Physics U9 Thermodynamics Test Review

Jayden and Ori are working as engineers to ensure a Mars rover's batteries do not freeze. To most effectively decrease the rate of heat transfer, they should (a)   when using insulation panels with a thermal conductivity of 0.017 W/m·°C and a temperature difference of 70°C between the batteries and the Martian environment.

• Use a thinner insulation panel

• Switch to a material with lower thermal conductivity

• Increase the exposed surface area

• Raise the external temperature

Answer explanation

Switching to a material with lower thermal conductivity reduces heat transfer more effectively than other options, ensuring the rover's batteries remain insulated from the extreme Martian cold.

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Match each heat transfer process to the scenario where it is primarily responsible for heating an object.

• Warm air rises and circulates in a room, distributing heat.

• Conduction

• A thermos keeps liquids hot by preventing heat loss.

• Insulation

• A metal spoon becomes warm after being placed in a hot cup of tea.

• Convection

• The Sun warms the Earth through electromagnetic waves.

• Radiation

Ori and Jayden are discussing how heat moves from the Sun to the Earth. Strategically, how would they determine which heat transfer method is occurring in this scenario?

• By checking if there is direct contact between the Sun and Earth

• By observing if fluids are involved in the transfer

• By identifying the presence of electromagnetic waves

• By measuring the temperature of the Earth's surface only

Answer explanation

Heat from the Sun to the Earth occurs via electromagnetic waves, specifically infrared radiation. There is no direct contact or fluid involvement, making this the key identifying factor.

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Kaylee pours herself a hot cup of coffee, but leaves it on the table. After some time, the coffee cools to room temperature. According to the Second Law of Thermodynamics, this process is best described as: (a)  

• Heat flows from the hot coffee to the cooler air

• The coffee maintains its energy but cools

• Heat flows from cooler air to the warm coffee

• The coffee cools because energy is transferred

Answer explanation

According to the Second Law of Thermodynamics, heat naturally flows from hotter objects to cooler ones. In this case, heat flows from the hot coffee to the cooler air, causing the coffee to cool down.

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Jayden builds a heat engine that absorbs 1,400 J of thermal energy from a hot reservoir and does 630 J of work. What is its efficiency?

• 30%

• 45%

• 40%

• 50%

Answer explanation

Efficiency is calculated as work done divided by thermal energy absorbed: Efficiency = (Work / Heat) x 100 = (630 J / 1400 J) x 100 = 45%. Thus, the correct answer is 45%.

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Jayden is heating a pot of ice on the stove. He notices that it takes much more energy to turn the melted water into steam than it did to melt the ice into liquid water. Why does the heat of vaporization for water exceed the heat of fusion?

• More hydrogen bonds must be broken during vaporization

• Water molecules move faster in the liquid phase

• Melting occurs at a lower temperature than boiling

• The molecular structure of ice requires less energy to change

Answer explanation

More hydrogen bonds must be broken during vaporization than during fusion. This is because vaporization requires overcoming the stronger intermolecular forces present in the liquid phase, resulting in a higher energy requirement.

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Kaylee and Adam are working on a science project and need to design a container to keep a sample at 20°C for 24 hours in a 0°C room. Which setup would best minimize heat transfer?

• A transparent glass container with large surface area

• A vacuum-sealed container with reflective inner walls

• A plastic container that conducts heat through convection

• A thin metal container that quickly distributes heat

Answer explanation

A vacuum-sealed container with reflective inner walls minimizes heat transfer by reducing conduction and convection, while the reflective walls help maintain temperature by reflecting thermal radiation, making it the best choice.

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Jada and JT claim to have invented a machine that converts 100% of absorbed heat into work. Which statement best refutes this claim using thermodynamic principles?

• The Third Law prevents any machine from reaching absolute zero.

• The Zeroth Law states that systems must remain at equal temperatures.

• The Second Law says some energy is always lost as unusable heat.

• The First Law says that energy cannot be created or destroyed.

Answer explanation

The Second Law of Thermodynamics states that not all absorbed heat can be converted into work; some energy is always lost as unusable heat. This directly refutes the claim of 100% efficiency.

Jayden and Willow observe a gas in a piston that completes a thermodynamic cycle and returns to its original temperature, pressure, and volume. What can be concluded about the system’s internal energy?

• It depends entirely on the type of gas used.

• It increased because work was done by the gas.

• It returned to its original value, meaning ΔU = 0 for one full cycle.

• It decreased because some energy left as heat.

Answer explanation

In a thermodynamic cycle, the system returns to its initial state, meaning the internal energy also returns to its original value. Therefore, the change in internal energy, ΔU, is zero for one full cycle.

Madi and Grayson are discussing what happens in a closed system when energy is evenly distributed and no more work can be done. This situation is known as:

• Heat Expansion

• Thermal Contraction

• The Heat Death of the Universe

• Energy Recycling

Answer explanation

The Heat Death of the Universe refers to the state where entropy reaches maximum, and energy is uniformly distributed, preventing any further work or energy transfer. This concept illustrates the ultimate fate of a closed system.

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