Heat and temperature. Making a thermometer

Transcription

1 Heat and temperature A bout four hundred years ago, it would have been impossible to tell the temperature of the air, the water or any other substance. That s because there was no such thing as a thermometer to measure temperature with. Objects were just described as hot, cold or warm. The meaning of temperature The words hot, warm, cool and cold are often used to describe temperature. The actual temperature of an object or substance is measured with a thermometer in degrees Celsius. For example, if a cup of hot chocolate is left sitting on a bench, it cools down. The fast-moving particles in the hot chocolate give up some of their energy to the air near the cup. The hot chocolate keeps cooling until it reaches room temperature. If chilled juice is left sitting on a bench, it warms up. The particles in the juice gain some energy from the warmer air near the glass. Left out of the fridge, the juice warms up until it reaches room temperature. The temperature of an object or substance depends on how fast the particles inside it are moving. The faster the particles move, the higher the temperature. Why temperature changes Heat is the energy that flows from a hotter object to a colder one. When an object receives heat energy, its particles move faster and its temperature rises. When an object gives up heat energy to another object, its particles move more slowly and its temperature drops. Heat continues to flow from hotter objects to colder objects until their temperatures are equal. The movement of heat then stops. Heat never flows from colder to hotter objects. During cold weather, snakes lie against rocks that have absorbed some heat from the sun. The fast-moving particles in the rocks transfer some of their energy to the snake, warg it up. Making a thermometer You will need: heatproof mat, Bunsen burner and matches gauze mat and tripod flaskstopper with hole in it glass tube retort stand bosshead and clamp food colouring water-based marker. Set up the equipment as shown in the diagram. Retort stand Water with added food colour Glass tube Stopper Clamp Flask Heatproof mat Gauze mat Tripod Bunsen burner 214 Science Alive for VELS Level 5

2 Go to worksheet 10.1: Introducing Anders Celsius Thermometers taking up space When an object receives heat, its particles move faster. The faster the particles move, the more space they take up. As the particles take up more and more space, the object expands. A thermometer works because the substance inside it takes up more space when it is heated. The substance used in most modern thermometers is alcohol, dyed red to make it easier to see. Column A very fine column rises from the bulb, up the thermometer. When the bulb is heated, the alcohol inside heats up too. As the alcohol expands, it has only one place to go up the column! The amount that the alcohol expands depends on its temperature. Higher temperatures make the alcohol expand further up the column. Scale The scale is designed so that the height of the alcohol in the column can be read as temperature. This thermometer would be useful for measuring temperatures between 0 and 100 C. It is measuring a temperature of 23 C. Bulb The dyed alcohol is stored in the bulb of a thermometer. 100 c 90 c 80 c 70 c 60 c 50 c 40 c 30 c 20 c 10 c 0 c REMEMBER 1. In what unit is temperature measured? 2. What happens to the particles in an object that receives heat energy? 3. What happens to the particles in an object as it cools? 4. What happens to the liquid in the bulb of a thermometer when it gets warmer? THINK 5. Are heat and temperature the same thing? Explain your answer. 6. Could the thermometer in the How it works section be used to measure the temperature inside a freezer? Explain your answer. 7. In which of these situations would heat travel from the first object to the second? Explain your answer. (a) From a glass block at 42 C to water at 50 C (b) From an oven at 120 C to an oven dish at 120 C (c) From an ice cube at 2 C to another ice cube at 4 C IMAGINE 8. Imagine that the makers of a thermometer made a mistake; the columns in their thermometers were all made a little too wide. How would that affect the accuracy of the thermometer? SKILLBUILDER 9. What temperatures do the following thermometers show? Use the retort stand and clamp to keep the flask and glass tube steady. Use a water-based marker to mark the level of coloured water in the glass tube. Light the Bunsen burner according to the rules on page Observe what happens to the level of coloured water in the glass tube while it is being heated. Record your observations. Turn the Bunsen burner off before the water boils. 2. Observe what happens as the water cools. Record your observations. 3. Explain how the equipment used in this experiment could be used to make a thermometer Heat, light and sound learning I CAN: explain what is meant by temperature and heat describe how a thermometer works describe how heat and temperature are related. 0

