunlight directly into electricity.
Solar thermal systems use mirrors or lenses to concentrate sunlight, generating heat for producing steam, which can drive turbines.

Advantages:

Abundant and eco-friendly energy source that does not emit greenhouse gases during operation.
Solar cells are made from silicon, which is the second most abundant element on Earth.
Helps reduce dependence on fossil fuels and lowers greenhouse gas emissions.

Disadvantages:

Requires significant land area to generate substantial electricity, especially for large-scale solar farms.
Solar energy is intermittent, dependent on weather conditions and daylight, necessitating storage or backup systems.
Initial installation costs for solar panels can be high.

Wind Energy:

Wind turbines convert the kinetic energy of the wind into mechanical energy, which is then transformed into electrical energy by a generator.

Advantages:

Clean and renewable energy source with no greenhouse gas emissions.
Wind turbines can be installed on land and offshore, providing flexibility in location.
Low operating costs after the initial investment.

Disadvantages:

Wind turbines are expensive to develop and install, requiring significant upfront capital.
Viable only in areas with strong and consistent winds.
Wind farms require large open spaces and can impact local wildlife, particularly birds.

Hydroelectric Power:

Water from rivers or reservoirs is directed to flow over turbines, converting the gravitational potential energy of water into mechanical energy, which is then converted into electricity.

Advantages:

Once established, hydroelectric plants have minimal environmental impact.
Provides a consistent and reliable source of energy due to the water cycle.
Renewable and efficient, with low operating costs after initial setup.

Disadvantages:

Construction of large dams can flood vast areas, destroying ecosystems and displacing communities.
Requires fast-flowing water, limiting its viability to specific geographical locations.
May alter the natural flow of rivers, affecting aquatic life and local ecosystems.

Geothermal Energy:

Geothermal power plants use heat from beneath the Earth’s surface to produce steam. This steam is used to drive turbines and generate electricity.

Advantages:

Provides a reliable energy source unaffected by weather conditions.
Does not produce greenhouse gas emissions during operation.
Abundant and can potentially provide energy for thousands of years.

Disadvantages:

Economically viable only in certain regions with accessible geothermal reservoirs.
May release toxic gases like hydrogen sulfide, which can be harmful in high concentrations.
Drilling for geothermal energy can cause minor earthquakes.

Biomass Energy:

Advantages:

Uses organic materials like wood, agricultural waste, and livestock manure, making it a sustainable resource.
Biogas produced from biomass is CO₂ neutral, as it recycles carbon dioxide already present in the atmosphere.
Helps in waste management by converting food waste and agricultural residues into energy.

Disadvantages:

Requires collaboration between farmers and industrial sites for effective implementation, which can be challenging.
The collection, processing, and transportation of biomass can be resource-intensive.
Large-scale use of biomass may lead to deforestation and habitat destruction if not managed sustainably.

Wave and Tidal Energy:

Devices like Wave Energy Converters (WECs) capture the kinetic energy of ocean waves, converting it into mechanical or electrical energy.

Advantages:

Captures kinetic energy from ocean waves, providing a continuous and renewable energy source.
Wave energy converters (WECs) can be deployed in coastal areas, offering an additional source of clean energy.
Can reduce dependence on fossil fuels by diversifying energy sources.

Disadvantages:

High cost of development and maintenance due to harsh marine environments.
Requires specific coastal locations with strong wave activity for optimal performance.
Potential impact on marine ecosystems and navigation.

7. Describe the processes by which energy is converted from one form to another with reference to fossil fuel energy, hydroelectric generation, solar energy, nuclear energy, geothermal energy, wind energy and biomass energy.

Energy Conversion:

Energy can be converted from one form to another through different processes, depending on the energy source.

Energy Conversion Processes for Different Energy Sources:

Fossil Fuel Energy:

Fossil fuels like coal, oil, and natural gas are burned in power plants. This combustion process converts the chemical energy stored in the fuel into thermal (heat) energy. The heat then boils water to produce steam, which drives turbines connected to generators, converting thermal energy into mechanical energy, and finally into electrical energy.

Hydroelectric Generation:

Hydroelectric power plants use the potential energy of stored water in a dam. When the water is released, it flows through turbines, converting its potential energy into mechanical energy. These turbines then drive generators, converting mechanical energy into electrical energy.

Solar Energy:

Solar panels (photovoltaic cells) convert sunlight directly into electrical energy. The solar cells capture the light energy from the sun and convert it into electrical energy through the photovoltaic effect.

Nuclear Energy:

In nuclear power plants, the nucleus of atoms like uranium undergoes a process called nuclear fission, releasing a large amount of thermal energy. This heat is used to produce steam, which drives turbines, converting thermal energy into mechanical energy, and then into electrical energy.

Geothermal Energy:

Geothermal power plants use the natural heat from beneath the Earth’s surface. Water or steam from underground reservoirs is brought to the surface, where it drives turbines connected to generators. This process converts the thermal energy from the Earth’s heat into mechanical energy, and finally into electrical energy.

