HEAT HISTORY. D. Whitehall

Transcription

1 1 HEAT HISTORY 18 th Century In the 18 th century it was assumed that there was an invisible substance called caloric. When objects got it was assumed that they gained caloric, therefore hot objects should be heavier than cold objects, but there was no evidence to prove this, hence this theory was untrue Count Rumford Count Rumford observed the boring of a gun barrel; he noticed a lot of heat was generated, while a small quantity of brass chips was removed from the barrel. He thought it was unlikely that all the heat was stored in the small chips. Further investigations showed that: (i) (ii) The longer the boring took place, the greater the amount of heat was produced. If the gun boring was done in a tank of water, the water becomes heated even though no flame was used. From his observations he concluded that: (i) Heat was created when mechanical work was done against friction (caused by the action of metal on metal). (ii) If heat can be created it could not a material substance J. P. Joules He conducted a series of experiments that heat was not a material substance. He also converted different types of energy into heat energy. He then measured the amount of energy created and produce, and found that they where in a constant ratio he was the first to conclude that heat was a form of energy. Today Today we believe that when an object gains heat, its molecules gain kinetic energy, and move and vibrate faster. We also believe that heat cannot be created or destroyed, but changed from one form to another.

2 2 CONDUCTION, CONVECTION AND RADIATION Heat can be transferred by three methods: (i) Conduction (ii) Convection (iii) Radiation Conduction This is the flow of heat through a material, without the movement or flow of the material itself. The experiment below demonstrates that there are materials which are good conductors of heat (poor conductors are called insulators). We also realize all metals are good conductors compared to others. How Heat is transferred in Conduction All materials transfer heat from molecule to molecule. As the material gains heat the molecules that are closer to the heat source gain kinetic energy. They vibrate more and bump into their neighbor and pass on some of their kinetic energy. The molecules become excited and vibrate vigorously as well. The passing of the energy from molecule to molecule does no involve the movement of molecules themselves. Metals however contain free electrons, which move independently throughout the metal. When a metal is heated, these free moving electrons move faster and diffuse themselves into the cooler parts of the metal. They transfer kinetic energy to the metal molecules by colliding with them; hence, the process of the transferring of energy is more quickly. Convection These are the flow of a liquid or gas caused by the change in density, in which the whole medium moves and carries heat energy with it. If we observe the experiment on page, we will observe purple streaks rising in the water above the crystals, and being carried to the far side of the beaker. This flow of water is called convection current. What Causes Convection Currents? The water at the bottom of the tank, close to heat source expands. As is expands it becomes less dense than the cooler surrounding water, so it rises. It moves away from the heat source and loses some of its heat energy to the surrounding and begins to cool. As it becomes denser, it sinks back to the bottom of the tank. Radiation This is the transfer of heat energy by means of electromagnetic waves. Radiation can take place without a material medium.

3 3 Read and make notes: PFC Page 161 (include diagrams) Convection currents in the air. Land and sea breezes. PFC Page 163 (include diagrams) The greenhouse effect The vacuum flask PFC Page 390 (include diagrams) Solar panel PFC Page Radiant heat (shiny polish surfaces vs dark matted surfaces)

4 4 EXPANSION OF SOLIDS, LIQUIDS & GASES Objects increase in size or expand, when they are heated, and contract when they are cooled. They are three types of expansion and they are: 1.) Linear Expansion: objects increase in length when heated. 2.) Superficial Expansion: objects increase in area when heated. 3.) Cubical Expansion: objects increase in volume. Why Solids Expand and Contract When solid are heated their molecules gain extra energy and vibrate violently, and need more room for movement. The molecules try to push their neighbors away against their mutual attraction. The increase in distance between the molecules causes expansion in all directions. If the solid has no room to expand, its molecules will produce a force of expansion. When a hot solid tries to cool down, its molecules try to return to their original positions. If the molecules are held too far apart and are not allowed to shrink, they pull on their neighbours and produce a tension or a force of contraction. Bimetallic Strip A bimetallic strip is made of two different metals, e.g. brass and iron, welded or riveted together. When cold the double strip is straight, fig. (a). As it is heated the brass expands more than the iron and so the brass forms the outside of a curve, and the iron on the inside, fig. (b). Bimetallic strips are used in thermo stats and many other mechanical switching circuits.

