Topic 3 - Conservation of energy COPY Flashcards
What is a closed system?
A physical system that doesn’t allow certain types of transfers in or out of the system
What equation is used to calculate the change in gravitational potential energy?
Change in GPE = Mass(kg) x Gravitational field strength(N/kg) x Change in vertical height(metres)
What equation is used to calculate the amounts of energy associated with a moving object (kinetic)?
Kinetic energy(J) = 1/2 x Mass(kg) x Velocity squared(m/s sqaured)
Describe Sankey diagrams.
- Sankey diagrams summarise all the energy transfers taking place in a process.
- The thicker the line or arrow, the greater the amount of energy involved.
Describe Energy Transfer diagrams
These diagrams ONLY show the useful transfers so they don’t show unnecessary ones such as heat from a bulb.
Describe the conservation of energy
Energy is always conserved – the total amount of energy present stays the same before and after any changes.
Analyse the changes involved in the way energy is stored when an object is projected upwards or up a slope
b a moving object hitting an obstacle
c an object being accelerated by a constant force
d a vehicle slowing down
e bringing water to a boiling an electric kettle
Kinetic energy is transferred to gravitational potential energy: the velocity of the object decreases as its height increases.
Analyse the changes involved in the way energy is stored when a moving object hitting an obstacle.
- Kinetic energy in the obstacle, heat energy and sound energy (which eventually is converted into heat energy) is created.
- If the object is immovable, then no kinetic energy is introduced into it.
Analyse the changes involved in the way energy is stored when an object is accelerated by a constant force.
-The source of energy providing the acceleration (usually chemical, heat or electrical) is transferred to kinetic energy in the object.
Analyse the changes involved in the way energy is stored when a vehicle slows down.
The source of energy providing the acceleration (usually chemical, heat or electrical) is transferred to kinetic energy in the object
Analyse the changes involved in the way energy is stored when water is boiled in an electric kettle
- Electrical energy input is converted to heat energy in the kettle. Some will be lost, not all the electrical energy is converted to heat energy within the water.
- The water will boil and the electrical energy will then be used in turning the water to steam, that is assuming that the kettle does not turn itself off then.
What happens when there are energy transfers in a closed systems?
When there are energy transfers in a closed system, there is no net change in the total energy in the system.
When and why do mechanical processes become wasteful?
- Mechanical processes become wasteful when the useful energy transfers into unnecessary energy such as heat.
- As this energy is then lost to the surroundings so the process has less energy.
Why is energy dissipated in all system changes?
- Any energy that is not transferred to useful energy stores is said to be wasted because it is lost to the surroundings.
- Electrical cables warming up are a good example of this. It is not useful to have hot wires behind a television as energy is dissipated to the surrounding air.
Explain ways of reducing unwanted energy transfer including through lubrication, thermal insulation.
- In a mechanical system, energy is dissipated when two surfaces rub together.
- Work is done against friction which causes heating of the two surfaces - so the internal (thermal) energy of the surfaces increases.
- Adding lubricant between the surfaces reduces this friction and so less heat is wasted, like on a conveyor belt for example.
+Also, if you want your coffee to remain hot it can be kept insulated in a flask so that heat isn’t lost to the surroundings. [Vacuum reduces the amount of energy transferred by conduction.]
Describe the effects of the thickness and thermal conductivity of the walls of a building on its rate of cooling qualitatively
- To reduce thermal energy transferred from a warm house, the walls can be built thicker, so the energy must travel further before it is transferred to the outside.
- Thermal energy transfers can be reduced further if there are two walls with an air gap between them, as air has a lower thermal conductivity than brick. This is known as a ‘cavity wall’.
How do you work out efficiency?
Efficiency = Useful energy transferred by the energy ÷ total energy supplied to the device.
How can efficiency be increased?
+For systems that are designed to transfer thermal energy, the efficiency can be improved by reducing the wasteful dissipation of thermal energy to the surroundings, for example by using insulation.
Describe the energy source - fossil fuels. (5)
Chemical energy store
Non-renewable, cheap to extract
Transport, heating, electricity generation (uses)
High energy - and still plenty of fuel to meet current demand, so can respond quickly to changes in demand [but slowly running out]
Releases CO2 (causes global warming) - adds to greenhouse effect.
Burning coal and oil can also release sulfur dioxide, which causes acid rain.
Describe the energy source - nuclear. (5)
Nuclear energy store
Non-renewable
Electricity generation (uses)
Very high power and clean
Radioactive waste (needs to be disposed of safely and decommission)
Could lead to disasters like Chernobyl [nuclear waste is highly dangerous]
Describe the energy source - biofuel. (4)
Chemical
Renewable
Transport, heating, electricity generation [uses]
Medium power
Carbon-neutral so low impact [if you’re growing plants at the rate you’re burning things.]
In some regions large areas have to be cleared to make room to grow bio-fuels resulting in loss of species, natural habitats.
Decay of burning cleared vegetation increases methane and CO2 emissions.
Describe the energy source - wind. (5)
Kinetic energy store
Renewable
Electricity generation - wind rotates the blades which turn the generator and produce electricity.
Very low power - initial costs high but running costs minimal.
Take up large areas that could be used for farming, people say windmills spoil their view.
Only works when its windy, so can’t always supply electricity, or respond to high demand.
Describe the energy source - hydroelectricity. (5)
Gravitational potential energy store - involves flooding a valley by building a big dam - rainwater is caught and allowed through turbines [no pollution]
Renewable
Electricity generation - Can immediately respond to increased electricity demand - more water can be let out through the turbines to generate more electricity.
Medium power - initial costs high but minimal running costs [generally a reliable resource]
Local habitats are affected by the large areas that need to be flooded to build dams.
Describe the energy source - geothermal. (5)
Internal energy store Renewable Electricity generation, heating Medium power Very low impact on the environment
