Selina Concise Solutions for ICSE Class 7 Physics Chapter 3 Energy

Points to Remember

Work - Work is said to be done if the applied force on the body moves it. If no motion takes place, no work is said to be done.

1. The amount of work done depends on two factors :
   (i) on the magnitude of the force applied (greater the force applied, greater is the work done), and
   (ii) on the distance moved in the direction of force (greater the distance moved, greater is the work done).
2. The work done by a force on a body is equal to the product of the force and the distance moved by the body in the direction of force, i.e. Work done = Force x distance moved in the direction of force Or W = F x d
3. The S.I. unit of work is joule (J), where 1 joule (J) = 1 newton (N) x 1 metre (m)
4. The energy of a body is its capacity (or ability) to do work. The energy of a body in a state is equal to the work done on the body to bring it to that state.
5. The S.I. unit of energy is joule (J).
6. Kinetic energy of a body is the energy possessed by it due to its motion. It is the energy stored when work is done to bring the body in motion.
7. Kinetic energy of a moving body depends on two factors :
   (i) on the mass of the body (greater the mass of the body, greater is its kinetic energy), and
   (iii) on the speed of the body (more the speed of the body, higher is its kinetic energy).
8. The potential energy changes into the kinetic energy when it is put to use.
9. In transformation of energy, the total sum of useful and non-useful energy obtained after conversion is equal to the energy converted, i.e. the total energy remains conserved.
10. According to the conservation of mechanical energy, if friction is neglected, the total sum of potential energy and kinetic energy remains constant. Examples are : motion of roller coaster, free vertical fall of a body etc.
11. The electricity obtained from the energy possessed by the flowing water is called the hydro-electricity.

Test Yourself

A. Objective Questions

1. Write true or false for each statement

(a) A man going up has potential energy and kinetic energy both.
Answer: True.
The man has potential energy because he is gaining height from the ground and kinetic energy because he is in the process of moving. Combined, these two forms of energy make up his total mechanical energy as he climbs.
Teacher's Tip: Use "P" for Position (Potential) and "K" for Kicking/Movement (Kinetic).
Exam Tip: To score full marks, identify that any moving object at a height possesses both types of energy.

(b) A gum bottle lying on a table has no energy.
Answer: False.
Correct - A gum bottle lying on a table has energy.
Even though it is stationary, the bottle has potential energy due to its elevated position on the table relative to the floor. If the bottle were to fall, this stored potential energy would be converted into kinetic energy.
Teacher's Tip: Think of height as a "battery" that stores potential energy until the object starts falling.
Exam Tip: Remember that an object at rest can still have energy if it is placed above the ground level.

(c) In an electric fan, electrical energy changes into the mechanical energy.
Answer: True.
The electrical energy from the power supply flows into the fan motor to create rotational movement in the blades. This physical motion of the blades is known as mechanical energy.
Teacher's Tip: Anything that plugs in and moves is usually converting electrical energy into mechanical energy.
Exam Tip: Mention the rotation of the motor or blades to specifically explain why it is mechanical energy.

(d) Potential energy changes into kinetic energy when it is put to use.
Answer: True.
Stored energy, like that in a stretched rubber band or a ball held high, is potential energy that turns into motion once released. This transformation allows the stored capacity to perform actual work.
Teacher's Tip: Think of potential energy as "waiting" energy and kinetic energy as "happening" energy.
Exam Tip: Give an example like a falling apple to illustrate this transformation clearly in your answers.

(e) One form of energy cannot be converted into another form.
Answer: False.
Correct - One form of energy can be converted into the other form.
Energy transformation is happening everywhere around us, such as a battery converting chemical energy into electrical energy. Without these conversions, machines and living organisms would not be able to function.
Teacher's Tip: Remember the phrase "Energy is a Transformer" because it is always changing its shape.
Exam Tip: Use the Law of Conservation of Energy to explain that energy can only be converted, not created or destroyed.

(f) There is always some loss of energy in conversion from one form of energy to another form, so the total energy is not conserved.
Answer: False.
Correct - There is always some loss of energy in conversion from one form of energy to the other form, so the total energy is conserved.
While energy might turn into "useless" forms like heat due to friction, it is never truly lost from the universe. The sum of useful energy and wasted energy always equals the total energy we started with.
Teacher's Tip: Wasted energy (like heat from a lightbulb) is still energy; it just isn't doing the job we wanted.
Exam Tip: Distinguish between "useful energy" and "total energy" to prove conservation is always true.

