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Energy graphs: (a) Figure 8.41 shows a graph of potential energy vs. interatomic distance for a particular molecule. What is the direction of the associated force at location A? At location B? At location C? Rank the magnitude of the force at locations A,B and C. (That is, which is greatest , which is smallest, and are any of these equal to each other?) For the energy level shown on the graph, draw a line whose height is the kinetic energy when the system is at location D.

(b) Figure 8.42 shows all of the quantized energies (bound states) for one of these molecules. The energy for each state is given on the graph, in electron volts ( 1eV=1.61019J). How much energy is required to break a molecule apart, if it is initially in the ground state? (Note that the final state must be an unbound state; the unbound states are not quantized.)

(c) At high enough temperatures, in a collection of these molecules there will be at all times some molecules in each of these states, and light will be emitted. What are the energies in electron volts of the emitted light?

(d) The "inertial" mass of the molecule is the mass that appears in Newton's second law, and it determines how much acceleration will result from applying a given force. Compare the inertial mass of a molecule in the ground state and the inertial mass of a molecule in an excited state10eV above the ground state. If there is a difference, briefly explain why and calculate the difference. If there isn't a difference, briefly explain why not.)

Short Answer

Expert verified

(a) The direction of the force is largest at the point A and it is at the+r direction. The direction of the force is the second largest at the point C and it is at the-r direction. The direction of the force is smallest at the point B as it is0 .

(b) The energy required to break the molecule apart is2.3鈥塭痴 .

(c) The energies in electron volts of the emitted light are1.2鈥塭痴 , 1.8鈥塭痴, 2.1鈥塭痴, 0.6鈥塭痴,0.9鈥塭痴 androle="math" localid="1661259297236" 0.3鈥塭痴 respectively.

(d) The inertial mass in the ground state is higher by about1.71035鈥塳驳 than the mass in the excited state. The reason for the difference is due to the difference in the energy levels.

Step by step solution

01

Identification of the given data

The given data can be listed below as:

  • The energy at the ground state is E1=2.3鈥塭痴.
  • The energy at the first excited state is E2=1.1鈥塭痴.
  • The energy at the second excited state is E3=0.5鈥塭痴.
  • The energy at the third excited state is E4=0.2鈥塭痴.
02

Significance of the energy graph

The energy graph is described as a graph in which an energy change occurs while a particular reaction occurs. It mainly includes transition, reactants and product states.

03

(a) Determination of the force’s direction along with drawing an energy level line

The equation of the force is expressed as:

F=螖鲍

Here,F is the force and 螖鲍is the change in the potential energy. As the force increases, the potential energy slope also increases.

The negative largest slope is being contained by the point A. The force is being cancelled out as the slope has a negative sign and it moves in the+r direction. Having a slope of 0, the force also becomes0 and the point B stays in the equilibrium position. The positive slope is being contained by the point C and the slope鈥檚 magnitude of point C is lesser than the slope鈥檚 magnitude of point A. As the point C has a positive slope and also a negative sign, it moves in the rdirection.

The diagram of the height of the kinetic energy when the system is at the location D has been drawn below:

In the above diagram, the red line is the line which adds up. The total energy is the summation of the kinetic and the potential energy.

Thus, the direction of the force is largest at the point A and it is at the+r direction. The direction of the force is the second largest at the point C and it is at the-r direction. The direction of the force is smallest at the point B as it is 0.

04

(b) Determination of the energy

The particle can go to a distance of r=if it is set free. In this case, the particle has zero energy who can go to that distance. That shows that the particle needs a large amount of kinetic energy from going to the excited state from the ground state.

The equation of the required energy is expressed as:

k+E1=0K=E1

Here,K is the required energy andE1 is the potential energy.

As the lowest energy according to the diagram in the question is 2.3鈥塭痴, then it will be considered as the potential energy.

Substitute2.3鈥塭痴 for E1in the above equation.

K=(2.3鈥塭痴)=2.3鈥塭痴

Thus, the energy required to break the molecule apart is2.3鈥塭痴 .

05

(c) Determination of the energies in electron volts

The equation of the energy of the emitted light in the first phase is expressed as:

P1=E2E1

Here,P1is the energy of the emitted light in the first phase,E2is the energy at the first excited state andE1is the energy at the ground state.

Substitute the values in the above equation.

P1=-1.1鈥塭痴-(-2.3鈥塭痴)=1.2鈥塭痴

The equation of the energy of the emitted light in the second phase is expressed as:

P2=E3E1

Here, P2is the energy of the emitted light in the second phase, E3is the energy at the second excited state and E1is the energy at the ground state.

Substitute the values in the above equation.

P2=-0.5鈥塭痴-(-2.3鈥塭痴)=1.8鈥塭痴

The equation of the energy of the emitted light in the third phase is expressed as:

P3=E4E1

Here, P3is the energy of the emitted light in the third phase, E4is the energy at the third excited state and E1is the energy at the ground state.

Substitute the values in the above equation.

P3=-0.2鈥塭痴-(-2.3鈥塭痴)=2.1鈥塭痴

The equation of the energy of the emitted light in the fourth phase is expressed as:

P4=E3E2

Here, P4is the energy of the emitted light in the fourth phase, E3is the energy at the second excited state and E2is the energy at the first excited state.

Substitute the values in the above equation.

P4=-0.5鈥塭痴-(-1.1鈥塭痴)=0.6鈥塭痴

The equation of the energy of the emitted light in the fifth phase is expressed as:

P5=E4E2

Here,P5is the energy of the emitted light in the fifth phase,E2is the energy at the first excited state andE4is the energy at the third excited state.

Substitute the values in the above equation.

