P1 - Energy

• A system is an object or group of objects
• There are changes in the way energy is stored when a system changes.
• The principle of the conservation of energy says that energy cannot be created or destroyed, only transferred/stored/dissipated
• Kinetic energy is the energy stored in a moving object. Stationary objects have no kinetic energy

  Calculated by: Ek = 0.5 * mass * velocity^2

  Kinetic Energy - Joules (J)

  Mass - Kilograms (Kg)

  Velocity - metres/sec (m/s)
• Elastic potential energy is the energy stored in a stretched spring or elastic object.

  Calculated by: Ee = 0.5 * spring constant * extension^2

  Ee = Elastic potential energy (J)

  Spring Constant = Newtons/metre (N/m)

  Extension = Metres (m)
• Gravitational Potential Energy is the energy stored in an object due to its position above the earth's surface. Due to force of gravity acting on an object.

  Calculated by: Ep = mass * grav field strength * height

  Mass - Kilograms (kg)

  Grav field strength - Newtons/kilogram (N/kg)

  Height - Metres (m)
• The specific heat capacity of a substance is the amount of energy required to raise the temperature of 1kg of the substance by 1*C
• SPECIFIC HEAT CAPACITY:

  change in thermal energy (J) = mass(kg) * SHC(J/kg*C) * temp change(*C)
• Thermal energy is energy stored due to an object's temperature
• A closed system is a system where energy cannot enter or leave
• Energy Transfers in a Pendulum:

  • The point where it stops swinging to one end is the point where it has the maximum GPE
  • As it begins to swing again, the GPE is transferred to KE
  • Maximum KE of the mass is at the lowest point of the swing
  • As mass swings back up, KE transfers to GPE
  • The point where it stops moving up is the maximum GPE of the mass
• Friction causes energy to be transferred to thermal energy stores.
• You can reduce unwanted energy transfers by reducing friction, and removing air particles
• A moving object hitting an obstacle

  • The object's energy is in the kinetic store to begin with because it is moving.
  • When the object collides with the obstacle, energy is converted to:
  • The kinetic store of the obstacle (making it move),
  • The thermal store of the object and the obstacle (the particles in the object and the obstacle vibrate more).
  • Some energy remains in the object's kinetic store as it moves away after the collision.
• Objects accelerated by constant force

  • Work is done by a force on an object.
  • This work is converted to the object's kinetic store.
• A vehicle slowing down

  • To begin with, the vehicle's energy is in the kinetic store.
  • The brakes do work slowing the car down. During this process, energy is dissipated (lost) through heat and sound.
• An object projected upwards

  • The object's energy is initially in the kinetic store as it moves upwards.
  • The object's energy is slowly transferred from the kinetic store to the gravitational potential store as it slows down and climbs higher.
  • Once the object reaches its highest point, all of its energy is in the gravitational potential store.
  • As the object falls again, energy transfers from the gravitational potential store to the kinetic store.
• Boiling water in an electric kettle

  • Energy transfers from the electrical store of the mains power supply to the thermal store of the water.
• More work is done if you:

  • push with the same force for a longer distance.
  • or push with a larger force for the same distance.
• • Work done = force x distance (moved along the line of action of the force).
• E = V x I x t

  (electrical energy = voltage x current x time)
• Examples of wasted energy:

  • Car engines - noise and heat
  • Wires - heat
  • Friction - heat
• Heat will be lost through the roof, windows, walls and floor. To reduce the energy transfer, we might:

  • Fit loft insulation (the roof).
  • Use double glazing (windows).
  • Use cavity wall insulation (walls).
  • Fit carpets (floors).
• • We can decrease the rate a house cools down by:
  • Having thicker walls
  • Having walls made from a material with a lower thermal conductivity.
  • The more heat that a material conducts (the higher its thermal conductivity is), the higher the rate of energy transfer by conduction across the material.
• • Lubrication reduces frictional forces.
  • A moving object normally has at least one frictional force acting against it. This causes some energy to be dissipated.
  • Lubricants can reduce the friction between the surfaces of objects that are rubbing together.
  • Lubricants are normally liquids.
• • Power (watts, W) = work done (joules, J) ÷ time (seconds, t)
• • Power (watts, W) = energy transferred (joules, J) ÷ time (seconds, t)