P1b: Keeping homes warm

Question 1

Describe conduction in most solids(3)

Answer 1

particles gain KE and vibrate more vigorously

particles collide with other particles

KE transferred to neighbouring particles when they collide

Question 2

Describe conduction in metals(2)

Answer 2

free electrons gain more KE from collisions as metal is heated

they transfer the KE very quickly as they travel through the metal
(check)

Question 3

Describe convection(4)

Answer 3

particles gain KE and move faster

fluid expands, therefore becoming less dense

warmer/less dense fluid rises, displacing cooler/more dense fluid

cooler/more dense fluid takes its place and gets heated ­and rises, creating a convection current

Question 4

Describe radiation(3)

Answer 4

all bodies emit and absorb radiant heat (infrared radiation)

radiant heat is radiated as infrared waves (electromagnetic wave)

doesn’t need a medium to travel through
(check)

Question 5

How does loft insulation reduce heat loss(6)

Answer 5

loft filled with insulating material(e.g. fibreglass) with tiny pockets of trapped air

silver foil fitted to insulating material

reduces heat loss through conduction and convection

• material and the air trapped are poor thermal conductors so slow down conduction
• pockets of air too small to form convective currents

foil good reflector of radiant heat, reflects back down to living space

Question 6

How does double glazing reduce heat loss(5)

Answer 6

2 panes of glass with vacuum or air at low pressure between them

air gap too small and reduced number of particles due to low air pressure ­ difficult to form convective currents

trapped air is poor thermal conductor so slows down conduction

vacuum prevents conduction and convection (no particles)

radiation can still occur

Question 7

How does cavity wall insulation reduce heat loss(4)

Answer 7

cavity filled with insulating material(e.g. fibreglass) with tiny pockets of trapped air

material and the air trapped are poor thermal conductors so slow down conduction

pockets of air too small to form convective currents

radiation can still occur

Question 8

How does cavity wall reduce heat loss(3)

Answer 8

air gap - air is a poor thermal conductor, slows down conduction

convection and radiation can still occur
-can be improved by insulating the cavity wall

Question 9

What is the sink in the context of energy lost from houses

Answer 9

the cooler surroundings

Question 10

Good emitters/absorbers of radiant heat

Answer 10

dark, matt surfaces

Question 11

Poor emitters/absorbers of radiant heat

Answer 11

light­coloured, shiny surface (they reflect the radiant heat away)

Question 12

Good reflectors of radiant heat

Answer 12

light­coloured, shiny surfaces

Question 13

Payback time equation

Answer 13

payback time = initial cost / savings per year

Question 14

How is a radiator designed to maximise energy transfer to the room?(3)

Answer 14

conduction - made of metal

radiation - silvered back to reflect radiant heat back into room; larger surface area

convection - placed near bottom of room to heat the whole room

Question 15

How is a boiler designed to maximise heat transfer to the water?(2)

Answer 15

convection - heating element at bottom so convection currents heat all the water

radiation - white walls to prevent heat loss