A rectangular piece of aluminium foil is situated in a uniform magnetic field of flux density 
, as shown in the figure below.
The magnetic field is normal to the face PQRS of the foil. Electrons, each of charge 
, enter the foil at right angles to the face PQTV.
On the figure above, shade the face of the foil on which electrons initially accumulate.
Explain why electrons do not continuously accumulate on the face you have shaded.
The Hall voltage 
developed across the foil in (a) is given by the expression
where 
is the current in the foil.
State the meaning of the quantity 
.
Using the letters on the figure above, identify the distance 
.
Suggest why, in practice, Hall probes are usually made using a semiconductor material rather than a metal.
