Efficiency

Question 1

what happens when there is a change in a system?

Answer 1

energy is transferred and some of that energy is dissipated.

Question 2

what is a rise in temperature in mechanical processes caused by? (2)

Answer 2

• transfer of wasteful energy in mechanical processes.

- energy is then dissipated into the system.

Question 3

what causes energy to be dissipated in a mechanical system?

Answer 3

-when two surfaces rub together.

Question 4

how is energy dissipated in a mechanical system? (3)

Answer 4

• work is done against friction which causes heating of the two surfaces
• so internal (thermal) energy store of surfaces increases
• this is then transferred to internal energy store of the surroundings

Question 5

useful and wasteful energy in an electrical kettle: (3)

Answer 5

Useful energy:
Energy that heats the water.

Wasted energy:
Internal (thermal) energy heating the kettle.
Infrared radiation transferred to the surroundings.

Question 6

useful and wasteful energy in an hairdryer: (5)

Answer 6

Useful energy:
Internal (thermal) energy heating the air.
Kinetic energy of the fan that blows the air.

Wasted energy:
Sound radiation.
Internal (thermal) energy heating the hairdryer.
Infrared radiation transferred to the surroundings.

Question 7

useful and wasteful energy in a light bulb: (2)

Answer 7

Useful energy:
Light radiation given out by the hot filament.

Wasted energy:
Infrared radiation transferred to the surroundings.

Question 8

useful and wasteful energy in a tv: (4)

Answer 8

Useful energy:
Light radiation that allows the image to be seen.
Sound radiation that allows the audio to be heard.

Wasted energy:
Internal (thermal) energy heating the TV set.
Infrared radiation transferred to the surroundings.

Question 9

describe a method of reducing wasted energy in a mechanical system: (1)

Answer 9

lubrication

Question 10

how does lubrication reduce the wasted energy in a mechanical system?

Answer 10

reduces friction between moving parts of machine., thereby reducing thermal energy transferred.

Question 11

describe a method of reducing wasted energy in a thermal-transferring system, and why would this specific method work in this instance?

Answer 11

• thermal insulation

- a material would not be able conduct heat, and thereby less thermal energy will be wasted

Question 12

what is the difference between an efficient and inefficient device?

Answer 12

an efficient device would waste very little of its input energy, whilst an inefficient device will waste a large quantity of its input energy

Question 13

what is efficiency?

Answer 13

How good a device is at transferring energy input to useful energy output

Question 14

what is the efficiency of a device the proportion of?

Answer 14

the energy supplied that is transferred in useful ways

Question 15

equation for calculating efficiency:

Answer 15

(useful output energy/ total input energy) (x 100)

Question 16

why is it not possible to have an efficiency greater than 1/ 100%? (3)

Answer 16

more energy is being transferred than is being supplied,
which would mean that energy is being created.
this breaks the law of conservation of energy

Question 17

how can the efficiency of any device be increased?

Answer 17

by reducing wasted energy transfers so more of the input energy is usefully transferred

Question 18

why is efficient heating of buildings important in reducing the amount of energy used?

Answer 18

this is because the amount of energy required for heating can be very large.

Question 19

where will thermal energy transfer from the inside of warm buildings into the surroudings?

Answer 19

thermal energy will transfer from inside warm buildings to the cooler surroundings

Question 20

which materials are most appropriate for building warmer houses? such as?

Answer 20

poor conductors (brick, wood, plastic & glass)

Question 21

why would a house built out of conducting materials be difficult to live in?

Answer 21

it would be extremely cold as energy would be able to leave the house easily.

Question 22

what is thermal conductivity?

Answer 22

a measure of how well a material conducts energy when it is heated

Question 23

how can thermal conductivity be applied to how many Watts are transferred through a material?

Answer 23

‘x’ amount of joules (J) of energy will flow per second through a cubic block of ‘x’ material (1 m × 1 m × 1 m) when the temperature difference between its sides is 10°C.

Question 24

how can we reduce the thermal energy transferred from a warm house? (4)

Answer 24

• walls can be built thicker
• so the energy must travel further before it is transferred to the outside.
• thermal energy transfers can be reduced further if there are two walls with an air gap between them
• as air has a lower thermal conductivity than brick. This is known as a ‘cavity wall’.

Question 25

why is it better to have a window made of two layers of glass with a layer of air trapped between them? (3)

Answer 25

-both glass and air are insulators because they have low thermal conductivities.
-layer of air has the lowest thermal conductivity and reduces the overall conductivity of the window unit.
(-since air and glass are both transparent people can still see through the window.)