Unit B: Understanding Common Energy Conversion Systems

Unit B: Understanding Energy Transfer TechnologiesChapter 6: Controlling Heat Transfer

IntroductionFire-walkers amaze tourists.They aim to convince their audiences that they have some sort of special abilities.But! Fire-walkers have learned to reduce the transfer of thermal energy from the red-hot coals to their feet.But how is this done?Moisture on the walkers feet vaporizes.

Ace Ventura: Pet Detective Ace Ventura https://www.youtube.com/watch?v=ZlAA0G_WrVIIntroduction*Hint: Cooks test to see if a skillet is hot enough by dropping water into it.The pan is hot enough if the water drops dance across the surface of the skillet.

IntroductionWe will re-visit the secrets of fire-walking later.

We control heat transfer every day.

Knowledge of differences in heat absorption is used in many ways.

6.1 Absorbing and Losing HeatRecall from Chapter 5:Water moderates the temperature of the land around it.In this chapter we will discuss another reason why that happens.Different materials absorb heat at different rates.This is referred to as heat absorption.6.1 Absorbing and Losing HeatSpecific Heat CapacityDifferent substances require different amounts of thermal energy to raise their temperature the same amount.This is true for all substances.Each substance requires a unique amount of heat gain or loss to change its temperature.6.1 Absorbing and Losing HeatSpecific heat capacity measures a substances ability to absorb or lose heatMeasured in joules per gram degrees Celsius

6.1 Absorbing and Losing HeatThe specific heat capacity of a substance can change depending on its state. For example, solid water (ice) has a lower specific heat capacity than liquid water. This has to do in part with the relative attraction between the molecules. Molecules that are close together, as in a solid, have a stronger attraction to each other. They require more energy to get moving.The joule is named after James Prescott Joule (18181899), a brewer and physicist. It is represented by the symbol J.

6.1 Absorbing and Losing HeatExample The specific heat capacity for water isWhat does this mean?One gram of water absorbs 4.19 joules of heat to raise its temperature 1COne gram of water loses 4.19 joules of heat to lower its temperature 1C

Regardless of the amount of water, the amount of heat needed to change the temperature does not change.

6.1 Absorbing and Losing HeatAnother example:The specific heat capacity of sand is

This means:To increase the temperature of 1 gram of sand by 1 C would require 0.66 J of energy.

6.1 Absorbing and Losing HeatConsider:SHC of water =

SHC of sand =

Which has a higher specific heat capacity?

6.1 Absorbing and Losing HeatWater!This means that it takes more energy to increase the temperature of water than it does to increase the temperature of sand. This is why sand on a sunny beach is much warmer than the shallow water nearby.

6.1 Absorbing and Losing HeatSubstanceSpecific Heat Capacity Water4.19Motor oil2.00Vegetable oil1.97Air0.995glass0.84sand0.66Iron 0.45copper0.386.1 Absorbing and Losing HeatWarming Up and Cooling Down with OceansRecall: oceans moderate shore areasAlso, water has a high specific heat capacity.Oceans store more heat energy or thermal energy than you might expect.

6.1 Absorbing and Losing HeatWith waters large SHC, water can absorb, store or release much more thermal energy than land.That is another reason for land heating and cooling quicker than lakes and oceans.SHC also affects climate in other ways:On a hot day, water will absorb heat. This slows down the rise of temperature in the surrounding area.At night, water will release heat. This slows cooling in the surrounding area.6.1 Absorbing and Losing HeatThe secrets of fire-walkingWater has a very high specific heat capacity (4.184 kJ/K kg), whereas coals have a very low one. Therefore the foot's temperature tends to change less than the coal's.Water also has a high thermal conductivity, and on top of that, the rich blood flow in the foot will carry away the heat and spread it. On the other hand, coal has a poor thermal conductivity, so the hotter body consists only of the parts of the coal which is close to the foot.

6.1 Absorbing and Losing HeatWhen the coal cools down, its temperature sinks below the flash point, so it stops burning, and no new heat is generated.Firewalkers do not spend very much time on the coals, and they keep moving.Calluses on the feet may offer an additional level of protection, even if only from pain; however, most people do not have calluses that would make any significant difference.

6.1 Absorbing and Losing HeatPractice!Check Your Understanding p. 110 #1-5

6.2 Keeping Heat at HomeRecall: heat moves from hot toward cold.This happens especially in the winter.Heat often leaks through windows, doors, and roofs. Proper insulation can help this problem.Insulation slows heat transfer.

6.2 Keeping Heat at HomeWith energy costs rising, Canadians want to make sure heat stays inside the house in winter. For Canadians, 50 to70 percent of energy costs go toward heating or cooling our homes. If insulation is inadequate, much energy is wasted. This is bad for the environment and for the home budget.

6.2 Keeping Heat at HomeWith insulation you get two benefits for the price of one.The same insulation that keeps heat in during the winter keeps heat out on a hot summer day.Knowledge of heat transfer teaches you how to keep your home warm in the winter and cool in the summer.

But what makes a good insulator?6.2 Keeping Heat at HomeInsulation reduces heat transfer. A good insulator is the opposite of a good conductor.

