Phy 135 Recitation #7

117. Why do you hear creaking and groaning sounds in a house, particularly at night as the air temperature changes?

As the temperature of the house decreases the length of various pieces of wood from which it is constructed will decrease as well. This change of length is what cause the various sounds.

104. When you touch a piece of metal and a piece of wood that are both at the same temperature the metal feels colder. Why?

Both the metal and the wood are at a lower temperature than your skin. Therefore, heat will flow from your skin to both the metal and the wood. The metal feels cooler, however, because it has a greater thermal conductivity. This allows the heat from your skin to flow easier to the metal than the wood. Your body perceives the greater loss of heat per unit time and thinks that the metal is colder.

119. After a machinist very quickly slips a hot, snugly fitting iron ring over a very cold brass cylinder, there is no way that the two can be separated intact by heating. Can you explain why this is so?

Brass has a higher thermal expansion than iron and therefore it expands and contracts more than iron for the same change in temperature. Since they are both good heat conductors (metals) and are in close contact with each other, one cannot be heated or cooled without also heating or cooling the other. If you try to separate them by heating the brass will expand more than the iron. (also you cannot separate them by cooling since the brass was cold when the hot iron ring was slipped over it. The brass will not shrink enough for separation)

105. In winter, why does the road surface on a bridge tend to be more icy than the road surfaces at either end of the bridge?

Energy radiated by roads on land (end of the bridge) is partly replenished by heat conducted from the warmer ground below the pavement. But there's an absence of thermal contact between the road surfaces of bridges and the ground, so they receive very little, if any, replenishing energy conducted from the ground. This is why road surfaces on bridges get colder than roads on land, which increases the chance of ice formation on bridges.

118. We learn by experience that we can loosen a metal lid that is stuck on a glass jar by pouring hot water over the lid. Why does this work?

Heating the glass jar and its metal lid to the same higher temperature results in a greater expansion in the lid than in the glass. As a result, the lid can become loose enough to turn. In fact, you could dip the entire system (lid and glass container) in hot water, and the different expansions of the metal and the glass will lead to the same result. Note that the same difference in the thermal expansion is used in making a good seal: lids are placed on jars when the contents are hot (canning)

106. You can hold your fingers beside the candle flame without harm, but not above the flame. Why?

Hot air travels upward by air convection and can burn your fingers. Since air is a poor conductor, very little heat travels sideways to your fingers.

101. Is it valid to say that a hot object contains more heat than a cold object?

No. Heat is not a quantity that one object has more of than another. Heat is the energy that is transferred between objects of different temperatures

114. Would evaporation be a cooling process if molecules of every speed had an equal chance to escape from the surface of a liquid?

No. If molecules of all speeds escaped equally easily from the surface, the molecules left behind would have the same range of speeds as before any escaped, and there would be no change of the liquid's temperature. When only the faster molecules can break free, those left behind are slower, and the liquid becomes cooler. Look at the speed distribution curves below. During evaporation the fast moving molecules escape first. The distribution of speeds is changed and there are more slower molecules and a lower kinetic energy which means a lower temperature.

116. Why is it advisable to allow telephone lines to sag when stringing them between poles in summer?

Telephone lines are longer in summer, when they are warmer, and shorter in winter, when they are cooler. They therefore sag more on hot summer days than in winter. If the telephone lines are not strung with enough sag in summer, they might contract too much and snap during the winter.

107. Very often you will see hawks and eagles gliding effortlessly without flapping their wings and even gain altitude. Explain how.

The birds experience Lift forces due to updrafts. Updrafts can also be caused by the sun heating the ground. The heat from the ground warms the surrounding air lowering its density, which causes the air to rise, via convection. The rising pockets of hot air are called thermals

113. In times past, on a cold winter night, it was common to bring a hot object to bed with you. Which would keep you warmer through the cold night-a 10-kg iron brick or a 10-kg jug of hot water at the same temperature? Explain.

The brick will cool off too fast and you will be cold in the middle of the night. Bring a jug of hot water with its higher specific heat to bed and you will make it through the night.

100. A cup of hot coffee is placed on a table. Is it in thermal equilibrium? What condition determines when the coffee is in equilibrium?

The coffee is not in equilibrium because the temperature is different from that of its surroundings. Over time the temperature of the coffee will decrease, until finally it is the same as room temperature. At this point it will be in equilibrium - as long as the room stays at the same temperature

103. Why do good wine glass have long stems?

The long stem of a wine glass helps to prevent heat from thermal conduction from your hand from warming the wine.

111. The specific heat of concrete is greater than that of soil. Would you expect a major-league baseball field or the parking lot that surrounds it to cool off more in the evening following a sunny day?

The soil in the field cools off faster than the concrete parking lot, because its temperature changes more for a given amount of heat loss.

108. A farmer turns on the propane burner in his barn on a cold morning and heats the air to 20 C (68°F). Why does he still feel cold?

The walls of the barn are still cold. The farmer radiates more energy to the walls than the walls radiate back, and he is chilled. (Inside your home or your classroom, you are comfortable only if the walls are warm, not just the air.) Remember air is a poor thermal conductor and doesn't warm the farm very much unless he is real close to the propane burner.

110. If a lighted match is held beneath a balloon inflated with air, the balloon quickly bursts. If instead, the lighted match is held beneath a balloon filled with water, the balloon remains intact, even if the flame comes in contact with the balloon. Explain.

The water has a larger specific heat value than air. Therefore it can take on a larger amount of heat energy with little change in temperature. Also the water has a higher heat (or thermal) conductivity and the heat of the flame will not be as localized as with the air filled balloon.

115. When you wash your car in the winter time with hot water you will very often get a thin layer of ice on the car? Explain

This is because evaporation is a cooling process. The warm water evaporates quickly since it is generally a very thin layer on the car. The latent heat of evaporation for each gram of water is 540 calories. This heat energy comes from the car surface which becomes very cold and freezes the last bit of water on it to a thin layer of ice.

112. Why does a piece of watermelon stay cool for a longer time than sandwiches do when both are removed from a picnic cooler on a hot day?

Water in the melon has a much higher water content than the sandwich. Since water has a higher specific heat (more "thermal inertia") it will take longer to cold off.

109. Which is likely to be colder-a night when you can see the stars, or a night when you cannot?

When the sun goes down the ground begins to cool. Since air is poor conductor most of cooling is by radiation loss. When there are no clouds the radiation passes directly through the atmosphere and a lot of ground cooling occurs. On a cloudy night the clouds absorb some of the radiation emitted by the ground and re - radiate some of it back to Earth not allowing it to cool as much

102. Why is it that you can place your hand briefly inside a hot pizza oven without harm, but you are burned if you touch the metal sides of the oven?

When your hand is in the air of the hot oven, you're not harmed because air is a poor conductor-heat doesn't travel well between the hot air and your hand. Air Also has a low specific heat capacity (meaning that it doesn't take much energy to change its temperature), so the total amount of thermal energy in the air that can be transferred to your skin is small. Touching the hot metal sides of the oven is another story, for metal is an excellent conductor and has a much larger specific heat capacity, so considerable heat flows into your hand.