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Physics: 6. Heat Please remember to photocopy 4 pages onto one sheet by going A3→A4 and using back to back on the photocopier Syllabus OP22 Recall that heat is a form of energy and that it can be converted into other forms of energy
OP23 Investigate and describe the expansion of solids, liquids and gases when heated, and contraction when cooled
OP24 Demonstrate the expansion of water on freezing
OP25 Measure the temperature of various solids and liquids at, above and below room temperature; determine the melting point of ice and the boiling point of water
OP26 Investigate the effect of pressure on the boiling point of water
OP27 Explain the difference between heat and temperature
OP28 Carry out experiments that involve changes of state from i. solid to liquid and liquid to solid ii. liquid to gas and gas to liquid
OP29 Plot a cooling curve and explain the shape of the curve in terms of latent heat
OP30 Appreciate that all hot bodies radiate heat
OP31 Carry out simple experiments to show the transfer of heat energy by conduction, convection and radiation; investigate conduction and convection in water
OP32 Identify good and bad conductors of heat and compare insulating ability of different materials.
Student Notes
‘ Hotness ’ in turn is a measure of how much the atoms are vibrating or jiggling. You may not think that all the atoms in every single object jiggle, but there you go; Physics is mad. Note: You must use the term ‘ hotness’ (how many people do you think actually know this?)
So why can’t we say that “ the temperature of an object is a measure of how hot or cold an object is”? Answer: Because ‘hot’ is a vague term; does it refer specifically to temperature ( jiggliness of the atoms), or to the amount of heat in the object? After all, a litre of water at 100^0 Celsius has twice as much
Temperature is a measure of the hotness of an object
heat as half a litre of water at 100^0 Celsius. ‘ Hotness ’ is the physicists’ way of overcoming this potential confusion. Personally I think we should just stop using the word temperature altogether and stick with jiggliness.
To show that heat is a form of energy you must be able to show that it can do work (because energy is the ability to do work). Work is done when you cause an object to move. So we need to show that heat can move something. Demonstration Alcohol in a thermometer expands and moves up the glass when heat is applied. Solids, liquids and gases expand when heated, and contract when cooled
Heat is a form of energy and it can be converted into other forms of energy
- Suck up water which is at about 80 0 C into a syringe so that the syringe is about one-quarter full.
- Cover the open end (watch out - it’s hot!) and pull back the handle to create a partial vacuum.
- Result: The water begins to boil! Explanation
- The air acts like a blanket which presses down on the water and makes it difficult for the water molecules to leave (‘jump out of’) the liquid and become part of the air.
- Higher pressure therefore results in a higher boiling point (the molecules need to have more energy/ move more rapidly to make the transition). Changes of state, the cooling curve and latent heat
- Heat a tube of wax to a high temperature (so that it is in liquid form) and as it cools note the temperature.
- Plot a graph of temperature against time. Result
- The temperature decreased at a steady rate until (in this case) it reached 43 0 C. This is when the wax began to change state from a liquid to a solid.
- It remains at this temperature until all the wax has solidified (in this case it took 4 minutes) and after that it began to drop in temperature again as the solid wax cooled down.
Explanation As the wax changes state from a liquid to a solid it gives out heat without cooling down. This heat is called latent heat because latent means ‘hidden’ and in this case it is not obvious where the heat is coming from.
Latent Heat is the heat taken in or given out whenever a substance changes state
To demonstrate convection currents in air Cut tissue paper into narrow strips; tie the strips together at one end using the piece of thread and hang them from a retort stand as shown over a hot-plate. Result: the tissue paper will begin to move as a result of the convection current generated by the hot-plate.
Radiation: Dark materials are better radiators of heat than shiny materials.
Demonstration
- Take two identical metal containers and paint one with one black and the other silver.
- Fill both with hot water.
- Using a thermometer and stop-watch note which container cools the quickest.
- The dark container cooled more quickly because it is a better radiator of heat.
Conductors and Insulators
To compare the insulating ability of different materials Demonstration
- Take two identical containers and wrap one in cotton wool.
- Fill both with hot water.
- Note which container cools more quickly than the other.
- The container which cooled more slowly had better insulating material.
Note: There are no maths problems in this chapter.
A thermos flask keeps hot liquids hot but also keeps cold liquids cold. Can you see why (look at the diagram on the right for a clue)?
A conductor is a substance which allows heat to flow through it easily
An insulator is a substance which does not allow heat to flow through it easily
