revision notes for WA3.pdf

7: WORK=ENERGY & POWER WORK DONE Work done is defined as the product of the force and distance in the direction of the force. Implication: There is a transfer of energy from one system to another system. This transfer of energy may change the total energy of a system. Formula: Work...

7: WORK=ENERGY & POWER WORK DONE Work done is defined as the product of the force and distance in the direction of the force. Implication: There is a transfer of energy from one system to another system. This transfer of energy may change the total energy of a system. Formula: Work done, WD = Force x distance moved in the direction of the force W. D =F x dist. (Joule, J) (N) (m) EXAMPLE W.D by 10 N force = 10 N x 5 m = 50 J W.D against gravity = 50 N x 3 m = 150 J WORK = ENERGY LOST OR ENERGY GAINED Example: W. D = F x d = 20 N x 3 m = 60 J It means 60 J of energy is transferred to the box and the box gained 60 J of energy. 7.1 HOWEVER NO WORK IS DONE IF… 1) the object does not move. OR 2) The force applied is perpendicular to the distance moved WORK DONE AGAINST FRICTION = Frictional force x distance moved = ENERGY LOST DUE TO FRICTION Energy Stores and Transfers: S.I unit of energy is Joules, J: Energy Transfer: It is useful to think of energy as a non-physical and invisible substance that can be transferred from one energy store to another. The total amount of energy remains constant. 7.2 Energy can be transferred from one energy store to another in four different pathways: mechanically (by a force acting over a distance); by heating (as a result of a temperature difference); by propagation of waves (both electromagnetic and mechanical); and electrically (by charges moving through a potential difference). Energy in Kinetic Store / Kinetic Energy (K.E) Kinetic energy is the energy a body posses due to its movement. Kinetic Energy = ½ mv2 7.3 Energy in Gravitational Potential Store/ Gravitational Potential Energy (G.P.E) Gravitational Potential Energy = mgh mass height of object from lowest level 10 m/s2 Important Note: h is always the change in vertical height from the lowest level: LAW OF CONSERVATION OF ENERGY The law of conservation of energy states that energy cannot be created or destroyed. Energy can be transferred from one store to another. Total energy of an isolated system is constant. Law of conservation of energy for quantitative questions (P.E)A + (K.E)A = (P.E)B+ (K.E)B (Assuming there is no energy loss due to friction/air resistance) 7.4 If there is energy transferred to non-useful stores due to friction and work done by external force, the formula will be as follow: (P.E)A + (K.E)A + W.D by External F = (P.E)B + (K.E)B + W.D against friction “Allowance” “Savings” “Expenses”/ “Spending” POWER Power is defined as the rate of doing work or the rate of energy transferred. In reality, There is energy transferred to non-useful stores like the internal store of the objects or the surrounding due to work done against friction/air resistance. 7.5 Energy Dissipation: No system is perfect. Whenever there is a change in a system, energy is transferred and some of that energy is dissipated. Dissipation is a term that is often used to describe ways in which energy is wasted. Any energy that is not transferred to useful energy stores is said to be wasted because it is lost to the surroundings. Electrical cables warming up are a good example of this. It is not useful to have hot wires behind a television as energy is dissipated to the surrounding air. In a mechanical system, energy is dissipated when two surfaces rub together. Work is done against friction which causes heating of the two surfaces - so the internal energy of the surfaces increases. EFFICIENCY, : The efficiency of a system is defined as the ratio of useful energy/power output to the energy/power input. OR The efficiency of a system is defined as the ratio of useful power output to the power input. OR 7.6  9: PRESSURE Pressure is the force acting perpendicularly per unit area of a surface. 𝑭𝒐𝒓𝒄𝒆(𝒘𝒆𝒊𝒈𝒉𝒕) 𝑭 𝑷𝒓𝒆𝒔𝒔𝒖𝒓𝒆 = = 𝒄𝒐𝒏𝒕𝒂𝒄𝒕 𝒂𝒓𝒆𝒂 𝑨 Note: Unit of pressure is Pa = N/m2 ≠ N/cm2 Inverse relationship between Contact Area and Pressure For constant force/weight, The greater the contact area, the smaller the pressure The smaller the contact area , the greater the pressure Maximum pressure = Weight / smallest contact area Minimum pressure = Weight / largest contact area 9.1  PRESSURE IN LIQUIDS Pressure due to a liquid column = hg h; Vertical height of liquid above that point. (m) : density of the liquid g: gravitational field strength, 10N/kg or 10ms-2 NOTE: LIQUID PRESSURE is independent of the cross sectional area, A. Point A and point B have the same pressure as they have the same depth of water above them. IMPORTANT KEY CONCEPTS IN PRESSURE: 9.2  Pressure at point Y = h g + Patm BAROMETER (To measure atmospheric pressure) Pressure at point X = Pressure at point Y Pressure at point X = Patm Pressure at point Y = Patm = h g h is about 760mm or 76cm Pressure at point X = Pressure at point Y Pressure at point X = Patm Pressure at point Y = Patm = h g + Ptrapped air h is LESS THAN 760mm or 76cm 9.3  Conversion from cm Hg to Pa (Use the formula P = hg) Assuming density of mercury, Hg = 13, 600 kg/m3 Example: Pressure = 76 cm Hg = (0.76m)(13 600)(10) = 103 000 Pa Conversion from Pa to cm Hg (Use the formula P = hg) Example: P = hg 100 000 = h (13 600)(10) h = 0.735 m h = 73.5 cm Hg HOW DENSITY OF THE LIQUID AFFECT THE HEIGHT Using denser liquid, the height is shorter h1 Using less dense liquid, the h2 height is longer Using the formula, Less dense Denser liquid P = ρgh liquid No change 9.4 MANOMETER: To measure gas pressure Pressure difference = diff. in height Either: Pgas – Patm = h cm Hg (Pgas > Patm) Or: Patm – Pgas = h cm Hg (Patm > Pgas) Using same liquid, same level same pressure: Pressure at point A = Pressure at point B Pgas = hρg + Patm Note: It is a good practice to always “draw” the pressure due to atmosphere or gas so that the atmospheric pressure or gas pressure is “visible”. 9.5 Hydraulic Systems Hydraulic systems work by using liquids (usually oil is used) under pressure. They make use of two properties of liquids. 1. Liquids are incompressible. 