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~SAUD~ PHYSICS notes o General physics • The density of an object only changes when the substance is heated or cooled. • An object sinks if its density is more than the liquid on which it is placed on, but floats if less dense. • When there is no air resistance the acceleration of any object is constant (constant= 10m/s) • Easier to lift an object on the moon because gravity is less on the moon. • Resultant Force. • Gravitational, electrostatic and magnetic forces can be applied from a distance, other than that you have to be in contact with the object to apply a force. • A force applied on an object can cause a change in speed, direction and shape. • Acceleration, velocity etc. is a vector quantity. • When an object moves in a circle it is accelerating and at constant speed at the same time; because the direction is changing but the speed is constant. • Centripetal force is caused by tension in the string (ball on a string), gravity (satellite in space), friction between the tires and the road (a car on the road). • For an object to be in equilibrium state the resultant force has to be equal or zero (forces up = forces down) and the resultant moment has to equal zero (clockwise moment = anticlockwise moment). • For the stability of an object it has to have a wide base and a low center of mass. • To find the center of mass draw two points A and B, 1 ~SAUD~ hang the object from each hole so that it can swing freely. Tie a weight to each point then draw the vertical line, the center of gravity is where the lines intersect. • When a car is going up a hill K.E. = Same, G.P.E = Increases, Chemical Energy (petrol) = Decreases and the total energy stays the same as energy is not lost, created or destroyed. • Internal energy = Thermal energy. • Fission = Process called when large molecules break down into small. Example: power station of uranium. • Fusion = Process called when small molecules builds into large molecule. The fusion process produces more energy than fission. Example: Sun. • The mass lost is converted into heat energy. • In barometers the space at the top is a vacuum, because air would prevent the mercury from rising. • The width of the barometer does not affect the height of mercury. • Water manometers are more accurate to measure gas pressure. • To get the total pressure, remember to add the atmospheric pressure. • The gas pressure can be converted from mm of Hg to Pa by using P = density*gravity*height, but remember to use the total pressure. • Pressure= force/area • The more the area, the more the force & the less the area the, the less the force. 2 ~SAUD~ Thermal Physics • The pressure is the same on all the walls of the containers as the molecules move in a random motion. • Temperature is proportional to K.E. + Molecules hit the walls harder and more often. • Brownian motion: The tiny, fast-moving air molecules, which are in a constant random motion, are hitting the larger smoke particles from all directions. • Evaporation: It is the vaporization of liquid, when the fast moving molecules/energetic molecule leave the surface without the temperature reaching its boiling point. • The greater the surface area, the more the evaporation of particles. • Evaporation occurs at surface of liquid: no bubbles. • Boiling occurs throughout the liquid: bubbles appear. • We feel cold after bath because the high energetic molecule evaporate leaving behind, dull/less energetic molecules. • Thermal expansion: molecules gain kinetic energy and force results in pushing each other further apart. • Volume = less area so the particles hit the walls harder and more often. • Mercury is used to measure high temperature. • Alcohol is used to measure low temperatures. • The capillary tube of a thermometer is narrow so a small change in temperature results in a large change in volume of the liquid. • Thermocouples have a low specific heat capacity this means they can measure rapidly changing temperatures and they don’t take much heat away from the object they 3 ~SAUD~ are measuring the temperature of them. • During melting or boiling the temperature stops rising because extra energy is needed, which is taken from the heat source, to overcome the strong forces of attraction which are holding the molecules together. This energy will not increase the speed of the molecules it will only break the bonds. • In an experiment the value for the latent heat of fusion is too low as energy is taken in from the surroundings which melt the ice. • In an experiment the latent heat of vaporization is too high as energy is lost to the air from the heater therefore not all the energy is used for evaporating the water. • The value for the latent heat of vaporization is much higher than the value for fusion because a large amount of energy is needed to break the bonds between the molecules to move them far apart • Energy= Mass x Specific heat capacity x Change in temp. • Energy to break bonds= Mass x Latent heat of fusion/vaporization. 