THE HALL EFFECT
Before getting into Hall current electrodynamics, we begin with some unit vector basics. Along each axes one can define unit vectors: x^, y^ and z^. Then the respective multiplications (by vector directions) yield: x^*y^ = z^, and y^*z^ = x^ and x^*z^ = y^. These rules will always apply for a right handed coordinate system.Thus, a vector cross product given by: A X B, must always have the directions attached by means of vectors, e.g.:
A(x^) x B(y^) = (A X B) (z^) = C(z^)
Now, as depicted in the accompanying diagram below, consider a slab of conducting material through which a current I flows as shown (in direction x^) so that I = I(x^).
We consider in turn the effect on positive and negative charge carriers, after having attached the coordinate system x, y, z and thence we specify (in addition to the current I(x^):
The magnetic induction: B = B(y^)
The velocity: v = v(-x^)
(Since the velocity of actual charges is opposite to the direction of conventional current flow)
The magnetic force (F = qvB) acting on a unit (+) charge deflects it toward the upper face, resulting in the accumulation of + charges there, and negative (-) charges on the bottom face.
Expressing the force with appropriate directions:
F(z^) = q v(-x^) x B(y^)
The opposite accumulation of charge (+ to bottom, - to top) gives rise to an electrical force that counteracts the magnetic. Eventually equilibrium occurs when:
Eq = qvB
At this point:
E = VH t
Where VH is the Hall potential difference.
Then:
(VH / t) q = qvB
Or, by directions:
qE(-z^) = q vB(z^)
or VH = Bvt
The drift velocity can be found from the basic definition of the current:
I = ne v A
Where A is the area A = Lw (length x width of box)
n = number density of charges (per cubic meter)
e = unit of electronic charge = 1.6 x 10-19 C
Solving for v:
V = I / neLw
Therefore, the Hall potential difference is:
VH = B{I/neLw} t = BI/ new
The magnetic induction: B = B(y^)
The velocity: v = v(-x^)
(Since the velocity of actual charges is opposite to the direction of conventional current flow)
The magnetic force (F = qvB) acting on a unit (+) charge deflects it toward the upper face, resulting in the accumulation of + charges there, and negative (-) charges on the bottom face.
Expressing the force with appropriate directions:
F(z^) = q v(-x^) x B(y^)
The opposite accumulation of charge (+ to bottom, - to top) gives rise to an electrical force that counteracts the magnetic. Eventually equilibrium occurs when:
Eq = qvB
At this point:
E = VH t
Where VH is the Hall potential difference.
Then:
(VH / t) q = qvB
Or, by directions:
qE(-z^) = q vB(z^)
or VH = Bvt
The drift velocity can be found from the basic definition of the current:
I = ne v A
Where A is the area A = Lw (length x width of box)
n = number density of charges (per cubic meter)
e = unit of electronic charge = 1.6 x 10-19 C
Solving for v:
V = I / neLw
Therefore, the Hall potential difference is:
VH = B{I/neLw} t = BI/ new
Example Problem:
If the magnetic induction B = 1.0 T, and a rectangular slab of material (such as shown) is for copper, with n = 10 29 /m3, find the Hall emf if I = 10A, and the width of the slab is 0.001 m.
Solution:
VH = BI/ new
= (1.0T) (10 A)/ {1029 /m3)(1.6 x 10-19 C) (0.001m)}
VH = BI/ new
= (1.0T) (10 A)/ {1029 /m3)(1.6 x 10-19 C) (0.001m)}
VH = 0.6 mV
Suggested Problems:
Suggested Problems:
1) The diagram below shows a slab of silver with dimensions: z1 = 2 cm, y1 = 1mm, carrying 200 A of current in the +x^ direction. The uniform B-field has a magnitude of 1.5 T. If there are 7.4 x 1028 free electrons per cubic meter.
Find:
a)The electron drift velocity
b)The magnitude and direction of the E-field due to the Hall Effect
c)The magnitude of the Hall EMF.
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