Hall’s experiment

Homework E: Due Friday, March 28, 2014 at conference.
Instructions: Please provide a brief verbal explanation of each step in your solution. State
where the formulas are coming from, and why they are applicable here. Use symbols and
formulae effectively defining their meaning and making it clear whether they are vectors or
scalars. Write legibly, and draw large and clearly labeled sketches.

Problem 1
Consider Hall’s experiment indicated in Fig. 1. The material is a metal with a density n of
charge carriers each with charge q and mass m. The Hall voltage VH is measured for a given
applied field B current I and conductor thickness t. From these variables the so called Hall
coefficient for the material is determined:
RH =

VH t
IB

(1)

(a) For negative charge carriers, indicate on a sketch the carrier drift velocity v and which
side of the conductor has positive and negative Hall voltage.
(b) For negative charge carriers, indicate on a sketch the carrier drift velocity and which
side of the conductor has positive Hall voltage.
(c) Show that the Hall coefficient RH =

1
.
nq

To do this you may need the following formulae

that you should derive or argue for: v = E/B , I = Jwt, and J = nqv .
(d) Calculate the Hall voltage for an experiment on a conductor where n = 3.7 × 1022 cm−3 ,
q = 1.602 × 10−19 C. The current I = 100 mA, the conductor thickness t = 0.1 mm,
and the magnetic field B = 0.5 Tesla.

Problem 2
Consider the Helmholtz coil configuration shown in Fig. 2. Here R = 0.2 m, I = 10 A,
and there are N = 100 windings in each coil. You can neglect the thickness of each coils
windings so that its dimensions are fully specified by R.
(a) Use Biot and Savart’s law to derive an expression for the magnetic field as a function
of displacement x from the center of the coils.
(b) Calculate the strength of the magnetic field in Tesla at the central point between the
coils where x = 0 and for x = ±R/2 and make a plot of B (x).

1

Figure 1: Experimental setup to measure the Hall coefficient for copper

Figure 2: Helmholtz coil configuration.
Problem 3
Consider the coaxial conductor shown in Fig. 3. Assume the current I flows in opposite
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directions in the inner and outer conductor respectively. You can assume the current is
uniformly distributed in the inner and outer conductors.
(a) Use Ampere’s law to determine the magnetic field B within the inner conductor, between the conductors, and outside the outer conductor. Make a plot of B as a function
of displacement from the center of the coax cable.
(b) Considering the Lorentz force on moving charge in a magnetic field, sketch the direction
of forces acting on points along the circumference of the inner and outer conductor
where current is flowing.