Bennett Mechanical Thermodynamics and Heat Questions and Answers 1 — Questions and Answers
Question 1: A bimetallic strip is constructed by bonding a strip of steel to a strip of copper. Copper has a higher coefficient of thermal expansion than steel. If the straight strip is uniformly heated, how will it respond?
- It will remain straight but will become slightly thicker.
- It will bend with the copper on the inside (concave side) of the curve.
- It will twist into a corkscrew shape.
- It will bend with the steel on the inside (concave side) of the curve. (Correct answer)
Correct answer: It will bend with the steel on the inside (concave side) of the curve.
When heated, both metals expand, but copper expands more than steel for the same temperature change. Because the two strips are bonded together, the only way to accommodate the copper strip becoming longer than the steel strip is for the entire assembly to bend. The longer material (copper) will form the outer, longer arc of the curve, forcing the shorter material (steel) to be on the inside of the curve.
Question 2: To most effectively heat a room using natural air circulation, where should a hot air vent be placed?
- In the center of the ceiling, blowing downwards.
- High on an interior wall.
- Low on a wall, near the floor. (Correct answer)
- In the exact center of the room on the floor.
Correct answer: Low on a wall, near the floor.
This placement utilizes the principle of convection. Hot air is less dense than cold air, causing it to rise. By introducing hot air near the floor, it rises and displaces the cooler, denser air at the top of the room. This displaced cool air then sinks, is drawn towards the vent to be heated, and continues the cycle, creating a natural circulation that efficiently warms the entire space.
Question 3: A person stands at an equal distance from four large, flat plates, all heated to the same high temperature. Which plate will radiate the most heat to the person?
- A shiny, polished silver plate.
- A rough, matte black plate. (Correct answer)
- A smooth, white-painted plate.
- A clear glass plate.
Correct answer: A rough, matte black plate.
The amount of thermal energy an object radiates is dependent on its surface properties. Dark, matte (non-shiny) surfaces are the most effective emitters and absorbers of thermal radiation. Therefore, the rough, matte black plate will radiate the most heat towards the person compared to the reflective or lighter-colored surfaces.
Question 4: On a day when the room temperature is 70°F (21°C), why does a metal doorknob feel colder to the touch than a wooden door?
- The metal is actually at a lower temperature than the wood.
- The wood retains latent heat better than the metal.
- The metal has a higher rate of thermal conductivity. (Correct answer)
- The metal has a smoother surface, increasing contact area.
Correct answer: The metal has a higher rate of thermal conductivity.
Both the doorknob and the door are at the same ambient room temperature. However, metal is an excellent thermal conductor, while wood is an insulator. When you touch the doorknob, it rapidly conducts heat away from your hand, creating the sensation of cold. The wood conducts heat much more slowly, so it doesn't draw heat from your hand as quickly and therefore doesn't feel as cold.
Question 5: Sweating cools the body down on a hot day. This cooling effect is primarily a result of:
- The high specific heat of water absorbing ambient heat.
- The evaporation of sweat, which requires energy taken from the skin. (Correct answer)
- Conductive heat transfer from the moving air to the sweat.
- The temperature of the sweat being lower than the body's temperature.
Correct answer: The evaporation of sweat, which requires energy taken from the skin.
The process of a liquid changing to a gas (evaporation) requires a significant amount of energy, known as the latent heat of vaporization. This energy is drawn from its immediate surroundings, which in this case is the skin. As sweat evaporates, it takes this thermal energy from the skin, thereby lowering the skin's temperature and cooling the body.
Question 6: A vacuum flask (thermos) keeps liquids hot by minimizing heat transfer. The vacuum between the inner and outer walls is MOST effective at preventing which forms of heat transfer?
- Radiation and Conduction
- Convection and Radiation
- Conduction and Convection (Correct answer)
- Evaporation and Conduction
Correct answer: Conduction and Convection
Both conduction (heat transfer through direct contact) and convection (heat transfer through fluid movement) require a medium (matter) to occur. By removing most of the air between the walls to create a vacuum, the flask effectively eliminates these two methods of heat transfer. Heat transfer by radiation is handled by the shiny, reflective surfaces of the walls.
