Wave Mechanics Quiz

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Questions and Answers

What does the phase velocity of a wave represent?

  • The speed at which the overall shape of the wave's amplitudes propagates
  • The speed of energy transport in the wave
  • The speed at which a point of constant phase travels (correct)
  • The maximum speed of particles in the medium

The group velocity of a wave is always equal to the phase velocity.

False (B)

Define group velocity in the context of wave mechanics.

The velocity at which the envelope of a group of waves travels.

The speed of a wave packet is called the ______.

<p>group velocity</p> Signup and view all the answers

Match the terms to their correct definitions:

<p>Phase velocity = Speed of a wave's phase Group velocity = Speed of the wave packet Wavelength = Distance between consecutive peaks Frequency = Number of cycles per second</p> Signup and view all the answers

In which situation is the group velocity usually higher than the phase velocity?

<p>In dispersive media (D)</p> Signup and view all the answers

A free particle has a defined wave function that oscillates with uniform frequency and wavelength.

<p>True (A)</p> Signup and view all the answers

What is the relationship between the group velocity and dispersion?

<p>Group velocity can depend on dispersion; in dispersive media, group velocity varies with frequency.</p> Signup and view all the answers

What equation is derived from the time-independent Schrödinger equation when the potential energy is zero?

<p>$V ightarrow 0$ (B)</p> Signup and view all the answers

The values of A and B in the wave function equation depend on time.

<p>False (B)</p> Signup and view all the answers

What does the wave function become for a particle in a box according to the given content?

<p>y(x) = Bsin(kx)</p> Signup and view all the answers

The constant k is related to the energy by the equation k = _____ where E is the energy, and m is the mass.

<p>sqrt(2mE)</p> Signup and view all the answers

Match the following states with their corresponding wave functions:

<p>n=1 = y(x) = sin(1x) n=2 = y(x) = sin(2x)</p> Signup and view all the answers

Which term best describes the eigen function of the state n=1?

<p>y(x) = sin(1x) (A)</p> Signup and view all the answers

The wave function y(r) for a free particle inside a box is normalized.

<p>True (A)</p> Signup and view all the answers

What condition must be satisfied for the wave function to exist at boundaries in a potential box?

<p>y(0) = 0 and y(L) = 0</p> Signup and view all the answers

What is the state of uncertainty associated with the particle described by the wave function?

<p>The particle has uncertainties both in position and momentum. (B)</p> Signup and view all the answers

The wave function can be normalized by ensuring that the integral of the wave function squared equals 1.

<p>True (A)</p> Signup and view all the answers

What is one property of eigen functions with respect to operators?

<p>Eigenfunctions maintain their form when acted upon by the operator.</p> Signup and view all the answers

In the context of the Lagrangian, a cyclic coordinate is one that does not appear explicitly in the Lagrangian, therefore the degree of freedom is _____ .

<p>reduced</p> Signup and view all the answers

Match the following items related to wave functions and their properties:

<p>Normalized wave function = Integral equals 1 Eigen function = Characteristic equation Cyclic coordinate = Does not appear in Lagrangian Degree of freedom = Independent motions of a system</p> Signup and view all the answers

Which of these is a normalized condition for a one-dimensional wave function?

<p>$\int \psi^* \psi ,dx = 1$ (D)</p> Signup and view all the answers

The degree of freedom for a system of three point masses that are fixed with respect to each other is nine.

<p>False (B)</p> Signup and view all the answers

Which statement about the Lagrangian is correct?

<p>The Lagrangian is a function of generalized coordinates and velocities. (D)</p> Signup and view all the answers

Flashcards

Expectation Value for Position

The average value of the position of a particle in a given state, calculated by integrating the product of the wavefunction, the position operator, and the complex conjugate of the wavefunction over all space.

Expectation Value for Momentum

The average value of the momentum of a particle in a given state, calculated by integrating the product of the wavefunction, the momentum operator, and the complex conjugate of the wavefunction over all space.

Time-Dependent Schrödinger's Equation

A mathematical equation that describes how the wavefunction of a quantum system evolves over time.

Time-Dependent Schrödinger's Equation for a Free Particle

A simplified version of the Schrödinger's equation applied to a particle that does not experience any potential energy.

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Time-Independent Schrödinger's Equation

A mathematical equation that describes the stationary states of a quantum system, meaning states that do not change over time.

