11.6 The Hall Effect

The Hall effect is a fascinating phenomenon in conducting materials exposed to magnetic fields. It reveals crucial information about charge carriers, helping us understand the behavior of electrons and holes in various materials.

Velocity selectors and Hall voltage calculations demonstrate practical applications of the Hall effect. These concepts are essential for understanding charge carrier dynamics and material properties, connecting electromagnetic theory to real-world applications in electronics and materials science.

The Hall Effect

Hall effect in conducting materials

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• Phenomenon observed in conductors and semiconductors placed in a magnetic field perpendicular to electric current flow
  • Generates a transverse electric field (Hall field) perpendicular to both current and magnetic field
  • Hall voltage is the potential difference across the conductor, perpendicular to current flow
• Caused by the Lorentz force acting on charge carriers
  • Deflects charge carriers (electrons or holes) to one side of the conductor, creating a charge imbalance
  • Charge imbalance results in the Hall voltage
• Significant in determining charge carrier properties in conducting materials
  • Sign of Hall voltage indicates type of charge carriers (electrons or holes)
  • Magnitude of Hall voltage is proportional to charge carrier density and mobility
• Hall coefficient ($R_H$) relates Hall voltage to current, magnetic field, and conductor thickness
  • $R_H = \frac{E_H}{J_xB_z} = \frac{1}{ne}$, where $E_H$ is Hall field, $J_x$ is current density, $B_z$ is magnetic field, $n$ is charge carrier density, and $e$ is elementary charge
  • Sign and magnitude of Hall coefficient provide information about charge carrier type and density (semiconductors, metals)

Force balance in velocity selectors

• Velocity selector uses combination of electric and magnetic fields to select particles with specific velocity
• Electric field ($\vec{E}$) and magnetic field ($\vec{B}$) applied perpendicular to each other and particle motion direction
• For particle with charge $q$ and velocity $\vec{v}$, electric force ($\vec{F}_E$) and magnetic force ($\vec{F}_B$) act on particle
  • Electric force given by $\vec{F}_E = q\vec{E}$
  • Magnetic force given by $\vec{F}_B = q\vec{v} \times \vec{B}$
• When electric and magnetic forces balance, particle passes through velocity selector undeflected
  • Occurs when $|\vec{F}_E| = |\vec{F}_B|$, or $|q\vec{E}| = |q\vec{v} \times \vec{B}|$
  • Simplifies to $|\vec{E}| = |\vec{v} \times \vec{B}|$ or $E = vB\sin\theta$, where $\theta$ is angle between $\vec{v}$ and $\vec{B}$
• Adjusting electric and magnetic field strengths selects particles with specific velocity (mass spectrometry, particle accelerators)

Calculation of Hall voltage

• Hall voltage ($V_H$) is potential difference across conductor in presence of magnetic field and electric current
• Calculation requires current ($I$), magnetic field strength ($B$), and charge carrier properties (density $n$ and elementary charge $e$) of conductor
• Hall voltage given by $V_H = \frac{IB}{ntq}$, where $t$ is conductor thickness and $q$ is charge of carriers (positive for holes, negative for electrons)
• Alternatively, use Hall coefficient ($R_H$) to calculate Hall voltage: $V_H = \frac{IR_HB}{t}$
  • Hall coefficient related to charge carrier density and elementary charge by $R_H = \frac{1}{nq}$
• To calculate Hall voltage:
  1. Identify given values for current ($I$), magnetic field strength ($B$), conductor thickness ($t$), and either Hall coefficient ($R_H$) or charge carrier density ($n$) and elementary charge ($q$)
  2. If Hall coefficient not given, calculate using $R_H = \frac{1}{nq}$
  3. Substitute values into appropriate equation: $V_H = \frac{IB}{ntq}$ or $V_H = \frac{IR_HB}{t}$
  4. Solve for Hall voltage ($V_H$), ensuring consistent units throughout calculation

Charge carrier dynamics and material properties

• Drift velocity: average velocity of charge carriers in response to an applied electric field
• Mobility: measure of how easily charge carriers move through a material in response to an electric field
  • Related to drift velocity and electric field strength
• Conductivity: measure of a material's ability to conduct electric current
  • Depends on charge carrier density, mobility, and charge