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CNMS ICSE - 7 Std
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Answers to textbook exercises

Prepared by: learnloophq@gmail.com

Chapter: 03. Energy

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CONNECT

Identify the source of energy used to run the objects given below. [battery, cell, petrol, wind, electricity]
Flashlight
cell
Motorcycle
petrol
Smartphone
battery
Toaster
electricity
Wind turbine
wind
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CHECK YOUR PROGRESS (Page 51)

Choose the correct option.

Energy stored in diesel is an example of
b. chemical energy.
The kinetic energy of a moving object depends on its
c. Both a and b.
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REASON CORNER (Page 52)

As rain falls, potential energy is converted to …
kinetic energy (as the raindrops fall and gain speed).
The energy that causes water to evaporate and form water vapour is known as …
heat energy (specifically solar heat energy).
When you rub hands, mechanical energy is converted into …
heat energy (due to friction between the hands).
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CHECK YOUR PROGRESS (Page 54)

Choose the correct option:

Bursting of crackers is an example of conversion of
b. chemical energy into heat energy.
When a ball is thrown vertically upwards and rising up, it possesses
c. both a and b.
Which of the following is an example of conversion of electrical energy into magnetic energy?
a. electromagnets
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REASON CORNER (Page 54)

When a ball strikes the ground, it bounces back several times. a. What is the potential energy and kinetic energy of the ball when you release it from a height?
When the ball is released from a height, its potential energy is at its maximum and its kinetic energy is zero. b. Why does the ball bounce back after striking the ground? From where does the ball get the kinetic energy for this?
The ball bounces back due to its elastic properties. Upon striking the ground, its kinetic energy is temporarily converted into elastic potential energy as it deforms, which is then quickly converted back into kinetic energy, pushing the ball upward.
Can machines create new energy? Why or why not?
No, machines cannot create new energy. According to the law of conservation of energy, energy can neither be created nor destroyed; machines can only convert energy from one form into another.
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MY QUESTION TIME

Frame a question on each of the following topics and discuss their answers.
a. different forms of energy
Question: What are three different forms of energy used in daily household devices, and how are they used?
Answer: Heat energy (used in ovens and geysers), light energy (used in bulbs to illuminate rooms), and chemical energy (stored in dry cells and batteries to run appliances).
b. conservation of energy
Question: How does a simple pendulum demonstrate the law of conservation of energy?
Answer: As the pendulum swings, its energy continuously converts from potential energy at the highest points of its swing to kinetic energy at the lowest point. Assuming no air resistance, the total mechanical energy remains constant at every point of its path.
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PREPPING FOR PISA

Study the graph given above and answer the following questions. a. Which country harnesses the maximum amount of tidal energy?
China b. What can you infer about the overall change in electricity generated by tidal energy in India from 2012-2021?
The electricity generated by India remained extremely low and relatively unchanged/stable over the period from 2012 to 2021 compared to other major countries.
The heights of tidal waves recorded at two instances X and Y are 7 m and 11 m, respectively. What can we infer about the amount of energy harnessed?
b. More tidal energy was harnessed at Y.
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EVACUATION

A. Choose the correct option.

Energy possessed by virtue of motion is
b. kinetic energy.
When work is done on a body, its energy
a. increases.
In microphones, sound energy is converted to
b. electrical energy.
Which energy is used by plants for photosynthesis?
d. light energy.
In a thermal power plant, energy is converted into energy.
a. heat, electrical
A person M climbing up the hill crosses paths with person N who is climbing down the hill. What energy do M and N possess?
d. M and N possess both kinetic and potential energy.

B. Fill in the blanks.

The SI unit of energy is ________.
joule (J)
In steam engines, heat energy is converted into ________ energy.
mechanical
1 calorie = ________ joules.
4.186 (or 4.2)
In an electric drill, mechanical energy is converted into ________ energy.
heat
Due to ________ energy, a magnet can attract iron towards itself.
magnetic
A moving pendulum has ________ energy.
mechanical (or kinetic and potential)

C. Write true (T) or false (F) against the following statements.

A water bottle lying on a table has no energy.
False (F) (It possesses potential energy by virtue of its position above the ground.)
When we burn a matchstick, chemical energy is released in the form of heat and light.
True (T)
The food that we eat has nuclear energy.
False (F) (The food we eat has chemical energy.)
During photosynthesis, electrical energy is converted into chemical energy.
False (F) (Light energy is converted into chemical energy.)
On a roller coaster, the amount of energy changes at every point.
False (F) (While the forms of energy change, the total mechanical energy remains constant at every point, assuming no friction.)

D. Name the following:

the capacity to do work
Energy
the work done in bringing a body to a state of motion
Kinetic energy
energy stored in the nucleus of atoms
Nuclear energy
the energy stored in matter
Chemical energy
splitting of the nucleus of an atom
Nuclear fission

E. Differentiate between:

kinetic energy and potential energy
Kinetic energy is the energy possessed by an object by virtue of its motion (e.g., a moving ball), whereas potential energy is the energy stored in an object by virtue of its position above the ground or due to a change in its shape (e.g., a stretched rubber band).
work and energy
Work is defined as the product of force and the displacement of a body in the direction of the force (&nbsp
math
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&nbsp), whereas energy is the capacity or ability of a body to do work.

