We first calculate the gravitational potential energy Mgh that the entire truck loses in its descent and then find the temperature increase produced in the brake material alone. This temperature is close to the boiling point of water.
If the truck had been traveling for some time, then just before the descent, the brake temperature would likely be higher than the ambient temperature. The temperature increase in the descent would likely raise the temperature of the brake material above the boiling point of water, so this technique is not practical.
However, the same idea underlies the recent hybrid technology of cars, where mechanical energy gravitational potential energy is converted by the brakes into electrical energy battery. Note that Example 2 is an illustration of the mechanical equivalent of heat. Alternatively, the temperature increase could be produced by a blow torch instead of mechanically.
Suppose you pour 0. Assume that the pan is placed on an insulated pad and that a negligible amount of water boils off.
What is the temperature when the water and pan reach thermal equilibrium a short time later? The pan is placed on an insulated pad so that little heat transfer occurs with the surroundings.
Originally the pan and water are not in thermal equilibrium: the pan is at a higher temperature than the water. Heat transfer then restores thermal equilibrium once the water and pan are in contact.
Because heat transfer between the pan and water takes place rapidly, the mass of evaporated water is negligible and the magnitude of the heat lost by the pan is equal to the heat gained by the water. The exchange of heat stops once a thermal equilibrium between the pan and the water is achieved.
Bring all terms involving T f on the left hand side and all other terms on the right hand side. Solve for T f ,. This is a typical calorimetry problem—two bodies at different temperatures are brought in contact with each other and exchange heat until a common temperature is reached.
Why is the final temperature so much closer to The reason is that water has a greater specific heat than most common substances and thus undergoes a small temperature change for a given heat transfer.
A large body of water, such as a lake, requires a large amount of heat to increase its temperature appreciably. This explains why the temperature of a lake stays relatively constant during a day even when the temperature change of the air is large. However, the water temperature does change over longer times e.
Which sample cools off the fastest? This activity replicates the phenomena responsible for land breezes and sea breezes. The heat transfer depends only on the temperature difference. Since the temperature differences are the same in both cases, the same 25 kJ is necessary in the second case. Figure 3. Related questions How can specific heat be used to identify substances?
How can the heat capacity of a lead sinker be determined? How do the specific heats of metals compare with water? Dissolving some potassium bromide in cm3 of water leads to a decrease in temperature of 3oC Is heat capacity larger for polymers?
Why is heat capacity an extensive property? Why does heat capacity change with temperature? If you leave a bucket of water outside in the sun in summer it will certainly get warm, but not hot enough to boil an egg. But, if you walk barefoot on the black asphalt of a street in the southern portion of the United States in August, you'll burn your feet.
Dropping an egg on the metal of my car hood on an August day will produce a fried egg. Metals have a much lower specific heat than water. If you've ever held onto a needle and put the other end in a flame you know how fast the needle gets hot, and how fast the heat is moved through the length of the needle to your finger. Not so with water.
The high specific heat of water has a great deal to do with regulating extremes in the environment. For instance, the fish in this pond are happy because the specific heat of the water in the pond means the temperature of the water will stay relatively the same from day to night. They don't have to worry about either turning on the air conditioner or putting on their woolen flipper gloves.
Also, for happy fish, check out our page on Dissolved Oxygen. Lucky for me, you, and the fish in the pond to the right, water has a higher specific heat than many other substances.
One of water's most significant properties is that it takes a lot of energy to heat it. If you'd like to learn more about the specific heat of water at the molecular level, check out this video on the specific heat of water from Khan Academy. Looking at water, you might think that it's the most simple thing around. Pure water is practically colorless, odorless, and tasteless. Some other examples of specific heat capacities are:.
Because it has a low specific heat capacity, lead will warm up and cool down quickly as it doesn't take much energy to change its temperature.
Brick will take much longer to heat up and cool down, its specific heat capacity is higher than that of lead so more energy is needed for the same mass to change the same temperature.
0コメント