3B4 Energy

Question 1

Define:

Energy

Answer 1

The capacity to do work or produce change.

Energy exists in many forms, including mechanical, thermal, electrical, and chemical.

Question 2

Define:

Work

Answer 2

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.

Question 3

Fill in the blank:

The SI unit of energy is the ______.

Answer 3

Joule (J)

One joule is equal to the energy transferred when a force of 1 newton is applied over a distance of 1 meter.

Question 4

True or False:

Energy can be created or destroyed.

Answer 4

False

According to the law of conservation of energy, energy can only be transferred or transformed.

Question 5

What is the relationship between work and energy?

Answer 5

Work is the process of energy transfer through force acting over a distance.

Work and energy share the same unit, the joule (J).

Question 6

Explain the difference between energy and power.

Answer 6

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.

Question 7

How are energy and motion related?

Answer 7

Energy is required to change an object’s state of motion or position.

Kinetic energy depends on an object’s velocity and mass.

Question 8

Why is energy considered a scalar quantity?

Answer 8

Energy has magnitude but no specific direction.

Scalars are described fully by their size or value, unlike vectors.

Question 9

Define:

Potential energy

Answer 9

Stored energy due to an object’s position or configuration.

Gravitational potential energy is calculated as PE = mgh.

Question 10

Define:

Kinetic energy

Answer 10

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.

Question 11

Fill in the blanks:

Kinetic energy depends on an object’s _____ and ______.

Answer 11

mass; velocity

Doubling the velocity quadruples the kinetic energy.

Question 12

Explain the relationship between height and potential energy.

Answer 12

Potential energy increases as height increases.

PE = mgh shows direct proportionality between h and PE.

Question 13

True or False:

An object at rest has kinetic energy.

Answer 13

False

Kinetic energy requires motion; a stationary object has only potential energy.

Question 14

What type of energy transformation occurs when an object falls?

Answer 14

Potential energy converts to kinetic energy.

Total mechanical energy remains constant (neglecting air resistance).

Question 15

How is mechanical energy defined?

Answer 15

The sum of kinetic and potential energy.

It describes the total energy in a system due to motion and position.

Question 16

What is the formula for total mechanical energy?

Answer 16

E_total =KE+PE

In the absence of non-conservative forces, total mechanical energy remains constant.

Question 17

Explain why a pendulum exhibits both kinetic and potential energy.

Answer 17

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.

Question 18

State the law of conservation of energy.

Answer 18

Energy cannot be created or destroyed; it can only be transferred or transformed.

This principle applies universally in isolated systems.

Question 19

Define:

Energy transfer

Answer 19

Movement of energy from one system or object to another.

Common forms include work and heat transfer.

Question 20

Fill in the blank:

Energy transfer can occur through _____ or heat.

Answer 20

Work

Energy transfer through work involves a force acting over a distance.

Question 21

Define:

Energy transformation

Answer 21

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.

Question 22

Fill in the blank:

When energy is transferred, the total energy in a closed system remains _______.

Answer 22

Constant

Energy transformations may result in heat, sound, or other forms.

Question 23

Define:

Non-conservative forces

Answer 23

Forces that cause energy to be dissipated, such as friction and air resistance.

Energy lost to non-conservative forces cannot be fully recovered.

Question 24

How does friction affect energy conservation?

Answer 24

Friction converts mechanical energy into heat, reducing total mechanical energy.

Friction is a non-conservative force.

Question 25

# Define: Energy efficiency

Answer 25

The **ratio of useful output energy to input energy**, expressed as a percentage. ## Footnote Efficiency = (Useful energy / Total energy input) × 100%.

Question 26

# True or False: Energy transformations occur with **100% efficiency** in all systems.

Answer 26

False ## Footnote Real-world processes often lose energy as heat or other forms.

Question 27

A machine produces 90 J of useful work from 120 J of input energy. **What is its efficiency**?

Answer 27

𝜂 = (useful output energy / input energy) ×100 𝜂 = (90 𝐽 / 120 𝐽) ×100 = 75 % ## Footnote Efficiency is always expressed as a percentage.

Question 28

What happens to **stored energy** when motion occurs?

Answer 28

Stored potential energy is **converted into kinetic energy** during motion. ## Footnote This follows the principle of conservation of energy.

Question 29

What happens to the energy of a ball as it falls freely under **gravity**?

Answer 29

**Potential energy decreases** while kinetic energy increases. ## Footnote The sum of kinetic and potential energy remains constant.

Question 30

Explain the energy transformation in a **hydroelectric dam**.

Answer 30

Gravitational potential energy of water **transforms into kinetic energy** and then electrical energy. ## Footnote This process is not entirely efficient due to losses like heat.

Question 31

What type of energy transformation occurs when using a **solar panel**?

Answer 31

Solar (radiant) energy is **converted** into electrical energy. ## Footnote Efficiency depends on panel quality and sunlight availability.

Question 32

Explain the energy transfer in a **car braking system**.

Answer 32

Kinetic energy is **converted** into thermal energy through friction in the brakes. ## Footnote This demonstrates energy dissipation in non-conservative processes.

Question 33

How is energy conserved in a **roller coaster**?

Answer 33

Potential energy at the highest point transforms into kinetic energy at the lowest point and vice versa. ## Footnote Friction reduces total mechanical energy over time.

Question 34

Describe the energy transformation in a **battery powering a flashlight**.

Answer 34

Chemical energy in the battery **converts** to electrical energy, then to light and heat. ## Footnote Energy transformations are guided by the device's design.

Question 35

# True or False: Energy can exist in **only one form** at a time.

Answer 35

False ## Footnote Most systems involve multiple forms of energy simultaneously.

