Energy transfer and transformation Transitional Reasoning Download PDF

Questions

1. 1.

   In an industrial waste-heat recovery system, a thermoelectric module absorbs 400 J of thermal energy conducted from an exhaust duct. The module converts a portion into 80 J of electrical energy to power an exhaust fan while the remaining 320 J conducts directly to an exterior heat sink. The electric fan imparts 60 J of kinetic energy to the circulating air, while internal resistance and friction warm its motor and casing. Which statement correctly evaluates the energy transfers, transformations, and conservation across this system?

   • A) Energy is not conserved across the overall process because only 60 J of useful kinetic energy is obtained from an initial 400 J of thermal energy, destroying the remaining 340 J.
   • B) The thermoelectric module transforms 80 J of radiant energy into electrical energy, which the fan then fully converts into 80 J of kinetic energy without any thermal dissipation.
   • C) The system transforms 400 J of thermal energy directly into mechanical work, with 80 J stored as chemical energy in the motor and 320 J lost permanently from the universe.
   • D) Thermal energy is partially transformed into electrical energy and then into kinetic energy, with a total of 340 J dissipated as thermal energy, strictly conserving the initial 400 J.
2. 2.

   In an investigation of energy transformations, a 0.050 kg steel pellet is fired horizontally into a suspended block of ballistic clay, which is initially at rest. The pellet embeds itself completely into the clay, causing the block-pellet combination to swing upward and gain 4.0 J of gravitational potential energy at its highest point. High-precision thermal sensors detect that the impact generated 16.0 J of internal thermal energy in the clay and pellet. Assuming air resistance is negligible, which statement correctly evaluates the energy transfers and transformations from just before impact to the peak of the swing?

   • A) The pellet initially possessed 20.0 J of kinetic energy; 16.0 J was transformed into internal thermal energy during the inelastic collision, and the remaining 4.0 J of kinetic energy was transformed into gravitational potential energy as the system swung upward.
   • B) The total mechanical energy of the system remained conserved at 20.0 J throughout the entire event, because the law of conservation of energy prevents mechanical energy from being converted into non-mechanical forms.
   • C) The pellet initially possessed only 4.0 J of kinetic energy that transformed into gravitational potential energy, while the 16.0 J of thermal energy was drawn into the clay from the surrounding air.
   • D) The pellet initially possessed 16.0 J of kinetic energy that was completely converted into thermal energy, while the 4.0 J of gravitational potential energy was generated independently by work done by gravity.
3. 3.

   At a pumped-storage hydroelectric facility, an electric pump consumes 1000 MJ of electrical energy to elevate water into an upper reservoir, storing 780 MJ as gravitational potential energy while the remaining 220 MJ is dissipated as thermal energy. During peak power demand, this stored water flows back down through a turbine-generator system, producing 600 MJ of electrical energy for the power grid. Assuming no water is lost from the system, which statement correctly evaluates the energy transformations and the conservation of energy across the complete operational cycle?

   • A) The turbine-generator transforms 1000 MJ of gravitational potential energy into 600 MJ of electrical energy, with 400 MJ permanently destroyed by mechanical friction.
   • B) The turbine-generator transforms 780 MJ of gravitational potential energy into 600 MJ of electrical energy and 180 MJ of dissipated thermal and sound energy, confirming that total energy across the complete cycle is conserved.
   • C) The system gains 400 MJ of kinetic energy during the generation phase because potential energy is fully converted into net electrical power without dissipation.
   • D) The pumping stage stores 1000 MJ as elastic potential energy in the reservoir, of which 600 MJ is converted into electrical energy and 400 MJ is retained as chemical potential energy.
4. 4.

   An archer expends 150 J of chemical energy from muscular activity to draw a bow, storing 120 J as elastic potential energy in the bow limbs while the remainder warms muscle tissues. Upon release, the bow transfers 96 J of kinetic energy to an arrow launched vertically upward, with the remaining stored energy dissipated as sound and internal thermal energy of the bow. Assuming air resistance on the ascending arrow is negligible, which statement correctly evaluates the energy transformations and the maximum gravitational potential energy gained by the arrow relative to the release point?

   • A) The arrow attains 120 J of gravitational potential energy at its highest point because all elastic potential energy from the drawn bow converts into projectile energy, with zero losses during release.
   • B) The arrow attains 72 J of gravitational potential energy at its highest point because internal thermal losses in the archer's muscles reduce the kinetic energy during ascent.
   • C) The arrow attains 96 J of gravitational potential energy at its highest point because its kinetic energy converts entirely into gravitational potential energy, while a total of 54 J is dissipated to the surroundings throughout the process.
   • D) The arrow attains 150 J of gravitational potential energy at its highest point because the law of conservation of energy requires the entire initial input to reside in the final mechanical state.
5. 5.

   During a field test of a green hydrogen generation system, a photovoltaic panel absorbs 500 J of radiant energy from sunlight and transfers 100 J of electrical energy to an electrolysis cell, with the rest warming the panel. The electrolysis cell uses this electrical energy to produce hydrogen gas, storing 75 J as chemical potential energy while the remaining portion warms the electrolyte solution. Which statement correctly evaluates the energy transformations and the overall conservation of energy across this system?

   • A) The system achieves an overall conversion efficiency of 20% from radiant to chemical potential energy, with 400 J of energy dissipated as thermal energy.
   • B) The system achieves an overall conversion efficiency of 15% from radiant to chemical potential energy, with 425 J of energy dissipated as thermal energy.
   • C) The system achieves an overall conversion efficiency of 80% because only 100 J of energy is transformed into intermediate electrical energy.
   • D) The system achieves an overall conversion efficiency of 75% because the ultimate energy yield is determined entirely by the performance of the electrolyzer.
6. 6.

