Heat, Light and Sound Heat, light and sound are forms of energy that have many applications in our lives. Heat Heat is a form of energy When petrol burns in a car engine it causes the gases in the cylinders to expand pushing the pistons down. Heat energy has been converted into kinetic energy (movement) which is used to propel the car forward Heat causes solids, liquids and gases to expand by making the molecules vibrate more and move apart Addition or removal of heat causes substances to change state Sublimation: changing directly from a solid to a gas without becoming a liquid e.g. CO 2 and Iodine (I 2 ) Expansion of solids when heated and contraction when cooled Make sure that the ball passes through the ring Heat the ball in a Bunsen flame Try to put the ball through the ring Result: the ball does not pass through the ring Cool the ball under the cold tap Try to put the ball through the ring Result: the ball passes through the ring Conclusion: Solids expand when heated and contract when cooled Expansion of liquids when heated and contraction when cooled Make sure there is liquid in the glass tube Gently heat the bottom of the flask Result: the liquid rises up the glass tube Allow the liquid to cool Result: the liquid level in the tube falls Conclusion: Liquids expand when heated and contract when cooled Expansion of gases when heated and contraction when cooled Heat the flask gently Result: Air bubbles emerge from the flask Allow the flask to cool Result: water enters and rises up the tube Conclusion: gases expand when heated and contract when cooled 1
Demonstrate the Expansion of Water on Freezing Fill a bottle with water Place in a freezer overnight Remove the bottle from the freezer Result: The bottle has burst Conclusion: Water expands when frozen Consequences of the anomalous expansion of water This property of water is very unusual in that it expands between 4 o C and 0 o C It becomes less dense and so floats on water This causes ice to form on top of lakes rather than at the bottom The ice on top insulates the water below and stops the lake freezing to the bottom This allows fish and other animals to stay alive The expansion of water in pipes can cause the pipes to burst when frozen The burst is not discovered until the ice melts Determine the Melting Point of Ice Determine the Boiling Point of Water Setup the apparatus as shown Use ice from the freezer (-5 o C) Record the temperature every 30 seconds Stir the ice Result: the temperature rises at first then stays steady at 0 o C while the ice melts Conclusion: the melting point of ice is 0 o C Setup the apparatus as shown Place the thermometer just above the water Heat the water Record the temperature every 30 seconds Result: The temperature rises steadily then stays steady at 100 o C while the water boils Conclusion: the boiling point of water is 100 o C Investigate the Effect of Pressure on the Boiling Point of Water Make sure the clip is open and boil the water Note the temperature at which it boils Remove the heat source Close the clip Cool the flask with cold water to reduce the pressure in the flask Note the temperature Result: the water boils at a lower temperature Conclusion: reducing the pressure on the liquid reduces its boiling point The Difference Between Heat and Temperature Heat is NOT the same as temperature Temperature is the degree of hotness or coldness of a body. It is measure using a thermometer Heat depends on mass, temperature does not An egg cup full of water and a bucket full of water can both be at the same temperature (80 o C) The bucket will melt far more ice than the egg cup so it has far more heat 2
Plot the Cooling curve of Stearic Acid Place a test tube of solid stearic acid into the beaker of hot water When it has melted completely remove the test tube of molten stearic acid and place it in a beaker of cold water. Record the temperature of the stearic acid every 30 seconds Plot the results as shown in the graph until it has all solidified Result: The stearic acid cools and the temperature drops steadily When it reaches 70 o C the temperature remains steady while it is solidifying, until all the acid has solidified Conclusion: while the temperature is steady the liquid is changing to a solid. The liquid is losing heat to the cold water. The heat produced (or needed) to change state is called Latent Heat As it is melting the heat is being used to change the state from solid to liquid As it is cooling the heat is being given out to the cold water The temperature remains steady at is its melting point Heat Transfer All hot bodies radiate heat Transfer of Heat Energy by Conduction Use rods that are the same thickness and length Pour boiling water into the tank Result: The thumbtacks fell off at different times Copper fell first, the Brass, Aluminium and finally iron Conclusion: solids conduct heat from particle to particle Different substances conduct at different rates copper was best and iron worst To show that Water is a Bad Conductor of Heat Use a coin to hold the ice at the bottom of the tube Heat the water at the top of the tube Result: The water boils at the top of the tube while the ice remains at the bottom Conclusion: water is a bad conductor of heat Transfer of Heat Energy by Convection Place a crystal of potassium permanganate in the bottom of the beaker using a glass tube Heat the beaker below the crystal Result: the crystal dissolves and the pink colour is carried around the beaker by convection. Hot water rises above the Bunsen and as it cools at the top it sinks Conclusion: heat is carried through a liquid by movement of heated molecules. This is called a convection current 3
