Reducing Unwanted Energy Transfers

Question 1

what is a sankey diagram

Answer 1

a diagram that shows the different energy transfers made by a device and therefore how efficient it is

Question 2

what would a sankey diagram for an electric motor with 80% efficiency look like

Answer 2

• the total energy supplied to motor would be at the beginning of the arrow (100J)
• the useful energy transferred to kinetic energy stores (80J) would be a thick arrow going straight
• the energy wasted to thermal energy stores (20J) would be a small arrow stemming off from the whole arrow, pointing down

Question 3

what does the thickness of the arrow in a sankey diagram represent

Answer 3

• how much energy is being transferred

- the length has nothing to do with it

Question 4

what are the 2 main methods of reducing the amount of energy thats wasted

Answer 4

• lubrication

- thermal insulation

Question 5

what does decreasing the amount of wasted energy mean

Answer 5

• a higher proportion of supplied energy is transferred to useful stores
• so the efficiency of the process is increased

Question 6

what force acts against anything that moves, not in a vacuum

Answer 6

a frictional force

Question 7

how does friction contribute to wasted energy

Answer 7

• friction transfers energy mechanically as work is done against friction
• to the thermal energy store of the objects involved
• which is then dissipated by heating to the surroundings

Question 8

explain how pushing a box on the floor generate friction and a lack of efficiency

Answer 8

• pushing box along the ground causes energy to be transferred mechanically
• to the thermal energy stores of the box and the ground
• this energy is then radiated away to the thermal energy store of the surroundings

Question 9

in what situation would be lubricants only be useful when reducing amount of wasted energy

Answer 9

if objects that are touching each other are involved

Question 10

how do lubricants reduce the amount of wasted energy

Answer 10

• they are used to reduce the amount of friction between the objects surfaces when they move
• because they are usually liquids like oil
• which can flow easily between objects and coat them

Question 11

what happens when one side of an object is heated and what does this cause

Answer 11

• the particles in the hotter part vibrate more and collide with each other
• this transfers energy from their kinetic energy stores to other particles, which then vibrate faster
• this is conduction

Question 12

what is thermal conductivity

Answer 12

a term used to describe how well a material transfers energy by conduction

Question 13

what is the difference in thermal conductivity of a metal and gases

Answer 13

• metals have high thermal conductivity

- gases have a low thermal conductivity

Question 14

in a building, what does a lower thermal conductivity of its walls result in

Answer 14

• a slower rate of energy transfer through them

- meaning the building will cool more slowly

Question 15

what are cavity walls

Answer 15

an inner and an outer wall with an air gap in the middle

Question 16

why do some houses have cavity walls

Answer 16

• because the air gap reduces the amount of energy transferred by conduction
• due to air having a lower thermal conductivity

Question 17

why do some houses have thicker walls

Answer 17

• it helps with the insulation of heat

- the thicker the wall, the slower the rate of energy transfer

Question 18

what is the reason for the difference in thermal conductivity between metals and gases

Answer 18

• it is due to how tightly packed the particles are, allowing for the kinetic energy to be passed on from one particle to another
• metal is denser, meaning it has more particles in the same volume so kinetic energy can be passed at a quicker rate