Question 10
A thin slice of
conducting material has its faces PQRS and VWXY normal to a uniform magnetic
field of flux density B, as shown in Fig.
9.1.
Fig. 9.1
Electrons enter the
slice at right-angles to face SRXY.
A potential
difference, the Hall voltage VH,
is developed between two faces of the slice.
(a)
(i) Use letters from Fig. 9.1 to name the two faces between which the
Hall voltage is
developed. [1]
(ii)
State and explain which of the two faces named in (a)(i)
is the more positive. [2]
(b)
The Hall voltage VH is
given by the expression
VH = BI / ntq.
(i)
Use the letters in Fig. 9.1 to identify the distance t.
[1]
(ii)
State the meaning of the symbol n.
[1]
(iii)
State and explain the effect, if any, on the polarity of the Hall
voltage when negative
charge carriers
(electrons) are replaced with positive charge carriers, moving in the same
direction towards the slice. [2]
[Total: 7]
Reference: Past Exam Paper – March 2018 Paper 42 Q9
Solution:
(a)
(i) PSYV and
QRXW
{The Hall
voltage developed should be developed on the faces perpendicular to both the
magnetic field and the direction of motion of the electrons.}
(ii)
electron moving in magnetic field deflected
towards face QRXW
so face PSYV is more positive
{The direction of the force on the electrons
can be determined from Fleming’s left hand rule.
Forefinger: field – downwards
Middle finger: current – to the right (opposite
to the flow of electrons)
Thumb: force – towards QRXW
The electrons flow towards face QRXW. So, face
QRXW becomes more negative while face PSYV becomes more positive.}
(b)
(i)
PV or SY or RX or QW
{t is the
thickness of the slice.}
(ii)
number of
charge carriers per unit volume
(iii)
Negative and positive charge (carriers) would
deflect in opposite directions. So no change in polarity.
{A positive
charge carrier would deflects towards face PSYV, again making it more positive.}
