Energy Transfers Flashcards
How many ways can thermal energy be transferred?
3
What determines the effectiveness of heating mechanisms?
The state of matter
What are the three heating mechanisms?
- Conduction: mainly in solids
- Convection: only in liquid and gases (can flow)
- Radiation: heating by IR radiation waves, in all objects
What makes a good conductor?
In good conductors, particles are close together, hence they are more dense, as this allows collisions between particles to occur more rapidly. Solids are good conductors and gases are very bad. They must have many delocalised electrons, that collide with different nuclei, transferring energy from one place to another, when materials are touching. This evenly distributes energy, to level the temperature difference.
Conduction in non-metals
When you heat one end of an object, particles start vibrating more. These particles collide with their neighbouring particles. Thermal energy is transferred through the solid by these collisions between the particles. This is known as conduction.
Conduction in metals
When a metal bar is heated at one end, the free delocalised electrons at that end diffuse faster through the metal. They collide with other free electrons. As a result, they transfer energy from the hot end of the metal, levelling the temperature difference. The free electrons also collide with the atoms and make them vibrate more.
Is the fluid the same as a liquid?
NO! Fluids include gases as well as liquids.
What floats, less dense or more dense?
Less dense floats on more dense
Convection current description
At the heat source, the liquid is heated, the particles gaining energy, and overcoming the attractive force between them, spacing out, and causing the liquid to expand. This results in it being less dense. The hot liquid then rises due to the lower density. Further away from the heat source, the liquid loses energy, cooling and contracting, the density increasing, as energetic particles are lost through evaporation. Finally, the cooled liquid sinks and falls due to increased density.
What is radiation?
Radiation is heating by IR radiation waves, in all objects.
What is infrared radiation?
Infrared radiation is similar to light, in the form of electromagnetic waves, that transfer thermal energy. It is able to travel through a vacuum, not requiring a medium (like sound waves), giving us heat from the Sun. It is emitted by all objects, varying based on the heat on the object.
Black surfaces and IR radiation
If a surface is a good absorber of IR radiation, it is also a good emitter of IR radiation. Black surfaces are the best emitters or absorber of IR radiation.
Shiny surfaces and IR radiation
Shiny surfaces are best at reflecting IR radiation
Underneath its white fur, polar bears have black skin. Explain this.
The white fur decrease the heat loss by radiation from the inside of the polar bear’s body, as white is a good reflector of infrared radiation or thermal energy, trapping or reflecting the thermal energy back inside the body, for the black skin to absorb, as black is a good absorber or emitter of infrared radiation or thermal energy.
Out of convection, conduction and radiation…
Which one is…
In fluids only
Convection
Out of convection, conduction and radiation…
Which one…
Stops in thermal equilibrium
Conduction and convection
Out of convection, conduction and radiation…
Which one…
Transfers heat energy from one place to another
Conduction, convection and radiation
Out of convection, conduction and radiation…
Which one…
Is reflected by shiny surfaces
Radiation
Out of convection, conduction and radiation…
Which one…
Can go through a vacuum
Radiation
Out of convection, conduction and radiation…
Which one…
Is best in metals
Conduction
Out of convection, conduction and radiation…
Which one…
Sets convection currents
Convection
Out of convection, conduction and radiation…
Which one…
Is when there is a temperature difference
Conduction and convection
Out of convection, conduction and radiation…
Which one…
Is absorbed and emitted best by dark objects
Radiation
Out of convection, conduction and radiation…
Which one…
Is through electromagnetic waves
Radiation
Out of convection, conduction and radiation…
Which one…
Requires contact between objects
Conduction
Out of convection, conduction and radiation…
Which one…
Is best in gases
Convection
Out of convection, conduction and radiation…
Which one…
Is best in solids
Radiation and conduction
Insulation in pans
Pans are made of metal, being a good conductor to evenly spread thermal energy. The handle is make out of an insulator (eg. rubber, wood, etc.) to avoid burns. The lid prevents the most energetic particles escaping (steam). This is made out of an insulator, to trap or retain heat in the pan.
What are radiators?
