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 Physics Paper 1, MAy/June. 2009
 Questions: 1 2 3 4 5 6 Main

Weakness/Remedies
Strength

Question 3

(a)   Diagram

Connect the circuit as shown in the diagram above. PQ is a potentiometer wire 100 cm long and Rs a  standard resistor of 5Ω

1. With the jockey J  not making contact with PQ, close the switch.  Read and record the ammeter reading I.  Open the switch.
2. Use the jockey to make contact with PQ at the 20 cm mark such that PJ = l = 20 cm.  Close the switch read and record the value I, of the ammeter.  Evaluate L-1.
3. Repeat the procedure for other values of l = 35, 50, 65 and 80 cm.  In each case determine the corresponding values of I and L-1.  Tabulate your readings.
4. Plot a graph of L-1 on the vertical axis and Ii on the horizontal axis starting both axes from the origin (0, 0).
5. From your graph deduce the value Io of Ii when L-1 = 0.
6. Evaluate  I0      .

I

1. State two precautions taken to ensure accurate results.

(b)    (i)      Define the e.m.f of a battery.
(ii)      A cell X of e.m.f. 1.00 V is balanced by a length of 40.0 cm on a potentiometer wire.
Another cell Y is balanced by a length of 60.0 cm on the same wire.  Calculate the
e.m.f. of Y.

_____________________________________________________________________________________________________
Observation

Questions on electricity are not usually popular among candidates due to insufficient and malfunctioning apparatus.  However, this one was attempted by many candidates and their performance was fair.

Many responding candidates failed to record the value of Io as demanded for in the question.
Few candidates confused Ɩ with Ii hence were calculating Ii-1 instead of L-1.

In part (a), candidates were required among other things, to:

1. Read and record to at least 1 decimal place the value of ammeter reading ;
2. Read and record to 1 decimal place five values of Ii ;

[Trend: L increases as I decreases]
(iii)       Evaluate and record to at least 3 decimal places five values of L-1 correctly;
(iv)       Record data in composite table showing Ɩ, I and I-1

(v) Correctly distinguish the graph axes, select reasonable scales, plot five points correctly and draw line of best fit;

(vi)       State any two of the following precautions in acceptable forms:

-      Ensured tight connections/cleaned terminals
-     Key removed after each reading
-     Parallax error avoided in reading ammeter
-     Avoided sliding jockey on potentiometer wire
-     Repeated readings shown in table.

(b)(i)    E.m.f. is the total work done in pushing one coulomb of electricity round an external resistor and the internal resistor of the source or the total energy obtained per coulomb from a cell or battery.

In part (b) (i) candidates’ responses were poor. In (b)(ii), most candidates could not give the correct formula.

The expected responses were as follows:

(b)(ii)   Ey   =   ly
Ex         lx

Ey     =   60
1.00        40

Ey  =  60  x 1. 00
40

Ey  =  1.50V.