F. Write one example of each of the following energy transformations.

mechanical energy into electrical energy
Dynamos or generators
electrical energy into chemical energy
Recharging of chemical batteries
light energy into heat energy
Solar cookers or solar water heaters
electrical energy into light energy
Electric bulbs or tube lights
electrical energy into mechanical energy
Electric fans or washing machines

G. Short-answer-type questions.

Define energy. What is its SI unit?
Energy is the capacity to do work. Its SI unit is the joule (J).
State the law of conservation of energy.
The law of conservation of energy states that energy can neither be created nor be destroyed; it can only be transformed from one form to another. The total amount of energy in an isolated system always remains constant.
What are the two types of mechanical energy? Give one example of each.
The two types of mechanical energy are:
Kinetic energy: e.g., a moving carrom striker.
Potential energy: e.g., water stored in a dam.
What do you mean by transformation of energy?
Transformation of energy (or inter-conversion of energy) is the process in which energy is converted from one form to another.
State two factors on which the kinetic energy of a moving body depends.
Kinetic energy depends on:
Mass of the object: Greater mass results in greater kinetic energy.
Speed of the object: Greater speed results in greater kinetic energy.

H. Long-answer-type questions.

How do we hear sound?
Sound is produced by vibrating bodies, which create sound waves in the surrounding air. When these waves travel through the air and reach our ears, they cause the membrane (eardrum) inside our ear to vibrate. These vibrations are converted into signals that are sent to our brain, allowing us to hear the sound.
Give one example for each of the following transformation of energy.
a. chemical energy to light energy
Burning of a matchstick / candle
b. electrical energy to heat energy
Electric geysers / ovens c. mechanical energy to electrical energy
Wind turbines / dynamos d. electrical energy to sound energy
Loudspeakers / electric bells e. light energy to chemical energy
Photosynthesis in green plants
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Higher Order Thinking Skills (HOTS)

The bob of a simple pendulum is swinging from its lowest position to its highest position. Which of the following statements is true about the motion of the bob?
c. Potential energy will attain its maximum value and kinetic energy will attain its minimum value.
Identify the energy or energy conversions that take place in the following cases. a. from A to B
Electrical/Mechanical energy of the motor is converted into Potential energy of the roller coaster as it is pulled to a higher elevation. b. from C to D
Potential energy is converted into Kinetic energy as the roller coaster goes down the slope.
An arrow, when shot from the bow, possesses kinetic energy. Where does the arrow get this kinetic energy from?
The arrow gets its kinetic energy from the elastic potential energy stored in the stretched bowstring. When the string is released, the work done to stretch the bow is transferred to the arrow, converting potential energy into kinetic energy.
The energy of a freely falling body is conserved. Explain.
When a body is held at a certain height, it possesses only potential energy. As it falls freely, its height decreases (reducing its potential energy) while its speed increases (increasing its kinetic energy). At any point during its fall, the loss in potential energy is exactly equal to the gain in kinetic energy. Since the sum of potential and kinetic energy (total mechanical energy) remains constant at every point in its path, the total energy is conserved.
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Learning by Doing

Name the form of energy possessed by the cell when it is at rest on top of the board.
Potential energy
Name the form of energy possessed by the cell as it rolls down.
Kinetic energy (and a decreasing amount of potential energy).
Explain the law of conservation of energy demonstrated by the above activity.
When the dry cell is placed at the top of the inclined board, it has potential energy by virtue of its position. As it rolls down, this potential energy is progressively converted into kinetic energy. Upon hitting the matchbox, this kinetic energy is used to do work in displacing the matchbox. This shows that energy was not created or destroyed, but merely transformed from potential energy to kinetic energy and then transferred to do work.
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Integrate - Chemistry

Ritika (Highly Active Football Player) vs Rohan (Sedentary Chess Player) Daily Diet Matrix

The table below outlines a suggested diet plan to meet their daily calorie needs. Ritika requires a high-calorie, carbohydrate-rich diet (~2500–3000 kcal) to fuel intensive physical exercise, whereas Rohan requires a balanced, lighter calorie-controlled diet (~1800–2000 kcal) to support brain activity and maintain a healthy weight.
Ritika (Football Player - Active)
​
Rohan (Chess Player - Sedentary)
​
Food Item
Approx. Calories
Food Item
Approx. Calories
Oatmeal with whole milk, banana, and honey
550 kcal
Oatmeal with skimmed milk and berries
350 kcal
Chicken/Paneer wrap with whole wheat tortilla
600 kcal
Mixed salad with grilled tofu and olive oil
400 kcal
Brown rice with mixed lentils and broccoli
650 kcal
Quinoa with steamed vegetables and dal
450 kcal
Handful of mixed nuts (almonds, walnuts) & raisins
350 kcal
Apple slices with a tablespoon of peanut butter
200 kcal
Whole-wheat salmon sandwich with avocado
550 kcal
Baked fish or low-fat cottage cheese with spinach
400 kcal
Total Daily Intake
~2700 kcal
Total Daily Intake
~1800 kcal
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