ExamLearn.ie Heat
Heat Heat has the ability to do work or move something. Thus, heat is a form of energy. Heat can change into other forms of energy and vice versa. Heat is measured in the unit of energy, the joule (J). Expansion and Contraction One common effect of heat is that it causes substances to expand. The opposite happens when objects lose heat; they contract. Disadvantages: long lengths of railway tracks need tapered joints to allow for expansion in hot weather. Advantages: thermometers work on basis that liquids expand when heated, and contract when cooled. Mandatory Experiment (a) To show that a solid expands when heated and contracts when cooled Apparatus required: metal ball and ring apparatus, Bunsen burner, tongs 1. Set up apparatus as shown in diagram. Check that ball just fits through ring when cool. 2. Using tongs to hold chain, heat ball over Bunsen flame for few minutes. 3. Using tongs again, try pass ball through ring. 4. Allow ball to cool. 5. Check to see if ball will now pass through ring. At normal temperature, metal ball just passed through ring. When heated, ball did not fit through ring. When cooled, the ball passed through ring again. Ball expanded when heated and contracted when cooled.
(b) To show that liquid expands when heated and contracts when cooled Apparatus required: round bottomed flask with stopper and narrow glass tube at top, water bath, Bunsen burner, tripod, wire gauze, water (with dye added) 1. Fill flask and narrow glass tube with water to high level. Mark level with elastic band. 2. Heat flask in water bath for few minutes. 3. Note new level of liquid in tube. 4. Turn off Bunsen burner. Allow liquid to cool. Observe level of liquid after several minutes. When heated, level of liquid in glass tube went up. When heat was removed, liquid level went back down. Liquids expand when heated and contract when cooled.
(c) To show that gas expands when heated and contracts when cooled Apparatus required: round bottomed flask with stopper and long narrow glass tube at top, retort stand, large beaker of water, hair dryer
1. Set up apparatus as shown in diagram. 2. Heat flask with hair dryer. 3. Note bubbles appear at end of glass tubing in large beaker. 4. Now, take away hair dryer. Allow flask to cool. Observe what happens. On heating, air in long glass tube expands. When air cools, water rises up through sloped glass tube. Gas (air) expanded when heated. When the air cools, it contracts. *We saw how as liquids cool down they contract. There is an exception water contracts when cooling, but only until it reaches 4 degrees Celsius. Here it begins to expand again. Disadvantage of water expanding at low temp: In cold weather, exposed water pipes often burst as the water inside freezes and expands. Fish can survive in lakes in very cold weather. Water is at its most dense at 4 degrees Celsius. Being dense, this warm water sinks to bottom, and colder water (less dense) floats to top. Heat Transfer There are three ways that heat can be transferred from one place to another. These are called conduction, convection and radiation. 1. Conduction method by which heat travels from particle to particle through a solid, e.g. along a metal spoon. An insulator is a substance, which does not allow heat to flow through it easily, e.g. gases. Mandatory Experiment *To show the transfer of heat by conduction and to compare the conductivity of various substances Apparatus required: rods of various substances (metals, glass, wood, or plastic could be used), metal container with holes and stoppers for inserting rods, Vaseline/wax, small nails 1. Set up apparatus as shown in diagram. 2. Vaseline/wax is put at end of each rod. Attach small nail to each rod using the Vaseline. 3. Pour boiling water into metal container, covering inside ends of the rods. Nails drop from rods when heat arrives. Different materials conduct at different rates. The rod that drops its nail first is best conductor. All the rods can then be put in order of conductivity as nails drop off.
Mandatory Experiment *To investigate conduction in water Apparatus required: boiling tube, wire gauze, metal tongs, Bunsen burner, water, ice 1. Trap lump of ice with wire gauze at bottom of boiling tube as shown in diagram. 2. Add tap water to boiling tube until it is two thirds full. 3. Hold boiling tube at angle with metal tongs. Heat water near top of boiling tube until it begins to boil. Observe what happens to the ice. The water at top of boiling tube boils. The ice at bottom does not even melt. Water is a poor conductor of heat. Heat is unable to pass from the top of water to ice at bottom.
2. Convection the transfer of heat through a liquid or gas when particles of liquid or as move and carry heat with them. A smoke box is used to show convection currents in air. A taper is lit and then blown out. It is then placed above the hole on right. The air above candle on left is heated. It expands, becomes less dense and rises. The colder air and smoke nearby move in to take its place. The process repeated and convection currents set up. Mandatory Experiment *To investigate convection in water
Apparatus required: large glass beaker, forceps, tripod, glass tubing, potassium permanganate, water, candle 1. Set up apparatus as shown in diagram. 2. Using forceps, drop crystal of potassium permanganate to corner of beaker above candle. 3. Heat beaker gently with candle, Bunsen burner. 4. Observe what happens. The crystal colours the water near it and 'currents' are seen spreading throughout the water. The water molecules near crystal are first to receive heat. This causes them to expand. They become less dense than molecules around them. Thus, they float to the top. Their path is shown by purple dye. Their place is then taken by denser colder water. This, in turn, is heated. This process is continued until all of liquid is heated. The moving currents of water are referred to as convection currents. 3. Radiation the rapid transfer of heat from a hot object without needing a medium, eg. heat directly from sun. Mandatory Experiment *To show transfer of heat by radiation and that a dull black surface radiates heat better than a bright shiny surface. Apparatus required: two cans of equal size (one polished, one black), two temperature sensors and data logging equipment, retort stand, hot water
1. Pour equal amounts of hot water into each can. 2. Put temperature sensor linked to data logging equipment into each can. Make sure both cans are put on nonconducting surface 3. Allow hot water stand in each can, stirring water from time to time 4. Compare graphs of temperature dropping with time for each can. s and conclusion The temperature falls faster in container with dark surface. Therefore, dark surface radiates heat better than bright surface. Temperature and heat Heat is a form of energy. It is measured in joules (J). Temperature is a measure of the level of heat in an object. It is measured in degrees Celsius. Heat flows from hotter objects to colder objects. With low pressure, means a lower boiling point. With higher pressure, means a higher boiling point. Latent Heat Latent heat is the heat taken in or given out when a substance is changing state without changing temperature. The effect of heat supplied during changes of state seems to be hidden. The temperature does not change. This hidden heat is referred to as latent heat.