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P2 Energy Transfer by Heating (Infrared Radiation (all objects emit (and…
P2 Energy Transfer by Heating
Conduction
thermal conductors allow thermal energy to move through them easily
Metals are good thermal conductors
when heated particles vibrate faster
vibrations pass easily through close solid particles
delocalised electrons can move and carry thermal energy
high thermal conductivity
Rate of thermal energy transfer
temperature difference across material
the bigger the temperature difference the higher the transfer rate
thickness of material
thicker materials reduce rate of energy transfer
thermal conductivity of material
higher thermal conductivity reduces rate of transfer by conduction
Insulation
thermal
insulators
do not allow thermal energy through them easily
Fibreglass and wool are good thermal insulators
because they trap air
energy transfer as low as possible
low thermal conductivity
material thick as practically possible
Required Practical
Testing Sheets of Material as Insulators
place a small beaker inside a larger beaker and pour boiling water into the small beaker
place a cardboard lid on top of the large beaker with a hole for the thermometer
place a thermometer through the lid so that the bulb is in the hot water
record the starting temperature of the water and start a stopwatch to record the temperature every 3 mins for 15 mins
repeat experiment using same volume of hot water and same starting temperature of hot water
use same mass of an insulating material to fill the gap between the beakers OR use different no. of layers of the same insulator
plot a cooling curve for the results - the more effective the insulator, the slower the water cools
Insulating Buildings
houses are heated by heaters, central heating systems or fireplaces
Solar Panels
absorb infrared radiation from the sun
solar cell panels generate electricity directly
solar heating panels heat water directly
reducing energy transfer rate from homes to surroundings reduces fuel bills
Loft Insulation
fibreglass traps air to reduce conduction
reduces energy transfer rate through roof
Cavity Wall Insulation
insulation traps air to reduce conduction
reduces energy transfer through outer wall of house
insulation between two brick layers (cavity)
Aluminium Foil
foil reflects (so reduces) infrared radiation transfer
foil between radiator panel and wall
reduces energy transfer through wall
Double Glazed Windows
dry air reduces conduction/ vacuum prevents convection
dry air/ vacuum between two glass panels
reduces rate of energy transfer through windows
Thick bricks with low thermal conductivity
reduces rate of conduction transfer through exterior walls
Infrared Radiation
part of the electromagnetic spectrum
waves with wavelengths just longer than visible light
can be detected by skin (heat)
can be detected by thermometer with blackened bulb
glass prism splits white light
beyond the red is infrared so temperature increases
all objects emit (and absorb) infrared radiation
the hotter an object, the more infrared emitted in a given time
emits infrared at the same rate it absorbs it
a body at constant temperature
a good absorber is also a good emitter
dark, matt surfaces are good emitters and absorbers
light, shiny objects are poor emitters and absorbers
radiation absorbed at greater rate than emitted
temperature increases
radiation absorbed at lesser rate than emitted
temperature decreases
Black Body Radiation
radiation emitted by a
perfect black body
absorbs all radiation that hits it
best possible emitter of radiation
black body radiation
no radiation reflected or transmitted
an object at constant temperature emits radiation across a
continuous
range of wavelengths
hotter objects emit more shorter wavelength radiation
peak intensity of the higher temperatures is at a shorter wavelength
the
intensity
of radiation increases at higher temperatures at every wavelength
Earth's Temperature
earth's temperature depends of rates of radiation absorption and emission
earth receives light and infrared radiation from the sun
some is absorbed by
earth's atmosphere
some is absorbed by
earth's surface
warms the ground
some is
reflected
back into space
ground warms and emits longer-wavelength infrared radiation
from earth's surface and atmosphere into space
greenhouse gases
absorbs and re-emits some infrared back to the ground
keeps earth warm
Specific Heat Capacity
when heated, the temperature of the substance rises depends
amount of energy supplied
the mass of substance
a greater mass requires more energy to increase temperature by 1°C
substance material
greater the specific heat capacity, the more energy needed to increase temperature
the amount of energy required to raise the temperature of 1kg of substance by 1°C with no state change
ΔE
=
mc Δθ
Energy Transferred
=
Mass
x
Specific Heat Capacity
x
Temperature Change
Required Practical
Measuring specific heat capacity
measure the mass of the material using a mass balance
place a thermometer and immersion heater into the block of substance and read the starting temperature
wrap the material in insulating material to reduce energy transfer to surroundings
connect a joulemeter and powerpack to the immersion heater
time for 30 minutes then read the no. of joules (on joulemeter) of energy supplied to material by immersion heater
read the final temperature of the material then calculate the temperature change (final - initial)
use the equation to calculate the specific heat capacity of the material
Sources of Inaccuracy
thermal energy to surroundings
insulator
with lower conductivity
fully submerge
the immersion heater
incorrect reading of thermometer
use an
electronic temperature probe
heat uneven through oil
stir
the oil
Storage Heaters
use electricity off-peak to heat concrete blocks
brick have high specific heat capacity
store lots of energy and heat up slowly
cools slowly to transfer thermal energy to room
