14.5 Conduction

14.5 Conduction

• The three methods of heat transfer in a fireplace are conduction, convection, and radiation.
• Most of the heat goes into the room because of radiation.
• At a slower rate, heat transfer occurs through the room.
• Through cold air entering the room around windows and hot air leaving the room by rising up the chimney, heat transfer occurs.

• In the three following modules, we examine the methods in some detail.

• As you hold a hot cup of coffee, heat transfers into your hands.

• The barista makes hot cocoa by steaming cold milk.

• Coffee can be reheated in a microwave oven.

• In the winter and the summer, insulation is used to limit the heat from the inside to the outside.

• As you walk barefoot across the living room carpet in your cold house, your feet feel cold as you step onto the kitchen tile floor.

• The carpet and tile floor are both at the same temperature.
• The difference in temperature between the tiles and the carpet is explained by the different rates of heat transfer: the heat loss during the same time interval is greater for skin in contact with the tiles than with the carpet, so the temperature drop is greater on the tiles.

• Some materials are able to produce more thermal energy than others.
• Good conductors of electricity, such as copper, aluminum, gold, and silver, are also good heat conductors.
• The hotter the body, the higher the molecule's energy is.
• The temperature difference affects the heat flux.

• You will get a worse burn from boiling water than from hot tap water.
• The net heat transfer rate falls to zero if the temperatures are the same.
• The number of collisions increases as the cross-sectional area increases.
• If you touch a wall with your hand, it will cool quicker than if you touch it with your finger.

• Molecules in two bodies have different averages.
• Energy is transferred from high-temperature regions to low-temperature regions when there is a collision at the contact surface.
• The molecule in the lower temperature region has low energy before it collides with the surface, but it has more energy after it hits it.
• A molecule in the higher temperature region has high energy before it collides with the contact surface, but its energy decreases after it collides with the surface.

• The thickness of the material transfers heat.
• There is a slab of material with different temperatures on either side.
• If that is greater, then the heat is transferred from left to right.
• The transfer of heat from the left side to the right side is accomplished by a series of collisions.

• The thicker the material, the longer it takes to transfer heat.
• This model explains why thick clothing is warmer than thin clothing in the winter.

• The rectangular bar is a representation of heat conduction through any material.

• The material's temperature is on the left and right.
• The surface area, temperature difference, and substance's conductivity are all related to the rate of heat transfer.
• The thickness is related to the rate of heat transfer.

• The heat transfer rate depends on the material properties.
• Experiments confirm that all four factors are included in a simple equation.

• The values of thermal conductivity are given in Table 14.

• An average thickness of 2.50 cm is the average thickness of a Styrofoam ice box.

• melting ice keeps the inside of the box cold.

• The question is about heat for a phase change and the transfer of heat.
• We must find the net heat transferred to find the amount of ice melted.
• The rate of heat transfer can be calculated with the help of time.

• The knowns should be identified.

• The unknowns should be identified.
• The mass of ice needs to be solved.
• The net heat that is transferred to melt the ice will need to be solved.

• Determine which equations to use.

• The result of 3.44 kg, or about 7.6 lbs, seems to be right based on experience.
• A bag of ice can weigh up to 10 lbs per day.
• Extra ice is required if you add warm food or beverages.

• Styrofoam is a poor conductor and is a good insulator.
• Wool, fiberglass, and goose-down feathers are good insulators.
• These all incorporate small pockets of air, taking advantage of the poor thermal conductivity of air.

• A combination of material and thickness can be manipulated to create good insulators.
• A good insulator will have a large ratio.

• The rate of heat transfer is proportional to that.
• A couple of representative values are 11 for 3.5-in-thick fiberglass batts and 19 for 6.5-in-thick fiberglass batts.
• Walls and ceilings are usually insulated with 3.5-in batts.
• In cold climates, thicker batts can be used.

• The fiberglass batt is used for insulation of walls and ceilings to prevent heat transfer between the inside of the building and the outside environment.

• The best thermal conductors are silver, copper, gold, and aluminum, and they are related to the density of free electrons in them.
• Good conductors are used to make cooking utensils.

• The water is boiling in the pan on the stove.
• The sauce pan has a thick bottom that is 14.0 cm in diameter.
• The water is being evaporated at a rate of 1.00 g/s.

• Conduction through the aluminum is the primary method of heat transfer here, and so we use the equation for the rate of heat transfer and solve for the temperature difference.