18094990 Physics Notes

Physics Notes: Mr. Lew Compiled by ME! ©2008-2009

Table of Contents Thermal properties of matter...............................................................................2 Light........................................................................................................ ..................3 Kinetic Model of matter.........................................................................................5 Energy, work and power........................................................................................6 Transfer of Thermal Energy...................................................................................8

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Physics Notes: Mr. Lew Compiled by ME! ©2008-2009

Thermal properties of matter 1. Atoms rotate and translate (move in a straight line), hence they have kinetic energy. 2. Atoms also have latent energy (potential energy) due to their bonds. 3. The internal energy of an object is made up of kinetic energy and potential energy. 4. Temperature measures internal energy. Symbol: ∪ 5. Why does evaporation produce cooling? During evaporation, the more energetic molecule (more kinetic energy) escapes from the water surface. The total amount of internal energy in the remaining molecules therefore decreases. Since temperature measures internal energy, if internal energy decreases, temperature will also decrease. (Cooling takes place) 6. Differences between evaporation and boiling 7. Factors affecting rate of evaporation: Evaporation Boiling 1. No bubbles seen Bubbles present 2. Occurs at any temperature Occurs at boiling point 3. Occurs on the surface Occurs throughout the liquid 4. Do not need external heat Needs external heat 5. Slow process Fast Process 8. Factors affecting rate of evaporation: • Temperature • Humidity • Surface area • wind • pressure • boiling point 9. Melts Steam Boiling/evaporation Freezes condensation Ice

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Physics Notes: Mr. Lew Compiled by ME! ©2008-2009

Light 1. Light • • •

moves via: Reflection(light bounces back) Refraction(Lights bends) Lenses(Light converses)

2. Reflection Laws of reflection: A. The incident ray, the reflected ray and normal, at the point of incident, all lie in the same plane. B. The angle of incident is equal to the angle of reflection. 3. A. θi θr

θi=θ 4. Characteristics of the image of a plane mirror: • Virtual • Same size • Laterally inverted • Upright • Object distance is the same as image distance 5. Char Mirror t

40m

40m

6. Refraction: The bending of light when it travels from one medium to the other. 7. Laws of refraction: I. The incident ray, the refracted ray, the normal, at the point of incidence, all lie in the same plane II. When light travel from one medium to another, the ration of the sine of angle of incident to the sine of angle of refraction is a constant. III. sinisinr=Constant IV. This constant is called the “refractive index”, also known as Snell’s law. Page 3 of 9

Physics Notes: Mr. Lew Compiled by ME! ©2008-2009 V. N=sinisinr 8. Less dense Denser 9. n=cv

denser: Bend towards normal. Less dense: Bend away from normal

C is speed of light in vacuum; V is speed of light in medium

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Physics Notes: Mr. Lew Compiled by ME! ©2008-2009

Kinetic Model of matter 1. Kinetic theory is used to explain: a. Why solid have a fixed shape and size b. Why matters expand when heated 2. Kinetic theory talks about molecules. These molecules are constantly vibrating. In liquid or gas, they move. 3. 4. Brownian motion Solid Liquid Gas Arrangement Molecules are Molecules are Molecules are packed, thus randomly packed very far apart. resulted in high without pattern. Molecules will fill density. The The molecules any available pattern is regular. are also further spaces. apart. Movement Can only vibrate Can vibrate and Can move freely, move within fixed randomly and at vessel high speed Forces Intermolecular Intermolecular Little or no forces forces are very forces are weak between strong molecules 5. Brownian motion Molecules move in a random and continuer motion. • Smoke particles move randomly • Smoke particles move continuously without stopping • Glass cell is enclosed and there isn’t wind. 6. What causes smoke particles to move? • Air molecules bombard smoke particles and cause them to move. • Air molecules move continuously and randomly 7. Questions: 1) What happens to the smoke particles when temperature rises? The smoke particle move faster 2) What would happen if the smoke particles are larger? The smoke particles will slow down 3) Why do we need smoke particles to study movement of air particles? Because air molecules are not visible to the naked eye 4) Why do solids expand when heated? When metal is heated, the molecules gain more kinetic energy and vibrate vigorously. When energy molecules vibrate vigorously, the solid will expand. Page 5 of 9

Physics Notes: Mr. Lew Compiled by ME! ©2008-2009

Energy, work and power 1. Work is said to be done when a force produces motion. 2. SI unit: Newton (Ν) SI unit for work: Joules (J) 3. Work done(J)=Force(N)×Distance in direction of the force(m) 4. Potential energy (PE) is the energy a body has because of its position. PE=massKg×gravity(m/s2)×height(m) 5. Kinetic energy (KE) is the energy a body has because of its position. KE=12×mass(Kg)×velocity(r2) 6. There are many forms of energy. Some examples include nuclear, solar, chemical, wind, electrical etc. 7. Principle of conservation of energy: Energy can neither be created nor destroyed. However, it could be change to another form. 8.

