Work and Power

• Work done is the amount of energy transferred when a force moves a body.
• Work done = Energy transferred
• Energy cannot be created or destroyed, it can only be transferred usefully, stored or dissipated.​

  Energy is measured in joules (J).
• Work done (J) = Force (N) x distance (m)
• Power is the rate at which energy is transferred.
• Power (W) = Work done (J) ÷ time (s)​

  The unit of power is the Watt (W). One W = 1 J/s.
• Gravitational potential energy (J) = mass (kg) x gravitational field strength (N/kg) x height (m).
• The gravitational field strength on Earth is 9.8 N/kg.
• Lifting an object in a gravitational field requires work. This causes a transfer of energy to the gravitational potential energy store of the raised object. The higher the object is lifted the more energy is transferred to this store.
• Anything that is moving has energy in it’s kinetic energy store. Energy is transferred to these stores when an object speeds up (accelerates) and is transferred away from this store when an object slows down (decelerates). The energy in the kinetic energy store depends on mass and speed. The greater the mass and the faster it’s going the more energy there will be in its kinetic energy store.
• Kinetic energy (J) = ½ x mass (kg) x velocity2 (m/s)
• GPE lost = KE gained
• When something falls energy from its gravitational potential energy store is transferred to it’s kinetic energy store. 
• When you apply force to an object you may cause it to stretch, compress or bend. Work is done when a force stretches or compresses an object and causes energy to be transferred to the elastic potential energy store of the object. An elastic object can be inelastically deformed if it doesn’t return to it’s original shape and length after the force has been removed.
• Hooke’s Law​

  The extension of a stretched spring (or other elastic object) is directly proportional to the force applied.
• Force (N) = spring constant (N/m) x extension (m)
• The spring constant depends on the material that you are stretching. A stiffer spring has a greater spring constant, so a larger force would be needed for a given extension.
• There is a limit to the amount of force you can apply to an object for the extension to keep increasing proportionally. These graphs show force against extension for elastic objects. There is a maximum force above which the graph curves, showing that extension is no longer proportional to force, shown on the graphs at P. This is known as the limit of proportionality. 
• Stretching or squashing an object can transfer energy to it’s elastic potential energy store. So long as the limit of proportionality has not been exceeded, energy in the elastic potential energy store of a stretched spring can be found using this equation.
• Elastic potential energy (J) = ½ x spring constant (N/m) x extension2 (m)
• Conduction is heat transfer in solids. The direction of energy always goes from hot to cold.​

  When one end of a solid rod is heated the particles gain more kinetic energy. They cannot move but instead vibrate more, colliding with their neighbours and passing on the energy.
• Gas and Liquids are poor conductors of heat because of the spacing between the particles.
• Metals are good conductors of heat because their structure contains free electrons which gain energy and help transfer it as they move.
• The higher the thermal conductivity of a material the higher the rate of energy transfer by conduction across the material.
• Convection is the transfer of thermal energy in fluids.​

  As a fluid is heated the particles gain kinetic energy and move around more. The space between individual particles increases causing the fluid to expand. The warmer and now less dense fluid will rise above the denser, cooler fluid. The denser cold fluid falls into the warm areas. In this way, convection currents that transfer heat from place to place are set up. 
• Insulation reduces the rate of energy transfer by heating. To make things more efficient it is important to reduce heat transfer. We can reduce heat transfer via conduction and convection by using a vacuum. A vacuum is a space with no particles. Both conduction and convection require particles for energy transfers. 
• Air is an excellent insulator so foams and fibrous materials are used to trap air as an insulator. 
• Heat transfer by convection can be reduced by using lids so that heated particles cannot rise, along with foams and fibrous materials which stop the movement of gases.