Title 02_VI-PHY.E-TEC_(WPE)_WS,KEY & ADD._VOL-2_44-66.pdf ✅

e-Techno Text Book VI-Physics (Vol-2) Nishith Multimedia India (Pvt.) Ltd. 44 WORK, POWER AND ENERGY CONTENTS: Work Types of Work done by a force Power Energy Potential Energy Kinetical Energy Conservation of Energy
e-Techno Text Book VI-Physics (Vol-2) Nishith Multimedia India (Pvt.) Ltd. 45 MEMO GRAPH J SI Unit J SI Unit J SI Unit Work, W Work, W W=F×S where F=Force(N) S=distance moved in the direction of the force (m) P=W/t=E/t where W=work done(J) E=energy converted(J) t-time(S) 1.Chemical 2.Radiant 3. Mechanical 4. Internal 5. Electrical 6. Nucleus By virtue of motion kinetic energy E =1/2mv x 2 By virtue of position Gravitation al energy E =mgh p The principle of conservation of energy Conservation of energy is freely falling body as example Energy, E is defined as Which are related to each other by of many forms such as is related to is related to
e-Techno Text Book VI-Physics (Vol-2) Nishith Multimedia India (Pvt.) Ltd. 46 WORK, POWER AND ENERGY James Prescott Joule (1818 - 1889) James Joule was born in Salford, England in 1818. He was a physicist who is credited with discovering the law of conservation of energy that states energy cannot be created or destroyed, but rather, only changed in its form. Two other German scientists, Hermann von Helmholtz and Julius von Mayer are also credited with this discovery. In 1840, Joule discovered that heat is produced in any electrical conductor and today this is known as Joule's Law. Joule also worked with William Thompson and found that the temperature of a gas falls when it expands without doing any work. This idea led eventually to the principle behind today's refrigeration and air-conditioning systems. Joule's name is used to describe the international unit of energy known as the joule. James Prescott Joule (1818 - 1889) KNOW YOUR SCIENTIST WORK POWER & ENERGY_SYNOPSIS-1 Scientific conception of Work : Everybody does some sort of work everyday. Reading, thinking or studying are examples of mental work. We do not recognise mental work in physics, no matter if it needs energy. Lifting a load, carrying a bucket of water, cooking, drawing water from a well, are all examples of physical work. In physics, only physical or mechanical work is recognised as work done. Physical work and mechanical work: Definition : Work is said to be done by a force when it produces motion. Example : When an engine moves a train along a railway line, it is said to be doing work. Mathematical Expression for work : If a force F acts on a body and moves it a distance S in the direction of the force then work done, W = FS . Initial position of body final position of body F S Units of work : The unit of work in C.G.S. system is ‘erg’. W = F . S = 1dyne × 1cm = 1erg erg : Work done is said to be one ‘erg’ if a force of one dyne displaces the body through a distance of 1 cm along the direction of force. 1 erg = 1 dyne × 1 cm = 1 g cm s–2 × cm or 1 erg = 1 g cm2 s–2
e-Techno Text Book VI-Physics (Vol-2) Nishith Multimedia India (Pvt.) Ltd. 47 The unit of work in S.I. system is joule (J) W = F . S = 1N × 1m = 1 joule Joule : Work is said to be one joule, if a force 1 newton displaces a body through a distance of 1 m along the direction of force. 1 J = 1 N × 1 m = 1 kg. 2 m s . m 1 J = 1 kg m2 s–2 Relation between Joule and erg : 1 J = 1 N × 1 m = 105 dyn × 100 cm = 107 dyn cm or 1 J = 107 erg. Types of Work done by a force : Work done by a force can be positive, negative or zero depending upon the directrion of force and direction of motion. (displacement) a. Positive work done : Work done by a force on a body (or an object) is said to be positive work done when the body is displaced in the direction of applied force. Here in the figure ,Force and displacement are in the same direction Example: The body falling towards the earth under the action of gravity has positive work done by the gravitational force. b. Negative work done : The work done by a force on a body is said to be negative work done when the body is displaced in a direction opposite to the direction of the force. Example: When brakes are applied on a moving vehicle, work done by the braking force is negative. c. Zero work : a) If S = 0 i.e., the body does not move from its position on the application of force, then W = 0 . Thus, no work is done by the force if it fails to displace the body. Ex : When a person pushes a wall but fails to move the wall, then work done by the force on the wall is zero. b) When a body moves in a direction perpendicular to the direction of the force no work is done by the force. Examples : i) When a person carrying a suitcase in his hand or on his head is walking hori zontally, the work done against gravity is zero. ii) No work is done on a body when it moves along a circular path. iii) Work done by the flying aeroplane is zero as the force and displacement are perpendicular to each other. Activity: Suspend a brass bob from an iron stand as shown by the position C. Place a matchbox very close to the brass bob. Now stretch the brass bob to position A which is at a higher level. Release the brass bob. It will be seen that as the brass bob hits the matchbox, it shifts from position 1 to position 2.