Efficient heat transfer Flashcards
What is efficiency, and how is it calculated?
efficiency is a measure of how much useful energy something transfers
efficiency = useful energy / total energy x 100%
(efficiency is a %)
What does the rate at which thermal energy is transferred through a wall of a house depend on?
1) the difference in temperature between the warmer interior and the colder exterior
2) the thickness of the walls
3) the material which the walls are made from
What can unwanted energy transfers be reduced by?
thermal insulation and lubrication
What does a material that is good at conducting energy have?
a high thermal conductivity
Name 6 ways to reduce friction
material (using smoother materials like ice, metal on iceskates)
bushing (using solid materials as an extra layer, such as Teflon on a pan)
lubricant (using a liquid/gas/powder to put between substances, e.g. oil in a car engine)
wheels (to put between rotating surfaces and reduce rolling resistance, such as bearings or skateboards)
magnetic levitation
acoustic levitation
What is conduction?
when hot particles transfer heat energy to their neighbours
What is convection?
what hot particles move and carry the heat energy with them
convection works because areas with different temperatures have different densities
hotter areas are denser because more heat increases the kinetic energy, causing the particles to vibrate more
this causes the liquid/gas to expand and become less dense
less dense areas rise above cooler, denser areas
What is radiation?
when thermal energy is emitted in the form of light
some radiation is visible, some isn’t
thermal energy can also be emitted by infa-red waves
What does a higher efficiency mean?
that the machine is better at transferring energy to useful forms
What do different materials have?
different relative thermal conductivities
How can energy efficiency by represented?
by a Sankey diagram
In a Sankey diagram, what is important about the arrows?
1) the thickness of the arrows shows how much energy is involved
2) the length of the arrows does not matter
3) useful energy transfers go left to right
4) wasted energy transfers go upwards/downwards
What do fridges need to do?
- stop heat energy getting in
- let heat energy out
What do fridges need to stop heat energy getting in?
- a thick door
- a door made of a material with a low thermal conductivity
- layered materials in the door
- no metal going through the door (it would let thermal energy in)
- a smaller surface area (flat)
What do fridges need to let thermal energy out?
- thermal energy leaves from the back of a fridge
- hot air leaves and is in contact with cold air (convection)
-a black material/colour that emits heat faster
-a matte material
(matte black is best, shiny white is worst)
-a bigger surface area (spikes, ridges, fins, etc)
-a material with a high thermal conductivity
What state does conduction happen best in and why?
best in solids
particles in a solid are close and strongly bonded
metals are especially good conductors
What states does conduction not happen well in and why?
liquids, and is almost non-existant in gases
the particles are far apart and not as strongly bonded
Describe why convection works/doesn’t work in the different states
it doesn’t happen in solids because the particles can’t move
it happens well in liquids/gases because the particles can move/move around each other
Which materials are best at absorbing and emitting radiation and which are worst?
black, matte materials are best
white, shiny materials are worst
What is special about radiation?
it doesn’t require particles so it can travel through a vacumn, such as space
radiation enables the sun to heat the Earth
Explain what a solid wall is, what a cavity wall is, why a cavity wall is better at keeping energy inside it and how it can be improved even further.
A solid wall is a wall made of a solid layer of bricks (used in old houses).
A cavity wall is a wall with a space inbetween two brick layers (used in modern houses). The space can be filled with insulation.
The cavity wall is better because it stops conduction from happening as much because of the layer of gas inbetween the layers. Conduction doesn’t work well in gases, but it does in solids.
Solid walls are more energy inefficient because the heat escapes through the walls.
Convection still happens in a cavity wall. The cavity wall can be improved by placing insulation such as foam in the space. This reduces conduction, convection and even radiation as there is no light.
Conduction is reduced because the foam has pockets of gas which cannot conduct.
As there are also solid parts, convection is reduced as well.
Explain 4 features of a thermos flask which keep the contents warm/cold
1) the stopper is made of an unconductive material
2) the outer layer is shiny and pale to reduce radiation
it is metal to protect the contents from being damaged
3) there are 2 glass layers, which are pale/shiny to stop radiation
they also reduce convection and aren’t as conductive as metal
4) there are vacumns inbetween the layers
this stops both conduction and convection as there are no particles, although radiation can still happen
A motor transfers 100 J of energy by electricity. 60 J are transferred as kinetic energy, 12 J as sound energy and 28 J as thermal energy.
Calculate the efficiency of the motor.
efficiency = useful energy / total energy x 100%
total useful energy = 60 J
efficiency = 60 J / 100 J x 100 % = 0.6 x 100% = 60%
Calculate the efficiency of a lamp that transfers 14 J of energy into useful light energy for every 20 J of electrical energy input.
efficiency = (14 J / 20 J) x 100% = 0.7 or 70%