With good insulation, the three forms of heat transfer are slowed.

6.2 Keeping Heat at HomeHeat convection and heat conduction can be minimized in two ways: First, create a partial vacuum between the areas to be insulated. This is done in vacuum bottles and in some double-glazed windows with an inert gas between the panes. Second, use trapped air. Still air provides 15 000 times better insulation than metal.

6.2 Keeping Heat at HomeSubstances used for insulation are chosen for their low thermal conductivity and their ability to trap air. Good conductors include cork, felt, cotton batting, magnesium carbonate, and spun glass or fibreglass. Asbestos is an excellent insulator but it is a health hazard and no longer used in construction.

6.2 Keeping Heat at HomeInsulators work best when the air is dry. Moist air acts as a much better heat conductor than dry air. To keep insulation dry, fibreglass is covered with plastic sheeting before drywall is installed. Without the plastic sheet, moisture from inside the house from cooking, bathing, and breathing would infiltrate the insulation and reduce its effectiveness.Radiant heat loss can be lessened by installing aluminum foil, semi-reflective windows, and metal roofing.

6.2 Keeping Heat at HomeR-valueAir transfers heat when it is moved by convection currents.Air is an excellent insulator when it is held still.R-value is a measure of how well an insulating material slows heat transfer.Materials with high R-values are better insulators than those with low R-values.i.e.) R-12 loses heat faster than one with R-166.2 Keeping Heat at HomeWhen materials are used together, the total R-value is the sum of the R-values of each material used.ExampleWalls in your home have 25 mm of expanded polystyrene and 25 mm of rigid urethane foam. What is the R-value of the insulation?3.96 + 7.50 = R 11.46Thickness of insulating materialApproximate R-Value25 mm of air space in a wall cavity2.0425 mm of air space with reflective surface on inside of wall cavity5.5425 mm of expanded polystyrene3.9625 mm of rigid urethane foam7.5025 mm of fibreglass4.2525 mm of solid wood1.2525 mm of wood shavings2.4225 mm of clay brick0.1125 mm of concrete0.19One thickness of glass1.00Thermal glass (2 thicknesses with air space)1.80Will Aluminum Foil Keep You Warm Lab?? Initial 30 Minutes Later Beaker Aluminum Wrapped Beaker 6.2 Keeping Heat at HomeOther Ways to Keep the Heat in Your HouseThe empty space between the inside and outside wall of a house is called a wall cavity.Filling the cavity with insulation stops convection currents.

6.2 Keeping Heat at HomeWhen insulation fills a wall cavity, there are many pickets of trapped, still air.

Foam pellets and poured insulation work in this way.

6.2 Keeping Heat at HomeWindows and Doors That Keep Heat InWindows and doors are two weak points when you try to keep a house warm.A single pane of glass is a poor insulator.Heat escapes quickly through glass.Leaks also develop around the panes and around the edge of the windows.

6.2 Keeping Heat at HomeOlder houses use double windows and doors called storm windows and storm doors to keep heat in.Todays exterior doors and windows use double glazing.This provides a space of still air.Insulation value of this still air in improved when air is mixed with a gas such as argon.

Unit Heat Loss Rates

6.2 Keeping Heat at HomeExterior doors used to be solid wood.Modern doors are cavities filled with insulation. Some are metal covered.To prevent heat transfer, there is a break in the metal between the inside and outside.

6.2 Keeping Heat at HomeDid You Know?Even the smallest crack (1.5 mm) around the outside of one window, your furnace may burn an extra litre of fuel per day!6.2 Keeping Heat at HomeControlling Heat TransferPizza parlours keep pizza warm by transporting it in insulated containers.As well as limiting heat transfer, the envelope must be washable.This limitation prevents the use of some materials.

6.2 Keeping Heat at HomeA vacuum bottle uses several of the same technologies that keep your house warm in order to keep food and beverages warm.Inside is a double glass jar. One jar is fitted inside the other similar to windows with a double pane.Some air is removed from between the two jars. That is where the name comes from. The space between is a partial vacuum.6.2 Keeping Heat at HomeRubber or plastic keeps the glass away from the outer case.The cap is insulated.

6.2 Keeping Heat at HomeKitchen and WorkshopLarge appliances (i.e. Stoves, refrigerators, freezers, even dishwashers) are all insulated.Insulation slows heat transfer, keeping ovens hot and freezers cold.What makes a good insulator?Non-metals (i.e. Wood and plastic)6.2 Keeping Heat at HomeExamples:Plastic and/or wooden handles on kitchen utensils or pots and pansAprons and oven mitts6.2 Keeping Heat at HomePractice!Check Your Understanding p. 119 #1-6

6.3 Keeping Yourself WarmThe same techniques that keep our houses warm, keeps our bodies warm.For example, on a cold day we may choose to wear several layers. We choose inner layers for their open weave and thickness.Air trapped in the material serves as insulation.A windproof outer layer keeps warm air from escaping.

6.3 Keeping Yourself WarmClothing insulates by holding air between fibres. The better clothes trap air, the better insulators they are. This is why thicker clothing tends to be warmer than thinner. Thicker clothing reduces air flow and maintains still air. Windproof material reduces or eliminates airflow through the fibres.

