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Chapter 10: Q67E (page 470)

Question: An LED is connected in series with a resistor and a power supply of variable voltage. The supply voltage starts from —5 V and increases linearly with time to a maximum of +5 V. The figure shows the supply voltage the voltage across the diode, and the current in the Circuit as functions of time. The LED does not glow for the whole time shown. (a) Explain the plots, and identify the period when the LED glows. (b) Approximately what color is the light produced by the LED?

Short Answer

Expert verified

Answer

(a) The band gap of the LED is 2.5e V .

(b) The light produced by the LED is cyan color light.

Step by step solution

01

Given data

The given diagram is shown in figure 1.

Figure 1

02

Concept of the energy of a photon

The expression to determine the wavelength of the light is, λ=hcV .

03

Determine the explanation for the plots and identify the period when the LED glows

(a)

The LED only glows when the current flows in the circuit and the time period is 15s<t<20s.

From the above graph the voltage required by the LED to turn on is, V= 2.5 e V .

Therefore, the band gap of the LED is V = 2.5 e V .

04

Determine the color of the light produced by the LED

(b)

Calculate the wavelength of the light.

λ=hcV=1240 eV.nm2.5 eV=496 nm

The above value shows that the light is of cyan color.

Therefore, the light produced by the LED is cyan color light.

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Most popular questions from this chapter

Formulate an argument explaining why the even wave functions in Fig 10.1 should be lower in energy than their odd partners.

The bond length of theN2molecule is 0.11nm, and its effect the spring constant is 2.3×103N/m .

(a) From the size other energy jumps for rotation and vibration, determine whether either of these modes of energy storage should be active at 300K .

(b) According to the equipartition theorem, the heat capacity of a diatomic molecule storing energy in rotations but not vibrations should be52R(3 translational +2rotational degrees of freedom). If it is also storing energy in vibrations. it should be72R(adding 2 vibrational degrees). Nitrogen's molar heat capacity is 20.8J/mol.K at 300K. Does this agree with your findings in part (a)?

The bonding of silicon in molecules and solids is qualitatively the same as that of carbon. Silicon atomic states become molecular states analogous to those in Figure 10.14. and in a solid, these effectively form the valence and conduction bands. Which of silicon's atomic states are the relevant ones, and which molecular state corresponds to which band?

By the “vector” technique of example 10.1 , show that the angles between all lobes of the hybridsp3states are 109.5°..

Question: Referring to equations(10-2), lobe I of the hybrid states combines the spherically symmetric s state with the state that is oriented along thez-axis. and thus sticks out in the direction (see Exercises 28 and 33), If Figure is a true picture, then in a coordinate system rotated counterclockwise about they-axis by the tetrahedral angle, lobe II should become lobe. In the new frame. -values are unaffected. but what had been values in the 2x -plane become values in the -plane. according tox=x'cosα+z'sinα and z'cosα-x'sinα, whereα=109.5o iscos-1(-13) , or .

(a) Show that lobe II becomes lobe I. Note that since neither the 2s state nor the radial part of the p states is affected by a rotation. only the angular parts given in equations (10-1) need be considered.

(b) Show that if lobe II is instead rotated about thez-axis by simply shifting φby±1200 . it becomes lobes III and IV.
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