A LevelPhysicsEdexcelTopic Questions9. ThermodynamicsBlack Body Radiation

Black Body Radiation (Edexcel A Level Physics)

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1a
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Infrared cameras are used to create images that show the infrared radiation emitted by objects.

The photographs show the same scene taken first with an ordinary camera and then with an infrared camera.

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Deduce whether the objects shown in the photographs would be expected to have peak emissions at infrared wavelengths. Your answer should include a calculation.

longest wavelength of visible red light ≅ 700 nm

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1b
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The photograph shows the result when someone tries to take an infrared photograph of the same scene through a window. The image does not show the outdoor scene but does show an image of the photographer.

State what can be concluded about glass and infrared radiation.

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1a
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The photograph shows a filament bulb.

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The filament is an emitter with 35% of the power output of a black body radiator of the same temperature.

When a potential difference (p.d) of 2.0 V is applied across the bulb, there is a current of 0.37 A in the filament.
Calculate the temperature of the filament.

surface area of filament = 3.9 × 10−6 m2


Temperature = ....................................................................

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1b
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In an experiment to investigate the efficiency of a filament light bulb a p.d. was applied. The p.d. and current were measured and the light bulb was observed. The p.d. was then increased and new measurements taken.

When a small p.d. is applied to the bulb, no light is visible. If the p.d. is gradually increased, the filament starts to glow and eventually appears white.

i)
Add to the graph to show the distribution of radiation from a black body at a temperature of 2026 K.

Your answer should include a calculation.

(5)

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ii)
Use your graph to explain why filament light bulbs are considered inefficient.

(2)

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2a
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An oximeter is a device used in hospitals to monitor the oxygen level in a patient’s blood.

In an oximeter, two light-emitting diodes (LEDs) are mounted opposite light sensors in a clip and attached to the patient’s finger. One of the LEDs produces red light and the other produces infrared.

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The intensity I of electromagnetic radiation received by the detector, after passing through a thickness x of blood, is given by



I space equals space I subscript 0 e to the power of negative mu x end exponent

where I subscript 0 is the intensity that would have been received if the blood were not present and mu is the attenuation coefficient of blood.

The red LED emits visible light of wavelength 650 nm and the infrared LED emits infrared of wavelength 950 nm.

The graph shows how mu varies with wavelength lambda for oxygenated blood and deoxygenated blood.

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I subscript 0for the infrared LED is 1.8 W m–2.
 
Calculate I for the infrared after passing through 1.4 mm of oxygenated blood.

I = .......................................................

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2b
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The oximeter determines I/I subscript 0 the fraction of radiation transmitted at each wavelength.

Deduce whether I/I subscript 0 will be smaller for the red or the infrared radiation if the blood is deoxygenated.

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2c
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It is suggested that ambient light could affect the readings produced by the oximeter.

Halogen lamps have a filament temperature of 3200 K.

Deduce whether the light from such a lamp would have a significant effect on the oximeter readings.

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