physipics

thermal energy transfers I

E1: Thermal energy transfers I

Syllabus / specification points:

• The Sun acts as a huge store of energy, emitting energy to space

• The Earth has a lower temperature than the Sun, and gains energy from it

• Energy transfers from a mug of hot energy to the cooler surroundings, by thermal processes – conduction, convection and radiation

• Loss of energy by a body can cause cooling

• Energy can become thinly spread in the large surroundings; we say it is dissipated

• Objects or systems can store, gain and lose energy; energy can transfer between them

thermal energy transfers II

‘higher rate’ of emission isn’t the same as ‘more’ emission  

E4.B: ‘higher rate’ of emission isn’t the same as ‘more’ emission  

Syllabus / specification points:

• Objects emit thermal radiation

• Emission transfers energy away from the emitting object

• An object at high temperature, such as gas in a flame, emits energy at a high rate (quickly)

• However, a cooler object that emits for a long time can, in the end, send out more energy

• The amount of energy emitted is not the same as rate of emitting 

balance and imbalance of thermal energy transfer  

E4.C: balance and imbalance of thermal energy transfer

Syllabus / specification points:

• Objects receive and emit energy, such as by thermal radiation

• The thickness of energy transfer arrows can represent RATE of energy transfer

• An object that receives energy at the same rate as it sends it out has a steady average temperature

• An object, such as a cool cake, receives energy faster than it sends it out when it is placed into a hot oven; it’s temperature then rises

• Just before the cake is taken out of the oven, when it’s fully baked, it has the same temperature as the oven

• It then emits energy at the same rate as it receives it, and its temperature is no longer changing

energy balance of a planet 

E4.D: balance and imbalance of thermal energy transfer

Syllabus / specification points:

• Venus, like Earth, has a complicated atmosphere and climate.

• But over a long period of time it’s temperature is steady.

• That is because of energy balance – getting energy from the Sun and sending (emitting) energy back out into space at equal rates.

• If the temperature on Venus changes, then it emits energy faster or more slowly. 

• Changes in temperature act against changes in temperature.

changing the atmosphere

E4.E: changing the atmosphere

Syllabus / specification points:

• Fossil fuels are made of very ancient plants (mostly) and animals 

• Fossil fuels provide stores of energy; we burn the fuels, but that releases carbon dioxide

• The energy was stored a long time ago

• It took millions of years to create all the world’s fossil fuels

• People are using up the fuels MUCH more quickly

• That is changing our atmosphere, by adding carbon dioxide

• The Earth receives energy from the Sun

• For natural energy balance, the Earth must emit energy back into space

• Carbon dioxide is a ‘greenhouse gas’ that makes it harder for energy to escape into space

• That is destroying the natural balance of equal rates of receiving and emitting energy

changing the atmosphere, the energy balance,  the temperature, the climate

E4.F: changing the atmosphere, the energy balance, the temperature

Syllabus / specification points:

• The Earth’s temperature stays steady when there is energy balance

• The Earth is warmed by the Sun and sends energy back into space

• Extra carbon dioxide from burning fossil fuels make it harder for energy to escape through the Earth’s atmosphere

• The rate of sending energy out can be slower than the rate of receiving it

• The imbalance between ‘energy in’ and ‘energy out’ produces rise in temperature of the atmosphere

higher and higher temperature?

E4.G: higher and higher temperature?

Syllabus / specification points:

• Natural energy balance is destroyed by rapid release of carbon dioxide from burning fossil fuels

• Humans are responsible for this

• If we stop using fossil fuels now then the temperature of the Earth’s atmosphere and oceans may stop increasing (though nobody knows exactly when)

* If we keep using fossil fuels than energy imbalance will get worse (and nobody knows exactly how much the average temperature will rise)

work and kinetic energy

E6: Work and kinetic energy

Syllabus / specification points:

• Work must be done to change motion (accelerate) and to overcome resistance to motion

• Energy is needed for doing work

• There must be a source of energy, such as fuel+oxygen; fuel may be a fossil fuel

• Moving objects have an energy store due to motion, called kinetic energy

• Energy transferred to overcome resistance becomes dissipated

how to calculate kinetic energy

EN7: how to calculate kinetic energy

Syllabus / specification points:

• An object with kinetic energy has the ability to do work on other objects with which it collides, and so it is a store of energy

• The quantity of stored energy depends on the mass of the body and its speed

• Kinetic energy = ¹/₂ mv²

work and extra gravitational potential energy

EN8: Work, extra height and extra gravitational potential energy

Syllabus / specification points:

• A falling object can do work, and so acts as a store of energy

• The extra energy it has due to the potential to fall is its gravitational potential energy, GPE

• At the end of a fall, the energy may become dissipated

conservation of energy

EN10: Conservation of energy

Syllabus / specification points:

• Energy cannot be destroyed

• The total energy at the end of every process is the same as at the beginning

• However, energy can become thinly spread by thermal processes, and become dissipated

conservation of energy: motion with + without resistive forces

E10.B: Conservation of energy during motion, with and without resistive forces

Syllabus / specification points:

• The total energy at the end of every process is the same as at the beginning, so if there are no resistive forces the work done by a cyclist produces a matching increase in kinetic energy

• Where there are resistive forces work must be done to overcome them, resulting in dissipation of energy

• When resistive force is the same size as the driving force all of the work done by the cyclist results in dissipation and there is no gain in kinetic energy

conservation of energy: motion without resistive forces

E10.C: Conservation of energy: falling without resistive forces

Syllabus / specification points:

• The Moon has less mass than the Earth, so has smaller gravitational field strength, g

• The total energy at the end of every process is the same as at the beginning, so if there are no resistive forces the gravitational potential energy at the start of a fall matches the kinetic energy at the end

conservation of energy: motion without resistive forces

E10.E: Conservation of energy: motion without resistive force

Syllabus / specification points:

• The total energy at the end of every process is the same as at the beginning, so if there are no resistive forces the gravitational potential energy at the highest point matches the kinetic energy at the lowest point

• A pendulum, such as a playground swing, loses and gains gravitational potential energy and kinetic energy, in turn

• When there are is no need for work to be done to overcome resistive forces then there is no dissipation

conservation of energy: motion with resistive forces

E10.F: Conservation of energy: motion with resistive force

Syllabus / specification points:

• The total energy at the end of every process is the same as at the beginning, so if there are resistive forces the gravitational potential energy at the highest point matches the total of dissipated energy plus kinetic energy at the lowest point

• There is loss of energy, by dissipation, during every swing, so the maximum gravitational potential energy and the maximum kinetic energy gradually become smaller

• A source of energy is needed to replace the dissipated energy

Copyright (c) David Brodie, 2019-2021