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Energy transfers

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Apuntes de la lección

Conservation of Energy

  • Energy is never created or destroyed; it is only transferred between stores.
  • The principle of conservation of energy states that total energy in a closed system remains constant.
  • Energy is measured in Joules (J).

Energy Pathways

  • Energy can be transferred via four pathways: mechanically, electrically, by heating, and by radiation.
  • Mechanical transfer: work done by a force moving an object over a distance.
  • Electrical transfer: work done when charges move through a circuit.
  • Heating: energy moves from a hotter object to a cooler object.
  • Radiation: energy transferred by light, sound, or other electromagnetic waves.

Work Done

  • Work done is another term for energy transferred from one store to another.
  • Work is done when a force causes an object to move.

Energy Stores and Pendulum Example

  • A pendulum has gravitational potential energy (GPE) at its highest point (e.g., 10 J).
  • As it falls, mechanical work transfers GPE to kinetic energy (KE); total energy remains 10 J.
  • At the lowest point, GPE is minimum and KE is maximum (10 J).
  • As it rises again, KE transfers back to GPE; at the opposite highest point, GPE = 10 J, KE = 0 J.
  • Energy is conserved throughout the swing.

Efficiency and Wasted Energy

  • Energy transfers are not 100% efficient; some energy is wasted.
  • Wasted energy often ends up in the thermal energy store of the surroundings (e.g., air, pendulum).
  • Air resistance causes the pendulum to eventually stop, transferring KE to thermal energy.

Energy-transfer pathways and pendulum motion

Diapositivas

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Preguntas de práctica

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  1. 1.Which of the following is the principle of conservation of energy?

    Easy
    • AEnergy can be created but not destroyed.
    • BEnergy can be destroyed but not created.
    • CEnergy cannot be created or destroyed; it can only be transferred from one store to another.
    • DEnergy can be created and destroyed.
  2. 2.What is the unit of energy?

    Easy
    • AWatt (W)
    • BJoule (J)
    • CNewton (N)
    • DPascal (Pa)
  3. 3.A pendulum at its maximum height has 10 J of gravitational potential energy. As it swings down, what happens to the total energy of the pendulum?

    Medium
    • AIt increases to more than 10 J.
    • BIt decreases to less than 10 J.
    • CIt remains constant at 10 J.
    • DIt becomes 0 J at the lowest point.
  4. 4.A book on a shelf has 30 J of gravitational potential energy. When it falls to the ground, what is the maximum kinetic energy it can have just before hitting the ground?

    Medium
    • A0 J
    • B15 J
    • C30 J
    • D60 J
  5. 5.In practice, the kinetic energy of a falling object just before it hits the ground is less than the initial gravitational potential energy. Why?

    Medium
    • ABecause energy is destroyed during the fall.
    • BBecause some energy is transferred to the thermal energy store of the surroundings due to air resistance.
    • CBecause the object loses mass as it falls.
    • DBecause gravitational potential energy cannot be fully converted to kinetic energy.
  6. 6.Which of the following is NOT a pathway for energy transfer?

    Easy
    • AMechanically
    • BElectrically
    • CChemically
    • DBy radiation
  7. 7.Energy transferred by light or sound is an example of which pathway?

    Easy
    • AMechanically
    • BElectrically
    • CBy heating
    • DBy radiation
  8. 8.What is meant by 'work done' in physics?

    Easy
    • AThe total energy stored in an object
    • BEnergy transferred from one store to another when a force moves an object
    • CThe force applied to an object
    • DThe power used by a machine

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