chapter 7

• energy stores

  energy is required for things to work

  there are 6 types of energy stores

  • elastic potential store (object stretched or compressed)
  • kinetic store (object in motion)
  • internal/thermal store (hot object)
  • gravitational potential store (object that is above the ground)
  • nuclear store (nuclei of atoms)
  • chemical potential store (food/fossil fuels/batteries)
• energy transfer


  the total amount the energy, no matter the amount transferred from one store to another, remains constant

  can be transferred mechanically, by heating, by propagation of waves, electrically
• energy transferred mechanically by force acting over a distance

  by the newton's second law of motion, the resultant force causes the cart to accelerate in the direction of the applied force

  before force is applied, the system has energy in its kinetic store. after the force is applied, more energy is transferred mechanically to the kinetic store due to mechanical work done by the applied force acting on the cart over a distance
• energy transferred by heating due to a temperature difference

  temperature difference between the hot surface of the pan and the butter

  before the electric stove is turned on, the system has energy in its internal store. after the electric stove is turned on, the energy in the internal store of the system increases due to the energy transferred by heating from the hotter object to the cooler object, due to temperature difference
• energy transferred by propagation of waves (electromagnetic waves)

  electromagnetic waves from the sun interact with the skin of the girl to create a sensation of warmth

  before the girl steps into the field, the system has energy in the internal store. after she is beneath the sun for a period of time, the energy in the internal store of the system increases due to the energy transfer from the sun into the system by propagation of electromagnetic waves.
• energy transferred by propagation of waves (mechanical sound waves)


  when cymbals are hit, the cymbals vibrate to and fro, causing air particles next to the vibrating cymbals to vibrate to and fro to produce sound waves.

  before hitting the cymbals together, energy in the kinetic store of the system is zero as they are at rest. after hitting the cymbals together, energy in the kinetic store of the system increases due energy transfer by the propagation of mechanical sound waves from the cymbals
• energy transferred electrically

  flow of electric current increases the temperature of the filament in the bulb, causing it to light up

  at initial state before the switch is closed, the system has energy in the internal store at room temperature. at final state after the switch is closed, the energy in the internal store of the system increases as energy in the chemical potential store of the battery is transferred electricity to the energy in the internal store of the system
• principle of conservation of energy


  energy cannot be created or destroyed

  energy can be transferred from one store to another

  total energy of an isolated system is constant
• work done


  work is only done when an object moves under the influence of a force

  has the same si unit as energy as work done is equivalent to energy transferred
• power

  A has more power than B as A can do the same amount of work more quickly

  A was able to do more work than B in the same amount of time hence A has more power than B

  defined as the work done or energy transferred per unit time