5 5 Expansion of Liquids When we first heat the round bottom flask, the liquid level drops, because glass is a poor conductor by heat, so the glass flask expands and increase the inside volume. The liquid which has not started to expand as yet drops to yet to fill the extra volume in the flask. Once the liquid become heated it expands rapidly and spills over the top. This demonstrates that cubical (volume) expansion of liquid is very large. The expansion of liquids which we see is called apparent expansion; the real expansion is greater than what we observed, because of expansion of liquid s container which takes up some of the liquid s expansion. Expansion of Water Most liquids contract as they cool and further contact when they reach their freezing point. Water, however, contracts as it cools from 100 C - 4 C, and expands between 4 C - 0 C. When a pond freezes over; the denser water (4 C) remains at the bottom of the pond. The less dense water 3 C - 0 C floats in layers above the denser water. The water on the surface is frozen, but floats because it is less dense than water below it (this is because it increases in volume). The density layers stop convection currents from spreading the heat. Since ice is a poor conductor of heat, the top layer of ice on the pond acts as an insulator blanket and reduces further heat loss. Aquatic animals and plants use this phenomenon to live in ponds during the winter.

6 6 Heat and Temperature Heat flows from a body of high temperature to one of lower temperature. The thermometer is used to measure temperature. There are two scales: (i) Celsius Scale ( C) (ii) Absolute/Kelvin Scale (K) Celsius Scale On this scale there is a lower fixed point which is called the ice point (temperature of melting pure ice, 0 C) and upper fixed point, called the steam point (temperature of steam just above boiling water, 100 C) Kelvin Scale This scale is used for temperatures which are colder than the freezing point of ice and higher than the boiling point of water. The lowest possible temperature is called absolute 0 which also known as 0K which is -273 C. Relationship between Celsius and Kelvin Converting Temperature Kelvin temperature = Celsius temperature T/K = C Examples: C K

7 7 Temperature Change Temperature change of 1 C = Temperature change of 1K Examples: (a) Initial temperature: 50 C 323 K Final temperature: 110 C 383 K Temperature change ( T): 60 C 60 K [ Temperature change ( T) = Final temperature Initial temperature] (b) Initial temperature: 80 C 353 K Final temperature: 10 C 283 K Temperature change ( T): -70 C -70 K [the negative sign (-) means that the object is lossing heat energy or is cooling] Read and make notes: PFC Page 170 (include diagrams) Different types of thermometers

8 8 Heat Capacity and Specific Heat Capacity Heat Capacity (C) This is the heat energy needed of an object to raise its temperature by one Kelvin (or one degree). The heat capacity of an object depends on: (i) (ii) the type of material the object is made of. the mass of the object. The formula for heat capacity (C) is: Heat Capacity = Heat Energy Temperature Rise C = E T Units: J/ C or J/K [N.B. - The heat capacity refers to the whole object] Specific Heat Capacity (c) The specific heat capacity of a substance is the heat energy needed to raise the temperature of 1kg of a substance by 1K (or one degree). The formula for specific heat capacity (c) is: Specific Heat Capacity = Heat Energy Temperature rise Mass c = E M T Units: J/(kg C) or J/(kgK)

9 9 We can arrange the formula to get: heat energy = mass specific heat capacity temperature change E = mc T [This formula is used to calculate the heat energy required to heat up a substance] The Relationship between Heat Capacity and Specific Heat Capacity The heat capacity is when you are talking about the entire / whole object. The specific heat capacity refers to 1 kg of the object. There is a relation which exists between the heat capacity and the specific heat capacity of an object. heat capacity = specific heat capacity mass C = mc Table showing specific heat capacity of some materials Substance Specific Heat Capacity J/(kg C) or J/(kgK) Water 4200 Aluminum (alloy) 880 Copper 380 Ice 2100 Nylon 1700 Glass 670 Lead 126 Marble 880

