Heat, current, solar, nuclear, and hydro-power are the various forms of energy. As per the law of energy conservation, “Energy can neither be created nor be destroyed. Although, it may be transformed from one form to another”. In a closed system, i.e. a system isolated from its surroundings, the system’s total energy is conserved.

The amount of energy in any system is determined by the following equation:

\(\begin{array}{l}U_{T}=U_{i}+W+Q\end{array} \)

Where,

  • UT – total energy of a system
  • Ui – initial energy of a system
  • Q – heat added or removed from the system
  • W – work done by or on the system

The change in the internal energy of the system is given by:

\(\begin{array}{l}\Delta U=W+Q\end{array} \)

The below video helps to revise the concepts in the chapter Work and Energy Class 9

Important Law of Conservation of Energy Questions with Answers

1. Give a common example where you can witness electrical energy transform into sound energy.

In loudspeakers, electrical energy transforms into sound energy.

2. When a bomb explodes, which form of the energy is converted to which form?

When a bomb explodes, chemical energy is converted to kinetic energy.

3. The heat added to or removed from the system is represented using the alphabet ______.

  1. W
  2. U
  3. Q
  4. H

Answer: c) Q

Explanation: The heat added to or removed from the system is represented by the alphabet Q.

4. State TRUE or FALSE: Conservation of energy applies only to isolated systems.

  1. TRUE
  2. FALSE

Answer: a) TRUE

Explanation: Conservation of energy applies to isolated systems.

5. In which device mechanical energy is converted into electrical energy?

In a generator, mechanical energy is converted into electrical energy.

6. Choose the correct answer: Solar panels can create solar energy.

  1. YES
  2. NO

Answer: b) NO

Explanation: Solar panels cannot create solar energy. They can harness solar energy from the sun and convert them into another type of energy, i.e. electric current.

7. When is the potential energy converted into kinetic energy?

  1. When solar power is converted into electricity.
  2. When dynamite explodes.
  3. When a block slides down a slope.
  4. When the object remains stationary.

Answer: c) When a block slides down a slope.

Explanation: A block has stored potential energy. When it slides, the potential energy is converted into kinetic energy.

8. What is the unit of energy?

  1. Radians
  2. Volt
  3. Joule
  4. Newton

Answer: c) Joule

Explanation: The SI unit of energy is the joule (J). It is named after James Prescott Joule, the English physicist.

9. Light bulbs transform _____ into light.

  1. Chemical energy
  2. Nuclear energy
  3. Sound energy
  4. Electrical energy

Answer: d) Electrical energy.

Explanation: Light bulbs transform electrical energy into light energy.

10. State true or false: Fingers hitting piano keys transfer energy from the hand to the keys.

  1. TRUE
  2. FALSE

Answer: a) TRUE

Explanation: The hand transfers energy to the piano through the fingers.

Practice Questions

  1. What is meant by energy?
  2. State law of conservation of energy.
  3. Give examples of potential energy transferred into kinetic energy.
  4. Can we destroy energy?
  5. Which is the formula to find the change in the system’s internal energy.

Watch the video below to understand if the work done is negative

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