(g) The energy of flowing water can be converted into electric energy (electricity).
Answer: True.
The kinetic energy of fast-moving water is used to spin large turbines, which then drive generators to produce electricity. This process is the foundation of hydroelectric power plants.
Teacher's Tip: Flowing water has kinetic energy because it is moving; stored water behind a dam has potential energy.
Exam Tip: Mention "Hydroelectricity" as the technical term for electricity generated from moving water.

2. Fill in the blanks

(a) An electric fan converts electrical energy into mechanical energy.
Answer: mechanical.
Electricity causes the motor inside the fan to spin, turning the static blades into moving parts. This conversion from current to motion is the definition of mechanical output.
Teacher's Tip: If it spins, rolls, or swings using electricity, the answer is mechanical energy.
Exam Tip: Ensure you spell "mechanical" correctly to get full marks in fill-in-the-blank sections.

(b) Cooking gas converts chemical energy into heat energy.
Answer: chemical.
The fuel stored in the gas cylinder contains energy in its molecular bonds, which is released as a flame when burned. This chemical reaction provides the heat necessary for cooking food.
Teacher's Tip: Any fuel like gas, petrol, or coal stores "chemical" energy.
Exam Tip: Remember that burning is a chemical process that releases stored energy.

(c) Energy possessed by a compressed spring is potential energy.
Answer: potential.
When you push a spring together, you are doing work to change its shape, and that work is stored inside it. As soon as you let go, that stored potential energy pushes the spring back out.
Teacher's Tip: Compressed or stretched things always have "Elastic Potential Energy."
Exam Tip: Specify that it is "potential" because the energy is stored due to the spring's distorted shape.

(d) The ability to do work is called energy
Answer: energy.
Energy is essentially the "currency" needed to perform any physical task or movement. Without energy, no force can be applied to move an object over a distance.
Teacher's Tip: Just like you need money to buy things, you need energy to do work.
Exam Tip: This is the most basic definition in physics, so memorize it word-for-word.

(e) The energy possessed by a body due to its position is called potential energy.
Answer: potential.
Potential energy depends on where an object is located, especially its height above the ground. The higher an object is placed, the more potential energy it has stored up.
Teacher's Tip: Position = Potential. Both start with the letter "P"!
Exam Tip: Mention "height" or "stretched state" as examples of "position" for a more complete answer.

(f) The energy possessed by a body due to its motion is called kinetic energy.
Answer: kinetic.
Every moving object, from a speeding car to a walking person, carries kinetic energy because of its speed. If an object stops moving, its kinetic energy becomes zero immediately.
Teacher's Tip: Think of "Kicking" a ball to give it "Kinetic" energy.
Exam Tip: Kinetic energy depends on both the mass of the object and its speed.

(g) Green plants convert light energy into chemical energy.
Answer: light.
During photosynthesis, plants capture sunlight and use it to create food in the form of sugar and starch. This process stores the sun's radiant energy as chemical energy inside the plant's tissues.
Teacher's Tip: Plants are like solar panels that store energy in the food they make.
Exam Tip: Use the term "photosynthesis" if you are explaining this conversion in a long answer.

(h) The S.I.unit of energy is joule
Answer: joule.
Named after James Prescott Joule, this unit measures the amount of work done or energy transferred. It is the standard unit used by scientists all over the world for consistency.
Teacher's Tip: Work and Energy are "twins"—they have the same definition and the same unit (Joule).
Exam Tip: Always use a capital 'J' when writing the symbol for Joule.

(i) An object falling freely from the roof of a multistorey building has potential energy and kinetic energy when halfway down the building.
Answer: potential, kinetic.
At the halfway point, the object still has some height left (potential energy) and has also gained significant speed (kinetic energy). As it continues to fall, the potential energy keeps turning into kinetic energy.
Teacher's Tip: At the top it's all potential; at the bottom it's all kinetic; in the middle it's both!
Exam Tip: For a freely falling body, the sum of these two energies remains constant if we ignore air resistance.

