Radiation
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Thermal energy is the transfer of thermal energy through electromagnetic waves.
All objects give o thermal radiation.
The hotter an object is, the more thermal radiation it emits.
Thermal radiation is the part of the electromagnetic spectrum called infrared.
Thermal radiation is the only way in which heat can travel through a vacuum.
It is the way in which heat reaches us from the Sun through the vacuum of Space.
The colour of an object a ects how good it is at emitting and absorbing thermal
radiation:
Reflecting
VERY POOR REFLECTOR
POOR REFLECTOR
REASONABLE REFLECTOR
VERY GOOD REFLECTOR
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Apart from colour, the surface area of an object also a ects the rate of thermal
emission of the object (greater surface area = more area for radiation to be
emitted from)
Thermal Equilibrium
As an object absorbs thermal radiation it will become hotter.
As it gets hotter it will also emit more thermal radiation.
The temperature of a body increases when the body absorbs radiation faster than it
emits radiation.
Eventually, an object will reach a point of constant temperature where it is absorbing
radiation at the same rate as it is emitting radiation.
At this point, the object will be in thermal equilibrium.
Investigating IR Radiation
Aims of the Experiment
The aim of the experiment is to investigate how the amount of infrared radiation
absorbed or radiated by a surface depends on the nature of that surface.
Variables:
o Independent variable = Colour
o Dependent variable = Temperature
Control variables:
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Identical flasks (except for their colour)
Same amounts of hot water
Same starting temperature of the water
Same time interval
Equipment List:
Resolution of measuring equipment:
Thermometer = 1°C
Stopwatch = 0.01 s
Method
1. Set up the four identical flasks painted black, grey, white and silver.
2. Fill the flasks with hot water, ensuring the measurements start from the same
initial temperature.
3. Note the starting temperature, then measure the temperatures at regular intervals
e.g. every 30 seconds for 10 minutes.
Analysis of Results
All warm objects emit thermal radiation in the form of infrared waves.
The intensity (and wavelength) of the emitted radiation depends on:
 The temperature of the body (hotter objects emit more thermal radiation)
 The surface area of the body (a larger surface area allows more radiation to be
emitted)
 The colour of the surface
Most of the heat lost from the beakers will be due to conduction and convection.
This will be the same for each beaker, as colour does not a ect heat loss in this way.
Any di erence in heat loss between the beakers must, therefore, be due to infrared
(thermal) radiation.
To compare the rate of heat loss of each flask, plot a graph of temperature on the
y-axis against time on the x-axis and draw curves of best fit.
The expected results are shown on the graph below:
Evaluating the Experiment
Systematic Errors:
Make sure the starting temperature of the water is the same for each material.
It is best to do this experiment in pairs to coordinate starting the stopwatch and
immersing the thermometer.
Use a data logger connected to a digital thermometer to get more accurate readings.
Random Errors:
Make sure the hole for the thermometer isn’t too big, otherwise the heat will escape
through the hole.
Take repeated readings for each coloured flask.
Read the values on the thermometer at eye level, to avoid parallax error.
Safety Considerations
Keep water away from all electrical equipment.
Make sure not to touch the hot water directly.
Run any burns immediately under cold running water for at least 5 minutes.
Do not overfill the kettle.
Make sure all the equipment is in the middle of the desk, and not at the end to avoid
knocking over the beakers.
Carry out the experiment only whilst standing, in order to react quickly to any spills.
Consequences of Thermal Energy Transfer
In real situations there is very rarely only one form of energy transfer.
Usually all three happen at once.
The diagram below illustrates all the three forms of thermal energy transfer occurring
simultaneously.
Thermal energy is transferred from hotter areas (the tea) to cooler areas (the cup,
hands and air) by the processes of:
Conduction; by direct contact between the tea and the solid sides of the cup
and also by direct contact from the cup to the surface it is sitting on.
Convection; from the surface of the co ee to the air directly above it.
Radiation; from the sides of the hot cup in all directions to the surrounding air.
To minimise the heat losses through these ways, devices like thermos flasks are created
to curb the di erent energy losses in these ways.
Thermos flask
Thermos flask, also known as a vacuum or Dewar-flask, is a double-walled glass vessel;
that minimizes the heat transfer between the fluid inside and the surrounding outside.
It keeps a hot liquid hot and a cold liquid cold for an extended period.
Key components of vacuum flask
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Outer casing: Outer casing made of plastic or stainless steel protects the glass
walls.
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The inner and outer layer of glass: The inner and outer portion of this glass layer
are coated with shiny (silver) reflective material reflects any incoming radiation
as well as prevent any thermal radiation from the liquid inside to escape through
radiation. It also prevents heat loss through conduction, since glass is a poor
conductor (great insulator).
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Vacuum space: This space maintained at almost a vacuum prevents heat loss
through convection and conduction.
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Plastic or cork stopper: The stopper made of plastic or cork acts as a seal to
prevent fluid from going out and as insulation for heat conduction and
convection.
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Support structure: The structure made of insulation material such as rubber
acts as a rigid connector that keeps the inner and outer flask rigidly in its place
as well as prevent heat loss through conduction.
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Additional insulation layer (Optional): The space between the glass vessel and
the metal or plastic case contains heat-insulating material like heat or plastic to
prevent heat transfer conduction further.