3B SCIENTIFIC PHYSICS Instrucciones De Uso página 7

•
Slowly turn the coarse setting knob till the pho-
toelectric current measured by the nanoamme-
ter is approximately 0.
•
Use the fine setting knob to optimise the calibra-
tion. Turn it round till the display oscillates be-
tween 0 and -0.
•
Take note of the back-EMF as set in this fashion
and record it as the critical voltage U
•
Repeat this measurement for the four other
LEDs.
•
After the experiment, close the plastic cover back
over tube for attaching the LED.
6.2 Determining Planck's constant h.
•
Work out the frequencies of the light from the
printed wavelengths λ using the formula
m
c = ⋅
8
where 3 10 .
s
•
Use the critical voltages U
e U ⋅ where e=1.6021x10
gies
0
•
Plot the values obtained on a graph of energy
against frequency.
•
Draw a straight line through the points and deter-
mine Planck's constant h from the gradient and the
work W from where the line crosses the Y axis.
Note: It is easiest to evaluate the results, particularly
those referring to the back-EMF, with the help of the
supplied Excel spreadsheet. This merely involves
entering the measured critical voltage into the rele-
vant table. Afterwards the gradient h and the y-axis
crossing point W can be read from a graph and any
discrepancies from quoted values can be calculated
by entering the quoted figures into the relevant cell
in the second table.
U e
0
eV
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
480
U ⋅
Fig.1 Critical energy e against frequency f
0
.
0
c
f =
λ
to work out the ener-
0
-19
C.
500 520 540 560
6.3 Proof that the critical voltage does not de-
pend on the light intensity.
•
Select an LED.
•
Set the light to maximum intensity and deter-
mine the critical voltage U
•
Reduce the intensity to zero in a set of steps and
determine the critical voltage U
7. Cleaning
•
Only use conventional mild washing-up liquid
and never aggressive detergents.
•
Make very sure that no moisture gets inside the
equipment.
•
Unplug the power supply to turn off the equip-
ment.
•
Clean the equipment using a slightly damp and
non-fluffy cloth.
8. Disposal
•
The packaging should be disposed of at local
recycling points.
•
Should you need to dispose of the equipment
itself, never throw it away in normal domestic
waste. Local regulations for the disposal of elec-
trical equipment will apply.
580 600 620 640
3
.
0
in each case.
0
f
THz