3. Match the following columns

Column A
(a) Running water
(b) Burning
(c) Energy
(d) Sound energy
Column B
(i) heat energy
(ii) vibrations
(iii) atom bomb
(iv) kinetic energy

Column A - Column B
(a) Running water - (iv) kinetic energy
(b) Burning - (i) heat energy
(c) Energy - (v) joule
(d) Sound energy - (ii) vibrations
(e) Nuclear energy - (iii) atom bomb
Answer: (a)-(iv), (b)-(i), (c)-(v), (d)-(ii), (e)-(iii)
This matching exercise shows how different physical phenomena are linked to specific energy types or units. For instance, sound is produced by vibrating particles, and running water carries energy because it is in motion.
Teacher's Tip: Look for keywords like "running" (motion) to find kinetic energy and "burning" to find heat.
Exam Tip: When matching, double-check that every item in Column A has exactly one partner in Column B.

4. Select the correct alternatives

(a) When we rub our hands
1. kinetic energy changes into potential energy
2. mechanical energy changes into heat energy
3. potential energy changes into kinetic energy
4. heat energy changes into mechanical energy.
Answer: 2. mechanical energy changes into heat energy.
The physical movement of rubbing (mechanical energy) creates friction between your palms, which generates warmth. This is why our hands feel hot after rubbing them together quickly.
Teacher's Tip: Friction is the "thief" that turns motion (mechanical) into heat.
Exam Tip: Always look for a temperature change to identify if heat energy is being produced.

(b) A ball rolling on the ground possesses
1. kinetic energy
2. potential energy
3. no energy
4. heat energy
Answer: 1. kinetic energy.
Because the ball is moving across the ground, it has energy due to its motion. Since it is on the ground level, its potential energy is considered to be zero.
Teacher's Tip: Rolling = Moving = Kinetic.
Exam Tip: If an object is "on the ground," its potential energy is zero, so only kinetic energy remains.

(c) The energy stored in an electric cell is
1. chemical energy
2. electrical energy
3. heat energy
4. mechanical energy.
Answer: 1. chemical energy.
Inside a cell or battery, chemicals react with each other to store energy until it is needed. When you turn on a device, this stored chemical energy is converted into electricity.
Teacher's Tip: Batteries and cells are "chemical packets" of energy.
Exam Tip: Distinguish between the "stored" form (chemical) and the "delivered" form (electrical).

(d) When a bulb lights up on passing current, the change of energy is
1. from electrical energy to heat energy
2. from electrical energy to light energy
3. from electrical energy to heat and light energy
4. from electrical energy to mechanical energy.
Answer: 3. from electrical energy to heat and light energy.
As electricity flows through the bulb's filament, it gets so hot that it glows. This results in both light for us to see and heat that we can feel if we touch the bulb.
Teacher's Tip: Old incandescent bulbs waste a lot of electrical energy as heat, while LEDs produce much less heat.
Exam Tip: Don't forget that "lighting up" involves both light and the side effect of heat.

(e) The correct statement is
1. Both work and energy have the same units
2. Potential energy of a body is due to its motion
3. Kinetic energy of a body is due to its position or state
4. Kinetic energy can change into potential energy, but potential energy cannot change into kinetic energy.
Answer: 1. Both work and energy have the same units.
Work is the transfer of energy, so they are essentially the same physical quantity measured in Joules. Statements 2, 3, and 4 are scientifically incorrect because they swap the definitions of kinetic and potential energy.
Teacher's Tip: Work and Energy are just two different ways of looking at the same Joules!
Exam Tip: Carefully read each choice; often "correct statement" questions have three very similar-sounding lies.

(f) According to law of conservation of energy, energy changes from one form to another form, but the total energy of that system
1. increases
2. decreases
3. alternates
4. remains the same
Answer: 4. remains the same.
The law states that energy is neither created nor destroyed, only transformed. This means the numerical value of total energy in a closed system is constant over time.
Teacher's Tip: Think of energy as an unchangeable total "score" that just moves between different players.
Exam Tip: Use the word "constant" or "remains the same" when defining the law of conservation.