P5=-0.2鈥塭痴-(-1.1鈥塭痴)=0.9鈥塭痴

The equation of the energy of the emitted light in the sixth phase is expressed as:

P6=E4E3

Here, P6is the energy of the emitted light in the sixth phase, E3is the energy at the second excited state and E4is the energy at the third excited state.

Substitute the values in the above equation.

P6=-0.2鈥塭痴-(-0.5鈥塭痴)=0.3鈥塭痴

Thus, the energies in electron volts of the emitted light are 1.2eV,1.8eV,2.1eV,0.6eV,0.9eVand 0.3eVrespectively.

06

(d) Determination of the comparison and explanation

The equation of the inertial mass of the molecule in the ground state is expressed as:

螖贰=mc2m=螖贰c2

Here,mis the inertial mass of the molecule, 螖贰is the change of the energy states andcis the velocity of light.

For the ground state,m1is the mass.

Substitute 2.3鈥塭痴 for 螖贰and 3108鈥尘/蝉for c in the above equation.

m1=2.3鈥塭痴(3108鈥尘/蝉)2=2.3鈥塭痴1.61019鈥塉1鈥塭痴(91016鈥尘2/s2)=3.61019鈥塉1鈥塳驳m2/s21鈥塉(91016鈥尘2/s2)=4.081036鈥塳驳

For the excited state ,m2is the mass.

Substitute 10鈥塭痴2.3鈥塭痴 for 螖贰 and 3108鈥尘/蝉for cin the above equation.

m2=10鈥塭痴2.3鈥塭痴(3108鈥尘/蝉)2=7.7鈥塭痴1.61019鈥塉1鈥塭痴(91016鈥尘2/s2)=1.21018鈥塉1鈥塳驳m2/s21鈥塉(91016鈥尘2/s2)=2.191035鈥塳驳

The equation of the difference between the mass of the ground and the excited state is expressed as:

m=m2m1

Here, mis the difference between the mass of the ground and the excited state, m1is the mass of the ground state and m2is the mass of the excited state.

Substitute the values in the above equation.

m=2.191035鈥塳驳-4.081036鈥塳驳=1.71035鈥塳驳

Thus, the inertial mass in the ground state is higher by about1.71035鈥塳驳 than the mass in the excited state. The reason for the difference is due to the difference in the energy levels.

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Most popular questions from this chapter


Assume that a hypothetical object has just four quantum states, with the following energies:

-1.0eV(third excited state)

-1.8eV(second excited state)

-2.9eV(first excited state)

-4.8eV(ground state)

(a) Suppose that material containing many such objects is hit with a beam of energetic electrons, which ensures that there are always some objects in all of these states. What are the six energies of photons that could be strongly emitted by the material? (In actual quantum objects there are often 鈥渟election rules鈥 that forbid certain emissions even though there is enough energy; assume that there are no such restrictions here.) List the photon emission energies. (b) Next, suppose that the beam of electrons is shut off so that all of the objects are in the ground state almost all the time. If electromagnetic radiation with a wide range of energies is passed through the material, what will be the three energies of photons corresponding to missing (鈥渄ark鈥) lines in the spectrum? Remember that there is hardly any absorption from excited states, because emission from an excited state happens very quickly, so there is never a significant number of objects in an excited state. Assume that the detector is sensitive to a wide range of photon energies, not just energies in the visible region. List the dark-line energies.

The mean lifetime of a certain excited atomic state is 5 ns. What is the probability of the atom staying in this excited state for t=10 ns or more?

The first excited state of a mercury atom is 4.9eV above the ground state. A moving electron collides with a mercury atom and excites the mercury atom to its first excited state. Immediately after the collision the kinetic energy of the electron is 0.3eV. What was the kinetic energy of the electron just before the collision?

N=1 is the lowest electronic energy state for a hydrogen atom. (a) If a hydrogen atom is in a state N=4, what is K+U for this atom (in eV)? (b) The hydrogen atom makes a transition to state N=2, Now what is K+U in electron volts for this atom? (c) What is energy (in eV) of the photon emitted in the transition from level N=4 to N=2? (d) Which of the arrows in figure 8.40 represents this transition?

Suppose we have a reason to suspect that a certain quantum object has only three quantum states.When we excite a collection of such objects we observe that they emit electromagnetic radiation of three different energies: 0.3eV(infrared), 2.0eV(visible), and 2.3eV(visible).

(a) Draw a possible energy-level diagram for one of the quantum objects, which has three bound states. On the diagram, indicate the transitions corresponding to the emitted photons, and check that the possible transitions produce the observed photons and no others. The energyK+U of the ground state is -4eV. Label the energies of each level ( K+U, which is negative).

(b) The material is now cooled down to a very low temperature, and the photon detector stops detecting photon emissions. Next a beam of light with a continuous range of energies from infrared through ultraviolet shines on the material, and the photon detector observes the beam of light after it passes through the material. What photon energies in this beam of light are observed to be significantly reduced in intensity ("dark absorption lines")? Energy of highest-energy dark line: eV Energy of lowest-energy dark line: eV

(c) There exists another possible set of energy levels for these objects which produces the same photon emission spectrum. On an alternative energy-level diagram, different from the one you drew in part (a), indicate the transitions corresponding to the emitted photons, and check that the possible transitions produce the observed photons and no others. When you are sure that your alternative energy-level diagram is consistent with the observed photon energies, enter the energies of each level (K+U, which is negative).

(d) For your second proposed energy-level scheme, what photon energies would be observed to be significantly reduced in intensity in an absorption experiment ("dark absorption lines")? (Given the differences from part (b), you can see that an absorption measurement can be used to tell which of your two energy-level schemes is correct).

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