Electricity and Magnetism (PHYS2239)

BSc, Physics - PHYS

Semester: Resit

Level: 200

Year: 2018

Answers (200 )
REPUBLIC OF CAMEROON THE UNIVERSITY OF BAMENDA
Peace-Work-Fatherland
P. O. BOX 39 Bambili
School/Faculty:FS Department:_Geology Lecturer(s): Mr Pokam
Course Code: _phys2239____Course Title: Electricity and magnetism
Date: _11-09-2018_______Halls: __ Room 5 FHS________TIME: 7:30-9:30
Instructions: Answer all questions showing all detailed calculations.
QUESTION I: ELECTRIC FIELD 6 MARKS
Consider a charged line of the length L, having a uniform linear charge density placed along the X-axis of the
Cartesian reference frame with its center at the origin.
1) Sketch the diagram showing the electric field at an arbitrary point P (x, y) in the (x, y) plane
created by the element of charge at the element dx’. [1mark]
2) Express the position vector r’ describing the distance of the point P from the charge element at
the position dx’ and deduce the square of that position. [1mark]
3) Express the element of the electric field at P point due to the element dx’. [1mark]
4) Using questions 1), 2) and 3 to deduce the different components of the electric field E. [1.5marks]
5) Calculate those different components in the case where the line charge is infinitely long. [1.5mark]
QUESTION D: ELECTRIC POTENTIAL 4 MARKS
Consider the disk of radius R carrying a uniform charge density at a point P at the distance Z (along the axis
of the disk).
1) Express the element of the potential field at the P point due to one ring of radius r with respect
to r, and Z. [2marks]
2) Deduce the expression of that electric potential at P point. [2marks]
QUESTION III: MAGNETISM 10 MARKS
A device known as a "scarch coil” consists of N = 50 turns of wire with a cross- sectional area A=m
2
and a
resistance R=25Ω as shown in fig [1] Such a device can be used to determine the value of a constant
magnetic field.
Figure 1
(a) In fig [1] the plane of the search coil is perpendicular to the unknown field. What is the flux of B at
this time? (3 mks)
(b) If now we quickly flip the search coil 90°. what is the new flux, of B? (2mks)
(c) What is the change in the flux, during the time ∆t required to flip the coil? (2mks)
(d) What is the induced emf in the coil? (1mks)
(c) Assuming the resistance to be only that of the scarch coil.what current is induced in the scarch coil
during the time ∆t? Express your answer in terms of B, A, R and ∆t. (1mks]
(e) Using the definition of current, rewrite the previous answer. (1mks)
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