B. Short/Long Answer Questions

Question 1: Define the term energy.
Answer: Energy is the capacity of doing work.
Energy is the invisible force that allows objects to move or change their state. Without it, the universe would be static and no work could ever be performed.
Teacher's Tip: Energy is like your body's "stamina" for playing sports.
Exam Tip: This is a 1-mark question, so keep the definition short and precise.

Question 2: State the unit of energy and define it.
Answer: The energy is measured in the same unit as work. Therefore the S.I. unit of energy is joule (symbol J).
A body is said to possess an energy of one joule if a force of 1 newton moves the body by a distance of 1 metre in the direction of force.
Another unit of energy is calorie (symbol cal) where 1 cal = 4.2 J. A bigger unit is kilo-calorie (symbol kcal) where 1 kcal = 1000 cal.
We use Joules for physics experiments, but we often use calories to measure the energy stored in the food we eat. One Joule is actually a very small amount of energy, roughly equivalent to lifting a small apple one meter up.
Teacher's Tip: Remember the conversion 1 cal = 4.2 J—it's very common in science.
Exam Tip: When defining a unit, always include the base units it is made from (like Newton and Metre).

Question 3: Name five different forms of energy.
Answer: The different forms of energy are :
(i) Mechanical energy
(ii) Heat energy
(iii) Light energy
(iv) Chemical energy
(v) Sound energy
(vi) Magnetic energy
(vii) Electrical energy and
(viii) Atomic energy or nuclear energy.
These forms of energy can be found in everyday items: heat from a stove, light from the sun, and chemical energy in our breakfast. Scientists group them based on how they affect matter.
Teacher's Tip: Use the acronym "MELCH" (Mechanical, Electrical, Light, Chemical, Heat) to remember the main ones.
Exam Tip: If asked for five, list the five you are most confident explaining with examples.

Question 4: What are the two kinds of mechanical energy.
Answer: The mechanical energy is found in two forms namely :
(a) The potential energy, and (b) The kinetic energy.
Mechanical energy is the sum of an object's motion and its stored position. For example, a rolling ball at the top of a hill has both potential and kinetic energy.
Teacher's Tip: Mechanical Energy = Potential Energy + Kinetic Energy.
Exam Tip: Always mention that these two together make up the "total mechanical energy."

Question 5: What is potential energy ? State its unit.
Answer: The energy of a body at rest is called the potential energy. It is defined as follows :
Potential energy of a body is the energy possessed by it due to its state of rest or position. Actually, it is the work spent in bringing the body to that state of rest or position.
It is written as P.E. or U.
The S.I. unit of potential energy is Joules.
Potential energy is "potential" because it has the possibility to do work in the future. For example, water held back by a dam has a high potential to create electricity once it is released.
Teacher's Tip: Think of it as "Stored Work."
Exam Tip: Use the symbol 'U' or 'P.E.' to represent potential energy in formulas.

Question 6: Give one example of a body that has potential energy, in each of the following : (i) due to its position, (ii) due to its state.
Answer: (i) Potential energy of a body is the energy possessed by it due to its state of rest or position. It is the energy stored when work is done on the body to bring it to that state or position.
(ii) Potential energy of a body in the raised (or lifted) position depends on two factors : (1) the mass of the body greater the mass of the body, greater is the potential energy of the body), and (2) the height of the body above the ground (greater the height of the body, greater is its potential energy.)
An example of position-based potential energy is a stone sitting on a cliff edge. An example of state-based potential energy is a tightly coiled clock spring or a stretched catapult.
Teacher's Tip: Position = height; State = stretched or compressed shape.
Exam Tip: For (ii), make sure to explain that the energy comes from the "work done" to change the object's shape or lift it.

Question 7: State two factors on which the potential energy of a body at a certain height above the ground depends.
Answer: The potential energy of a body in the raised position depends upon the following two factors :
(a) The mass of the body: Greater the mass of the body, greater is the potential energy of the body.
(b) Its height above the ground : Higher the height of the body, greater is its potential.
Mathematically, potential energy is represented by P.E. = mgh, where 'm' is mass and 'h' is height. This means doubling either the weight of the object or its height will double its stored energy.
Teacher's Tip: Think of a heavy brick vs. a feather at the same height; the brick has way more potential energy!
Exam Tip: List mass and height as the primary factors and gravity as the constant factor.