   An emergency beacon is powered by a manual hand crank connected to an internal electrical generator. An operator performs 60.0 J of mechanical work on the crank, producing 45.0 J of electrical energy while internal friction warms the mechanism. The circuit delivers this electrical energy to an LED module, which emits 18.0 J of radiant energy as visible light while the remaining portion warms the housing and adjacent air. Which statement correctly evaluates the energy transfers and transformations occurring throughout this process?

   • A) Mechanical energy is transformed into electrical energy and subsequently into radiant energy, with a total of 42.0 J transferred to the surroundings as thermal energy to satisfy energy conservation.
   • B) Mechanical energy is transformed entirely into radiant energy and electrical energy, with only 15.0 J dissipated as thermal energy because light emission produces no thermal dissipation.
   • C) Mechanical energy is transformed into electrical energy, but the law of conservation of energy is violated because the final useful radiant output is less than the input work.
   • D) Mechanical energy is transformed into electrical energy and then into radiant energy, with a total of 27.0 J dissipated as thermal energy because mechanical friction is fully converted into electrical energy.
7. 7.

   In an investigation of acoustic energy harvesting, a sensitive piezoelectric module absorbs 8.0 J of acoustic energy from air pressure oscillations over a fixed testing interval. The module directs this energy to charge a storage capacitor, accumulating 1.6 J of electrostatic potential energy while warming the module housing and the adjacent air. Which statement accurately identifies the forms of energy involved and accounts for the final distribution according to the law of conservation of energy?

   • A) The sound waves deliver radiant energy that is converted into chemical energy in the capacitor at an efficiency of 80%, with the remaining 1.6 J dissipated as heat.
   • B) The sound waves deliver mechanical energy that is converted entirely into electrostatic potential energy, with the 6.4 J of thermal energy originating from external heat absorbed from the room.
   • C) The sound waves deliver mechanical wave energy that is partially converted into electrical potential energy at 20% efficiency, with 6.4 J dissipated as thermal energy to the surroundings.
   • D) The sound waves deliver thermal energy that is converted into electrical potential energy at 20% efficiency, while the remaining 6.4 J is permanently eliminated from the total energy of the universe.
8. 8.

   In an engineering test, a compressed horizontal spring storing 50.0 J of elastic potential energy is released to launch a 0.50 kg test block along a rough horizontal track. A high-speed sensor detects that the block detaches from the spring with a speed of 12.0 m/s, after which friction brings the block to a complete stop. Which statement correctly evaluates the energy transfers and transformations throughout this process?

   • A) During the launch, 36.0 J of elastic potential energy is transformed into kinetic energy while 14.0 J is dissipated thermally, and the entire 36.0 J of kinetic energy is subsequently transferred to the thermal store of the surroundings as the block stops.
   • B) The full 50.0 J of elastic potential energy is initially transformed into kinetic energy, of which 14.0 J is destroyed by friction before the block comes to rest.
   • C) During the launch, 14.0 J of elastic potential energy is transformed into kinetic energy, leaving 36.0 J permanently stored within the spring after the block detaches.
   • D) The block acquires 72.0 J of kinetic energy from the expanding spring, requiring 22.0 J of additional work to be done on the spring by track friction.
9. 9.

   An automated hoist powered by a rechargeable battery lifts a 1.0 kg container vertically through 1.5 m at a constant speed in a testing chamber where $g = 10\text{ m/s}^2$. Measurements show that the battery supplies 25.0 J of energy during the lift, while thermal sensors indicate that the motor casing and cables experience a measurable temperature rise. Which statement correctly evaluates the energy transfers and transformations in this system?

   • A) Energy stored chemically in the battery is transferred electrically to the motor, where 15.0 J is transferred mechanically to increase the gravitational potential store of the container and 10.0 J is dissipated to the surroundings as thermal energy and sound.
   • B) Energy stored chemically in the battery is transferred mechanically to the container, where 15.0 J is transformed into kinetic energy of the container and 10.0 J is stored as elastic potential energy in the lifting cables.
   • C) Energy stored electrically in the motor is transferred by radiation to the container, with only 10.0 J increasing its gravitational potential energy because 15.0 J of energy is permanently destroyed by friction.
   • D) Energy stored thermally in the battery is transferred electrically to the motor, causing both the kinetic energy and gravitational potential energy of the container to increase by 12.5 J each.
10. 10.

    An experimental model car equipped with a regenerative braking system descends an incline at a constant speed. An onboard data logger reveals that for every 200 J of gravitational potential energy lost during the descent, 140 J is stored as chemical energy in the battery, while the remaining energy is transferred to the surroundings as thermal energy. Which statement correctly evaluates the energy transfers and transformations occurring in this system?

    • A) Gravitational potential energy transforms into chemical energy and thermal energy, fully satisfying the conservation of energy because the total energy lost equals the total energy gained and dissipated.
    • B) The vehicle's kinetic energy transforms directly into chemical energy in the battery, meaning gravitational potential energy is conserved without any thermal dissipation.
    • C) Energy is not conserved in this process because the useful energy recovered in the battery is strictly less than the initial gravitational potential energy lost.
    • D) Thermal energy generated by friction transforms into gravitational potential energy to maintain the vehicle at a steady descent speed.