Energy

• Important stores of energy: kinetic, gravitational potential, chemical, elastic potential, thermal (internal), nuclear, electrostatic, magnetic.
• Work done = force x displacement
• Energy is the ability to do work.
• The law of conservation of energy: Energy cannot be created or destroyed, it can only be transferred from one form to another
• Different ways of transferring energy from one store to another: mechanically, electrically and by heating or radiation
• Work means transfer or energy
• Gravitational field strength (g) is 10N/kg on Earth
• Transfer by heating: energy transferred between objects when there is a difference in temperature
• Transfer electrically: energy transferred by an electric current
• Transfer by radiation: energy transferred by a wave
• Transfer mechanically: energy transferred through the action of a force
• Kinetic energy is energy due to motion
• GPE = mass x gravitational field strength x height OR GPE = mgh
• Weight = mass x gravitational field strength
• GPE can also = weight x height
• KE = $1/2 mv^2$or half mass times velocity squared
• Speed must be in the units: m/s
• kinetic energy is inversely proportional to gravitational potential energy
• The speed does not depend on the mass because the mass cancels out
• When objects are at rest they do not have any kinetic energy but may still have some gravitational potential energy if they are above ground level.
• If an object has no kinetic or gravitational potential energy, it will have zero total energy.
• Work done = energy transferred
• Work is done on an object when it is lifted up, and work is done by gravity/weight when it falls
• Mechanical work done can be calculated as W = Fs
• Power is the rate of transfer of energy or the rate of doing work
• Power = energy transferred/ time taken or work done / time taken
• Force = mass x acceleration
• Often wasted energy is transferred to the thermal store of the surroundings, either mechanically by the force of friction or radiated
• Efficiency (percentage) = useful energy or power output / total energy or power output (x 100)