4 ~SAUD~ o Waves, Light & Sound • Waves transfer energy from one place to another. • There are 2 types of waves: Transverse and Longitudinal waves • Transverse wave: A wave in which the particles vibrate perpendicular, to the direction of movement. A complete wave consists of 1 crust and 1 trough. E.g. Light waves • Longitudinal wave: A wave in which the particles vibrate back and forth in the direction of movement. It consists of compressions and rarefactions. E.g. Sound waves • Compressions are the area in a wave where the particles are closer to each other and the pressure in that area is highest. • Rarefactions are the area in a wave in which the particles are far apart and the pressure is least. • The larger the frequency and amplitude the greater the energy of the wave. • When a wave gets refracted, its Speed decreases, Wavelength decreases, Frequency = Same. • When a wave gets reflected, it’s Speed = Same, Wavelength = Same, Frequency = Same. • When a wave gets diffracted, it’s Speed = Same, Wavelength = Same, Frequency = Same. • Radio waves are easily diffracted because they have a large wavelength. • In the spectrum of light all the colors have the same speed, but different wavelengths & frequencies. • Objects that produce light are called luminous objects and the object which uses the light from source to reflect and make it see is called illuminated objects. • When a light wave travels from a less dense material to a more dense material, the refractive index is more than 1. 5 ~SAUD~ • When a light wave travels from a more dense material to a less dense material, the refractive index is less than 1. • A fish in a pond appears to be higher than it actually is because light refracts. • Total incidence angle is always = Total reflection. • Refractive index = speed of light in air/speed of light in that material. • Remember that when the ray is passing from less dense to more dense the formula applied is Refractive index = sin i/ sin r • But when it passes from more dense to less dense we use Refractive index = sin r/ sin i. • Reflective index = 1/sin C (C stands for critical angle). • Total internal reflection is when the ray is travelling to a less dense material from a more dense material and also if the incidence angle is above 42 degrees. • The center of a lens is called its optical center, C. • The line through C at right angles to the lens is called the principle axis. • The fatter the lens, the stronger it is and the shorter its focal length. • Parallel light, e.g. Sun, must be used to find the focal length of a lens. • When the object is beyond 2F the image comes in between F and 2F, real, inverted and smaller than the object. • When the object is in between F and 2F the image comes beyond 2F, real, inverted and same size as the object. • When the object is before F the image appears in, real, inverted and smaller than the object. • Dispersion of white light occurs because each fraction of white light has a different wavelength, so they are slowed 6 ~SAUD~ down by different amounts. • A spring is used to represent longitudinal waves. The speed of sound in: Air 330 m/s Water 1400 m/s Wood 4000 m/s • Ripple tank can be used to see transverse wave. • When a loudspeaker moves out the air is compressed, when it moves in the air is rarefied. • Frequency = number of vibrations per second. • Speed (m/s) = Frequency (Hz) x Wavelength (m) • The speed of sound increases as the particles move closer together. • Gamma Rays, X-Rays, Ultra-Violet are Carcinogenic. Wave Use 1.Gamma Rays  Sterilize food & equipment, Treat Cancer 2.X-Rays  Go through body to check for broken bones 3.Ultra-Violet  Suntan, Make fluorescent materials glow 4.Light  Allows you to see 5.Infrared  Heating, Remote Control, Mobile phones, Night sight 6.Microwaves  Send messages, Cook food 7.Radio & T.V.  Send messages 7 ~SAUD~ o Electricity & Magnetism • Ferrous (magnetic) metals are Iron, Steel, Nickel & Cobalt. • The true test for a magnet is that it repels another magnet. • When a ferrous metal is magnetized the domains are turned so that they are lined up. • Making Magnets: Stroking, Electrically (D.C.) & Induction. • Demagnetizing a magnet: Heating, Hammering, and Electrically (A.C.). • Uses of permanent magnets: Compasses, Door catches. • Uses of electromagnets: Scrap yards, Electric bell, and Electric relay. • Conductors have free moving electrons in them, insulators don’t. • In electrostatic experiments metals cannot be used as they are good conductors of electricity and don’t stay charged. • In ammeters and voltmeters Red terminal = Positive, Black terminal = Negative. • Current in a series circuit always remains same but gets divided in parallel. • Voltage in a series gets divided but
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