Science: Energy Thermodynamics and Heat Transfer
Thermal energy, the laws of thermodynamics, and mechanisms of heat transfer
Science: Energy Thermodynamics and Heat Transfer
Thermal energy, the laws of thermodynamics, and mechanisms of heat transfer
Science - Grade 9-12
- 1
Define temperature and thermal energy. Then explain how they are related but not the same.
Think about the difference between an average and a total amount.
Temperature is a measure of the average kinetic energy of the particles in a substance. Thermal energy is the total internal energy associated with the motion and interactions of all the particles in a sample. They are related because hotter objects usually have particles moving faster, but they are not the same because thermal energy also depends on the amount of matter present. - 2
State the first law of thermodynamics in your own words and give one everyday example that shows this law.
The first law of thermodynamics says that energy cannot be created or destroyed, only transferred or changed from one form to another. One everyday example is a toaster, where electrical energy is converted into thermal energy that heats the bread. - 3
A 2.0 kg block of metal is heated and absorbs 9000 J of energy. If the block's temperature increases by 15 degrees C, what is its specific heat capacity?
Use the equation Q = mc delta T and solve for c.
The specific heat capacity is 300 J/kg degrees C. Using c = Q divided by m times delta T, c = 9000 J divided by (2.0 kg times 15 degrees C) = 300 J/kg degrees C. - 4
Explain the difference between conduction, convection, and radiation. Give one example of each.
Conduction is the transfer of thermal energy through direct contact between particles, such as a metal spoon getting hot in soup. Convection is the transfer of thermal energy by the movement of fluids, such as warm air rising in a room. Radiation is the transfer of energy by electromagnetic waves, such as heat from the Sun reaching Earth. - 5
A 0.50 kg sample of water cools from 80 degrees C to 30 degrees C. How much thermal energy does it lose? Use c = 4186 J/kg degrees C.
A negative value for Q means energy leaves the water.
The water loses 104650 J of thermal energy. Using Q = mc delta T, Q = 0.50 kg times 4186 J/kg degrees C times (30 - 80) degrees C = -104650 J. The negative sign shows energy is lost, so the amount of energy lost is 104650 J. - 6
Describe thermal equilibrium. What happens to net heat flow when two objects reach thermal equilibrium?
Thermal equilibrium occurs when two objects in contact reach the same temperature. At that point, there is no net heat flow between them because energy is being transferred equally in both directions. - 7
A student says, "Cold flows from the ice cube into the drink." Explain what is wrong with this statement.
Heat moves from higher temperature to lower temperature.
The statement is incorrect because cold does not flow as a substance. Thermal energy flows from the warmer drink to the colder ice cube. As the drink loses thermal energy and the ice gains it, the drink cools and the ice melts. - 8
What does the second law of thermodynamics say about the direction of natural processes and energy transfers?
The second law of thermodynamics says that natural processes tend to move toward greater entropy and that thermal energy naturally flows from warmer objects to cooler objects unless external work is done. This means energy transfers have a preferred direction in real systems. - 9
A heat engine takes in 1200 J of energy from a hot source and releases 750 J to a cold sink. How much work does the engine do?
For a heat engine, input energy is split into work and waste heat.
The engine does 450 J of work. Using energy conservation, work output equals heat in minus heat out, so W = 1200 J - 750 J = 450 J. - 10
Calculate the efficiency of the heat engine in the previous problem.
The efficiency is 37.5 percent. Efficiency equals work output divided by heat input, so efficiency = 450 J divided by 1200 J = 0.375, or 37.5 percent. - 11
Why can no heat engine be 100 percent efficient according to the second law of thermodynamics?
A heat engine must reject some energy to a colder place.
No heat engine can be 100 percent efficient because some energy must always be transferred to a colder reservoir as waste heat. The second law of thermodynamics prevents all absorbed thermal energy from being converted completely into useful work. - 12
A copper rod and a wooden rod are left in the same room overnight. In the morning they have the same temperature, but the copper rod feels colder. Explain why.
The copper rod feels colder because copper is a much better conductor of thermal energy than wood. It transfers energy away from your hand more quickly, so your skin cools faster even though both rods are at the same temperature. - 13
During a phase change such as melting or boiling, why can a substance absorb energy without increasing in temperature?
Think about where the energy goes besides particle speed.
During a phase change, the absorbed energy is used to break or weaken intermolecular attractions rather than increase the average kinetic energy of the particles. Because temperature depends on average kinetic energy, the temperature stays constant until the phase change is complete. - 14
A refrigerator removes 500 J of thermal energy from its inside and releases 650 J to the room. How much work does the refrigerator require?
The refrigerator requires 150 J of work. By energy conservation, the energy released to the room equals the energy removed from inside plus the work input, so W = 650 J - 500 J = 150 J. - 15
Name one way insulation reduces heat transfer in a house, and identify which method or methods of heat transfer it mainly reduces.
Air can slow energy transfer when it cannot move easily.
One way insulation reduces heat transfer is by trapping pockets of air in walls or attic spaces. This mainly reduces conduction because air is a poor conductor, and it also reduces convection by limiting the movement of air.