3 Heat transfer: conduction T he metals used to make pots and pans are very good conductors of heat. This helps to ensure that the heat from the flame or hotplate is evenly spread. To understand how heat is transferred through an object by conduction you need to look inside the object. Looking inside an object The particles in a solid are packed very closely together. They can vibrate on the spot, but they cannot move from one place to another. If some of the particles are heated, they cannot move along the object to transfer heat to the whole object. When particles are heated (for example, with a flame), they start to move more quickly. When the fast-moving particles collide with other particles, they cause nearby particles to start vibrating more quickly as well. Bunsen burner flame Direction of heat transfer High temperature Low temperature Eventually, as particles keep colliding with others, some of their energy is transferred along the object. This process is known as conduction. Conduction Heat travels by conduction when fast-moving particles collide with other particles, making them move faster. Heat can travel by conduction through objects, or from one object to another. Heat travels by conduction at different speeds, depending on the type of substance. Heat travels more quickly in solids than in liquids or gases. Conduction occurs more quickly when the particles in an object are closer together. Solids are usually very good conductors of heat because the particles in them are packed closely together. Gases are the poorest conductors because the particles in them are far apart. The particles in a solid are packed closely together. If some particles receive heat energy and begin to move faster, they collide easily with other particles nearby and pass the heat energy along. The particles in liquids are further apart than the particles in solids. When some particles receive heat energy and start to move faster, they collide with other particles. But the distance between the particles means that there are fewer collisions. So, heat is transferred by conduction more slowly in a liquid than in a solid. The particles in a gas are far apart. Heat does not travel easily by conduction through gases.

4 Go to worksheet 10.2: When warm feels cold Observing conduction in solids You will need: heatproof mat, Bunsen burner and matches tripod copper rod iron rod glass rod wax ruler stopwatch. Set up the equipment as shown at right. Light the Bunsen burner and start the stopwatch. Time how long it takes for the last of the three pieces of wax to melt completely on the copper rod. Record your results. Repeat for the other two rods. Make sure that the last piece of wax is the same distance from the heated end of each rod. 1. How can you tell that heat travelled along the length of the rods? 2. Through which rod did heat travel fastest? 3. Why should the wax be placed at the same distance from the heated end of each rod? 4. When conduction happens easily inside an object, the object is called a good conductor of heat. Which of the rods was the best conductor of heat? Wax pieces Tripod Heatproof mat Metal rod Bunsen burner REMEMBER 1. In which type of object (solids, liquids or gases) does heat travel fastest by conduction? 2. What effect does heating an object have on the speed of the particles inside it? SKILLBUILDER 6. The following is a table of results collected during an experiment similar to the one on this page. Material Time taken for last piece of wax to melt (s) Rock 8.0 Copper 6.5 THINK 3. Draw two diagrams of the particles inside a metal. In the first diagram, show how the particles would move before they are heated with a Bunsen burner. In the second diagram, show how the particles would move while they are being heated with a Bunsen burner. 4. Can heat travel by conduction through a vacuum, where there are no particles? Explain your answer. 5. Why does heat travel faster by conduction in some objects? Heat, light and sound Brick 11.0 Silver 5.0 Aluium 7.7 List the items in the table from the best conductor of heat to the poorest conductor of heat. INVESTIGATE 7. What is a poor conductor of heat called? 8. Name two substances that are poor conductors of heat. Find out what they are used for. learning I CAN: describe how heat is transferred by conduction explain why solids transfer heat by conduction more quickly than liquids and gases.

