HW 8

What is the role of "loose" electrons in heat conductors?

"Loose" electrons move quickly and transfer energy to other electrons that migrate through the material.

Why is it incorrect to say that, when a hot object warms a cold object, temperature flows between them?

Boyle's law, it's the law of thermodynamics heat does not travel from a colder to a hotter. Heat is energy and travels

On a cold day, why does a metal doorknob feel colder than the wooden door?

Compared to the door or handle (at room temperature) your hand is the warmer body. The heat transfer from your hand to the metal handle is relatively fast but to the wood of the door it is much slower. So the handle feels colder than the door.

If you hold one end of a piece of metal against a piece of ice, the end in your hand soon becomes cold. Does cold flow from the ice to your hand? Explain.

Energy "flows" from higher to lower temperature, from your hand to the ice. It is the energy, heat, flowing from your hand that produces the sensation of coolness. There is no flow from cold to hot; only from hot to cold.

In which direction does thermal energy flow between hot and cold objects?

Energy flows from warmer objects to cooler objects

Wood is a better insulator than glass, yet fiberglass is commonly used as an insulator in wooden buildings. Explain.

Fiberglass batting contains a great deal of trapped air. Since trapped air is an excellent insulator, fiberglass batting can be used for efficient insulation. Wood is a better insulator than solid glass because wood, too, contains "empty" air spaces; it is porous.

What role does temperature have in the direction of internal energy flow?

Heat flows spontaneously from high temperature to low temperature

How does heat differ from internal energy, or are they two terms for the same thing?

Heat is internal energy that flows from hot to cold locations. They are not two terms for the same thing.

Does a good insulator prevent heat from escaping or slow its passage?

Insulation delays heat transfer.

Does a hot object contain internal energy, or does it contain heat?

Internal energy

If you touch the metal sides in an oven with your bare hand, you're in trouble. But hold your hand briefly in the oven air and you're okay. What does this tell you about the relative conductivity of metal and air?

Most liquids and gases are poor conductors of heat, air is a very poor conductor

Why does the direction of coastal winds change from day to night?

The direction of winds changes with changes in land and water temperatures. Air flow reverses as relative temperatures reverse.

Which has the greater amount of internal energy: an iceberg or a cup of hot coffee? Defend your answer.

The iceberg has the greater internal energy ("amount of heat"), because it has so very much more mass than the cup of coffee. This is true even though the temperature of the coffee is much higher.

Is it possible for heat to flow between two objects with the same internal energy? Can heat flow from an object with less internal energy to one with more internal energy? Defend your answers.

The two objects may consist of one with a higher temperature and smaller number of the molecules, and the other with a lower temperature and a larger number of molecules. In that case it is possible for both objects to have the same internal energy, but heat will still flow from the object with the higher temperature to the one with the lower temperature. Internal energy depends on both the number of molecules of material and the temperature of the material. Heat will flow naturally from the object with the higher temperature to the object with the lower temperature. The object with the high temperature may or may not be the object with the higher internal energy.

Why are materials such as wood, fur, feathers, and even snow good insulators?

They have many air spaces, air is a good conductor

Explain why a firewalker can step quickly without harm on red-hot coals with bare feet.

Wood is a good insulator even when it's red hot; therefore, very little thermal energy is transferred to the feet.