2. The pressure exerted by liquids in all directions are equal. F1 F2 KEY CONCEPT IN EXPLANTION Pressure in piston 1 is equal to pressure in piston 2 Since area of piston 1 is much smaller than piston 2 Force on piston 1, F1 is smaller than force on piston 2, F2 as Pressure = force / area 9.6 9.7  11: Thermal Processes I. There must be a difference in temperature for the transfer of energy to take place II. Energy is transferred from a region of high temperature to a region of low temperature III. At thermal equilibrium, all regions are at the same temperature and there is no net transfer of energy. Three processes by which energy may be transmitted ▪ conduction ▪ convection ▪ radiation What is conduction?? (More effective in solid) Conduction is the process by which energy is transmitted through a medium from one particle to another through molecular vibrations without the transfer of the medium. How Conduction Works ?? (More effective in solids as solid particles are closely packed together) Molecules at the hot end of a solid material absorb energy and start to vibrate vigorously. These molecules vibrate so much that they collide with neighboring molecules. During these collisions, some kinetic energy is passed on to the neighboring molecules. This motion repeats throughout the whole object. Why metal are good conductors of heat ?? Metals have free electrons. These electrons can move freely to other parts of the solid so energy is transferred by free electrons diffusion as well as molecular vibration hence transferring energy to other parts more rapidly. Why the marble floor feels cooler than the carpet? Concept: Marble is a better thermal conductor than carpet. More energy is conducted from the person to the marble. 11.1 Convection (mainly for liquids or gases) Convection is the transfer of energy by means of fluid movement (in liquids or gases), due to a difference in density. How Convection works?? Convection occurs because of change in the density of a fluid substance when it Is heated/cooled. When a fluid is heated, that part of the fluid expands. Since the mass of the fluid remains constant, this hotter region of the fluid becomes less dense as compared to colder region of the fluid which is more dense. The less dense fluid will then rise while the colder region sinks. This creates a convection current in the fluid that transfer energy throughout the fluid. How Convection Works?? (Shorter version) Convection occurs because of change in the density of a fluid when it is heated. When a fluid is heated, it expands and the hotter region of the fluid becomes less dense and rises. While the colder region of the fluid is more dense and sinks. This process repeats and creates a convection current that transfer energy throughout the fluid and increasing its internal energy. Common Question: Why is it not effective to place the heating coil of the kettle at the top ? As warm water is less dense, very little convection current will be formed below the coil to transfer energy throughout the whole liquid. Water is a poor thermal conductor. Convection in refrigerator Convection occurs because of change in the density of a fluid. The fluid near the freezer will contract when cooled. The colder region of the fluid is more dense and sinks. While the warmer region of the fluid is less dense and rises. This process repeats and creates a convection current that causes the content to lose internal energy and a drop in its temperature. 11.2 Radiation Radiation is the transfer of energy in the form of electromagnetic waves without the aid of a medium. All bodies emit electromagnetic radiation. The radiation does not require a medium to be transmitted. This is how the Earth is warmed by the Sun. It can take place in vacuum and at any temperature. Note: Radiation is not heat, but rather a wave that only changes into heat when absorbed. Dull & dark surfaces are GOOD (fast) absorber and emitter of radiation. Shiny & White surfaces are POOR (slow) absorber and emitter of radiation. DULL & BLACK SURFACES ALWAYS GAIN OR LOST RADIATION AT A FASTER RATE THAN SHINY & WHITE SURFACE. Black, Black, White, White, Dull Shiny Dull Shiny Best (fast) Worst (slow) absorber and absorber and emitter of emitter of radiation radiation Rate of heat radiation is affected by : surface colour and texture of the surface surface temperature: higher temperature, faster rate of heat radiation surface area: greater area, faster rate of heat radiation 11.3 Application in Thermal / Vacuum Flask The stopper will reduce the energy losses through convection and evaporation as it prevent the hot air from escaping. The space between the two walls is a vacuum. This can stop/reduce energy transfer by conduction and convection as there are no particles/medium to transfer the energy. The shiny bright silvering coated glass surfaces can reduced the heat lost by radiation as silver surface colour is a poor absorber and emitter of radiation Application in Double-layered window / door/wall Trapped air Concept: The layer of trapped air between the window or door or wall help to reduce rate of energy transfer to the surroundings by conduction as air is a poor thermal conductor. 11.4 Tips for Application Questions: material ---→ conduction surface colour/texture/surface area ----→ radiation ___________________, thereby increases/decreases the rate of energy transfer by conduction/convection/radiation. 11.5  Key Concept in Thermal Processes How energy is transferred in conduction, convection and radiation? Conduction Convection Radiation Energy is transferred Energy is transferred by movement of in the form of Energy is transferred electromagnetic gas or liquid through molecular waves without the particles due to vibration and collision aid of a medium. differences in with neigbouring density. molecules. can be transferred through vacuum. Rate of absorption /emission of radiation is Molecular Vibration affected by : Free electrons in the metal (i) surface colour and texture (ii) surface temperature: higher temperature, faster rate of heat radiation (iii) surface area: greater area, faster rate of radiation (In metals) Molecular Vibration Free Electron Diffusion 11.6