CIE A Level Physics (9702) 2019-2021

Revision Notes

25.3.3 The Hall Effect

Hall Voltage

  • The Hall voltage is a product of the Hall effect
  • Hall voltage is defined as:

The potential difference produced across an electrical conductor when an external magnetic field is applied perpendicular to the current through the conductor

  • When an external magnetic field is applied perpendicular to the direction of current through a conductor, the electrons experience a magnetic force
  • This makes them drift to one side of the conductor, where they all gather and becomes more negatively charged
  • This leaves the opposite side deficident of electrons, or positively charged
  • There is now a potential difference across the conductor
    • This is called the Hall Voltage, VH
  • An equation for the Hall voltage VH is derived from the electric and magnetic forces on the charges
  • The voltage arises from the electrons accumulating on one side of the conductor slice
  • As a result, an electric field is set up between the two opposite sides
  • The two sides can be treated like oppositely charged parallel plates, where the electric field strength E is equal to:

Hall Voltage equation 1

  • Where:
    • VH = Hall voltage (V)
    • d = width of the conductor slice (m)
  • A single electron has a drift velocity of v within the conductor. The magnetic field is into the plane of the page, therefore the electron has a magnetic force FB to the right:

FB = Bqv

  • This is equal to the electric force FE to the left:

FE = qE

qE = Bqv

  • Substituting E and cancelling the charge q

Hall Voltage equation 2

  • Recall that current I is related to the drift velocity v by the equation:

I = nAvq

  • Where:
    • A = cross-sectional area of the conductor (m2)
    • n = number density of electrons (m-3)
  • Rearranging this for v and substituting it into the equation gives:

Hall Voltage equation 3

  • The cross-sectional area A of the slice is the product of the width d and thickness t:

A = dt

  • Substituting A and rearranging for the Hall voltage VH leads to the equation:

Hall Voltage equation 4 Hall Voltage equation 5

  • Where:
    • B = magnetic flux density (T)
    • q = charge of the electron (C)
    • I = current (A)
    • n = number density of electrons (m-3)
    • t = thickness of the conductor (m)
  • This equation shows that the smaller the electron density n of a material, the larger the magnitude of the Hall voltage
    • This is why a semiconducting material is often used for a Hall probe
  • Note: if the electrons were placed by positive charge carriers, the negative and positive charges would still deflect in opposite directions
    • This means there would be no change in the polarity (direction) of the Hall voltage

Exam Tip

Remember to use Fleming’s left-hand rule to obtain the direction the electrons move due to the magnetic force created by the magnetic field.

Measuring Magnetic Flux Density using a Hall Probe

  • A Hall probe can be used to measure the magnetic flux density between two magnets based on the Hall effect
  • It consists of a cylinder with a flat surface at the end
  • To measure the magnetic flux density between two magnets, the flat surface of the probe must be directed between the magnets so the magnetic field lines pass completely perpendicular to this surface
  • The probe is connected to a voltmeter to measure the Hall voltage
  • If the probe is not held in the correct orientation (perpendicular to the field lines), the voltmeter reading will be reduced
  • Since the Hall voltage is directly proportional to the magnetic flux density, the flux density of the magnets can be obtained
  • A Hall probe is sensitive enough to measure even the Earth’s magnetic flux density

Worked example: Using a Hall probe

Measuring_Magnetic_Flux_Density_using_a_Hall_Probe_Worked_Example_-_Using_a_Hall_Probe_Question, downloadable AS & A Level Physics revision notes

      • The Hall voltage depends on angle between the magnetic field and the plane of the probe
      • The Hall voltage reaches a maximum when the field is perpendicular to the probe
      • The Hall voltage is zero when the field is parallel to the probe

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