### Question 11

(a)  Define:

•  the coulomb;
•  resistance
• (b)        State the three effects of an electric current.

(c)       State the standard international colour convention for the insulating material   covering the following electrical wires in a three-pin plug:

•  live;
•  neutral;
•  earth.
•

(d)

A water melon of mass 5.0 kg is suspended on a uniform rod of mass 4.0 kg and 4.0 m  long as illustrated in the diagram above.  If the rod is in equilibrium by the action of the force between the charges + q and –q, calculate the:

•  anticlockwise moment;
•  value of q                  [ g = 10 ms-2]

• ### Observation

Part (a):  Very popular question among the candidates and performance was average.  However, some candidates rather than defining were showing the mathematical relationship between charge, current and time.  Some even defined resistance as the ratio of voltage to current.

Part (b):  The effect of magnetic and heating effects proved more popular than chemical effect in the candidate’s responses.  Performance was average.

Part (c):  Most candidates were able to give the correct colours as required by the question.  However, few candidates still reflected some level of ignorance on the topic.  Performance was average.

Part (d):  Many candidates failed to perform to expectations.  Most candidates that attempted this question got the anticlockwise moment but majority failed to apply the concept of physics to the clockwise moment.  This led to their inability to get the value of q.  Performance was poor.

The expected answers are:

11.(a)(i)  Coulomb
The coulomb is the quantity of (electric) charge passing through a section / point in  a                      conductor when a steady current of 1 ampere passes for  1 second.
Resistance

This is the opposition offered to the passage of an electric current in a conductor.

(b)   Effects of electric current
-  Magnetic
-   Heating/ thermal
-   Chemical

(c)   Electrical wire insulating colour

 Wire Colour Live Neutral Earth Brown /Red                    Blue / Black                    Green / Yellow/Green and Yellow

(d) (i) Anticlockwise moment  =  5gl1           OR      (i)      Anticlockwise moment  =  5gl1
=    50N                                                                   =    50Nm
(ii)Clockwise moment  =    4gl2 +                     (ii)   Clockwise moment  =   4gl2 +
= 4 x 10 x 1 +                                                                    = 4 x 10 x 1 +
=        Nm                                                                  =  40 + 4.315x   q2                           Sum of clockwise moments  =                                                         Sum of clockwise moments  =  anticlockwise moments                                                                                             anticlockwise moment
=   50                                                    40 + 4.315x   q2     = 50

q =             C                                                                      q  =      4.814 x C