10 10 Examples: (i) How many joules of heat are required to raise the temperature of 550 g of water from 12 o C to 18 o C? (remember the specific heat of water is 4200 J/kg o C) (ii) 8750 J of heat are applied to a piece of aluminum, causing a 56 o C increase in its temperature. The specific heat of aluminum is J/kg o C. What is the mass of the aluminum? (iii) A 250 g sample of water with an initial temperature of 98.8 o C loses 7500 joules of heat. What is the final temperature of the water? (Remember, final temp = initial temp - change in temp and that specific heat capacity of water 4200 J/kg o C)

11 11 Latent Heat and Specific Latent Heat Latent heat is hidden heat. That changes the state of an object without causing a temperature change. For example: Latent heat changes ice at 0 C to water at 0 C. State Of Matter GAS Change of State Latent Heat of Vaporization LIQUID Change of State Latent Heat of Fusion SOLID Latent Heat of Fusion (L) The latent heat of fusion of a solid is the heat required to change a solid to a liquid without a temperature change. latent heat of fusion = heat energy needed to melt all of it. L = E Units: Joules (J)

12 12 Specific Latent Heat of Fusion ( ɭ ) The specific latent heat of fusion of a solid is the heat required to change 1kg of it, from a solid to a liquid without any temperature change. specific latent heat of fusion = heat energy mass ɭ = E m Units: J/kg We can rearrange this formula, to obtain a formula for heat energy: heat energy = mass specific latent heat E = m ɭ Latent Heat of Vaporization (L) The latent heat of vaporization of a liquid is the heat required to change a liquid to a gas, without a temperature change. Specific Latent Heat of Vaporization ( ɭ ) The specific latent heat of vaporization of a liquid is the heat required to change 1kg of it, from a liquid to a gas without any temperature change. Heat Formulas We now have two formulas to use to determine the heat energy: (i) E = mc T (this heat energy causes a change of temperature) (ii) E = m ɭ (this heat energy causes a change of state, but no temperature change)

13 13 Examples: (i) An ice lolly has a mass of 100g, if the specific latent heat of fusion of ice is J / kg; calculate the amount of heat needed to melt the lolly. (ii) Calculate the heat energy required to convert 4 kg of ice at -25 C, to stem, at 100 C, given that specific heat capacity of water is 4 200J/(kg C), the specific heat capacity of ice is 2 100J/(kg C), the specific latent heat of fusion of ice is J/kg, and the specific latent heat of vaporization of water is J/kg.

14 14 Determining the Specific Heat Capacities of Metals and Liquids by Experimentation There are two types of experiments we can use to determine the specific heat capacity of a metal or a liquid. Electrical Method We set up the experiment as shown in the diagram above. We then determine the mass of the material. We use a thermometer and measure the initial temperature of the material. Next we supply a known amount of energy to the material and we measure the temperature rise in the material. We use a heater of known power supply and use the heater for approximately five (5) minutes. We can use the formula below to determine how much energy was sent to the material. heat energy supplied = power of heater time E = Pt We the find the temperature change of the material by using temperature change ( T) = initial temperature final temperature T = T final T initial Final we use the formula below to calculate the specific heat capacity of the material specific heat capacity = heat energy supplied temperature change mass c = Pt m T

15 15 Method of Mixtures This is the most common practical used to find the specific heat capacities of solids and liquids. We usually add a hot solid (or a hot liquid) of known temperature to a cold liquid and determine the final temperature. We assume that all the heat from the hot substance goes to the cooler one if we can reduce heat loss by using insulation. We then use the formula below to determine the specific heat capacity of the substance heat loss by solid = heat gained by liquid m solid c solid (T solid T final ) = m liquid c liquid (T final T liquid ) Where: m solid = mass of the solid c solid = specific heat capacity of solid m liquid = mass of liquid c liquid = specific heat capacity of liquid T solid = initial temperature of solid T liquid = initial temperature of liquid T final = final temperature of mixture

16 16 Example: Find the specific heat capacity (c) of aluminum by the following procedure below: (i) heat a 0.5 kg mass of aluminum in boiling water (100 C). (ii) put 1kg of water at 20 C in an insulated container. (iii) quickly transfer the hot aluminum into the water. (iv) stir and record the final temperature of the mixture, which is 28 C (given that the specific heat capacity of water is 4 200J/(kg C) ).