6.3 Keeping Yourself WarmSome of the warmest winter clothing contain down.Birds grow fluffy, down feathers.These feathers keep birds warm.Down is often quilted between the outer shell and inner lining of a vest or jacket.When down is fluffed, it holds air in place.The jacket material keeps air from blowing through the down and changing cold air for warm.

6.3 Keeping Yourself WarmRecall: a dry body is a warm body.Vigorous activity causes sweat. Water causes cooling as it evaporates.When working or playing outside, you want to protect yourself from getting and staying damp.When you start to sweat, remove one layer, possibly another.This allows heat to transfer out and your body to stay at the right temperature.When you stop moving you can replace those layers.6.3 Keeping Yourself WarmOn a cold day, up to 40 percent of body heat is lost through the head. Covering the head is important for heat retention. When a person becomes overheated in winter, a hat is often the first piece of clothing removed to cool the body and reduce perspiration.

6.3 Keeping Yourself WarmPeople of the NorthInuit designed clothing is the warmest.Traditionally, Caribou Inuit clothes have two complete suits on cold days.The inner set is worn with fur next to the body.Body moisture is transferred through the fur and through the leather skin.

6.3 Keeping Yourself WarmCaribou fur is dense; individual hairs are hollow.Air trapped between and inside the hair provides insulation.

The outer parka is worn with the fur outside this is important for very cold days.The parka hood traps air in front of the wearers face.Frigid air is warmed before being breathed. This protects the wearers lungs.6.3 Keeping Yourself WarmDuring extremely cold weather, water vapour from the lungs can condense and freeze on peoples faces and clothing.The hoods are designed to prevent condensation.The edge of the hood, where ice might form, is trimmed with fur. Ice does not stick to wolverine, wolf, or some dog fur.

6.3 Keeping Yourself WarmCaribou and Copper Inuit wear up to four layers on their feet in winter.Seal skin is preferred to boots because it is waterproof.Inuit parkas are much larger than we might expect.The large size allows wearers to bring their arms inside this warm space.

6.3 Keeping Yourself WarmDid You Know?Some motorcyclists have clothing with built in heaters!Heating elements are sewn into the clothing and warm areas of the body such as the torso where a lot of heat can be lost.The clothing runs off of a motorcycle battery and uses less power than a headlight.The amount of heat can be controlled using a palm-sized computer.6.3 Keeping Yourself WarmDid You Know?People outside in winter protect themselves from frostbite. Frostbite usually freezes hands, feet and the face. One way to warm a cold hand is to tuck it into your armpit for a few minutes!6.3 Keeping Yourself WarmKeeping CoolEver notice that people who live in warmer climate areas where more clothes?This is because they want to minimize heat transfer.They wear long, thick robes to protect the body from the Suns rays.These clothes are usually light in colour to help reflect heat and to allow body heat to escape.

6.3 Keeping Yourself WarmOven mitts work in a similar manner.Their quilted material reduces heat transfer.Some include reflective material that also reduces heat transfer.

6.3 Keeping Yourself WarmDressing for Intense Heat or ColdFirefighters and deep-sea divers have to deal with major changes in temperature.But how?

6.3 Keeping Yourself WarmFirefighters suits are made of a special material. Many contain flame retardant chemicals.When flames or sparks come into contact with the suit, the fabric chars but does not burn.The charred material produces a layer of insulation that protects the firefighter from too much heat.

6.3 Keeping Yourself WarmFirefighters can suffer from heat stroke if their body temperature increases too much.Material on the inside of their fire suit absorbs body moisture.This helps to keep them cool.Firefighters must monitor their own bodies.If they get too hot, they could suffer from heatstroke even if its in the winter!

6.3 Keeping Yourself WarmThe fabrics used in firefighters suits are tested to determine their fire resistant qualities.They are tested by Scientists to see if they are safe enough for firefighters to wear.This is done by dressing a mannequin in fire gear and setting it on fire.These mannequins are equipped with sensors to measure the rate and amount of heat transfer from the fire.6.3 Keeping Yourself WarmEven in hot climates, temperatures deep underwater can be as cold as winter.As a result, a divers suit should fit snugly.Tight diving suits prevent cold water next to the skin from causing cooling by conduction.If cold ocean water moved in and out of the suit, a diver would soon be cold.Water could pick up heat from the divers body and carry it off into the ocean.6.3 Keeping Yourself WarmDive suits are made of neoprene that has bubbles of nitrogen trapped in the fabric.The more gas trapped in the fabric, the higher the thermal value.These neoprene suits are well insulated to keep body heat inside.

6.3 Keeping Yourself WarmDive suits also have hoods for the same reason that winter parkas have hoods.Underwater, a great amount of heat can be lost from a divers head.Some dive suits have titanium added to the side of the fabric that is next to the skin.The shiny titanium reflects heat back into the body.

6.3 Keeping Yourself WarmPractice!Check Your Understanding p. 125 #1-6

ReviewComplete review questions on pg. 126

Hand-in Assignment #6