Physics Paper 2, MAy/June. 2009

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Questions:	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	Main

General Comments
Weakness/Remedies
Strength

Question 14

(a)        State two essential differences between a moving coil galvanometer and a d.c. generator.
(b)        Explain the term eddy currents and state two devices in which the currents are applied.
(c)        State the principle on which the potentiometer is based when it is functioning.
(d)       A source of e.m.f. 110 V and frequency 60Hz is connected to a resistor, an inductor and a capacitor in series. When the current in the capacitor is 2A, the potential differences across the resistor is 80 V and that across the inductor is 40 V. Draw the vector diagram of the potential differences across the inductor, the capacitor and the resistor.
Calculate the:
(i)         potential difference across the capacitor;
(ii)        capacitance of the capacitor;
(iii)       inductance of the inductor. [π = 3.14]

_____________________________________________________________________________________________________

Observation

In part (a), responses were generally poor, indicating that the teaching of the principles underlying the operations of these devices was most likely neglected in schools.
In part (b), not many candidates succeeded in recognizing that eddy currents are those that are induced in a conductor (plate/block) subjected to a varying magnetic field. Giving at least one valid additional information was also an uphill task to many. Performance in it was poor.
Part (c) was the most poorly attempted while performance in part (d) was satisfactory.

The expected answers are as follows:

(a) Differences between a d. c. generator and a moving coil galvanometer.

D. C. generator	moving coil galvanometer
converts mechanical energy to electrical energy	converts electrical energy to mechanical energy.
uses split rings or commutator	uses hair springs.
rotation of coil is continuous	rotation of coil is incomplete
uses carbon brushes (as terminals)	uses jeweled bearings (as terminals)

(b) Eddy currents are currents induced in a conductor when subjected to varying magnetic field.
Any valid additional information e.g.

eddy current flows in a circular path or closed loops;
eddy current generates heat;
eddy current cannot flow through gaps or slots;
The currents move in such a direction as to oppose the change producing them.

Devices in which eddy currents are applied

pointers of sensitive electric meters
sensitive mass balances
brakes in large electric motors
speedometers in automobiles
detection of cracks in railway tracks
detection of metals

When a steady current is allowed to pass through a uniform wire, equal lengths of the wire will have equal potential differences.

The p.d across a length of a wire is (directly) proportional to the length provided the wire has a uniform cross section.

Vector diagram showing
   VL   with arrow
VC   with arrow
   VR   with arrow

                          V² = VR² + (VL - VC)²

                        110² = 80² + (40 – Vc)²

                        110² -80² = (40 – Vc)²

(40 – Vc ) =

(110 + 80) (110 – 80)

40 – VC = + 75.5

VC = 40 + 75.5

VC = 115.5V

            (ii)        VC = I X c
                       Xc = Vc /   =   1 /
                                    I             2π¦C                      2
                        ∴ C = I/2π¦VC          =    2 x 3.14 x 60 x 115.5

                        C = 0.0000459                         C = 45.9 x 10-6 F or 46mF