Section 4: Thermal Energy

Question 1

How does heat transfers occur

Answer 1

From a region of high temp to a region of low temp

Question 2

Conduction

Answer 2

-Particles in a solid can vibrate but not move around
-Particles in a liquid and gas can move around
-As you heat a substance its particles gain kinetic energy:
In a solid particles vibrate more
In a liquid/gas particles move faster

Question 3

Convection

Answer 3

• Requires particles to move around (can’t happen in solids)
  1. Part of substance heated- particles gain KE
  2. Heated substance expands- particles get further apart
  3. The density of substance decreases
  4. The less dense substance rises above the cooler, denser substance
  5. Cooler, denser substance replaces that which has risen

Question 4

Radiation

Answer 4

• electromagnetic wave can travel through a vacuum
• ALL objects both emit and absorb radiation all the time
• If an object emit more than it absorbs its temp falls (opposite rises)
• If an object emit and absorbs at the same rate it stays the same

Question 5

Payback time

Cost efficiency

Answer 5

Payback time = cost of installation / savings per year

Cost efficiency = (Saving per year x time)-cost

Question 6

Why is heat transferred

Answer 6

Temperature difference

Question 7

More efficient lightbulb

Answer 7

Wastes less energy

Question 8

The vacuum flask

Answer 8

Conduction:
-there are no particles in a vacuum so conduction can’t take place
-the cap and many layers also help to insulate the flask as plastic is an insulator
Convection:
-there are no particles in a vacuum so can’t take place
-filling flask all the way also stops convection currents within the air in the flask
Radiation:
-the inner and outer silvered surface reflects back the electromagnetic waves so radiation is stopped from escaping

Note: hottest part just below surface and in the middle of water

Question 9

Loft insulation

Answer 9

• reduces heat lost by conduction
• fibre glass mats are placed around the loft to trap air (poor conductor) and stops air from circulating and creating a convection current
• costs a few hundred pounds

Question 10

Cavity wall insulation

Answer 10

• outer layer of bricks which are durable
• inner layer of thermal bricks which have excellent insulation properties
• gap between 2 layers filled with fibre glass matting to stop convection currents (foam)
• if house already built you can drill a hole in one brick and put foam inside

Question 11

Double glazing

Answer 11

• gap between 2 panels of glass (with air sucked out to create a vacuum) to stop conduction
• gap very small so convection doesn’t occur
• doesn’t stop much heat radiation, only absorbs/reflects some
• very expensive, thousands

Question 12

Carpets/underlay

Answer 12

• made of wool, good insulator

- air gets trapped in wool

Question 13

Draught excluders

Answer 13

• rolls of sponge
• blocks air from coming under the door
• brush fibres stop air currents coming in the letterbox

Question 14

How houses lose heat

Answer 14

Mostly by walls and roof

Question 15

Examples of conduction

Answer 15

• lighting a match on flames

- pan on a stove

Question 16

Examples of convection

Answer 16

• radiator
• pan on a hot stove with water:
  1. heat conducts through pan
  2. warmer water rises, cooler water descends
  3. circular convection currents
• water boiler:
  1. water heated in the boiler
  2. cooler water gets pushed down to the boiler to be heated
• sea and land breeze:
  1. land heats up faster during the day
  2. land cools faster at night

Question 17

Factors affecting radiation

Answer 17

1. *Surface Area *of the object
   Greater SA -> more radiation
2. Temperature difference between object and surrounding
   Greater temp difference-> more radiation emitted
3. Surface, colour/texture

Best emitters/absorbers: Matt black, Shiny black
Worst: shiny white/silver

Question 18

Conduction in non-metals

Answer 18

Non-metal:

• particles gain energy and vibrate more
• vibrations make particles collide against one another and share kinetic energy

Question 19

Conduction in metals

Answer 19

Metal:

• conducts better because some electrons are detached from the atoms and free to move around. They help spread KE by colliding with atoms.
• more delocalised electrons, better conductors e.g silver