5 Heat transfer: convection U nless you have run out of hot water in the middle of a shower, you may not have given much thought to how the water was heated in the first place. Hot water comes out of the shower head, but it wasn t always hot. Heat has to be transferred to the water from somewhere else. Heat does not travel through liquids by conduction very well. Another heat transfer method, convection, is responsible for heating the water that comes out of your shower. Hot water systems Ready to use A pipe connects the water at the top of the water tank with the shower and every other hot-water tap. If this water is not used, it cools and sinks to the bottom, where it is heated again. Cold water in Cold water enters the hot-water system through a pipe that takes it to the bottom of the tank. Rising Hot water rises because its particles are further apart than those in cold water; it is less dense than cold water. As the hot water rises, cold water continues to move to the bottom of the water tank. Sinking The particles in the cold water move more slowly than the particles in the warmer water. The cold water sinks because its particles are close together. Cold water is more dense than hot water. Heating The flame heats the cold water at the bottom of the tank. The particles begin to move more quickly and spread out. Gas flame If you have a gas hot-water system, there is a flame at the bottom of the water tank. The flame heats the water near the bottom of the tank. An electric hot-water system has elements, similar to those in a kettle, inside the water tank. 218 Science Alive for VELS Level 5

6 Convection currents Like solids, the particles in liquids and gases take up more space when they are heated because they move faster. But, unlike solids, the particles in liquids and gases can move freely. The movement of particles in liquids and gases is called convection. The rising and sinking of water in the hot-water system is an example of a convection current. Convection currents can form in air as well as water because warm air is less dense than cold air. So, the warm air rises and the cold air sinks (where it is heated again). Dropper with food colouring Drinking straw Heatproof mat Heater Coloured convection currents You will need: 250 ml beaker heatproof mat, Bunsen burner and matches tripod and gauze mat food colouring with dropper drinking straw. Fill the beaker with water. Place it over the Bunsen burner as shown below. Beaker Water Gauze mat Tripod Bunsen burner Carefully drop about three drops of food colouring down the straw. Slowly remove the straw, making sure not to disturb the water. Light the Bunsen burner according to the rules on page (a) Draw a diagram to show what happens to the food colouring as the water is heated. (b) Why does this happen? 2. What is the purpose of the food colouring in this experiment? Heat, light and sound REMEMBER 1. List two sources of energy that are used to heat the water in hot-water systems. 2. Which is more dense hot or cold water? 3. When water is heated by a flame at the bottom of a container, why does water near the flame rise? THINK 4. Why can t heat travel through a solid by convection? 5. Is this an effective way to heat water in a saucepan? Explain your answer in terms of convection currents. 6. Why is the water that leads to hot-water taps taken from the top of a hot-water tank? 7. Gas wall furnaces usually push warm air out near the floor. Why not higher up? 8. Why doesn t the smoke from a factory keep rising forever? INVESTIGATE 9. Convection currents can occur in fluids. What does the word fluid mean? 10. Find out what a convection oven is and how it works. 11. The newest technology in hotwater systems is the continuousflow system. This system heats water only when hot water is required. Find out how continuous-flow hot-water systems work. What advantages do continuous-flow hot-water systems have over the type described on the opposite page. learning I CAN: Gas flame describe how the convection currents in liquids and gases are formed explain how a hot-water system uses convection currents to heat water.