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Time-Independent Schrödinger's Equation for a Free Particle

A simplified version of the time-independent Schrödinger's equation applied to a particle that does not experience any potential energy.

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Time-Independent Schrödinger's Equation with Potential

The Schrödinger's equation applied to a system that experiences potential energy.

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Time-Independent Schrödinger's Equation in Three Dimensions

The Schrödinger's equation extended to describe systems in three-dimensional space.

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Total Energy in a Box

The total energy of a particle is equal to its kinetic energy since its potential energy is zero.

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Time-Independent Schrödinger Equation for a Particle in a Box

The time-independent Schrödinger equation for a particle in a box, where the potential energy is constant.

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General Solution of Schrödinger's Equation

The general solution to the time-independent Schrödinger equation for a particle in a box, where A and B are arbitrary constants.

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Boundary Conditions for a Particle in a Box

The boundary conditions imposed on the wave function to ensure it is physically realistic.

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Determining A and B

The process of determining the values of the constants A and B in the wave function using the boundary conditions.

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Normalized Wave Function

The normalized wave function for a particle in a box, where the probability of finding the particle within the box is equal to one.

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Eigenfunctions of the Particle in a Box

The eigenfunctions representing different energy states of a particle in a box.

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Expectation Value Calculation

The process of calculating the expectation value of an observable, such as position or momentum, for a given state of the particle in a box.

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Wave Function with Definite Momentum

A wave function that describes the probability of finding a particle at a given position. In this context, the particle has a definite momentum but uncertainty in position. This means we know the particle's momentum, but we can't predict its exact location.

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Generalized Coordinate

A type of coordinate used to represent a system's motion. It's independent of any specific reference frame and simplifies describing complex movements.

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Cyclic Coordinate

A coordinate in a Lagrangian system that doesn't appear explicitly in the Lagrangian. This makes it easier to simplify the system's description by ignoring the coordinate.

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Degrees of Freedom

The number of independent parameters that are needed to completely describe the state of a system. In this case, the system has six independent parameters or degrees of freedom.

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Eigenfunction

A function that, when acted upon by an operator, returns a multiple of itself. This multiple is called the eigenvalue, and the function is called an eigenfunction. The eigenfunction remains unchanged except for a scaling factor after the application of the operator.

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Normalization of Wave Function

The process of normalizing a wave function by ensuring that the probability of finding the particle in all possible locations adds up to 1. In this context, the normalization condition is that the integral of the squared magnitude of the wave function is equal to 1.

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Expectation Value

The average value of a quantity, like position or momentum, calculated using the wave function and the corresponding operator. This tells us the most likely value of the quantity we're measuring.

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Study Notes

Quantum Mechanics Chapter at a Glance

  • Probability density, P(x,t), is the probability per unit length of finding a particle near coordinate x at time t. Expressed as P(x,t) = |ψ(x,t)|² dx.
  • Position operator represents the space coordinate.
  • Linear momentum operator, p, is -ih(d/dx) in one dimension, and more complex in three dimensions.
  • Potential energy operator: V(x)
  • Total energy operator: E
  • Kinetic energy operator: Eₖ
  • Angular momentum operator: Describes angular momentum from classical mechanics. Components are Lₓ, Ly, Lz.
  • Schrödinger equation in one dimension: (h²/2m)(d²ψ/dx²) + V(x)ψ = Eψ
  • Schrödinger equation in three dimensions: (h²/2m)∇²ψ + V(r)ψ = Eψ
  • Wave function of a free particle with constant momentum and energy
  • Operators in quantum mechanics are linear. An operator â is linear if it satisfies two conditions: â(cψ₁) = câψ₁ and â(ψ₁ + ψ₂) = âψ₁ + âψ₂.
  • Commutation of operators: Two operators commute when their commutator is zero. [â, b] = âb - bâ = 0.

Expectation Values

  • Expectation value of position (x): ∫ψ*(x,t)xψ(x,t)dx
  • Expectation value of momentum (p): ∫ψ*(x,t)(-ih(d/dx))ψ(x,t)dx

Other Important Concepts

  • Time-dependent Schrödinger equation
  • Time-independent Schrödinger equation
  • Degeneracy of energy levels
  • Equation of Continuity
  • Uncertainty relation (Heisenberg's)
  • Planck's Law of Radiation

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