energy Flashcards
define a system
an object or group of objects
what happens when a system changes
energy is transferred - this can either be into or away from a system, between different objects or between different types of energy stores within a system
what happens when energy is transferred to an object
the energy is stored in one of the object’s energy stores
list all energy stores
kinetic, thermal, chemical, gravitational potential, elastic potential, electrostatic, magnetic and nuclear
define kinetic energy store
energy stored in a moving object
define thermal energy store
all objects have thermal energy stores; the higher the temperature, the more thermal energy it stores
define chemical energy store
energy stored in substances that can release the energy by a chemical reaction; this includes food, fuels and even our muscles – e.g., chemical energy stored in muscles is transferred to kinetic energy
define gravitational potential energy store
it’s stored in objects with mass and are inside a gravitational field
define elastic potential energy store
it’s stored in stretched or compressed objects
define electrostatic potential energy store
stored in objects with an electric charge interacting with another electric charge
define magnetic energy store
stored in magnets that are interacting with other magnetic material
define nuclear energy store
energy stored in atomic nuclei released in nuclear reactions
define a closed system
a system where neither matter nor energy can enter or leave and so the net change in total energy is always zero
what ways can we transfer energy
- mechanically; doing work by exerting a force on an object and moving it
- electrically; a current moving through a potential difference can do work
- by heating; energy can be transferred from a hotter object to a colder object and through radiation when energy is transferred by waves
describe the energy changes when an object is projected upwards
the chemical energy store in the person’s muscle decreases and the energy is transferred to the kinetic energy stores of the object. as its altitude increases, the kinetic energy stores decrease as kinetic energy is transferred to gravitational potential energy, so the gravitational potential energy stores increase. some of the kinetic energy is dissipated as thermal energy to the surroundings due to air resistance acting on the object
describe the energy changes when a moving object hits an obstacle
the kinetic energy decreases as it is transferred to the thermal energy stores of the obstacle due to friction. the thermal energy stores of the obstacle increase, leading to the energy being dissipated
describe the energy changes when a vehicle slows down
kinetic energy is transferred to the thermal energy store in the brake pads. the energy is dissipated as kinetic energy stores of the car decrease and thermal energy stores in the breaks increase due to work being done by friction
describe the energy changes when bringing water to a boil in a kettle
energy is transferred from the electrostatic stores in the wires to thermal energy in the heating element. this thermal energy is transferred to the thermal energy stores in the water, causing it to boil
why is no device 100% efficient
some energy is always dissipated when an energy transfer takes place – the energy is wasted/stored in a way that is not useful (dissipation is usually by energy transfer to the thermal energy stores of the surroundings)
how does the law of conservation of energy explain why total energy in a closed system is always the same
no energy can enter or leave the closed system, and, since energy cannot be created or destroyed, the total energy is always the same
what do you assume when using the elastic potential equation
that the limit of proportionality has not been exceeded
properties of energy
it can be transferred usefully, stored or dissipated, but cannot be created or destroyed
define power
the rate at which energy is transferred or the rate at which work is done
what is an energy transfer of 1 joule per second equal to
a power of 1 watt
give an example that illustrates the definition of power
a powerful machine does not necessarily exert a strong force – a powerful machine is one that transfers a lot of energy in a short space of time
e.g., two identical cars that are racing on the same distance along a straight race track to a finish line - the car with the more powerful engine will reach the finish line faster because it will transfer the same amount of energy, but over less time
another example is two electric motors both lifting the same weight through the same height, but the one with the greater power rating lifts it faster
the main factors that affect the rate of energy transfer
thermal conductivity and thickness of the walls
describe the relationship between the rate of energy transfer and thermal conductivity
the higher the thermal conductivity of a material, the higher the rate of energy transfer by conduction across the material; i.e. the lower the thermal conductivity, the more well-insulated a house is
feature of most houses in relation to thermal conductivity
most houses have an internal and external wall with a cavity in between, which forms a relatively high thermal conductivity
how can you reduce unwanted energy transfer via thermal conductivity
- by adding cavity wall insulation which has a very low thermal conductivity; this reduces cost of heating because less energy is wasted
- double-glazed windows have a lower thermal conductivity because of the layer of insulating air between the two layers of glass, which reduces energy transferred by conduction
- loft insulation e.g. fibreglass has air gaps that give it a low thermal conductivity
- draught excluders can stop draughts blowing in and out, reducing energy transfer by convection
describe the relationship between the rate of energy transfer and thickness of walls
the thicker the walls, the slower the rate of unwanted thermal energy transfer; i.e. an ideal house has walls with a low thermal conductivity and very thick walls
how do lubricants work to reduce unwanted energy transfers
whenever something moves, frictional forces always act on it that cause energy to be dissipated and transferred to thermal energy stores of the surroundings; lubricants reduce friction between forces when they move so that less energy is transferred to the surroundings
what are lubricants
substances (usually liquids) that coat objects and flow between the moving parts
how do we reduce
- friction
- air-resistance
FRICTION: reduced by lubricants
AIR-RESISTANCE: reduced by streamlining
define useful devices
devices that transfer energy from one energy store to a useful energy store
what is the exception to no devices being 100% efficient
electric heaters, because all the energy is transferred to the useful thermal energy stores
how can efficiency of a device be increased
by lubrication or insulation depending on the intended energy transfer
name the fossil fuels and their uses
- coal; used for heating and electricity generation
- oil; used for transport and electricity generation
- gas; used for heating and electricity generation
list the renewable energy resources
nuclear, biofuel, wind, hydroelectricity, tidal, geothermal, solar and waves
define renewable energy resource
an energy resource that is being replenished as it is used up so it will never run out
define non-renewable energy resource
an energy resource that is not being replenished as quickly as it is used up, so will run out one day
pros and cons of renewable energy
PROS:
- they will never run out
- they don’t produce greenhouse gases because they don’t release CO₂
- they don’t create radioactive waste products
CONS:
- they still generate pollution
- low-efficiency levels
- can be unreliable
- generally expensive
pros of nuclear fuel
- they don’t produce greenhouse gases because they don’t release CO₂
- it’s more energy rich than fossil fuels
what fuels can be used for nuclear power
uranium (or plutonium)
cons of nuclear fuel
- toxic nuclear waste to dispose of
- a fire or explosion at a nuclear power station would cause significant environmental damage
cons of wind power
- it isn’t always windy
- it’s expensive to build
cons of hydroelectric power
- can damage habitats as it involves damming rivers
cons of solar power
- it isn’t always sunny
- expensive to build
cons of geothermal energy
- only certain places can access enough heat
cons of tidal energy
- only two tides a day
cons of wave energy
- waves vary; not a consistent source of energy
- they can only be built in certain places
pros and cons of biomass
PROS:
- carbon neutral
CONS:
- destruction of habitats
define specific heat capacity
the specific heat capacity of a substance is the amount of energy required to raise the temperature of one kilogram of the substance by one degree Celsius
