CISCE Class VII Physics PDF

UNIVERSAL EDUCATION CISCE 2023-24 CLASS VII PHYSICS Ch. 3 Energy Select the...

UNIVERSAL EDUCATION CISCE 2023-24 CLASS VII PHYSICS Ch. 3 Energy Select the correct alternative: 1. When we rub our hands: a. kinetic energy changes into potential energy b.mechanical energy changes into heat energy c.potential energy changes into kinetic energy d.heat energy changes into mechanical energy 2. A ball rolling on the ground possess a. kinetic energy b. potential energy c. no energy d. heat energy 3. The correct statement is a. Both work and energy have the same units b. Potential energy of a body is due to its motion c. Kinetic energy of a body is due to its position or state d.Kinetic energy can change into potential energy, but potential energy cannot change into kinetic energy 4. According to law of conservation of energy, energy changes from one form to another form, but the total energy of that system a. increases b. decreases c. alternates d. remains the same 5. The S.I. unit of energy is............................ a. metre b. joule c. fermi d. mole 6. Cooking gas converts..................... energy into heat energy. a. kinetic energy b. electrical energy c. chemical energy d. heat energy 7. Green plants convert...................... energy into chemical energy. a. light energy b. electrical energy  c. mechanical energy d. no energy 8. The energy possessed by a body due to its position is called..................... energy. a. kinetic energy b. light energy c. electrical energy d. potential energy 9. The energy stored in an electric cell is a. chemical energy b. electrical energy c. heat energy d. mechanical energy 10. The incorrect statement is: a. A gum bottle lying on a table has no energy. b. A man going up has potential energy and kinetic energy both. c. In an electric fan, electrical energy changes into mechanical energy. d. One form of energy cannot be converted into another form. Q.5 State the energy changes that occur in the following a. The unwinding of a watch spring - potential energy changes into kinetic energy b. An electric generator Kinetic energy to electric energy c. Burning gas to cook food - Chemical potential energy is converted into thermal energy Q. 6 Give one example for each of the following transformations of energy a. Electrical energy to mechanical energy electric fan b. Electrical energy to light energy tubelight c. Chemical energy to heat energy burning of coa d. Chemical energy to sound energy fire crackers e. Potential energy to kinetic energy- water stored behind dam to flowing water Unit Review page 34 1. Write true or false for each statement. i. In an electric fan, electrical energy converts only to kinetic energy False ii. There is never a loss of energy in conversion from one form of energy to another state, so the total energy is always conserved. True iii. Potential energy changes into kinetic energy when it is put to use.- True iv. Wind energy can be converted into electrical energy. True v. A stone thrown from a cliff converts kinetic energy into potential energy. True 2. Match the following columns  Column A Column B 1. Kinetic energy a. Heat energy 2. fire b. atoms 3. sound energy c. running car 4. nuclear energy d. vibration 1. c, 2. a, 3. d, 4. b 3. Fill in the blanks. i. Kinetic energy is due to motion of the body ii. ii. When fireworks are lighted in a celebration then chemical energy changes to heat, light and sound energy iii. The sum total of the potential and the kinetic energy process by a body is equal to one or conserved 4. Answer the following questions. a. Name three different types of energy and describe each with suitable examples. Ans. Energy exists in many different forms. Examples of these are: light energy, heat energy, mechanical energy, gravitational energy, electrical energy, sound energy, chemical energy, nuclear or atomic energy and so on. Each form can be converted or changed into the other forms. Mechanical energy it is the total kinetic energy and potential energy possessed by an object in other words it is energy because of the movement or position or both of an object for example water stored in a dam and rolling ball have mechanical energy. Sound energy: sound energy is created when an object vibrates. These vibrations cause a regular disturbance in the medium which is in the form of waves. For example, musical instruments Thermal or heat energy the energy contained in a hot substance this called the thermal energy. Objects with higher temperatures have a greater thermal energy. Eg. Water heater b. Give one example of energy conversion from electrical to sound energy. Ans. In an Electric bell and loudspeaker electrical energy is converted into sound energy. c. State the factors that affect the kinetic energy of the body. Ans. The kinetic energy of a moving object depends on two factors: The mass of the body - More the mass of the body, the higher is the kinetic energy The speed of the body- More the speed of the object, the higher is the kinetic energy. d. A football is on the first floor of your house and another identical football with the same mass and volume is kept on the second floor which of the two footballs has greater potential energy? Ans. The football with the same mass and volume which is kept on the second floor has greater potential energy. e. Ball is falling freely from a height state the kind or kinds of energy process by  the ball when it is i. At the highest point Potential energy (max) +Kinetic energy (0) ii. Just the middle Potential energy (E/2)+Kinetic energy (E/2) iii. At the ground.