3B4 Energy Flashcards
Describe the forms of mechanical energy, and explain how energy is conserved, transferred, or used to do work.
Define:
Energy
The capacity to do work or produce change.
Energy exists in many forms, including mechanical, thermal, electrical, and chemical.
Define:
Work
The transfer of energy through the application of force over a distance.
Work is calculated as W=F⋅d⋅cosθ, where θ is the angle between force and displacement.
Fill in the blank:
The SI unit of energy is the ______.
Joule (J)
One joule is equal to the energy transferred when a force of 1 newton is applied over a distance of 1 meter.
True or False:
Energy can be created or destroyed.
False
According to the law of conservation of energy, energy can only be transferred or transformed.
What is the relationship between work and energy?
Work is the process of energy transfer through force acting over a distance.
Work and energy share the same unit, the joule (J).
Explain the difference between energy and power.
Energy is the capacity to do work, while power is the rate at which work is done.
Power is measured in watts (W), where 1 W = 1 J/s.
How are energy and motion related?
Energy is required to change an object’s state of motion or position.
Kinetic energy depends on an object’s velocity and mass.
Why is energy considered a scalar quantity?
Energy has magnitude but no specific direction.
Scalars are described fully by their size or value, unlike vectors.
Define:
Potential energy
Stored energy due to an object’s position or configuration.
Gravitational potential energy is calculated as PE = mgh.
Define:
Kinetic energy
The energy an object has due to its motion.
It is given by the formula KE = ½mv², where m is mass and v is velocity.
Fill in the blanks:
Kinetic energy depends on an object’s _____ and ______.
mass; velocity
Doubling the velocity quadruples the kinetic energy.
Explain the relationship between height and potential energy.
Potential energy increases as height increases.
PE = mgh shows direct proportionality between h and PE.
True or False:
An object at rest has kinetic energy.
False
Kinetic energy requires motion; a stationary object has only potential energy.
What type of energy transformation occurs when an object falls?
Potential energy converts to kinetic energy.
Total mechanical energy remains constant (neglecting air resistance).
How is mechanical energy defined?
The sum of kinetic and potential energy.
It describes the total energy in a system due to motion and position.
What is the formula for total mechanical energy?
Etotal =KE+PE
In the absence of non-conservative forces, total mechanical energy remains constant.
Explain why a pendulum exhibits both kinetic and potential energy.
As it swings, energy constantly shifts between these two forms, with kinetic energy reaching its maximum at the bottom and potential energy peaking at the endpoints.
The total mechanical energy of a pendulum remains constant in an ideal scenario.
State the law of conservation of energy.
Energy cannot be created or destroyed; it can only be transferred or transformed.
This principle applies universally in isolated systems.
Define:
Energy transfer
Movement of energy from one system or object to another.
Common forms include work and heat transfer.
Fill in the blank:
Energy transfer can occur through _____ or heat.
Work
Energy transfer through work involves a force acting over a distance.
Define:
Energy transformation
One type of energy turns into another type of energy.
Examples include electrical energy creating thermal energy or chemical energy in gasoline transforming into mechanical energy.
Fill in the blank:
When energy is transferred, the total energy in a closed system remains _______.
Constant
Energy transformations may result in heat, sound, or other forms.
Define:
Non-conservative forces
Forces that cause energy to be dissipated, such as friction and air resistance.
Energy lost to non-conservative forces cannot be fully recovered.
How does friction affect energy conservation?
Friction converts mechanical energy into heat, reducing total mechanical energy.
Friction is a non-conservative force.
Define:
Energy efficiency
The ratio of useful output energy to input energy, expressed as a percentage.
Efficiency = (Useful energy / Total energy input) × 100%.
True or False:
Energy transformations occur with 100% efficiency in all systems.
False
Real-world processes often lose energy as heat or other forms.
A machine produces 90 J of useful work from 120 J of input energy. What is its efficiency?
𝜂 = (usefuloutputenergy / inputenergy) ×100
𝜂 = (90 𝐽 / 120 𝐽) ×100 = 75 %
Efficiency is always expressed as a percentage.
What happens to stored energy when motion occurs?
Stored potential energy is converted into kinetic energy during motion.
This follows the principle of conservation of energy.
What happens to the energy of a ball as it falls freely under gravity?
Potential energy decreases while kinetic energy increases.
The sum of kinetic and potential energy remains constant.
Explain the energy transformation in a hydroelectric dam.
Gravitational potential energy of water transforms into kinetic energy and then electrical energy.
This process is not entirely efficient due to losses like heat.
What type of energy transformation occurs when using a solar panel?
Solar (radiant) energy is converted into electrical energy.
Efficiency depends on panel quality and sunlight availability.
Explain the energy transfer in a car braking system.
Kinetic energy is converted into thermal energy through friction in the brakes.
