ConceptConceptDocsDocuments

CAIE A-Level Physics 9.1.4 Using I = Anvq

Practise using I = Anvq to identify quantities and units, calculate current, drift speed or carrier density and interpret dimension or carrier changes.

Syllabus
2028–2030
Course
Physics 9702
Level
AS

Exam points

  • identify the meanings and units of A, n, v and q in I = Anvq
  • calculate current, drift speed, carrier number density, cross-sectional area or carrier charge using I = Anvq
  • interpret how cross-sectional area, carrier density or drift speed changes affect current in a conductor

9.1.4—Use, for a current-carrying conductor, the expression I = Anvq question 1

[Maximum number: 5]

Question (a)

(a)

A metal wire has length L and cross-sectional area A, as shown in Fig. 6.1.

Fig. 6.1

Fig. 6.1

I is the current in the wire,
n is the number of free electrons per unit volume in the wire,
v is the average drift speed of a free electron and
e is the charge on an electron.

[ 3 ]

Question (i)

(i)

State, in terms of A, e, L and n, an expression for the total charge of the free electrons in the wire.

[ 1 ]

Question (ii)

(ii)

Use your answer in (i) to show that the current I is given by the equation

I = nAve.
[ 2 ]

Question (b)

(b)

A metal wire in a circuit is damaged. The resistivity of the metal is unchanged but the cross

sectional area of the wire is reduced over a length of 3.0 mm , as shown in Fig. 6.2.

Fig. 6.2

Fig. 6.2

The wire has diameter d at cross-section X and diameter 0.69 d at cross-section Y . The current in the wire is 0.50 A .

[ 2 ]

Question (i)

(i)

Determine the ratio
 average drift speed of free electrons at cross-section Y average drift speed of free electrons at cross-section X\frac{\text { average drift speed of free electrons at cross-section } \mathrm{Y}}{\text { average drift speed of free electrons at cross-section } \mathrm{X}}.
ratio =

[ 2 ]
All question bank results loaded