Question 36

# Fill in the blank: **Energy transformations** in nature often result in some energy being converted into \_\_\_\_\_.

Answer 36

Heat ## Footnote Heat is often considered a less useful form of energy in practical applications.

Question 37

Why is **heat** often considered a less useful form of energy in practical applications?

Answer 37

Heat energy is **often dispersed and difficult to harness** efficiently. ## Footnote According to the Second Law of Thermodynamics, entropy causes energy to become more spread out and less available for work.

Question 38

What kind of energy transformation occurs when a **wind turbine** generates electricity?

Answer 38

Kinetic energy of wind is converted into mechanical energy, then electrical energy. ## Footnote Efficiency depends on turbine design and wind speed.

Question 39

How does energy conservation apply to a pendulum in a **vacuum**?

Answer 39

Total mechanical energy **remains constant** without air resistance. ## Footnote Vacuum eliminates non-conservative forces like friction.

Question 40

Explain how **regenerative braking** conserves energy in electric vehicles.

Answer 40

Captured kinetic energy during braking is **converted back into electrical energy**, which is then stored in the battery. ## Footnote This improves energy efficiency and reduces waste.

Question 41

How is energy transferred during **photosynthesis**?

Answer 41

Light energy is converted into chemical **energy stored in glucose**. ## Footnote This process powers most life on Earth.

Question 42

What happens to **water molecules in a tea kettle** when heat is applied?

Answer 42

The **water molecules gain energy** and move quicker. ## Footnote This results in an increase in temperature of the water.

Question 43

# Fill in the blank: **Work** is done only when the force has a component along the \_\_\_\_\_\_ \_\_\_ \_\_\_\_\_\_.

Answer 43

direction of motion ## Footnote If the force is perpendicular to motion, no work is done.

Question 44

# True or False: Work is a **scalar quantity**.

Answer 44

True ## Footnote While force and displacement are vectors, work is a scalar derived from their dot product.

Question 45

What is the **SI unit** of work?

Answer 45

Joule (J) ## Footnote One joule is equivalent to one newton-meter.

Question 46

Calculate the work done when a **10 N force** moves an object **5 m** in the direction of the force.

Answer 46

W= 10N⋅5m=50J ## Footnote Work is the product of force and displacement when aligned.

Question 47

What determines if **work is positive**?

Answer 47

If the external force and the object's displacement are in the **same direction**, it is positive. ## Footnote Examples include pushing an object on a smooth surface and throwing a stone forward.

Question 48

Describe a scenario where **work is zero** despite the application of force.

Answer 48

* Pushing against a wall that doesn’t move. * A box pulled across the floor. * A person holding a suitcase. ## Footnote Without displacement, no work is performed.

Question 49

# Fill in the blank: When a **force opposes motion**, the work done is \_\_\_\_\_\_.

Answer 49

Negative ## Footnote Negative work indicates energy is removed from the system.

Question 50

Explain the **work-energy theorem**.

Answer 50

The work done on an object **equals the change** in its kinetic energy. ## Footnote This links force, motion, and energy transfer.

Question 51

How does the **work-energy theorem** apply to a car accelerating from rest?

Answer 51

The work done by the engine on the car is **equal to the change in its kinetic energy**. ## Footnote Work (W) = ΔKE = ½mv² - 0, where the initial kinetic energy is zero since the car starts at rest.

Question 52

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?

Answer 52

The box’s kinetic energy **increases by 80 J**. ## Footnote W = F × d = 20 × 4 = 80 J. The work-energy theorem states that the net work done equals the change in kinetic energy.

Question 53

How does the work-energy theorem explain a **braking car**?

Answer 53

The work done by **friction reduces the car’s kinetic energy** to zero as it comes to a stop. ## Footnote Negative work is done since the force of friction opposes motion, decreasing KE.

Question 54

A **roller coaster** descends from a height of **20 m**. How does the work-energy theorem apply?

Answer 54

The work done by gravity converts potential energy into kinetic energy as the coaster descends. ## Footnote The total mechanical energy remains constant, assuming negligible friction and air resistance.

Question 55

Explain why no work is done by a force when there is **no displacement**.

Answer 55

**Work depends on both force and displacement**; without displacement, work is zero. ## Footnote This is true regardless of the magnitude of the force applied.

Question 56

# Define: Power

Answer 56

**Rate at which work is done** or energy is transferred. ## Footnote Power is measured in watts (W), where 1W=1J/s.

Question 57

How is **average power** calculated?

Answer 57

P=W/t ## Footnote where: W is work t is time Power quantifies how quickly energy is transferred.

Question 58

What is the **unit of power**?

Answer 58

Watts (W), equivalent to Joules per second (J/s). ## Footnote Named in honor of engineer James Watt.

Question 59

What is the power output of a machine that does **500 J of work in 10 seconds**?

Answer 59

P= 500J/10s =50W ## Footnote Power quantifies the rate of doing work.

Question 60

How does **power** relate to **efficiency**?

Answer 60

Efficiency is the **ratio of useful power output** to total power input. ## Footnote It is often expressed as a percentage.

Question 61

# True or False: A faster machine performing the same amount of work has a **higher power**.

Answer 61

True ## Footnote Power depends on both work and the time taken to do it.

Question 62

What is the formula for **power** in terms of **velocity**?

Answer 62

Power (P) = F v cos θ ## Footnote where: F is the force applied, and v is velocity

Question 63

What is the significance of a **power rating** when using electrical devices?

Answer 63

It indicates the **maximum energy that can be handled** without risk of damage. ## Footnote Using a device with a lower power rating than the outlet can lead to short-circuiting.