CLASS seventh chapter 3 physics ENERGY and it's types

Different forms of energy

1. ENERGY

2. WORK When the force applied on a body moves it, work is said to be done by the force on the body. Work is measured as, Work = Force X Displacement in the direction of force. The S.I unit of energy is joule(J). 1 J work is said to be done when a force of 1 N moves a body by a distance of 1m in the direction of force.

3. Relation between work and energy When work is done on the body its energy increases. In other words, the work done on the body is stored in it in the form of energy. But if work is done by the body its energy decreases. In other words, energy is spent when the body does work

4. RELATIONSHIP BETWEEN ENERGY AND WORK To do more amount of work, we need more amount of energy. Hence, We can say that there is a direct relationship between work and energy. Similarly, the work done on a body is changing its state is said to be energy possessed by the body.

5. E.g when a body is moved upwards work is done against the force of gravity and the body at the height is said to possess energy. Similarly, if a body initially at rest is made to move, work is done on the body and the body in motion is said to posses the amount of energy equal to the work done on the body producing motion in it.

6. ENERGY Energy is the capacity to do work A body is said to possess an energy of one joule if one joule work is done to bring the body in that state i.e 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. 1 cal = 4.2 J or 4.816 J Bigger unit is kilo-calorie (kcal) where 1 kcal=1000 cal

7. DIFFERENT KINDS OF ENERGY Mechanical Energy Heat Energy Light Energy Chemical Energy Sound Energy Magnetic Energy Electrical Energy Atomic Energy or Nuclear Energy

8. MECHANICAL ENERGY The energy possessed by a body due to its state of rest or state of motion is called mechanical energy It is found to be in the two forms Potential & (2) Kinetic Energy The total mechanical energy of a body is the sum of potential and kinetic energies.

9. HEAT ENERGY The energy released when we burn anything like coal , oil , wood or gas is called heat energy. Steam possess heat energy and it capable to do work. It is our common experience that if we heat some water in a kettle with a lid on it, we notice that as the water begins to boil, the steam makes the lid to lift up. This actually the heat energy of the steam which moves the lid up.

10. LIGHT ENERGY Light is a form of energy in the presence of which other objects are seen. We do not feel light energy because ordinary light does not move the object around us. But strong beam of light can move small particles like electron. Plants convert light energy into chemical energy by the process of photosynthesis.

11. CHEMICAL ENERGY The energy possessed by fuel such as coal, oil, gas etc is called chemical energy. The chemical energy of petrol or diesel is capable to move a car or truck.

12. SOUND ENERGY The energy responsible for the sensation of hearing is called sound energy. A vibrating body is the source of sound energy We can hear sound when the frequency of sound is in between 20 Hz and 20,000 Hz

13. MAGNETIC ENERGY The energy possessed by a magnet is called magnetic energy. Magnets can attract an iron nail from a distance and thus the force exerted by the magnet makes the nail to move towards the magnet.

14. ELECTRICAL ENERGY When two dry bodies are rubbed together they get charged and gain electrical energy. A comb when rubbed on dry hair can attract small bits of paper and cause them to move towards the comb.

15. ATOMIC ENERGY The energy stored in atom is called atomic energy. Atomic energy cam be used for destructive purposes. For peaceful purpose, atomic reactors are used to produce electrical energy from atomic energy.

16. For destructive purpose, atom bomb used for destroying the enemy in a war. Actually atomic energy is the energy released due to loss in mass when either a heavy nucleus splits into two light nuclei or two light nuclei combine to form one nucleus under certain circumstances. This is why atomic energy is also called nuclear energy.

17. KEY POINTS The sun is a major source of energy. The energy which we receive on earth from the sun is called the solar energy. Nowadays solar energy is used for making solar cooker, solar cells etc. Efforts are being made to obtain electricity by using solar panels from solar energy. The Government of India is also promoting to produce electricity for household purposes by using solar cooker .

