Conservation and dissipation of energy

• Energy can be stored in a variety of different energy stores.
• Energy is transferred by heating, by waves, by an electric current, or by a force when it moves an object.
• When an object falls and gains speed, its store of gravitational potential energy decreases and its kinetic energy store increases.
• When a falling object hits the ground without bouncing back, its kinetic energy store decreases. Some or all of its energy is transferred to the surroundings - the thermal energy store of the surroundings increases, and energy is also transferred by sound waves.
• Energy cannot be created or destroyed.
• Conservation of energy applies to all energy changes.
• A closed system is a system in which no energy transfers take place out of or into the energy stores of the system.
• Energy can be transferred between energy stores within a closed system. The total energy of the system is always the same, before and after, any such transfers.
• Work is done on an object when a force makes the object move.
• Energy transferred = work done
• Work done is W = Fs where F is the force and s is the distance moved (along the line of action of the force).
• Work done to overcome friction is transferred as energy to the thermal energy stores of the objects that rub together and to the surroundings.
• The gravitational potential energy store of an object increases when it moves up and decreases when it moves down.
• The gravitational potential energy store of an object increases when it is lifted up because work is done on it to overcome the gravitational force.
• The gravitational fiel strength at the surface of the Moon is less than on the Earth.
• The change in the gravitational potential energy store of an object is ΔPE = mgΔh
• The energy in the kinetic energy store of a moving object depends on its mass and its speed.
• The kinetic energy store of an object is E=1/2mv^2
• Elastic potential energy is the energy stored in an elastic objects when work is done on the object.
• The elastic potential energy stored in a stretched spring is E=1/2ke^2, where e is the extension of the spring.
• Useful energy is energy in the place we want it and in the form we need it.
• Wasted energy is the energy that is not useful energy and is transferred by an undesired pathway.
• Wasted energy is eventually transferred to the surroundingd, which become warmer.
• As energy dissipates (spreads out), it gets less and less useful.
• The efficiency of a device = useful energy transferred by the device / total energy supplied to the device x 100
• No energy transfer can be more than 100% efficient.
• Machines waste energy because of friction between their moving parts, air resistance, electrical resistance and noise.
• Machines can be made more efficient by reducing the energy they waste. For example, lubrication is used to reduce friction between moving parts.
• Electricity and gas and/or oil supply most of the energy you use in your home.
• Electrical appliances can transfer energy in the form of useful energy at the flick of a switch.
• Uses of everyday electrical appliances include heating, lighting, making objects move (using an electric motor), and producing sond and visual images.
• An electrical appliance is designed for a particular purpose and should waste as little energy as possible.
• Power is the rate of energy transfer of energy.
• Power wasted by an appliance = total power input - useful power output
• The power of an appliance is P = E/t
• Efficiency of an appliance = useful power out/total power in x 100