Engineering Physics Viva



Experiment No………Objective-Determination of Planck’s constant using Wien’s distribution law.Apparatus Used: Solar cell (Photo-voltaic cell- Celliniun type), Optical Filters (Blue, Green andRed), Convex Lens. Power supply with voltmeter & ammeter and DMM. Figure-1Brief Theoretical Description & Formulae Used: In this experiment we make use of Wien’sdistribution law for the experimental determination of Planck’s constant. The Wien’s law is anapproximate form of Planck’s distribution formula in the high frequency (or equivalently short wavelength) limit: uλdλ=8πhcλ5exp?-hcλkB Td? …………… (1)u (?) d? = Energy per unit volume emitted by the black-body within the wavelength interval ? to ? + d?kB = is the Boltzmann’s constantT = Absolute temperature of the black-bodyh = is Planck’s constant to be determined by the experiment.In the present experiment, we idealize a tungsten filament (bulb) as a perfect black body, described by Wien’s distribution law. The radiation emitted by the filament is measured by means of a photovoltaic cell. Typically the photo-current obeys the following relationship: Iλ,T= Bλ exp?-hcλkB T …………………(2) logeI=logeBλ-hcλkBT ...............................(3)We compare above equation with an equation of a straight line y=C-mx …………………(4)Thus variation of loge I with 1/T comes out as a straight line; calculating slope of this allows oneto determine h. Slope=m= hcλkB ……………….(5)Temperature of the black-body can be obtained as follows,At temperature T the relation with filament resistance is RTR0 = TT01.2 ……………..(6)At room temperature this relation reads: RTR0 = TRT01.2 ………………(7)Here RT is resistance of the filament at temperature TR and R0 is at T = 273K. RDR0 = TDT01.2 ……………………(8)Here RD = VDID is the draper voltage, the minimum voltage at which the filament just startsglowing and TD is draper temperature which is 800 K for tungsten.Dividing Eq.(7) by (8) we obtain: RR= RDTR8001.2 ……………………….(9)Now dividing Eq. (6) by (7) T= TR RTRR0.833 ………………………(10)Here TR is the room temperature should be noted by room thermometer. Procedure:? Note draper voltage and draper current from power supply.? Align the radiation source (light bulb), convex lens, filter and the radiation detector on the optical bench. In this one first bring all the components namely bulb, lens, filter and detector (solar cell) at the same height.? Next we focus radiation coming from the bulb in such a way that the maximum amount of light passes through lens and filter.? After fixing the filter, we adjust the lens and solar cell in such way that the maximum amount of radiation in received by the center of the black strip on the detector.? We connect digital multimeter across the detector and use in ammeter mode (Range 2000 μA).? We keep changing the voltage (VT) across the filament and note corresponding current (IT ) and IμObservation Table: RD = …… Ω TR = ……. KWavelength ? = …… °AS.No.VT in (volt)IT in (Ampere)I in (μA)RT =VTIT T= TR RTRR0.833 1TlogeI1.2.3.4.5.6.Above set of observations should be taken for three wavelengths by replacing the filter. Calculations: From the plot loge I versus 1/T the slope is calculated asSlope=ΔlogeIΔ1TComparing Eq. (5) with (11) the value of Planck constant can be calculated as:h=slope ?λkBcResult:Standard Value: h = 6.626 × 10-34 Joule-secPercentage error- Percentage error = Standard value-Observed valueStandard value x 100%Precautions & Sources of Errors:1. Special care should be taken while measuring the VT, particularly the draper voltage.2. The bulb should be connected with power supply with thick wires so the resistance of the of the wires do not contribute in the RT and RD.3. Radiation from the bulb should be properly focused. ................
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