Topic 8 ENERGY - FORCES DOING WORK

• Work done


  The amount of energy transferred 
• conservation of energy.


  Energy can't be created or used up.
•  system


  is an object or a group of objects.

  Systems have energy.
• Energy is measured in joules (J).
• Energy is stored in different energy stores.

  Energy is transferred when a system changes.
• Energy can be found in different stores: chemical, elastic potential, electrostatic, gravitational potential, kinetic, magnetic, nuclear
• closed system

  • where energy cannot enter or leave.

  It can only be transferred usefully, stored, or dissipated by the objects within the system.

  Energy cannot be created or destroyed.

  So the total amount of energy in a closed system always stays the same.
• how to calculate effieicency

  
• There are 3 main ways that the energy of a system can be changed:

  • The first way to change the energy of a system is through work done by forces.
  • The second way to change the energy of a system is in electrical equipment.
  • The third way to change the energy of a system is by heating.
• Work is done when a force moves an object through a distance.
• work done(E) = force(F) X distance moved in direction of force(d)
• Joules(J) = Newtons(N) x metres(M)
• How to increase work done?

  increase force on object
• Another way to increase work done?

  increase distance moved by object
• i joule of work is done when force of 1N causes displacement of 1m
• work done against friction causes temperature rise
• example: if apple falls it does work as its weight(force) moves through distance. It also works against another force called friction. The air resistance increases temperature of apple
• work done against any frictional force acting on object causes rise in temperature of object
• how is work done by forces changing energy of system?

  e.g. if you push box across floor energy is transferred from chemical energy store in arms to kinetic energy store into box(and to thermal energy store of box and surroundings)
• how is electrical equipment changing energy of system?

  e.g. when you start electric scooter, energy is transferred from chemical energy store of its battery to kinetic energy store of scooter(and thermal energy store of surroundings) by electrical pathway
• how is heating changing energy of system?

  e.g. if you boil pan of water on gas hob energy is transferred from the chemical energy of gas to thermal store of the water(and those of the pan and surroundings)
• how to increase efficiency of energy transfer?

  reducing amount of dissipated energy
• p = E/T
• Power(w) = work done(J) x time taken(s)
• dissipated energy is travelled to less useful energy stores
• heavier faster objects have more kinetic energy than lighter objects
• What is power


  rate at which energy is transferred(rate of energy transfer)
• 1 watt = energy transfer of 1 joule per second (j/s)
• example. when hairdryer has power of 1500 w when switched on it transfers 1500 joules of energy every second
• Efficiency= useful energy transferred/total energy supplied