Conservation and Dissipation of Energy


Changes in Energy Stores

- 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 the energy is also transferred by sound waves.

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Conservation of Energy

- energy cannot be created or destroyed 

- conservation of energy applies to all energy changes

- a closed system is an isolated 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 closed system. the total energy of the system is always the same, before and after, any such transfers

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Energy and Work

- work is done on an object when a force makes the object move

- energy transferred = work done

- work done = force x 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

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Gravitational Potential Energy Stores

- 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 field strength at the surface of the moon is less than on the surface

- the change in gravitational potential energy store of an object is:

    change in object's gravitational potential energy store = weight x change of height

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Kinetic Energy and Elastic Energy Stores

- 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: 

    kinetic energy= 1/2 x mass x speed²

- elastic potential energy is the energy stored in an elastic object when work is done on the object

- The elastic potential energy stored in a stretched spring is:

    elastic potential energy = 1/2 x spring constant x extension²

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Energy Dissipation

- useful energy is energy in the place we want it and 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 surroundings which becomes warmer

- as energy dissipates (spreads out) it gets less and less useful


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Energy and Efficiency

- 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 reistance and noise

- machines can be made more efficient by reducing the enrgy they waste, for example, lubrication is used to reduce friction between moving parts

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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

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Energy and Power

- power is the rate of transfer of energy

- the power of an appliance is:

    power = enrgy transferred to appliance / time taken for energy to be transferred 

- efficiency of an appliance is:

    efficiency = useful power out / total power in ( x100%) 

- power wasted by an appliance = total power input - useful power output

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