Science: Energy Thermodynamics and Heat Transfer Worksheet

Question 1

Define temperature and thermal energy. Then explain how they are related but not the same.

Accepted Answer

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.

Question 2

State the first law of thermodynamics in your own words and give one everyday example that shows this law.

Accepted Answer

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.

Question 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?

Accepted Answer

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.

Question 4

Explain the difference between conduction, convection, and radiation. Give one example of each.

Accepted Answer

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.

Question 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.

Accepted Answer

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.

Question 6

Describe thermal equilibrium. What happens to net heat flow when two objects reach thermal equilibrium?

Accepted Answer

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.

Question 7

A student says, "Cold flows from the ice cube into the drink." Explain what is wrong with this statement.

Accepted Answer

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.

Question 8

What does the second law of thermodynamics say about the direction of natural processes and energy transfers?

Accepted Answer

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.

Question 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?

Accepted Answer

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.

Question 10

Calculate the efficiency of the heat engine in the previous problem.

Accepted Answer

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.

Question 11

Why can no heat engine be 100 percent efficient according to the second law of thermodynamics?

Accepted Answer

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.

Question 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.

Accepted Answer

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.

Question 13

During a phase change such as melting or boiling, why can a substance absorb energy without increasing in temperature?

Accepted Answer

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.

Question 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?

Accepted Answer

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.

Question 15

Name one way insulation reduces heat transfer in a house, and identify which method or methods of heat transfer it mainly reduces.

Accepted Answer

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.

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Science: Energy Thermodynamics and Heat Transfer

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