Question 11
(a) Define:
(i) reactance;
(ii) impedance in an a.c. circuit.
The diagram above illustrates an a.c. generator. When the coil is rotated, an e.m.f. is induced in the coil.
(i) Explain why an e.m.f. is induced.
(ii) State the purpose of the slip-rings.
(iii) Name and state the law used to determine the direction of the induced current.
(iv) State two ways to increase the induced e.m.f.
(c) A lamp is rated 12V, 6 W. Calculate the amount of energy transformed by the lamp in 5 minutes.
Observation
Part (a)(i): Candidates performance was good. Many candidates did well while some candidates could not complete and so lost mark. Also, in the definition, some candidates were not specific about the type of current. were unable to correctly define the term dielectric.
(ii) Many candidates did not mention the type of current or the reactive components. Performance was fair
Part (b)(i): Candidates performance was low.
(ii): Some candidates mistook slip rings for split rings to loss mark.
(iii): Some candidates instead of Lenz’s law stated Fleming’s law;
Part (c): Performance was fair. Many candidates had no problem with the calculation but failed during substitution.
The expected answer:
(a)(i) Reactance
It is the opposition offered to an a.c by an inductor/capacitor
(ii) Impedance
It is the overall opposition offered to an a.c in an R-L, R-C or R-L-C circuit
OR
It is the combined opposition to an a.c by both reactive and resistive components
(b)(i) Emf is induced because the coil cuts magnetic flux/magnetic lines of force
(ii) Slip rings ensure that the direction of current is reversed in each half of the rotation
of the coil
(iii) Name and State Lenz’s law
Lenz’s law
[
It states that the direction of the induced current is such as to oppose the change producing it
(iv) Increased Induced emf
Ensure:
- increased speed of rotation
- increased number of turns in the coil
- increased area/diameter of coil
- efficient core design
(c) E = Power x Time
= 6 x 5 x 60
= 1800 J