Question 8: Two bodies A and B of masses 10 kg and 20 kg respectively are at the same height above the ground. Which of the two has the greater potential energy ?
Answer: The body B having mass 20 kg has the greater potential energy. This can be explained as follows :
P.E. = mgh.
For both the bodies gravity and height are same so the body with greater mass possesses greater potential energy.
Since body B is twice as heavy as body A and they are at the same height, body B has exactly double the potential energy of body A. Mass is a direct multiplier in the potential energy formula.
Teacher's Tip: More mass = More "Oomph" when it falls.
Exam Tip: Always write out the formula P.E. = mgh to support your reasoning in physics problems.

Question 9: A bucket full of water is on the first floor of your house and another identical bucket with same quantity of water is kept on the second floor. Which of the two has greater potential energy ?
Answer: A bucket full of water kept on second floor has the greater potential energy. This can be explained as follows :
P.E. = mgh
Mass of both bucket and the gravitational force are same, so the body at greater height will possess more potential energy.
Because the second floor is higher than the first floor, the 'h' (height) value in our formula is larger for that bucket. Even though the buckets are identical, the extra work required to carry one up to the second floor is stored as extra energy.
Teacher's Tip: The harder you work to lift something, the more energy it stores!
Exam Tip: Clarify that "identical" means the mass is constant, so only height makes the difference.

Question 10: Define the term kinetic energy. Give one example of a body which possesses kinetic energy.
Answer:
The energy of a body in motion is called its kinetic energy. It is defined as follows :
Kinetic energy of a body is the energy possessed by it due to its state of motion.
Actually, it is the work done on the body bringing it to the state of motion. In short form it is written as K.E. or K.
Example : In a swinging pendulum moving to and fro, the bob has the kinetic energy.
Kinetic energy is what makes things crash or push other objects when they collide. For example, a moving bowling ball has kinetic energy that it uses to knock down pins.
Teacher's Tip: If it's moving, it's Kinetic. No movement = No Kinetic Energy.
Exam Tip: Mention that kinetic energy is equal to the "work done" to get the object up to its current speed.

Question 11: State two factors on which the kinetic energy of a moving body depends.
Answer: The kinetic energy of a moving body depends on the following two factors :
(a) The mass of the body - Greater the mass of the body, higher is its kinetic energy.
(b) The speed of the body - More the speed of the body, higher is its kinetic energy.
A fast car has more kinetic energy than a slow car, and a heavy truck has more kinetic energy than a small car moving at the same speed. Speed is actually even more important than mass in the mathematical formula.
Teacher's Tip: Speed and Mass are the "power duo" for kinetic energy.
Exam Tip: Use the formula K.E. = 1/2 Mv² to show that speed is squared, making it very influential.

Question 12: Two toy-cars A and B of masses 500 g and 200 g respectively are moving with the same speed. Which of the two has the greater kinetic energy?
Answer: The toy car ‘A’ of mass ‘500 gm’ has the greater kinetic energy. This can be explained as :
K.E. =1/2 Mv²
Here, both the cars are moving with same speed. So the car with greater mass will possess greater kinetic energy.
Since the speeds are identical, the car with more mass (500g vs 200g) naturally carries more energy of motion. Car A would be much harder to stop than Car B.
Teacher's Tip: Think of stopping a rolling tennis ball vs. a rolling bowling ball at the same speed.
Exam Tip: Be sure to mention that "speed is constant" so the comparison rests entirely on mass.

Question 13: A cyclist doubles his speed. How will his kinetic energy change: increase, decrease or remain same ?
Answer: When a cyclist doubles his speed. His kinetic energy increases four times.
K.E. = 1/2 Mv²
When v = doubles the K.E. quadruples.
In the formula, the speed is squared (v²), so 2 x 2 = 4. This is why going fast is much more dangerous in accidents—the energy increases much faster than the speed does.
Teacher's Tip: Speed squared means if speed goes up by 3, energy goes up by 9 (3²)!
Exam Tip: Always show the 2² = 4 logic to explain why doubling speed quadruples energy.