18. TWO FORMS OF MECHANICAL ENERGY The energy possessed by a body due to its state of rest or state of motion, is called mechanical energy. Mechanical Energy is of two forms 1. Potential Energy 2. Kinetic Energy

19. POTENTIAL ENERGY Potential energy of a body is the energy possessed by it due to its state of rest or position. Actually it is equal to the work done in bringing the body to that state of rest or position. When a body is taken from earth’s surface to height, work has to be done on the body against the force of gravity on it. The work done in the body is stored in the form of potential energy. This energy also called gravitational potential energy.

20. EXAMPLES A wound up watch spring has potential energy because of the wound up state of its coils. As the spring unwinds itself, it moves the arms of the watch. A compressed spring has potential energy stored in it which equal to the work done on the spring in bringing it to the compressed state.

21. 3. A stretch rubber band has potential energy. It does not work restoring itself to its original state . A pebble placed on the stretched rubber catapult, is thrown away when it is released to restore its original state. In stretching a rubber band, work is done. This work is stored in the rubber band in the form of potential energy.

22. 4. A hammer at a height has potential energy stored in it. The hammer has this energy because of its position at a height. In figure, the hammer is dropped on a nail fixed on a piece of wood. It drives the nail into the wood as shown in figure.

23. 5.In figure a falling stone when reaches a pan, attached at one end of a pulley, lifts up a weight at its other end because of its potential energy. 6. Water at a height has potential energy stored in it. Falling water from a height can be used to do work such as turning a wheel.

24. FACTORS AFFECTING THE POTENTIAL ENERGY Potential energy of a body depends upon the following two factors: (a) The mass of the body: Greater the mass of the body, greater the potential energy of the body. (b) Its height above the ground: Higher the height of the body, greater is its potential energy. Formula: mass X height X Gravity = mgh Or Weight X height = mg X h= mgh

25. KINETIC ENERGY 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.

26. EXAMPLE 1 . A fast moving stone has the capacity of breaking a window pane when it strikes the pane. The fast moving stone has kinetic energy. 2. A falling hammer ( i.e in motion) when strikes a nail fixed on a wooden block moves in further into the block. Thus moving hammer has kinetic energy and it does work on the nail.

27. 3. In a swinging pendulum, moving to and fro, the bob has kinetic energy. 4. In a river or sea, the flowing water has kinetic energy which makes wooden boat or a log of wood to move in it. 5. A bullet fired from a gun, a rolling ball, an apple falling from a tree etc all have kinetic energy.

28. FACTORS ON WHICH KINETIC ENERGY DEPENDS Kinetic energy of a moving body depends on the following two factors: (a)The mass of the body: Greater the mass, higher is the kinetic energy. (b)The speed of the body: More the speed of the body, more is its kinetic energy. Formula : ½ X mass X speed 2 = ½ x m x V 2

29. CONVERSION OF POTENTIAL ENERGY INTO KINETIC ENERGY Potential energy changes into kinetic energy when it is put to use. Examples ( 1) A hammer at a height has potential energy due to its lifted or raised position. When a hammer is dropped from that position, it begins to fall. The falling hammer has kinetic energy. Thus the potential energy stored in the hammer in its raised position changes into kinetic energy when the hammer is falling. The kinetic energy does work on the nail as the hammer strikes the nail and causes the nail to move into the wood.

30. ( 2) The wound up watch spring has potential energy because of its wound up state. As the spring unwinds itself, the potential energy changes into kinetic energy. The kinetic energy does work in moving the arms of the watch. (3) A stretch bow has potential energy because of its stretch position. When the stretch bow is released, the potential energy of the bow changes into its kinetic energy. This kinetic energy does work on the arrow and makes the arrow to move.

31. (4 ) A compressed spring has potential energy in it due its compressed state, when the compressed spring is released, the potential energy changes into its kinetic energy which does work on the ball placed on it and makes the ball to fly away.