This demonstrates energy dissipation in non-conservative processes.
How is energy conserved in a roller coaster?
Potential energy at the highest point transforms into kinetic energy at the lowest point and vice versa.
Friction reduces total mechanical energy over time.
Describe the energy transformation in a battery powering a flashlight.
Chemical energy in the battery converts to electrical energy, then to light and heat.
Energy transformations are guided by the device’s design.
True or False:
Energy can exist in only one form at a time.
False
Most systems involve multiple forms of energy simultaneously.
Fill in the blank:
Energy transformations in nature often result in some energy being converted into _____.
Heat
Heat is often considered a less useful form of energy in practical applications.
Why is heat often considered a less useful form of energy in practical applications?
Heat energy is often dispersed and difficult to harness efficiently.
According to the Second Law of Thermodynamics, entropy causes energy to become more spread out and less available for work.
What kind of energy transformation occurs when a wind turbine generates electricity?
Kinetic energy of wind is converted into mechanical energy, then electrical energy.
Efficiency depends on turbine design and wind speed.
How does energy conservation apply to a pendulum in a vacuum?
Total mechanical energy remains constant without air resistance.
Vacuum eliminates non-conservative forces like friction.
Explain how regenerative braking conserves energy in electric vehicles.
Captured kinetic energy during braking is converted back into electrical energy, which is then stored in the battery.
This improves energy efficiency and reduces waste.
How is energy transferred during photosynthesis?
Light energy is converted into chemical energy stored in glucose.
This process powers most life on Earth.
What happens to water molecules in a tea kettle when heat is applied?
The water molecules gain energy and move quicker.
This results in an increase in temperature of the water.
Fill in the blank:
Work is done only when the force has a component along the ______ ___ ______.
direction of motion
If the force is perpendicular to motion, no work is done.
True or False:
Work is a scalar quantity.
True
While force and displacement are vectors, work is a scalar derived from their dot product.
What is the SI unit of work?
Joule (J)
One joule is equivalent to one newton-meter.
Calculate the work done when a 10 N force moves an object 5 m in the direction of the force.
W= 10N⋅5m=50J
Work is the product of force and displacement when aligned.
What determines if work is positive?
If the external force and the object’s displacement are in the same direction, it is positive.
Examples include pushing an object on a smooth surface and throwing a stone forward.
Describe a scenario where work is zero despite the application of force.
- Pushing against a wall that doesn’t move.
- A box pulled across the floor.
- A person holding a suitcase.
Without displacement, no work is performed.
Fill in the blank:
When a force opposes motion, the work done is ______.
Negative
Negative work indicates energy is removed from the system.
Explain the work-energy theorem.
The work done on an object equals the change in its kinetic energy.
This links force, motion, and energy transfer.
How does the work-energy theorem apply to a car accelerating from rest?
The work done by the engine on the car is equal to the change in its kinetic energy.
Work (W) = ΔKE = ½mv² - 0, where the initial kinetic energy is zero since the car starts at rest.
A 5 kg box is pushed with a force of 20 N over 4 m on a frictionless surface. What is the change in its kinetic energy?
The box’s kinetic energy increases by 80 J.
W = F × d = 20 × 4 = 80 J. The work-energy theorem states that the net work done equals the change in kinetic energy.
How does the work-energy theorem explain a braking car?
The work done by friction reduces the car’s kinetic energy to zero as it comes to a stop.
Negative work is done since the force of friction opposes motion, decreasing KE.
A roller coaster descends from a height of 20 m. How does the work-energy theorem apply?
The work done by gravity converts potential energy into kinetic energy as the coaster descends.
The total mechanical energy remains constant, assuming negligible friction and air resistance.
Explain why no work is done by a force when there is no displacement.
Work depends on both force and displacement; without displacement, work is zero.
This is true regardless of the magnitude of the force applied.
Define:
Power
Rate at which work is done or energy is transferred.
Power is measured in watts (W), where 1W=1J/s.
How is average power calculated?
P=W/t
where:
W is work
t is time
Power quantifies how quickly energy is transferred.
What is the unit of power?
Watts (W), equivalent to Joules per second (J/s).
Named in honor of engineer James Watt.
What is the power output of a machine that does 500 J of work in 10 seconds?
P= 500J/10s =50W
Power quantifies the rate of doing work.
How does power relate to efficiency?
Efficiency is the ratio of useful power output to total power input.
It is often expressed as a percentage.
True or False:
A faster machine performing the same amount of work has a higher power.
True
Power depends on both work and the time taken to do it.
What is the formula for power in terms of velocity?
Power (P) = F v cos θ
where:
F is the force applied, and
v is velocity
What is the significance of a power rating when using electrical devices?
It indicates the maximum energy that can be handled without risk of damage.
Using a device with a lower power rating than the outlet can lead to short-circuiting.