Question 14: Name the form of energy which a wound up watch spring possess.
Answer: A wound up watch spring has the potential energy because of its wound up state. As the spring unwinds itself, the potential energy changes into the kinetic energy. This kinetic energy does work in moving the arms of the watch.
The energy is "stored" in the tight coils of the metal spring when you turn the watch key. This elastic potential energy is released slowly to keep the clock ticking at a steady pace.
Teacher's Tip: Wound-up springs and stretched rubber bands are classic examples of stored potential energy.
Exam Tip: Explain the conversion from potential to kinetic to show a deeper understanding of the process.

Question 15: Can a body possess energy even when it is not in motion ? Explain your answer with an example.
Answer: Yes, a body possesses energy even when it is not in motion ;
Consider a body raised to a certain height say h. It its velocity is zero. Kinetic energy will be zero but the body will have.
P.E. = mgh
Thus, a body may possess energy even though it is not in motion.
This "not in motion" energy is called potential energy, which is stored work. A compressed jack-in-the-box or water in a high tank are perfect examples of stationary objects full of energy.
Teacher's Tip: Motion is just one way to have energy; position and state are the other ways!
Exam Tip: Use the term "Potential Energy" as the specific name for energy held by a stationary body.

Question 16: Name the type of energy (kinetic or potential) possessed by the following :
(i) A moving cricket ball.
(ii) A stone at rest on the top of a building.
(iii) A compressed spring.
(iv) A moving bus.
(v) A bullet fired from a gun.
(vi) Water flowing in a river.
(vii) A stretched rubber band.
Answer:
(i) Kinetic energy.
(ii) Potential energy.
(iii) Potential energy.
(iv) Kinetic energy.
(v) Kinetic energy.
(vi) Potential energy. (Note: flowing water also has kinetic energy; the textbook identifies the capacity of water at a height/state as potential).
(vii) Potential energy.
Moving items like cricket balls, buses, and bullets always have kinetic energy. Stationary items that have been lifted, stretched, or compressed store their energy as potential energy.
Teacher's Tip: If it's "at rest" but has potential to move, it's Potential.
Exam Tip: Be careful with flowing water; usually, it is "Kinetic" because it is moving, but the question may be referring to its source at a height.

Question 17: Give one example to show the conversion of potential energy to kinetic energy when put in use.
Answer: The example to show the conversion of potential energy to kinetic energy when put in use is :
A stone at a height has the potential energy due to its lifted or raised position. In the figure below when the stone is dropped from that position, it begins to fall. The falling stone has the kinetic energy. Thus, the potential energy stored in the stone in its raised position changes into the kinetic energy when the stone is falling. This kinetic energy does work on the nail as the stone strikes the nail and makes the nail to move into the wood.
As the stone falls, it loses height (potential energy) and gains speed (kinetic energy). When it hits the nail, all that kinetic energy is used to push the nail deeper into the wooden block.
Teacher's Tip: Falling = Converting P.E. to K.E.
Exam Tip: Describe the "striking" moment to explain how energy is finally used to do work.

Question 18: State the energy changes that occur in the following :
(i) The unwinding of a watch spring.
(ii) Burning coal while operating a steam engine.
(iii) Lighting of a torch bulb.
(iv) An electric generator (or dynamo).
Answer:
(i) Potential energy to kinetic energy.
(ii) Chemical energy of coal changes to heat energy of the steam. Heat energy changes into mechanical energy.
(iii) Chemical energy into light and heat energy.
(iv) Electrical energy change into mechanical energy. (Note: Mechanical energy of rotation changes into electrical energy in a generator).
These transitions show that energy is never static; it is constantly flowing from one type to another to get things done. In a torch bulb, for example, the chemical energy in the batteries turns into electricity and finally light.
Teacher's Tip: Always look for the "Input" energy and the "Final Result" energy.
Exam Tip: For a steam engine, mention all three steps (Chemical -> Heat -> Mechanical) for a perfect score.

Question 19: Energy can exist in several forms and may change from one form to another. Give two examples to show the conversion of energy from one form to another.
Answer: The examples that show the conversion of energy from one form to another are :
(1) In a steam engine, the chemical energy of the coal first changes into the heat energy of the steam. Then heat energy of steam changes into the mechanical energy which makes the train to move.
(2) In an electric motor (or in fan), the electrical energy changes into the mechanical energy. This energy rotates the axle of motor (or the blades of the fan).
Every machine we use is built specifically to perform these transformations. In a fan, the input is electrical current and the desired output is the mechanical wind-making rotation.
Teacher's Tip: Use a "flowchart" style answer (A -> B -> C) to explain multi-step conversions.
Exam Tip: Clearly label the input energy and the output energy in your description.

