chap 5 - energy

• energy stores

  • energy is required for things to work
  • SI unit of energy = joules (J)
  • a system consists of different parts working together
  • gravitational potential store of an object is actually an energy store of the object-Earth system
• GPE of an object is actually an energy store of the object
• types of energy store

  • elastic potential store
  • thermal store --> heat energy
  • kinetic store
  • gravitational potential store
  • chemical potential store
  • nuclear store
• energy transfer

  > non-physical and invisible substance can be transferred from one energy store to the other

  > total amount of energy remains constant

  > 4 ways :

  • : mechanically- force acting over distance
  • heating- as a result of temp. difference
  • propagation pf waves- by electromagnetic and mechanical
  • electrically- by charges moving through a potential difference
• energy transferred mechanically by a force acting over a distance

  • eg:

  > cart of mass m moving horizontally at a constant speed of 1.0m/s along a smooth surface (no friction). when force is applied, the speed of cart increased to 2.0m/s.

  = this shows that the energy transferred from energy store in the person to the energy in the cart.

  by newton´ s second law of motion, the reactant force f caused the cart to accelerate in the direction of force applied
• energy transferred by heating due to temp. difference

  • eg:

  > small block of solid butter on top of electrical stove. before electrical stove is on, the butter has energy in its internal store

  = energy is transferred by conduction from pan to the butter due to a temp. difference
• energy transferred by propagation of waves

  • electromagnetic waves

  > eg: girl kicking a ball in an open field on a sunny day. Before she step onto the field, her skin has energy in its internal store, energy is being transferred by propagation of electromagnetic waves

  = after being in the sun for some time, energy in the internal store in her skin increased.
• energy transferred by the propagation of waves

  • mechanical sound waves

  >eg: boy hitting two cymbals together

  - before hitting the cymbals together, the ear drums of the boy has zero kinetic energy store . after hitting the cymbals together, the energy in the kinetic store increased through the energy transferred by the propagation of mechanical sound waves through the air
• energy transferred electrically by an electric current

  > eg: bulb in simple circuit

  - before the switch is closed, the bulb has energy in its internal store. Energy is transferred electrically from the chemical potential store of the battery

  = energy is also transferred by light and infrared waves tot he surroundings
• other examples of energy transfer

  • eg: burning of charcoal

  > before burning the charcoal, it has energy in the chemical potential store. Energy are transferred by propagation of electromagnetic waves. When charcoal is burning, energy are transferred to the internal store of the charcoal

  = energy in chemical potential store of charcoal decreases
• calculating the size if an energy store

  • energy in kinetic store

  > formula --> E = 1/2 x mass x velocity^2

  (anything that moves --> VELOCITY )
• calculating the size of an energy store

  • energy in the gravitational potential store

  > formula --> E= mass x gravitational potential store x height

  ( anything that has height )
• principle of conservation of energy

  • eg: if you strike a match, you will get a burning flame

  > energy in the chemical potential store of the match head is transferred by heating to the internal store of the surroundings

  = total amount of energy transferred is the same

  = states that the amount of energy cannot be created or destroyed
• principle of conservation of energy and the Ideal pendulum

  • so when the ball is held to the left (point A), it has a max. GPE due to the height above the line
  • when its released, it swings downwards with increasing speed due to CONSTANT FORCE OF ITS WEIGHT & TENSION OF STRING-->the height at point A decreases, energy is transferred MECHANICALLY from GPE at A to KE at B(middle of pendulum)
  • from B to C (right side), speed decreases from the max. at B to zero at C due to RESULTANT FORCES OF ITS WIGHT & TENSION IN THE STRING.-->energy is being transferred MECHANICALLY from KE at B to GPE at C
• principle of conservation of energy

  • there is no change in the GPE if the height is constant
  • flow of electric current increases the temp. of the filament wire of the lights and causses them to lit up
  • energy i transferred from the lights to the surroundings by electromagnetic waves
  • reduce in height= decrease
  • height decrease, GPE decrease = speed decrease
• work done by a force

  • constant force of an object is the product of the force and the distance moved by an object in the direction of force
  • object not moving = no work done
  • work done = energy transferred
  • direction of force and direction of motion must be the same
  • FORMULA --> work done = force x distance
  • SI UNIT-->joule (J)
• power

  • power = work done or energy transferred per unit time
  • SI unit--> watt (W)
  • FORMULA--> P = energy divide by time
• energy

  • measurement of the ability to do work --> never destroyed, just transferred
  • KE is the only energy an object has cuz of its motion --> calculate KE = work done formula