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Some of my experiments
Károly Piláth dr.
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Physics lessons can be good, if the teacher feels
good. So I chosed three experiments of my kit
which are very nice for me, and I got good
feedback from my students who are not
interested in physics.
17.08.2015.2
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My first experiment is try to visualize
the quantized behavior of light.
17.08.2015.3
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Some Theory
17.08.2015.4
When we shine light on the photocathode, we have
some possibilities of detecting energy of the
electrons which are emitted the photocathode.
One of them if the electrons charge up the
capacitor, then an electric field develops in the
photocell which will stop the electrons.
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17.08.2015.5
If we use the Einstein equation, the result will be that the voltage measured at the terminals of the capacitor (UC) will be proportional to the frequency of the illuminating light.
ℎ ∙ 𝑓 =1
2∙ 𝑚𝑒 ∙ 𝑣
2 +𝑊 = 𝑈𝐶 ∙ 𝑒 +𝑊
𝑈𝐶 ∙ 𝑒 = ℎ ∙ 𝑓 −𝑊
Where𝑾 is the binding energy𝒇 is frequency of the illuminating light
𝑈𝐶 =ℎ
𝑒∙ 𝑓 −
𝑊
𝑒
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Setup the UOUT
17.08.2015.6
If we add UW to the voltage of the capacitor and we
use the operational amplifier as a voltage
follower, the output voltage of the amplifier is equal
𝑈𝑂𝑈𝑇 = 𝑓 ∙ℎ
𝑒
𝑈𝑊 =𝑊
𝑒
𝑓 ∙ℎ
𝑒
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An example
17.08.2015.7
If the wavelength of the illuminating light
is 650 nm, then 𝑈𝑂𝑈𝑇 =1,9V
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The next example
17.08.2015.8
If the wavelength of the illuminating light
is 470 nm, then 𝑈𝑂𝑈𝑇 =2.64V
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A few words about the LED’s
A LED only starts to emitting
photons if the voltage of the
current source is higher than forward voltage (𝑉𝑓) of the
given color of LED.
17.08.2015.9
𝑒 ∙ 𝑉𝑓 = h ∙ 𝑓𝐿𝐸𝐷
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The complete theoretical circuit
17.08.2015.10
We constructed a device
which is able to recognize the color
of the (illuminating) light!
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Schematic of the circuit is a
little bit complex
17.08.2015.11
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The next experiment is a
simple applied energy
transformation
17.08.2015.12
The goal of this
experiment is to show
that energy can be
neither created
not be destroyed,but it transforms from
one form to another.
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The chemical energy of the gas can
be converted into heat energy.
The converter is a simple gas lighter.
The type of lighter is BelFlam Jet.
17.08.2015.13
The heat energy can be converted
directly into electric energy.
The converter is a Peltier element
with a heat sink.
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17.08.2015.14
The light energy can be converted
directly into electric energy. The
converter is a solar cell.
The electric energy can be
converted into mechanic energy.
The converter is a simple electric
motor.
The electric energy can be
converted into light energy.
The converter is a power LED.
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17.08.2015.15
In the classroom
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The last experiment is an
„Infrared cinema”
17.08.2015.16
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In this experiment I make a special
projector in infrared range.
17.08.2015.17
I use an infrared LED with a cheap,
plastic collimator lens (eBay) as a light
source
The parallel light of the light source goes
through the film, which is printed on a
clear film and bonded onto a plastic
ruler
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The projecting lens
17.08.2015.18
Or a ball-shaped bottle filled with
some liquor also can do.
The projecting lens can be traditional
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17.08.2015.19
Place the projector lens before the film.
The projector lens projects the image on the screen.
The real inverted infrared image is on the screen.
The image which is invisible
for us can be visible with the
use of an infrared webcam.