Radiators transfer heat to their surroundings in all three forms of heat transfer:
- By IR radiation to its surroundings
- By conduction to the air in contact with it
- By convection as the air around it sets up a convection current
What is cavity wall insulation?
In cavity wall insulation, is a foamy material between to walls, reducing conduction, as it traps air pockets, that are bad conductors. It also reduces convection, as the air has nowhere to rise to, trapped.
What is double glazing?
Double glazing is when gas is trapped between the inner and outer pane of a window. This reduces convection, as the air is trapped, preventing a convection current from being formed. This also reduces conduction, as gas is a poor conductor. To improve this, if there was the funding, a vacuum could be placed between the two panes.
What is roof insulation?
This is similar to wall insulation, with a foamy material covering the roof and loft. As well as preventing conduction and convection, it sometimes has a reflective coating, reducing radiation.
How to reduce heat transfer by conduction?
Use a vacuum, a stopper, a cork or air spaces. You could also separate the two materials.
How to reduce heat transfer by convection?
You could use a vacuum, a stopper, trapped air pockets, or a lid.
How to reduce heat transfer by radiation?
You could use mirrored or shiny surfaces.
How can you test for insulation?
Use different forms of insulation on a beaker filled with hot water (eg. padding, a lid, or nothing)
IV: Method of insulation
DV: Temperature of water overtime
CV: Volume of water, Material of container, Starting temperature, Time interval
Draw the convection current of water in a pan
N/a
How is convection used in everyday life?
- Fans
- Blood circulation in warm blooded animals
- Rising of a hot air balloon
- Radiators
- Ovens
- Sea and Land breeze
- Refrigerators
- Chimneys
- Shower steam
- Attic temperature
- Magma movement
Principle of Conservation of Energy
Energy cannot be destroyed or created, it can only be transferred from one store to another.
Label on a rollercoaster where the highest GPE, the lowest Kinetic Energy, and a mixture of both is
N/a
On a rollercoaster, where do you go the fastest, if it is upright?
The centre of mass, where force is applies
In exams, what assumption should you make about efficiency?
A machine is 100% efficient
What are state questions?
Questions where you have already calculated or should know the answer. No calculations are done for these
A golf ball has a mass of 50g and an astronaut transfers 56J of energy to it, by hitting it. At its highest point, the ball gained 12J of GPE.
State the kinetic energy of the ball at its highest point.
Transferred energy = 56J
GPE = 12J
Assume it is 100% efficient.
56 - 12 = 44
44J of kinetic energy
Suggest why a ball would travel further on the Moon, where gravitational field strength = 1.6 N/kg, than on Earth.
- On the Moon, the gravitational field strength is much lower than that on Earth, meaning in E = 1/2mv^2, more height would be achieved with the same amount of energy, as 1.6 < ~10.
- Lack of air resistance
- Longer time of flight
Efficiency
A measure of how good a device is at transferring energy from one store to another useful store of interest.
Efficiency formula
Useful energy output/Total energy input x 100%
What happens if an object has many useful energy outputs?
You add them together.
Energy efficiency rating
Most appliances have an energy efficiency rating (A-G), A being most efficient, meaning lower energy bills and less wasted energy.
Sankey diagrams
Sankey diagrams show whether a machine transfers energy efficiently (without much waste) or inefficiently.
Rules for drawing a Sankey diagram
- Length of the arrow does not matter
- Width of the arrow does matter and must be proportionate and to a suitable scale
- Useful transfers go left to right
- Width of the arrow show the proportion of energy transferred
- Wasteful transfers go up and down
- No titles
- Can be square or curved
- Add labels with the energy store, energy transfer, and amount of energy (J)
- Always use a pencil, ruler and graph paper
What is the unit of efficiency?
Decimals or percentage
Draw a Sankey diagram for a lightbulb that uses 25J of electricity in one second, producing 10J of light and wasting the rest as heat.
N/a
Draw a Sankey diagram for a TV, that uses 200J of electricity every 10 seconds, 2J going to sound as radiation, and 2J going to light energy, while the rest is wasted in thermal energy.
N/a
Which energy store does most wasted energy go to?
Thermal energy store as heat
