Energy change and transfers in a systems

• The energy transferred from one system to another can be stored as potential or kinetic energy.
• Write down the equation which links energy transferred, power and time.

  power = energy transferred / time
• Energy efficiency = useful output/total input x 100%
• Describe how the design of a glass vacuum flask keeps the liquid inside hot.

  glass is a poor conductor so reducing energy transfer by conduction glass reduces energy transfer by conduction
• Describe how the design of a vacuum flask keeps the liquid inside hot.

  both conduction and convection particles  so stops energy transfer between the two walls by conduction and convection
• Describe how the design of a vacuum flask wiht silvered surfaces keeps the liquid inside hot.

  silvered surfaces reflect infrared radiation accept heat for infrared silvered surfaces are poor emitters of infrared radiation
• How does the size of the ears help to keep foxs warm in a cold environment?

  small surface area ears are small is insufficient so reducing energy radiated
• Explain how heat is transferred through the glass wall of a boiling tube.

  particles vibrate with more electric energy and energy is transferred by collisions with other particles

   
• What are closed systems?

  Systems where neither matter nor energy can enter or leave.
• What is the net change in the total energy of a closed system?

  Always zero.
• What are the four main ways energy can be transferred between stores?

  Mechanically, electrically, by heating, by radiation
• Mechanically

  An object moving due to a force acting on it
• Electricity

  A charge (current) moving through a potential difference
• Heating

  energy transfer that increases the kinetic energy of particles
• Radiation

  Energy transferred by light or sound waves
• What is work done?

  energy transferred
• Energy can be transferred usefully, stored or dissipated, but cannot be created or destroyed
• Energy transferred when light bulb is turned on

  
• Closed systems don’t exchange with their surroundings
• Energy transfers can occur in closed systems.However, since energy cannot exchange with the surroundings, there will be no net change to the total energy in a closed system.
• Thermal energy is transferable.
• The rate of transfer depends on thermal conductivity
• Conduction is the main form of heat transfer in solids. This is because the particles are close by and can pass on vibrations easily, which is harder in a gas. Heat in liquids and gases is normally transferred by convection and radiation, which we will touch on below.
• Heat is usually transferred by conduction in solids. In liquids and gasses, convection and radiation are more important. 
• Convection – radiators transfer heat by convection. Air near the radiator is heated, which causes it to become less dense. Therefore this warm, less dense air rises up the room, and the cold, denser air above it sinks down. The cold air can now be heated by the radiator. This cycle goes on and on, and a convection current is formed. The heat is spreading through the room via this current
• Convection currents

  
• Radiation 


  involves transfer of heat via infra red waves. The sun provides heat to the earth due to radiation.
• Forms of heat transfer

  
• Double glazed windows


  thickenin the windows reducing thermal conductivity. As the thermal conductivity decreases, the rate of thermal energy transfer will also decrease.
• Power is the rate at which energy is transferred
• power is the watt (W)
• Power equation

  
• Word done power equations

  
• Evaporation system

  
• We can have two types of energy systems: open or closed.
• Open systems are able to exchange energy and matter with their surroundings
• A Closed energy system is unable to exchange energy and matter with their surroundings. 
• 8 types of energy stores

  
•  Work done is equal to the energy transferred by an action.