Science - Energy - including equations (physics)

• Energy is is never used up . It is just transferred between different energy stores and different objects .
• When energy is transferred to an object , the energy is stored in one of the object's energy stores . Energy stores are :

  • Thermal energy store
  • Kinetic energy store
  • Gravitational potential energy stores
  • Elastic potential energy stores
  • Chemical energy stores
  • Magnetic energy stores
  • Electrostatic energy stores
  • Nuclear energy stores
• Energy is transferred mechanically (by a force doing work) , electrically (work done by moving charges ) , by heating or radiation (example: light or sound )
• • A system is a single object or a group of objects
  • When a system changes , energy is transferred . It can be transferred into or away form the system , between different objects in the system or between different energy stores .
  • Closed systems are systems where neither matter nor energy can enter or leave . The net change in the total energy of a closed system is always zero .
• Energy can be transferred by heating :

  • For example : boiling water in a kettle - think of the water as the system . Energy is transferred to the water (from the kettle's heating element ) by heating , into the water's thermal energy store (causing the temperature of the water to rise)
  • You could also think of the kettle's heating element and the water together as a two-object system . Energy is transferred electrically to the thermal energy store of the kettle's heating element , which transfers energy by heating to the water's thermal energy store
• • Work can be done when current flows (work is done against resistance in a circuit) or by a force moving an object .

  For example :

  • The initial force exerted by a person to throw a ball upwards does work . It causes an energy transfer from the chemical energy store of the person's arm to the kinetic energy store of the ball and arm .
  • A ball dropped from a height is accelerated by gravity . The gravitation force does not work . it causes energy to be transferred from the ball's gravitational potential energy store to its kinetic energy store
• • Work done is just another way of saying energy transferred.
• • Examples of work done :The friction between a car's brakes and it's wheels does work as it slows down . It causes energy transfer from the wheel's kinetic energy to the thermal energy store of the surroundings .
  • In a collision between a car and a stationary object , the normal contact force between the car and the object does work . It causes energy to be transferred from the car's kinetic energy store to other energy stores - like elastic potential and thermal energy stores of the object and the car body . Some energy might also be transferred by sound waves .
• Kinetic energy stores :

  • Anything that is moving has energy in its kinetic energy store . Energy is transferred to this store when an objects speeds up and is transferred away form this store when an object slows down .
  • The energy in the kinetic energy store depends on the object's mass and speed . The greater the mass and the faster it's going , the more energy there will be in its kinetic energy store .
• • The formula for kinetic energy :

  KE = 1/2 mv(squared)

  kE is kinetic energy (J)

   1/2 mv2 means 1/2 x m x v(squared)

  M is Mass (kg)

  V2 is (Speed)squared (m/s)squared

  Example:

  A car of mass 2500kG is travelling at 20 m/s .

  Calculate the energy in its kinetic energy store :

  KE = 1/2mv(squared)

  KE = 1/2 x 2500 x 400 = 500 , 000 J
• Raised objects store energy in Gravitational potential energy stores .

  • Lifting an object in a gravitational field requires work . This causes a transfer of energy to the gravitational energy (g.p.e) store of the raised object . The higher the object is lifted , the more energy is transferred to this store .
  • The amount of energy in a g.p.e store depends on the object's mass , height and strength of the gravitational field the object is in
• • Use this equation to find the change in energy in an object's gravitational potential energy store for a change in height :

  $Ep =$$mgh$

  Ep is g.p.e (J)

  M is mass (kg)

  G is gravitational field strength (N/Kg)

  H is height (m)
• Falling objects also transfer energy :

  • When something falls , energy from its gravitational potential energy store is transferred to its kinetic energy store
  • For a falling object when there's no air resistance :

  Energy lost from the g.p.e store = enrgy gained in the kinetic energy store

  • In real life , air resistance acts against falling objects - it causes some energy to be transferred to other energy stores , e.g the thermal energy store of the objects and surroundings
• Stretching can transfer energy to elastic potential energy stores :

  Stretching or quashing an object can transfer energy to its elastic potential energy store . So long as the limit of proportionality has not been exceeded , energy in the elastic potential energy
• Stretching can transfer energy to elastic potential energy stores: Stretching or quashing an object can transfer energy to its elastic potential energy store . So long as the limit of proportionality has not been exceeded , energy in the elastic potential energy of a stretched spring can be found using :$Ee =$$1/2KE(squared)$Ee is Elastic potential energy (J)

