conservation of energy Flashcards
equation to calculate change in gravitational Potential energy when an object is raised above ground
change in GPE = m * g * change in h
joule kg n/kg m
equation to calculate the amounts of energy associated with a moving object
KE = 0.5 * m * v squared
Joule 0.5 kg speed squared
interpret diagrams to represent energy transfers
- boxes show stores
- arrows show transfers
(looks like box with energy store written, arrows showing way of transferring energy ((ex mechanically)), arrow goes to other energy transfers, ex kinetic energy -> mechanically -> thermal energy store of ball)
*can be more arrows and boxes
explain what is meant by conservation of energy
- energy can be stored, transferred between stores and dissipated
-BUT IT CAN NEVER BE CREATED OR DESTROYED - total energy in a closed system has no net change
Analyse the changes involved in the way energy is stored when
a system changes for an object projected upwards or up a slope
- ball works against gravitational force
- so energy transferred mechanically from kinetic energy store of ball
- to its gravitational potential energy store
Analyse the changes involved in the way energy is stored when
a system changes a moving object hitting an obstacle
ex bat hitting ball
- energy in kinetic energy store
- some energy transferred mechanically to ball’s kinetic energy store
- some energy is transferred mechanically to thermal energy stores of bat and ball (and surroundings by heating)
- rest carried away by sound
Analyse the changes involved in the way energy is stored when
a system changes an object being accelerated by a constant force
ex rock dropped from cliff
- assuming no air resistance, gravity works on rock, so rock constantly accelerates towards the ground
- energy transferred mechanically from rock’s gravitational potential energy store
- to its kinetic energy store
Analyse the changes involved in the way energy is stored when
a system changes a vehicle slowing down
(without braking)
- energy in kinetic energy store of car transferred mechanically (due to friction between tires and road)
- then by heating, to the thermal energy stores of the car and road
Analyse the changes involved in the way energy is stored when
a system changes bringing water to boil in an electric kettle
- energy transferred electrically from mains to heating element of kettle
- then by heating to the thermal energy store of water
Explain that where there are energy transfers in a closed
system there is no net change to the total energy in that system
- total energy in a closed system has no net change
because - closed system is a system (collection of objects) that can be treated on its own without any energy being exchanged to or from the surroundings
- if energy incr. or decr. its not closed
- you can make it into a closed system by incr. number of things you treat as part of it
ex pan of water heating on hob = not closed, but pan, gas and oxygen that burn to heat it and their surroundings are a closed system
Explain that mechanical processes become wasteful when they
cause a rise in temperature so dissipating energy in heating the
surroundings
- mechanical (force acting on object) work done means frictional forces need to be overcome
- ex moving parts rubbing together and air resistance
- energy needed to overcome frictional forces is transferred to thermal energy stores of whatever is doing the work and the surroundings
- this energy isn’t useful and so is dissipated
Explain, using examples, how in all system changes energy is
dissipated so that it is stored in less useful ways
- energy is only useful when it is transferred from one store to a useful store
- not useful energy gets dissipated (often to surroundings or thermal energy stores)
ex - - radio or speakers - electrical work is transferred into useful sound waves
- thermal energy is dissipated = rise in temperature (internal energy store) of the surroundings
ex - - tumble dryer - electrical work is transferred into useful internal (thermal) energy - helps dries clothes
- energy is dissipates wastefully by sound waves = rise in temperature of surroundings
*dissipated = energy is spread out so “lost”
Explain ways of reducing unwanted energy transfer
through lubrication
- lubrication reduces energy transferred by friction
- something moves = at least one friction force acting against it
- this transfers energy mechanically to thermal energy store of objects involves - then dissipated by heating to surroundings
- lubricants can be used to reduce friction between object’s surfaces when they move
- usually liquids like oil so they can flow easily between objects and coat them
Explain ways of reducing unwanted energy transfer
through thermal insulation
- insulation reduces rate of energy transfer by heating
- fabrics like wool and cotton are good insulators
- as usually low density or contain pockets of trapped air
- makes them effective as air is a poor conductor
- Surrounding a warm object with a material that contains trapped air will reduce the rate at which it loses heat
Likewise, surrounding a cold object with such a material will reduce the amount of heat reaching the object
Describe the effects of the thickness and thermal conductivity of
the walls of a building on its rate of cooling qualitatively
(qualitatively = good or bad something is )
- in a building the lower the thermal conductivity of its walls the slower the rate of energy transfer through them (building cools down slower)
- cavity walls with air gap in middle reduces amount of energy transferred by conduction as air has a low thermal conductivity
- thicker walls hep = slow energy transfer