Transfer of Heat Energy by Radiation Switch on the infra-red lamp Result the temperature in the flask rises Conclusion: heat travels to the flask by radiation Radiation does not need particles. It can travel through a vacuum Heat from the sun reaches Earth through the vacuum of space by radiation Compare the Insulating Ability of Different Materials Pour boiling water into both containers Record the temperature every two minutes for half an hour Repeat using different materials to insulate the container Result: the temperature falls more slowly in the insulated container Conclusion: Insulating materials prevent heat passing through Different materials have different insulating abilities The slower the temperature drops the better the material is at insulating Light Light is a form of energy and that it can be converted to other forms of energy Plants use photosynthesis to change light energy to chemical energy Solar cells convert light energy to electrical energy which can then be used to drive a fan (kinetic energy) Objects that produce light are said to be luminous Luminous objects include: the sun, light bulbs, fire Objects that do not produce light are non-luminous are seen as a result of light reflected from them Speed of Light: 300 million metres per second Show that Light Travels in Straight Lines Line up the three holes using a metal rod Light the candle and look through Result: you see the candle Move one of the cards Result: you do not see the candle Conclusion: Light travels in straight lines How Shadows are Formed Use a small bright light source Place the key between the light source and the screen Result: a shadow forms on the screen The shadow has the same shape as the key Conclusion: A shadow results from light travelling in straight lines being blocked by an opaque object such as a key 4
Producing a Spectrum in the diagram Use a ray box to produce the beam of white light Shine the ray of white light through a prism Result: a spectrum forms on the screen Conclusions: The white light is dispersed (spread out) by the prism White light consists of red, orange, yellow, green, blue, indigo and violet light Red is bent least and violet is bent most This group of colours is called the spectrum Reflection and Refraction of Light Reflection of Light by a Plane Mirror and use of Ray Diagram Place a plane mirror on a sheet of paper Draw a line (N) perpendicular to the mirror Shine a ray of light to the point where this line meets the mirror Mark the position of the ray of light using four dots as shown Draw the rays using a ruler Measure the angles A and B Result: angles A and B are the same Conclusion: the reflected ray bounces off the mirror at the same angle as the original ray The image in a mirror is reversed Ambulance is written in reverse on the front so it appears the right way round in car mirrors Demonstrate the Operation of a Simple Periscope The two lenses are set at 45 o in the tube The first mirror bends the light 90 o The second mirror bend the light 90 o in the opposite direction Result: the path of the light has been moved by the distance between the two mirrors Conclusion: a periscope can be used to see around corners or over the heads of people The image in a periscope is the right way round due to double reflection Refraction of Light from Air to Glass and Glass to Air in the diagram Draw the outline of the glass block with a pencil Shine a ray of light at the glass block at an angle Mark the position of the ray into the glass with two dots as shown Mark the position of the ray out of the glass with two dots as shown Remove the block of glass Draw the two rays and then join them Result: the ray bends one way as it enters from air to glass The ray bends the other way as is goes from glass to air Conclusion: Light refracts in opposite directions as it goes from glass to air and from air to glass The ray coming out is parallel to the ray entering 5
The Refraction of Light from Air to Water and Water to Air Water to Air Place a coin in an empty basin Adjust your position till the coin is just hidden Get a friend to fill the basin with water Result: The coin becomes visible as the basin is filled Conclusion: Light is bent as it goes from water to air Air to Water Fill a beaker with water plus a little dye View it from the side Darken the room Shine a ray of light into the water Result: the ray of light can be seen bending as it enters the water Conclusion: Light is bent as it passes from air to water Show Refraction of Light through a Lens; When light passes through a lens it is refracted (bent) A convex lens brings the rays together at one point (the focus) A concave lens spreads the rays apart Demonstrate the Operation of a Magnifying Glass Take a sample of print Place a magnifying glass over the print Result: The print appears bigger Conclusion: a magnifying glass makes things appear bigger Applications Reflection: mirrors Refraction: to focus images using lenses in microscopes, spectacles and telescopes Sound Sound is a form of energy It is produced by vibrations (of your vocal cords or a loudspeaker) To Show that Sound is a Form of Energy Switch on the signal generator Result: the polystyrene ball moves when the sound is turned on Conclusion: Sound is a form of energy and is changed into kinetic energy when the ball moves 6
Show that Sound Transmission Requires a Medium Switch on the bell Turn on the vacuum pump Result: as the air is removed the sound gets weaker Conclusion: sound needs a medium to travel Ultrasound uses echoes to study babies in the womb without harming them Sonar uses echoes to measure the depth of water or locate submarines The time lag between seeing and hearing the same event As lightning jumps from a cloud to earth thunder is formed The speed of light (300,000,000 ms -1 ) is so big that the flash reaches the viewer almost instantaneously Sound is much slower (340 ms -1 )so it takes a measurable time to reach the viewer (about 4 seconds for every 1500 m) Reflection of Sound Show that Echoes are Reflected Sound Find a large open space with a high wall Clap your hands once, loudly Result: after a short delay the sound is heard a second time Conclusion: the second sound is caused by the sound reflecting off the wall and returning to you This is called an echo Hearing The inner ear detects sound Sound is measured in decibels Very loud sounds can damage your ears o Don t play music too loudly. o The ringing in your ears after a rock concert is a sign of damage Continuous loud noise can also damage your ears so if you are operating a machine wear ear protectors 7