7 Heat transfer: radiation T he Sun provides energy to the Earth. Without heat from the Sun, the Earth would be far too cold for humans to live on. Heat from the Sun must travel through space to reach the Earth. The heat does not travel by conduction or convection because there are too few particles in space to vibrate or to move between the Sun and the Earth. Heat from the Sun reaches the Earth by radiation. A different way to travel Heat that travels by radiation is called radiant heat. Radiant heat travels very quickly because it does not rely on the movement of particles to move energy from one place to another. The heat from the Sun takes about eight utes to reach the Earth, but would never reach us by conduction or convection. Striking a surface Radiant heat is similar to light. When radiant heat strikes a surface, it can be reflected, transmitted or absorbed. Most surfaces do all three, but some surfaces are better reflectors than others. Some surfaces are better absorbers of heat and others transmit more heat. Transmitted heat Transmitted radiant heat Clear objects, like glass, allow light and radiant heat to pass through them. The temperature of these objects does not increase quickly when heat reaches them by radiation. Radiated heat Absorbed heat Absorbed radiant heat Dark-coloured objects tend to absorb light and radiant heat. Their temperatures increase quickly when heat reaches them by radiation. Reflected heat Reflected radiant heat Shiny or light-coloured surfaces tend to reflect light and radiant heat away. The temperature of these objects does not change quickly when heat reaches them by radiation. Place your hand near the base of the globe of a lamp. Turn on the lamp. You feel the heat from the globe almost instantly. Heat does not travel through air easily by conduction. So, the heat does not reach your hand by conduction. Convection currents travel upwards, so the heat does not reach you by convection. The heat has reached your hand by radiation. Infra-red scanners All objects, including the human body, radiate some heat. The human body usually radiates more heat than the environment around it. That s why infra-red scanners are useful for finding people lost at sea, in bushland or even buried under a collapsed building. Infra-red scanners detect the radiant heat cog from the human body. 220 Science Alive for VELS Level 5

8 Go to worksheet 10.3: Make a solar oven Radiant heat You will need: bar radiator or microscope lamp 3 metal cans of the same size and shape lids to fit the three cans black and white paint 3 thermometers (or 3 temperature probes and a data logger). Paint one can white, one black and leave the third with an unpainted, shiny surface. Pour equal amounts of cold tap water into each can. Place lids on the cans, with holes for the thermometers to stand in. Make sure the lids are the same colour as the cans. Measure the starting temperature of the water in each can. Record your results. Place the three cans at the same distance from the radiator or lamp. Turn on the power to the heat source. Black Record the temperature of the water in each can every 2 utes for 14 utes. Radiator 1. Why was the temperature of the water measured before starting to heat the water? 2. How did the temperature of the water in each can change during the experiment? 3. Which cans were better absorbers and which were better reflectors of radiant heat? How can you tell? 4. Why was it important to use cans that were the same size? White Police use infra-red scanners to locate crials at night. Crials can t hide from the scanners. The scanners can distinguish between objects that are only 0.2 C different in temperature. Police choose to use scanned images in black and white rather than colour because it is difficult to tell the people from the objects in the coloured images. REMEMBER 1. Which method of heat flow is the only one for which particles are not needed? 2. List three different things that can happen to radiant heat when it reaches a surface. 3. List two uses of infra-red scanners. THINK 4. How do we know that heat travels from the Sun to Earth by radiation? 5. Why is it almost impossible for crials to hide from infra-red scanners? Shiny INVESTIGATE 6. Look at the back surface of a radiator like the one used in the experiment on this page. (a) What colour is the surface? (b) Why is the back surface this colour? 7. The change in temperature of water inside different containers was investigated in the experiment on this page. The surfaces were shiny, black and white. Design and perform an experiment to investigate the heat-absorbing properties of different colours. Which colours absorb more heat? How can you tell? The image of a crial formed by an infra-red scanner. Each shade of grey represents a different temperature. The crial looks white because he is the hottest object in the area Heat, light and sound learning I CAN: explain, in basic terms, how heat travels from the Sun to the Earth. describe similarities in the way heat and light behave when they strike different surfaces. Checkpoint

