Question 11
Part II. Candidates are expected to answer any three questions from this part.
- State:
- three ways by which the emf of an ac generator can be increased.
- how an ac generator could be modified to produce dc
- (i) Define the root mean square value of an alternating current.
(ii) Explain the term resonant frequency as it relates to a series R – L - C
circuit.
- A particle of charge – 6 C released from rest in a uniform electric field E moves a distance of 5 cm. If its kinetic energy is 4 x 10-3 J, calculate the:
- work done by the electric field;
- potential at its initial position;
- magnitude of the electric field intensity
Observation
Part (a): This was a popular question among the candidates and they had little or no problem with this question.
Part (b): Most candidates failed to state the definition correctly. Some interchanged the positions of the alternating current and direct current by defining it thus: “the alternating current that would dissipate the same energy as was dissipated by the direct current” hence the lost the marks.
The explanation of resonant frequency was fairly handled. Most candidates however, stopped at definition and so missed the mark for additional information.
Part (c): Performance was very poor because most respondent did not recognise that work done by the electric field is equivalent to the kinetic energy of the particle. Most candidates only scored for the correct formulae W = qV and E = V/d.
The expected answers are:
(a)(i) Emf of a.c generator
- increasing the number of turns in the coil of the armature.
- increasing the strength of the magnetic field
- increasing the speed of rotation of the armature
- ensuring that the core is made of soft iron.
(ii) Modification of a.c generator
By replacing the slip rings with split ring (commutator)
b(i) Root-mean-square value
The direct/steady current that would dissipate the same amount of energy in a resistor
as is dissipated by the alternating current.
OR
The direct/steady current that would dissipate the same energy or quantity of heat in
the same time in the same resistor.
(ii) Resonant frequency
The frequency at which minimum impedance occurs.
OR
The frequency at which inductive reactance = capacitive reactance.
OR
The frequency at which maximum current is obtained from such a circuit.
Additional information
e.g. - 2= at resonance.
- measured in hertz (Hz)
(c)(i) Work done = kinetic energy = 4.0 x 10-3J
(ii) W = QV
OR
V =
= = 6.67 x 102 V
(iii) E = = = 13340 NC -1 / Vm-1