Question 20: Give one relevant example for each of the following transformation of energy :
(i) Electrical energy to heat energy.
(ii) Electrical energy to mechanical energy.
(iii) Electrical energy to light energy.
(iv) Chemical energy to heat energy.
(v) Chemical energy to light energy.
Answer:
(i) In an electric heater, oven, geyser, toaster etc., the electrical energy changes into heat energy.
(ii) An electric generator. (Wait: Motor is the correct example for Electrical -> Mechanical).
(iii) Tube light or bulbs.
(iv) Burning of wood, coal etc.
(v) Fire crackers burst.
Household appliances are designed for specific energy shifts: a toaster turns current into heat, while a lightbulb turns it into light. Natural fuels like wood store chemical energy that becomes heat and light when lit.
Teacher's Tip: If you see "Heater" or "Oven," the output is always Heat energy.
Exam Tip: For (v), you can also mention a "Glow-stick" as a more modern example of chemical-to-light conversion.

Question 21: What do you mean by conservation of mechanical energy? State the condition when does it hold.
Answer: This means “The total MECHANICAL ENERGY (P.E + K.E) of an isolated system at any instant is equal to the sum of kinetic energy and the potential energy.”
Condition : Condition under which the mechanical energy is conserved is “WHEN THERE ARE NO FRICTIONAL FORCES.” In other words the mechanical energy is conserved strictly in vacuum where friction due to air is absent.
In a perfect system, energy simply swaps between stored (potential) and moving (kinetic) without any being "stolen" by heat. On Earth, we usually have a small amount of loss due to air resistance and floor friction.
Teacher's Tip: "Conservation" means "Keeping it safe" — the total amount never changes.
Exam Tip: Mention "friction-free" or "vacuum" to explain the specific condition where conservation is perfect.

Question 22: Give one example to show that the sum of potential energy and kinetic energy remains constant if friction is ignored.
Answer: During the vertical fall of ball, if friction due to air is neglected, the total sum of potential energy and kinetic energy at each point of its path remains same.
As the ball drops, it loses P.E. and gains K.E. in exact equal amounts. If you calculated the total (P.E. + K.E.) at the top, the middle, and just before hitting the floor, the number would be identical.
Teacher's Tip: Imagine energy as a bucket of water; you can pour it into different glasses (K.E. or P.E.), but you still have the same amount of water.
Exam Tip: Use a diagram of a falling ball at points A, B, and C to help explain this concept visually.

Question 23: A ball is made to fall freely from a height. State the kind/ kinds of energy possessed by the ball when it is
(a) at the highest point
(b) just in the middle
(c) at the ground.
Answer:
(a) Potential energy.
(b) Potential energy + Kinetic energy.
(c) Kinetic energy.
At the top, the ball has maximum height but zero speed. At the bottom, it has zero height but maximum speed. In between, it has a mix of both height and speed.
Teacher's Tip: The "Exchange Rate" of a falling object is 1 P.E. unit for 1 K.E. unit.
Exam Tip: Make sure to explain *why* it has each type (height vs speed) at each point.

Question 24: State the changes in form of energy while producing hydro electricity.
Answer: The water in motion in a river or sea has the kinetic energy. The energy possessed by the flowing water is called the hydro energy. The most important use of hydro energy is to produce electricity from it.
The potential energy of water stored high in a dam turns into kinetic energy as it rushes down pipes. This kinetic energy then turns the turbine (mechanical energy), which finally creates electrical energy.
Teacher's Tip: Hydroelectricity is a chain reaction: Potential -> Kinetic -> Mechanical -> Electrical.
Exam Tip: Describe the role of the turbine and generator to get maximum marks for this answer.

Production of hydroelectricity
The flowing water of river is collected in a dam at a high altitude. The water stored in the dam has the potential energy. When water from dam falls on the water turbine, the potential energy of the water stored in dam changes into its kinetic energy and this kinetic energy of water is transferred to the blades of turbine as the kinetic energy which rotates the turbine. As the turbine rotates, it rotates the armature of the generator (or dynamo) to produce electricity.