Physics Paper 2, Nov/Dec. 2012
 Questions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Main
Weakness/Remedies
Strength

Question 14

(a)   Define magnetic flux.

(b)    Sketch a diagram to illustrate magnetic lines of force for a bar magnet placed in a
uniform earth’s magnetic field  with its north pole pointing north.
Indicate the neutral points.

(c)    State two ways in which the earth’s magnetic field is important to life on the earth
surface.

(d)    List two applications of electromagnets

(e)    In a certain house, three ceiling fans each of 80 W, an air-conditioner rated 1500 W are operated for 6 hours each day.  Seven lamps each rated 40 W are switched on for 10 hours each day.  The home theatre 100 W and television set 80 W are also operated each day for 5 hours after which they are kept in the standby mode.  In the standby mode, the power consumption is 5% of the power rating.  Calculate the:

1. Total energy consumed in the house for 30 days in kWh;
2. Cost of operating all the appliances for 30 days at N10.00 per kWh.

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P

1.    The description of magnetic flux posed some problem to candidates who failed

to see it as basically a collection of (imaginary) lines of force in a magnetic field.

1.  This part called for a sketch of a given magnetic field.  It was poorly reacted to  by candidates.  Some sketches were rough while some did not indicate the correct direction of the arrows on the magnetic lines.

1. Most candidates were able to state ways in which magnetic field is important to life on earth.  Performance was fairly alright.
1. Listing of applications of electromagnets was well answered by virtually all the respondent candidates.  Performance was good.

1. The numerical part was poorly tacked most respondent candidates mishandled the standby mode part of the question.  Performance was poor.

The expected responses are:
(a)       Magnetic flux consists of lines which represent the direction and strength of a magnetic field at any point in the field.

(b)

correct shape of lines of force
correct arrows / directions
two  neutral points P correctly indicated

(c)         Importance of earth’s magnetic field to life on earth
e.g
-           Navigation by sailors and pilots
-           Plotting of contour lines by surveyors
-           shielding of dangerous radiations / cosmic rays from the sun

(d)       Application of electromagnetic
e.g
-           loud speakers
-           door bells
-           automatic switches / relays
-           separation of magnetic materials from non magnetic materials
-           lifting of heavy magnetic materials
e)

 Appliance Quantity Power rating / W Standby power / W Total Watt KW KWh Ceiling fan 3 80 - 240 0.24 1.44 Air conditional 1 1500 - 1500 1.50 1.50 Lamps 7 40 - 280 0.28 2.80 Home theatre 1 100 5 100  5 0.100     0.005 0.5   0.095 Television set 1 80 4 80        4 0.080 0.004 0.4   0.076 14.311

NOTE: Standby mode was for 19 hours

1. Energy consumed in a day               =  14.311 kWh

Energy consumed in 30 days           =   14.311 x 30
=   429.33 kWh

(ii)   Cost of operating for 30 days
1 KWh                         N10.00
429.33KWh               429.33 x 10

=          N 4293.30