Efficient heat transfer

Question 1

What is efficiency, and how is it calculated?

Answer 1

efficiency is a measure of how much useful energy something transfers

efficiency = useful energy / total energy x 100%

(efficiency is a %)

Question 2

What does the rate at which thermal energy is transferred through a wall of a house depend on?

Answer 2

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

Question 3

What can unwanted energy transfers be reduced by?

Answer 3

thermal insulation and lubrication

Question 4

What does a material that is good at conducting energy have?

Answer 4

a high thermal conductivity

Question 5

Name 6 ways to reduce friction

Answer 5

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

Question 6

What is conduction?

Answer 6

when hot particles transfer heat energy to their neighbours

Question 7

What is convection?

Answer 7

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

Question 8

What is radiation?

Answer 8

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

Question 9

What does a higher efficiency mean?

Answer 9

that the machine is better at transferring energy to useful forms

Question 10

What do different materials have?

Answer 10

different relative thermal conductivities

Question 11

How can energy efficiency by represented?

Answer 11

by a Sankey diagram

Question 12

In a Sankey diagram, what is important about the arrows?

Answer 12

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

Question 13

What do fridges need to do?

Answer 13

• stop heat energy getting in

- let heat energy out

Question 14

What do fridges need to stop heat energy getting in?

Answer 14

• 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)

Question 15

What do fridges need to let thermal energy out?

Answer 15

• 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

Question 16

What state does conduction happen best in and why?

Answer 16

best in solids

particles in a solid are close and strongly bonded

metals are especially good conductors

Question 17

What states does conduction not happen well in and why?

Answer 17

liquids, and is almost non-existant in gases

the particles are far apart and not as strongly bonded

Question 18

Describe why convection works/doesn’t work in the different states

Answer 18

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

Question 19

Which materials are best at absorbing and emitting radiation and which are worst?

Answer 19

black, matte materials are best

white, shiny materials are worst

Question 20

What is special about radiation?

Answer 20

it doesn’t require particles so it can travel through a vacumn, such as space

radiation enables the sun to heat the Earth

Question 21

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.

Answer 21

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.

Question 22

Explain 4 features of a thermos flask which keep the contents warm/cold

Answer 22

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

Question 23

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.

Answer 23

efficiency = useful energy / total energy x 100%

total useful energy = 60 J

efficiency = 60 J / 100 J x 100 % = 0.6 x 100% = 60%

Question 24

Calculate the efficiency of a lamp that transfers 14 J of energy into useful light energy for every 20 J of electrical energy input.

Answer 24

efficiency = (14 J / 20 J) x 100% = 0.7 or 70%

Question 25

Explain why it is not possible for any device to be 100% efficient.

Answer 25

(any example can be used)

a kettle transfers most of the electrical energy to thermal energy for heating the water in a kettle
but some thermal energy is always transferred to the surroundings
instead of the water, so it must be less than 100% efficient

thermal energy is almost always wasted

Question 26

What should you ALWAYS do when answering a question that involves an equation?

Answer 26

• write out the equation if needed
• substitute the values
• show your working

Question 27

What should you do in questions that ask you to explain a concept?

Answer 27

give an example