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Example Questions
Example Question #1 : Other Principles Of Quantum Mechanics
A proton is confined to a one-dimensional box of length . It has an energy equal to that of a photon with a wavelength of
. What excited state is the proton in? (Remember, the first excited state is where
since the ground state is
).
Not enough information to solve the problem.
Ground state.
Fourth excited state.
Second excited state.
Second excited state.
The energy of the quantum system in the state is given by
where is Planck's constant,
is the mass of the proton and
is the length of the box. The energy of a photon is given by
where is the frequency,
is the speed of light and
is the wavelength. Setting these equal we can solve for
,
Since the ground state is , the proton must be in the second excited state.
Example Question #1 : Other Principles Of Quantum Mechanics
The expectation value of a particle in a quantum system tells us what about the particle?
The exact location of the particle
The momentum of the particle
The most probable location of the particle
If the particle exists or not
The energy of the particle
The most probable location of the particle
From a statistical standpoint, the expectation value of the position, , can only tell us the most probable location of the particle. A central idea in quantum mechanics is that we can never really know exactly where a particle is as a function of time, but rather where we are most likely to find the particle if we choose to observe it.
Example Question #1 : Other Principles Of Quantum Mechanics
If a particle in a quantum system is bounded, this means what about the calculated particle energy?
The energy must be negative and continuous.
The energy of the particle can be anything, positive or negative.
There is nothing constraining the energy of the particle.
The energy must be positive and continuous.
The energy is described by discrete energy levels, and the particle can only have energy that corresponds to these levels, positive or negative.
The energy is described by discrete energy levels, and the particle can only have energy that corresponds to these levels, positive or negative.
A key characteristic of bound-state systems is the quantization of the energy into discrete energy levels. The energy can be negative or positive, but the particle can only have an energy that corresponds to one of these energy levels, nothing other. An example is the energy of a hydrogen atom, where the energy levels are given by:
These discrete numbers come from the energy of an electron, which is the fundamental charge.
Example Question #4 : Other Principles Of Quantum Mechanics
The square modulus of the wavefunction, given as , contains what information about a particle in a quantum system?
represents the probability distribution of the particle as a function of time
represents the energy of the particle as a function of time
represents the time-averaged position of the particle as a function of time
represents the position of the particle as a function of time
represents the square of the particle's position as a function of time
represents the probability distribution of the particle as a function of time
By definition, represents the probability distribution of a particle in a quantum system as a function of time. It is used to calculate the expectation value of other observables, such as position, momentum, current, angular momentum, just to name a few.
All AP Physics 2 Resources
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