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P1: CONSERVATION AND DISSIPATION OF ENERGY (Changes in Energy Stores (When…
P1: CONSERVATION AND DISSIPATION OF ENERGY
Changes in Energy Stores
Energy can be transferred in a variety of 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 it energy is transferred to the surroundings- the thermal energy store of the surroundings increases, and energy is also transferred by sound waves
Conservation of Energy
Energy cannot be created or destroyed
Conservation of energy applies to all energy changes
A closed energy system is a system in which no energy transfers take place out of or into 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
Energy and Work
Work is done on an object when a force makes the object move
Energy transferred = work done
Work done to overcome friction is transferred as energy to the thermal energy to the thermal energy stores of there objects that rub together and to the surroundings
Gravitational Potential Energy Stores
The GPE store of an object increases when it moves down
The GPE store of an object increases when it is lifted up because work is done on it to overcome the gravitational force
The gravitational field strength at the surface of the moon is less than on the Earth
change in GPE = mass x gravitational field strength x change in height
Kinetic Energy and Elastic Energy Stores
The energy in the kinetic energy store of a moving object depends on its mass and speed
Kinetic energy store = 1/2 x mass x speed
Elastic potential energy is the energy stored in an elastic object when work is done on it
Elastic potential energy = 1/2 x spring constant x extension
Energy Dissipation
Useful energy is the energy in the place we want it and in the form we need it
Wasted energy is energy that is not useful energy and is transferred by an undesired pathway
Wasted energy is eventually transferred to the surroundings, which become warmer as a result
As energy dissipated (spreads out) it gets less and less useful
Energy and Efficiency
No energy transfer can be over 100% efficient
Machines waste energy because of friction between their moving parts, 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
efficiency= (useful energy / total energy supplied by the device) x 100
Electrical Appliances
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 sound and visual images
An electrical appliance is designed for a particular purpose and should waste as little energy as possible
Energy and Power
Power is the rate of energy transfer
Power = energy / time
Power wasted by an appliance = total power input - useful power input
The more powerful an appliance is, the faster the rate at which it transfers energy