# Homework Help: Hall Potential Difference

1. Nov 14, 2007

### PFStudent

1. The problem statement, all variables and given/known data

12. A strip of copper ${150{{.}}{\mu}m}$ thick and ${4.5{{.}}mm}$ wide is placed in a uniform magnetic field ${\vec{B}}$ of magnitude ${0.65{{.}}T}$, with ${\vec{B}}$ perpendicular to the strip. A current ${i = 23{{.}}A}$ is then sent through the strip such that a Hall potential difference $V$ appears across the width of the strip.

Calculate $$V$$. (The number of charge carriers per unit volulme for copper is $$8.47{\times}{10^{28}}$$ electrons/$$m^{3}$$).

2. Relevant equations

$$q = n_{e}e, {{.}}{{.}}{{.}}{n_{e}} = \pm1, \pm 2, \pm 3,...,$$

e $\equiv$ elementary charge

$$e = 1.60217646 {\times} 10^{-19}{{.}}C$$

$${n_{e}} = {\pm}N, {{.}}{{.}}{{.}}{N} = 1, 2, 3,...,$$

$${N_{V}} = \frac{n_{e}}{V}$$

$${n_{e}} = {N_{V}}{V}$$

$${N_{V}} = {\frac{BI}{{\Delta{V}}{le}}}$$

Where, $${\Delta{V}}$$ is the Hall potential difference.

3. The attempt at a solution

This seems like a straight forward problem, here is how I worked through it.

$${\Delta{V}} = {\frac{BI}{{N_{V}}{le}}}$$

Let $I$ = current, ${\Delta}{V} = V$, and $\left({\frac{N}{V}}\right)$ be the number of charge carriers per unit volume. So, since we're dealing with electrons,

$${N_{V}} = -{\left({\frac{N}{V}}\right)}$$

Where,

${\left({\frac{N}{V}}\right)} = {{8.47}{\times}{10^{28}}}$ electrons/${m^{3}}$

However, since they gave me two distances: thickness (${t}$) and width (${w}$); which one is ${l}$?

I thought at first, it was the width because isn't that how the Hall potential difference is defined, as the potential across the width of a strip?

The book used the thickness, so I am wondering why?

Any help is appreciated.

Thanks,

-PFStudent

Last edited: Nov 14, 2007
2. Nov 14, 2007

### Staff: Mentor

To understand why the formula for Hall voltage contains thickness (not width) you need to review its derivation. If your text isn't clear on the matter, check this out: http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/hall.html" [Broken]

Last edited by a moderator: May 3, 2017
3. Nov 27, 2007

### PFStudent

Hey,

Thanks for the link Doc Al.

So, I noticed they always used the thickness in their examples,..so in a nutshell is that how the Hall Potential is defined?

That is to say, in defining the Hall Potential--if we consider a thin strip (say like copper) of thickness ${t}$, width ${w}$, and length ${l}$; in a magnetic field ${\vec{B}}$ perpendicular to the face of the strip, carrying a current ${I}$ through one end of the strip and out the other.

Then there is a potential difference ${\Delta{V}}$ as a result of the electric force ${{\vec{F}}_{E}}$ and magnetic force ${{\vec{F}}_{B}}$ opposing each other due to the orientation of the two fields. This is a result of the magnetic field pushing the positive charges to one side of the thin strip. The placement of the positive charges on one side of the strip creates an electric field with the negative charges left on the other side of the strip. Resulting in the creation of a potential difference ${\Delta{V}}$ known as the Hall Potential Difference. Given as,

$${\Delta{V}} = {\frac{BI}{{N_{V}}{te}}}$$

Is that all right?

Thanks,

-PFStudent

Last edited by a moderator: May 3, 2017
4. Nov 27, 2007

### PFStudent

Hey,

Yea, still a little unclear on this.

Any help would be appreciated.

Thanks,

-PFStudent

5. Nov 27, 2007

### rock.freak667

The hall voltage is established when the resultant force exerted on the charge carriers is zero...that is when $F_E=F_B$

6. Nov 27, 2007

### PFStudent

Hey,

Ok, that makes sense but that does not quite explain why the Hall Potential is dependent on the thickness ${t}$, of the strip. Why is that?

Thanks,

-PFStudent

7. Nov 27, 2007

### rock.freak667

Well it all comes from the derivation of the formula
$$F_E=F_B$$
=> $$Ee=Bev$$
$$E=Bv$$
Re:$$E=\frac{V_H}{d}$$ where d= thickness

$$\frac{V_H}{d}=Bv$$
Re: $$v=\frac{I}{nAe}$$

$$\frac{V_H}{d}=\frac{BI}{nAe}$$
so that:

$$V_H=\frac{Bd}{nAe}$$
now A=td where l is the length of the material
$$V_H=\frac{Bd}{ntde}$$
$$V_H=\frac{B}{net}$$
so t is actually the length which is basically the width

8. Nov 27, 2007

### Staff: Mentor

Sorry I didn't get back to this sooner. In a nutshell, while the Hall voltage is defined across the width of the strip, it only depends on the thickness of the strip not the width. If you check out the derivation on the link I gave, you'll see how it comes about.

Good.

I'd say that:
$$E=\frac{V_H}{W}$$, where W = width, not thickness.

I can't quite follow this. I'd rewrite it this way:

$$E = vB$$

$$\frac{V_H}{W} = vB$$
(where W is width)

$$v = \frac{I}{n e A}$$

where A is cross-sectional area = Width*thickness = Wt, so:
$$\frac{V_H}{W} = \frac{I B}{n e Wt}$$

Note how the width cancels out, leaving the dependence on thickness:
$$V_H = \frac{I B}{n e t}$$

9. Nov 27, 2007

### rock.freak667

Well I guess width and thickness would depend on which way the electrons are flowing in the conducting material...as in my diagram..d as in the height of the conducting material.