  1/2kE(squared) is 1/2 x spring constant (N/m) x Extension(squared)(metres squared)
• Specific heat capacity


  how hard it is to heat something up
• 
  

  • Different materials Have different specific heat capacities
  • More energy needs to be transferred to the thermal energy store of some materials to increase their temperature than others
  • Materials that need to gain lots of energy in their thermal energy stores to warm up also transfer loads of energy when they cool down again
  • They can 'store' a lot of energy
• Specific heat capacity

  The amount of energy needed to raise the temperature of 1 kg of a substance by 1°C
• Equation linking energy transferred to specific heat capacity

  Change in thermal energy = mass(kg) x Specific heat capacity x Temperature change
• Investigating specific heat capacities

  1. Measure mass of material

  2. Wrap in insulating layer

  3. Insert thermometer and heater

  4. Measure initial temperature

  5. Turn on power supply and start stopwatch

  6. Take temperature and current readings every minute for 10 minutes

  7. Turn off power supply

  8. Calculate energy transferred to heater

  9. Plot graph of energy transferred vs temperature

  10. Find gradient of straight part of graph

  11. Calculate specific heat capacity
• Conservation of energy principle : Energy can be transferred usefully , stored or dissipated but can never be created or destroyed .
• When energy is transferred between stores , not all of the energy is transferred usefully into the stores that you want it to go to . Some energy is always dissipated when an energy transfer takes place .
• Dissipated energy is sometimes called wasted energy because the energy is being stored in a way that is not useful (usually energy has been transferred into thermal energy stores ) .

  For example :

  A mobile phone is a system . When you use the phone , energy is usefully transferred from the chemical energy store of the battery in the phone . But some of this energy is dissipated in this transfer to the thermal energy store of the phone .
• Describing energy transfer for closed systems : Example :

  A cold spoon is dropped into an insulated flask of hot soup , which is then sealed . You can assume that the flask is a perfect thermal insulator so the spoon and the hot soup form a closed system . Energy is transferred from the thermal energy store of the soup to the useless thermal energy store of the spoon (causing the soup to cool down slightly ). Energy transfers have occurred within the system , but no energy has left the system - so the net change in energy is zero .
• • Power is the rate of energy transfer , or the rate of doing work .
  • Power is measured in watts . One watt = 1 joule of energy transferred per second .
  • P = E/T or P = W/T

  P is power (watts)

  E is energy transferred (J)

  W is work done (J)

  T is time (s)
• Lubrication reduces frictional forces
• • Whenever something moves , there's usually at least one frictional force acting against it . This causes some energy in the system to be dissipated , for example , air resistance can transfer energy from a falling object's kinetic energy store to its thermal energy store .
  • For objects that are being rubbed together , lubricants can be used to reduce the friction between the object's surfaces when they move . Lubricants are usually liquids (like oil) so they can flow easily between objects and cot them
• Heating can occur by conduction and convection
• When an object is heated , energy is transferred to the kinetic energy stores of its particles . This causes the particles to vibrate more and collide with each other . During these collisions , energy is transferred between the particles' kinetic energy stores . This is called conduction
• Thermal conductivity is a measure of how quickly energy is transferred through a material in this way . Materials with a high thermal conductivity transfer energy between their particles at a faster rate .

  If the particles are free to move (e.g in a gas/liquid) the particles moving faster means that the space between individual particles increases . This causes the density of the region being heated to decrease .
• Because liquids and gases can flow , the warmer and less dense region will rise above denser , cooler regions . So energetic particles move away from hotter to cooler regions . This is called convection .
• Insulation reduces the rate of energy transfers by heating
• To prevent energy losses through heating :

  • Have thick walls that are made from a material with a low thermal conductivity .The thicker the walls and the lower their thermal conductivity , the slower the rate of energy transfer will be (so the building will cool more slowly)
  • Use thermal insulation
• Heat insulation

  • Cavity walls with an air gap
  • Cavity wall insulation with foam
  • Loft insulation
  • Double glazed windows with air gap
  • Draught excluders around doors and windows
• Cavity walls

  • Made of an inner and outer wall with an air gap in the middle
  • The air gap reduces the amount of energy transferred by conduction through the walls
• Cavity wall insulation

  • The cavity air gap is filled with a foam
  • Can reduce energy transfer by convection in the wall cavity
• Loft insulation

  • Can reduce convection currents (a cycle where air particles are constantly being heated, rising, cooling then sinking) being created in lofts
• Double glazed windows

  • Have an air gap in the middle two sheets of glass
  • Prevent energy transfer by conduction through the windows
• Draught excluders

  • Reduce energy transfers by convection around doors and windows
• Most energy transfers involve some waste energy . The input energy is usually a thermal energy store . The less energy is 'wasted' in this energy store , the more efficient the device is said to be . You can improve he efficiency of energy transfers by insulating objects , lubricating them or making them more streamlined .