1. Energy

• Closed System


  A system that experiences no net change in its total energy when energy transfers occur within it.
• System

  A single, or group of objects
• Conservation of Energy


  The law that energy can only be transferred, stored or dissipated, never created or destroyed
• Energy stores

  Elastic potential, gravitational potential, kinetic, thermal, magnetic, electromagnetic, chemical, nuclear
• Elastic potential energy


  The store of energy that stretched or compressed objects experience. It is directly proportional to the stiffness constant and to the square of the extension or compression
• Gravitational potential energy


  The store of energy that all raised matter has. It is directly proportional to the mass of the object, the distance that it is risen and the gravitational field strength at that point.
• Energy unit


  Joule, J
• Kinetic energy


  The store of energy that all moving matter has. It is directly proportional to the object’s mass and to the square of its velocity.
• Power unit


  Watts, W
• Specific Heat Capacity


  The amount of energy required to raise the temperature of 1kg of a substance by 1°C.
• Magnetic energy


  Magnetic materials interacting with each other have energy in their magnetic store
• Electrostatic energy


  Objects with charge (like electrons and protons) interacting with one another have energy in their electrostatic store
• Chemical energy


  Chemical reactions transfer energy into or away from a substance's chemical store
• Nuclear energy

  Atomic nuclei release energy from their nuclear store during nuclear reactions
• Thermal energy


  All objects have their thermal store, the hotter the object, the more energy it has in this store
• Energy Transfer Pathways


  Energy is transferred between stores through transfer pathways : Mechanically, Electrically, Heating, Radiation
• Mechanical working


  When a force acts on an object (eg. pulling, pushing, stretching, squashing)
• Electrical working


  A charge moving through a potential difference (eg. current)
• Heating (by particles)


  Energy is transferred from a hotter object to a colder one (eg. conduction)
• (Heating by) Radiation


  Energy transferred by electromagnetic waves (eg. visible light)
• Energy is transferred from the hot coffee to the mug to the cold hands


  An example of an energy transfer is a hot coffee heating up cold hands

  
• Energy transfer examples


  Different types of energy transfers occur all the time in various everyday circumstances, for example : when an object is projected upwards, when a moving object hits an obstacle, when an object is accelerated by a constant force, when a vehicle speeds up or slows down, when water is brought to a boil in an electric kettle
• An object projected upwards 


  Before ball is thrown upwards, person has chemical energy, when ball is thrown some of energy is transferred to kinetic energy of the ball as it moves, as height increases kinetic energy is transferred into gravitational potential energy

  
• A moving object hitting an obstacle

  The car has kinetic energy as it moving, if the car hits an obstacle the speed will decrease quickly which decreases kinetic energy, the kinetic energy is dissipated to the thermal store of the surroundings

  
• A vehicle being accelerated by a constant force


  When a vehicle is stationary (not moving), it has chemical energy (of the fuel), when the vehicle speeds up or accelerates, energy is transferred to the kinetic energy of the car

  
• A vehicle slowing down


  When the vehicle is moving, it has kinetic energy, as it decelerates (slows down) energy is transferred to the thermal energy of the surroundings (dissapated), the energy is transferred by the heating pathway due to friction

  
• Boiling water in a kettle


  When electric kettle boils water, thermal energy inside the kettle is transferred to the water, by heating (pathway) into the water's thermal energy store,

  
• Kinetic energy


  If an object accelerates, energy is transferred to its kinetic store , If an object slows down, energy is transferred away from its kinetic store

  Calculation : ½ ×m×v^2 (m:mass (kg), v:velocity (m/s)
• Example question of Kinetic Energy : Calculate the kinetic energy stored in a vehicle of mass 1200 kg moving at a speed of 27 m/s.

  
  

  Step 1: List the known quantities

  • Mass of the vehicle, m = 1200 kg
  • Speed of the vehicle, v = 27 m/s

  Step 2: Write down the equation for kinetic energy

  EK = ½ mv2

  Step 3: Calculate the kinetic energy

  EK = ½ × 1200 × (27)2

  EK = 437 400 J

  Step 4: Round the final answer to 2 significant figures

  EK = 440 000 J (Final Answer)
• Another example question of Kinetic Energy

  A car of a mass of 2500kg is travelling at 20 m/s, calculate its kinetic energy

  • $1/2mv^2$ -> 1/2 x 2500 x 20^2 = 500,000J (Final Answer)
• Gravitational potential energy


  If an object is lifted up, energy is transferred to its gravitational store, if an object falls, energy will be transferred away from its gravitational potential store

  Calculation : $mgh$ -> mass x gravitational field strength x height
• Example of GPE

  A man of mass 70 kg climbs a flight of stairs that is 3 m higher than the floor. Gravitational field strength is approximately 9.8 N/kg. Calculate the energy transferred to the man's gravitational potential energy store.

   

  Step 1: List the known quantities

  • Mass of the man, m = 70 kg
  • Gravitational field strength, g = 9.8 N/kg
  • Height, Δh = 3 m

  Step 2: Write down the equation for gravitational potential energy

  ΔEP = mgΔh

  Step 3: Calculate the gravitational potential energy

  ΔEP = 70 × 9.8 × 3

  ΔEP  = 2058 J
• Gravitational field strength

  GFS on Earth is 9.8 N/Kg

  The GFS on the Moon is less on Earth (easier to lift a mass on the Moon than on Earth)

  The GFS on a gas giant is more than the Earth (harder to lift a mass on a gas giant than on Earth
• Elastic potential energy

  When a spring is stretched (or compressed), work is done on the spring which results in the transfer of elastic potential energy of the spring

  When the spring is released, energy is transferred away from its elastic potential store

  Calculation : $1/2ke^2$ k=spring constant, N/m, e=extension (metres) m
• A mass is attached to the bottom of a hanging spring with a spring constant of 250 N/m. It stretches from 10.0 cm to 11.4 cm.

  Calculate the elastic energy stored by the stretched spring.

  1. Find out the extension of the spring (final length - original lenth) -> 11.4 - 10.0 = 1.4cm
  2. Convert from cm to m : 0.014m
  3. k=250 N/m , e=0.014m
  4. Use equation ½ ke2
  5. Substitute the numbers in : 1/2 x 250 x (0.014)2
  6. Answer : 0.0245 J (Round to 2sf) -> 0.025J
• Specific Heat Capacity

  The amount of energy required to raise the temperature of 1kg of a substance by 1°C
• Different substances have different Specific Heat Capacities

  • If a substance has a low specific heat capacity, it heats up and cools down quickly, which means it would take less energy to change its temperature
  • If a substance has a high specific heat capacity, it heats up and cools down slowly, which means it would take more energy to change its temperature
• Change in Thermal Energy calculation

  $mcΔT$

  m=mass (Kg) , c=specific heat capacity (J/kg°C) , T= change in temperature, degrees Celsius (°C)
• Different Specific Heat Capacities

  SHC of Water : 4200 J/Kg°C

  SHC of Aluminium : 900 J/Kg°C

  SHC of Copper : 389 J/Kg°C
• Specific Heat Capacity calculation

  change in thermal energy / (mass x temperature change)