9 Insulation I nsulators, such as a thermos flask or a thick jacket, control the transfer of heat. Some materials are better at controlling heat that is moving by conduction; others slow heat transfer by convection, and some by radiation. Slowing down the flow of heat is the key to keeping drinks cool in the summer and warm in the winter. On a hot day, heat flows from the hot environment to a cold drink, until they are both at the same temperature. On a cold day, heat flows from a warm drink to the cooler environment, until their temperatures are equal. Thermos flask Heat does not travel into or out of a thermos flask very easily. For this reason, a thermos flask can be used to keep cold drinks cold and warm drinks warm. A thermos flask slows heat transfer by conduction, convection and radiation. Silver surface The silver surfaces facing the inside of the container reflect radiant heat back into the container. Silver surfaces facing the outside of the container reflect radiant heat away from the container. Heat and the body The temperature of the human body is about 37 C. When the air temperature is much less than this, heat moves from your body to the environment. In very hot weather, heat moves from the environment to your body. Absorbing radiant heat When sunlight strikes any object, including the human body, the object absorbs some radiant heat. Losing radiant heat All objects give off some radiant heat. The amount depends on the temperature around the object. On a hot day, an object does not transfer as much radiant heat away as in the cold weather. Stopper The stopper is made from materials that do not allow much heat to move through them by convection or conduction. Protective case Air gap Air does not allow much heat to travel through it by conduction. Shock absorber Empty space Nearly all particles are taken out from between these layers. Without particles, heat cannot move through the walls by conduction or convection. Foam pads keep the glass bottle in place and absorb impacts. Convection Convection currents form when the heat from your body warms the air next to it. The air rises, taking some of your body heat away with it. Convection currents can form only in air that is free to move. Unless your body is touching a very hot or a very cold object, you won t gain or lose heat by conduction very easily. Most of the movement of heat near your body happens by convection and radiation. 222 Science Alive for VELS Level 5

10 100 c 90 c 80 c 70 c 60 c 50 c 40 c Fabric choices Clothes keep you warm in winter by stopping your body heat from escaping. Fabrics made from natural fibres, like wool, are good insulators of heat. Natural fibres contain small pockets of air. Air allows heat to travel through it by convection only if it is free to move. By having only Go to worksheet 10.4: Antarctic fashion designer small pockets of air, convection currents cannot be set up. However, air can flow easily through thin material, keeping you cool in summer. Loose fitting clothes allow more convection currents to form. The convection currents help heat to escape from your body. Reducing heat loss You will need: 4 identical soft-drink cans 4 thermometers plasticine foam stubby holder polystyrene packaging sticky tape lamb s wool or woollen cloth. Draw up a table like the one below. Cover No cover Stubby holder Polystyrene Wool 0 2 Temperature ( C) Put one can in the stubby holder, wrap one in wool and another in polystyrene. Make sure the covers are equal thicknesses. Leave the fourth can bare. Pour equal amounts of Thermometer hot water into each can. Place a thermometer in each can, standing it up with the plasticine. Quickly read the temperature of the water inside each can. Record the temperatures of the water every 2 utes for 12 utes. Plasticine holding thermometer in place 1. Draw a line graph of your results. Plot time on the horizontal axis and temperature on the vertical axis. Use a different coloured line for each can cover. 2. Which can cover is the best insulator? Explain your answer. 3. The covers used in this experiment slow the transfer of heat by two processes. (a) What are the two processes? (b) Explain how each of the covers slows down heat transfer in the two methods from part (a). 4. Why was a can left without a cover? 5. Why should the thickness of each cover be the same? REMEMBER 1. How does a thermos flask stop heat escaping by radiation from a hot drink? 2. List two features of a thermos flask that reduce heat loss by: (a) conduction (b) convection. 3. In what two main ways does heat move between your body and the environment? THINK 4. It is especially important for the stopper at the top of a thermos flask to stop heat loss by convection. Why? 5. How does a thermos keep liquids cool? Heat, light and sound 6. What is wrong with this statement: A thick coat keeps the cold out? CREATE 7. Design and test a container that will keep ice from melting as long as possible. The container needs to be easily carried and built from items found around your home. 8. Use the information on these two pages to design: (a) a summer outfit that keeps you cool (b) a winter outfit that keeps you warm. learning I CAN: describe how insulators control the transfer of heat explain how a thermos flask stops the transfer of heat to (and from) the liquid inside it.