Part (a) A lot of candidates attempted this question. Many could not answer the question correctly. Some wrote about lighting conductor instead of earthing.
Part (b) This question was well attempted by majority of the responding candidates
Performance was above average.
Part (c) This part was fairly attempted by the candidates. Some candidates however lost marks due to their carelessness e.g
- not putting arrows on the lines
- length of the lines not relative to the magnitudes of the values they represent
- right angles not indicated.
Many candidates could not give the correct sinusoidal equation for V in terms of time t.
The expected answers are:
(a) The ground wire is a safety device
In the event of a short circuit occurring in the appliance the dangerous current,
instead of passing through our body when we touch the appliances, gets sent
safely to the earth (ground) through the grounding wire because the ground has
much lower resistance than our body does .
(b) Because they have two basic defects namely
- polarization
- local action
(c)(i)
Correct vector diagram
Correctly relative magnitude
(c) (i) Frequency of the secondary output = 50Hz
( V2rms = VR2 + (Vc –VL) 2
= 302 + (70 – 30)2
= (900 + 1600) = 2500
Vrms = 50V
(ii) ( XL = 2
= 7Ω
Xc =
= = 1 Ω
Z = R2 + (XL - Xc)2
82 + (7 – 1)2 = 64 + 36
= 10 Ω
I =
= = 5.0 A
) P = I2R
= 52 x 8
= 200 W
(iii) Vo = Vrms
= 50 x
= 70.7V
2 = 2 x
= 1000t
V = Vo Sin 2
V = 70.7 sin 1000t