At point B, the speed is the fastest. 9. List of some energy conversion: • Microphone---sound energy to electrical energy • Electric cooker---electrical energy to heat energy • Dynamo---mechanical energy to electrical energy • Loudspeaker---electrical energy to sound energy • Iron---electrical energy to heat energy • Solar panel---solar energy to electrical energy 10.Energy is the capacity to do work 11. Efficiency: Efficiency=useful energyinput energy×100% 12.A bullet of mass 100g hits a block of wood. How high up will the block of wood swing up? KE of bullet= 12×mr2 = 12×1001000×22=0.2J The KE of bullet turns into PE of the block of wood. PE=mgh 0.2=0.6×10h ∴H=0.2÷0.6×10=3.33 Page 6 of 9

Physics Notes: Mr. Lew Compiled by ME! ©2008-2009 13.Power is the rate/time of doing work. SI unit: Watt(W) 14.Power can be calculated using: A. P=WDt B. P=Et C. P=F×V 15.1KW=1000W 1mW=1000000W 1gW=1000000000W 1NW=0.001W 16.A boy takes 10 seconds to reach the top of the stairs. Find the power exerted by the boy given that height of each step of the stairs is 2m and consist of four steps. P=wdt =50010×8100=4W 17.A 60W lamp transfers half its electrical energy to light energy. How much light energy does it emit in 10s? E=PT=60×10=600 ∴energy emitted=600÷2=300J

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Physics Notes: Mr. Lew Compiled by ME! ©2008-2009

Transfer of Thermal Energy 1. Three methods to transfer heat: • conduction(solid, liquid, gas) • convection(liquid, gas) • radiation(solid, liquid, gas, vacuum) 2. Examples of good conductors are metals. Examples of poor conductors (Insulator) are glass, plastic, water, air, wood, etc. 3. Mechanism of conduction: a) When A is heated, thermal energy is supplied to the molecules. b) The molecules gain more kinetic energy and vibrate more and faster. c) The more “energetic” molecules collide more often against their neighboring molecules, passing some energy to their neighbor. d) The neighboring molecules become more energetic and pass on the energy to the molecules further away. e) The process is repeated until the energy has reach B. f) This is how energy is conducted from A to B.

Test-tube

Wire gaze

Heat

Ice

4. Thermal energy always flows from higher temperature to lower temperature. 5. Water boils at the surface but the ice will not melt. This is due to: • Water is a poor conductor of heat; • Convection cannot occur 6. Convection is the movement of heated molecules. 7. Convection current is form when hot water, being less dense, rises and when colder water, being denser, sinks. Since water crystals are colourless, purple crystals are added to make the water purple in colour so that the convection current can be seen. 8. Heat can also be transmitted through radiation, in the form of electromagnetic waves. Heat is also called infra-red waves. Objects that give off heat are called emitter. Cold air, less dense, sinks. 9. Shiny, silvered surface are poor conductor of heat and poor absorbers of heat. 10.Black, dull surfaces are good conductor of heat and good absorbers of heat. 11.Questions: a) Why do we wear light-coloured clothes under hot sunlight? Light-coloured clothes are poor absorbers of heat. Page 8 of 9

Physics Notes: Mr. Lew Compiled by ME! ©2008-2009 b) 2 cups, filled with water are placed under the sun. One of the cups is shiny and the other is black and dull. Which cup of water is warmer? Why? Black and dull cup, as black colour is good absorbers of heat. c) The 2 cups are then placed in a room. After some time, which cup shows a lower temperature? The black and dull cup, because black are good emitters of heat. 12. Factors that affect the rate of radiation: • Colour texture of the surface. • A hotter surface temperature will result in faster emission of heat. • The bigger the surface area is, it will result in faster emission of heat. 13. Applications of good conductors • Cooking utensils • Soldering iron rods 14. Applications of bad conductors • Handles of appliances and utensils • Table mats • Saw dust • Wooden ladles

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