Topic 3 - Conservation of energy Flashcards
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 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 a system changes, including:
a an object 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
A) Kinetic energy is transferred to gravitational potential energy: the velocity of the object decreases as its height increases.
B) Kinetic energy in the obstacle (assuming it is not an immovable object), heat energy & sound energy (which eventually is converted into heat energy). If the object is immovable, then no kinetic energy is introduced into it.
C) The source of energy providing the acceleration (usually chemical, heat or electrical) is transferred to kinetic energy in the object.
D) The source of energy providing the acceleration (usually chemical, heat or electrical) is transferred to kinetic energy in the object
E) 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 (physical system that doesn’t allow certain types of transfers in or out of the system)?
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.
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.
Chemical energy store Non-renewable Transport, heating, electricity generation (uses) High power Releases CO2 (causes global warming)
Describe the energy source - nuclear
Nuclear energy store Non-renewable Electricity generation (uses) Very high power Radioactive waste (needs to be disposed of safely)