32. CONVERSION OF ENERGY Energy can neither be created nor destroyed but can be converted from one form to the other

33. EXAMPLES ( 1) In a steam engine chemical energy of coal first changes into heat of a steam. Then the heat changes into mechanical energy which causes train to move. (2) In electric motor or fan the electrical energy changes into mechanical energy. The energy rotates the axle of motor (or blades of a fan).

34. (3 ) In electric heater, oven , toaster, geyser the electrical energy changes into heat energy. (4) The electric cell in use, the chemical energy changes into electrical energy. (5) In a glowing bulb electrical energy changes into light and heat energy. (6) An electric bell while ringing the electric energy changes into sound energy.

35. ( 7) Electric generator or dynamo mechanical energy changes into electrical energy. (8) In microphone sound energy changes into electrical energy. (9) In loudspeaker electrical energy changes into sound energy. (10) Plants during photosynthesis light energy received from sun changes into chemical energy of food. (11) When fire crackers burst chemical energy changes into heat sound and light energy.

36. CONSERVATION OF ENERGY Energy can neither be created or destroyed but can transform one form to any other form. While transformation of energy takes place some energy is wasted as a un-useful form of energy such as heat. The sum of useful and un-useful energy will be constant. If 100J of electrical energy is converted 70 J of mechanical energy, the remaining 30 J of energy is converted to heat and sound energy. In absence of friction the sum of potential and kinetic energy always constant i.e Total mechanical energy always constant.

37. KEY POINTS In transformation of energy when one form of energy changes into other form of energy, a part of energy is converted into non-useful form which is called dissipation of energy. e.g When a bulb is glowing electrical energy gets converted to light energy. But a part of energy is wasted as heat which is dissipation of energy .

38. ROLLER COASTER

39. FALLING BODY

40. HYDROELECTRICITY

41. The water collected in a dam at a height has potential energy stored in it. When it is made to fall on a turbine, the potential energy changes to kinetic energy which is transferred to the turbine to rotate it. By connecting the turbine to the armature of a dynamo, electricity (called the hydroelectricity) is produced.

42. Two bodies having masses 50 kg and 100 kg placed at a same height. Which one should have more potential energy? Ans . Mass of 100 kg, more mass more potential energy Two bodies having same mass are placed but placed at 50m and 100m. Which one has greater potential energy 2 nd mass , as the height is more.

43. Name the type of energy(Kinetic or potential) possessed by the following Moving cricket ball- A stone at rest at the top of a building- A compressed spring- A moving bus- A bullet fired from a gun- Water flowing in a river- A stretched rubber band-

44. State the energy change in the following : Unwinding of watch spring- Burning coal while operating a steam engine- Lighting of torch bulb- A electric generator or dynamo-

45. Give one relevant example for each of the following transformation Electrical to heat energy-Electric Heater/Toaster/Geyser Electrical to mechanical energy- Electric Fan/Mixer-Grinder Electrical to light energy-Electric Bulb/Tube light Chemical to heat energy- Burning Coal/ Fire Craker

46. Questions from CONCISE PHYSICS A ball is made free to fall from a height. State the kind/kinds of energy possessed by the ball when it is ( i ) at the highest point (ii) just in the middle (iii) At the ground

47. What do you mean by conservation of mechanical energy? State the condition when it holds. In absence of friction the sum of potential and kinetic energy always constant i.e Total mechanical energy always constant. In absence of friction it holds.

48. Two bodies having masses 500 g and 100 g having same speed. Which of them having higher kinetic energy? Why? First body having mass 500 g will have more kinetic energy. More the mass more the kinetic energy if speed is constant. Two bodies having same mass have speed 3 m/s and 4m/s. Which one should have more kinetic energy? Why? Second body will have more kinetic energy. More the speed more kinetic energy if mass is constant.

49. Can a body posses energy when it is not in motion? Explain your answer with an example. Yes, A stone which is at rest placed in a height will have potential energy . A bucket filled with water placed in first floor. The same bucket with equal amount of water is placed in second floor. Which one should have more potential energy? Explain. The bucket placed at second floor will have more potential energy because it has more height than the bucket placed at first floor and mass is constant for both the buckets..