- Potential energy (0) + Kinetic energy (E) Important Terms and Definitions Work: Work is defined as transferring energy into an object so that there is some displacement Energy: Energy is defined as the ability to do work. Potential Energy: Potential energy is energy stored in an object due to its position relative to the ground or due to the changes in its dimensions. Kinetic Energy: Kinetic energy is defined as the energy that is possessed by an object due to its motion Law of conservation of energy: The law of conservation of energy states that the amount of energy is neither created nor destroyed. For example, when you roll a toy car down a ramp and it hits a wall, the energy is transferred from kinetic energy to potential energy. Dissipation of energy: In transformation of energy when one form of energy is changed to another useful form of energy apart of energy a part of energy gets converted with the non-useful form which is called dissipation of energy. for example, to light a bulb the electrical energy is converted into light energy but a part of electrical energy converts into heat energy which is radiated to warm the bulb. This part of electrical energy is the dissipated part. f. Define work and energy with one example of each Ans: Work is defined as transferring energy into an object so that there is some displacement. E.g. A group of girls playing badminton, work is said to be done. Energy is defined as the ability to do work. E.g. The capability to play badminton is energy g. What is the amount of work if a force of 5 N is applied on a wall? Ans. Work = F x S , When we apply force on the wall there is no displacement of the object so s = 0. Work = 5 x 0 = 0. Hence zero amount of work is done. h. Give two sources of chemical energy Ans. Wood, coal i. All energy change on the earth start with the sun. Justify Ans. The sun is the ultimate source of energy on the Earth. It is sun that creates all the energy changes. Solar energy is trapped by plants during photosynthesis. They get converted to coal that we use in the power station to produce electricity. The water cycle is also a result of solar energy. In some cases, we use solar energy directly in solar cells which are part of watches, calculators, and artificial satellites. The use of solar cookers and solar heaters are other examples. j. Name the two kinds of mechanical energy and describe them with an example each. Ans. The two kinds of mechanical energy are potential energy and kinetic energy. Potential energy is energy stored in an object due to its position relative to the ground or due to the changes in its dimensions. Example top of waterfall stores gravitational potential energy Kinetic energy is defined as the energy that is possessed by an object due to its motion example of kinetic energy and airplane has enormous kinetic energy in flight because of its high velocity and huge mass. k. Define the term kinetic energy. Give an example of a body that possesses kinetic energy. Ans. Kinetic energy is defined as the energy that is possessed by an object due to its motion example of kinetic energy and airplane has enormous kinetic energy in flight because of its high velocity and huge mass. Test Yourself: Choose the correct option: a. The strong wind can turn the blades of a windmill because it possesses- Ans. kinetic energy b. If an object moves higher, then its gravitational potential energy- Ans. increases c. If the speed of an object doubles, its kinetic energy becomes- Ans. four times d. The change of energy from one form to another is called _____ of energy Ans. Transformation e. If a spring is compressed, its potential energy. Ans. Increased f. Which of the following is not an example of potential energy? Ans. a moving car g. When a ball is thrown vertically upwards its velocity keeps on decreasing. What happens to its kinetic energy when it reaches the maximum height? Ans. kinetic energy is zero h. For an object to have gravitational energy only, it must be- Ans. At an elevated position with respect to the ground i. If a ball falls from a height then the sum of its kinetic and potential energy after it falls through a distance is- Ans. equal to the potential energy at h