Chapter 3: Problem 27
What possibly can be the ratio of the de Broglie wavelengths for two electrons having the same initial energy and accelerated through \(50 \mathrm{~V}\) and \(200 \mathrm{~V}\) ? (a) \(3: 10\) (b) \(10: 3\) (c) \(1: 2\) (d) \(2: 1\)
Short Answer
Expert verified
The ratio is \( 2:1 \), so the correct option is (d).
Step by step solution
01
Understanding de Broglie Wavelength
The de Broglie wavelength \( \lambda \) for a particle is given by the formula \( \lambda = \frac{h}{p} \), where \( h \) is Planck's constant and \( p \) is the momentum of the particle. For an electron, its momentum \( p \) can also be related to its kinetic energy.
02
Relating Kinetic Energy and Momentum
When an electron is accelerated through a potential \( V \), it gains kinetic energy \( eV \). The kinetic energy and momentum \( p \) are related by the formula \( KE = \frac{p^2}{2m} \). By rearranging terms, we get \( p = \sqrt{2m \cdot KE} = \sqrt{2m \cdot eV} \).
03
Expressing de Broglie Wavelength
Substituting \( p = \sqrt{2m \cdot eV} \) into the de Broglie wavelength formula, we have \( \lambda = \frac{h}{\sqrt{2meV}} \). This means the wavelength is inversely proportional to the square root of the potential \( V \), \( \lambda \propto \frac{1}{\sqrt{V}} \).
04
Calculating the Wavelength Ratio
The ratio of the wavelengths for electrons accelerated through \( 50 \text{ V} \) and \( 200 \text{ V} \) is given by \( \frac{\lambda_{50}}{\lambda_{200}} = \frac{\sqrt{200}}{\sqrt{50}} = \sqrt{\frac{200}{50}} = \sqrt{4} = 2 \).
05
Choosing the Correct Option
The computed ratio is \( 2:1 \), which corresponds to option (d). Thus, the ratio of de Broglie wavelengths for the electrons is \( 2: 1 \).
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Kinetic Energy
In physics, kinetic energy is the energy that a body possesses due to its motion. It is defined mathematically as \( KE = \frac{1}{2}mv^2 \), where \( m \) is the mass of the object and \( v \) is its velocity.
When dealing with electrons, the situation becomes quite interesting, as their kinetic energy changes with acceleration through a potential difference. We often describe this change in energy using the formula \( KE = eV \), where \( e \) is the electron’s charge and \( V \) is the potential difference they are subjected to.
When dealing with electrons, the situation becomes quite interesting, as their kinetic energy changes with acceleration through a potential difference. We often describe this change in energy using the formula \( KE = eV \), where \( e \) is the electron’s charge and \( V \) is the potential difference they are subjected to.
- This concept is crucial as it links the electrical potential energy, a form of energy storage, to kinetic energy, manifesting as apparent electron motion.
- As kinetic energy increases, so does an electron's velocity, which directly affects its momentum and subsequently its de Broglie wavelength.
Electron Acceleration
Acceleration occurs when a force is applied to an object, causing a change in motion. When electrons are subjected to an electric field, they experience acceleration. In this context, the electric field is provided by a potential difference.
- The potential difference works like a driving "push," nudging the electron forward and causing it to zip across the field with increasing velocity.
- As an electron accelerates, the gain in velocity translates into increased kinetic energy as per the relation \( KE = \frac{1}{2}mv^2 \), emphasizing how acceleration influences energy dynamics.
Potential Difference
The potential difference, sometimes described as voltage, is a measure of the work needed to move a charge from one point to another within an electric field. It is expressed in volts (\( V \)) and represents the potential energy per unit charge.
- For electrons, a higher potential difference means more force exerted upon them, translating to greater acceleration and higher kinetic energy.
- In our exercise, knowing the potential difference allows us to compute changes in kinetic energy, which in turn affects the de Broglie wavelength of the electron.
Momentum
Momentum is a key property that describes the motion of an object. It is calculated as the product of an object's mass and velocity: \( p = mv \). In the context of electrons, managing their momentum is essential to understanding phenomena like diffraction and interference.
- Due to their high speeds and tiny masses, electrons exhibit behaviors characterizing the dual particle-wave nature of matter.
- As electrons gain momentum through acceleration, their de Broglie wavelength changes, portraying their wave-like properties.