Wind Energy:

Wind turbines capture the kinetic energy of moving air. The wind turns the blades of the turbine, which spins a rotor connected to a generator, converting the kinetic energy of wind into mechanical energy and then into electrical energy.

Biomass Energy:

Biomass, like plant materials and animal waste, is burned or decomposed to release chemical energy. This energy is then converted into heat, which can be used to produce steam for driving turbines. The turbines generate mechanical energy, which is then converted into electrical energy.

8. Describe the process of electricity generation by drawing a block diagram of the process from fossil fuel input to electricity output.

Electricity generation from fossil fuels involves a series of energy conversions, starting with the chemical energy stored in fuels like coal, oil, or natural gas. This process transforms the chemical energy into thermal, mechanical, and finally electrical energy through several stages.

Fossil Fuel Input: Fossil fuels like coal, oil, or natural gas are burned in a furnace, releasing stored chemical energy.

Boiler: The heat from burning fossil fuels is used to convert water into steam in a boiler.

Turbine: The high-pressure steam is directed onto turbine blades, making the turbine spin. This converts thermal energy into mechanical energy.

Generator: The spinning turbine is connected to a generator, which converts mechanical energy into electrical energy.

Electricity Output: The electrical energy is then transmitted through power lines for use in homes, industries, and businesses.

9. Define power. What is the relation of its SI unit with horse power?

Power:

Power is defined as the rate at which work is done or the rate at which energy is converted from one form to another. It measures how quickly work is completed over time.

Formula:

Quantity:

Power is a scalar quantity.

Unit:

The SI unit of power is the watt (W). One watt is defined as one joule of work done per second.

Relation of Watt to Horsepower:

For practical purposes, especially in engines and motors, a larger unit of power known as horsepower (hp) is often used. The relationship between horsepower and watts is:

10. What is efficiency? Why is it important for cars or electronic devices to be designed with high efficiency? Why efficiency of machines can never be unity or 100%?

Efficiency:

Efficiency refers to how well a machine or device converts input energy into useful output energy or work.

Efficiency ‘η’ is the ratio of useful work output ” to the total work input ”.

Efficiency in terms of energy, where” is energy output and” is total energy input.

Efficiency has no units because it is a ratio of the same quantities, so the units cancel out.

Efficiency given in percentage as follows:

Importance of High Efficiency:

It is important for cars, electronic devices, and machines to be designed with high efficiency because:

Optimal Use of Resources: Efficient machines use less energy to perform the same task, minimizing resource wastage.
Cost Savings: High-efficiency devices consume less power, leading to lower energy bills for consumers.
Environmental Impact: By reducing energy wastage, high-efficiency devices help decrease carbon emissions and reduce the environmental footprint.

Efficiency Can Never Be 100%:

Machines can never achieve 100% efficiency due to energy losses, primarily in the form of heat, caused by factors like friction. These losses occur in nearly all energy conversion processes, such as:

Friction: In machines, friction between moving parts converts some energy into heat, which is not useful for the machine’s purpose.
Other Losses: Other forms of energy dissipation, like vibrations and sound, also contribute to less-than-perfect efficiency.

For example, an incandescent bulb only converts about 5% of electrical energy into light; the rest is wasted as heat.

11. Explain by drawing energy flow diagrams through steady state systems such as Filament lamp, a power station, a vehicle traveling at a constant speed on a level road.

Energy flow diagrams help visualize how energy is transferred and converted in various systems operating in a steady state.

Filament Lamp:

The filament lamp converts electrical energy into light and heat. The energy flow diagram for a filament lamp is as follows:

Electrical Energy: The input energy supplied to the lamp.
Light Energy: A small portion of electrical energy is converted into visible light.
Heat Energy: A significant portion is wasted as heat due to the resistance of the filament.

Power Station (Hydroelectric Power Plant):

A hydroelectric power station converts the potential energy of stored water into electrical energy, with several intermediate energy transformations. The energy flow diagram for a hydroelectric power plant is:

Gravitational Potential Energy: Water stored in a high dam has potential energy due to its height.
Kinetic Energy: Water is released from the dam, flowing through pipes (penstocks), converting potential energy into kinetic energy.
Mechanical Energy: The fast-flowing water spins the blades of a turbine, converting kinetic energy into mechanical energy.
Electrical Energy: The rotating turbine drives a generator, which converts mechanical energy into electrical energy, which is the output.

Vehicle Traveling at a Constant Speed on a Level Road:

A vehicle moving at a constant speed converts chemical energy from fuel into kinetic energy, with some losses as heat and sound. The energy flow diagram for a vehicle is:

Chemical Energy: Fuel is combusted in the engine.
Thermal Energy: Combustion generates heat to power the engine.
Kinetic Energy: The vehicle’s movement at a constant speed.
Heat Energy: Some energy is lost as heat from the engine and exhaust.
Sound Energy: Additional energy is lost as sound due to the engine and tires.