waecE-LEARNING
Physics Paper 3, Nov/Dec 2009  
Questions: 1 2 3 Main
General Comments
Weakness/Remedies
Strength

 

 

 

 

 

































 

Question 3

(a)  Diagram

Five constantan wires of different diameters marked J, K, L, M and N, each of the length 1.0m are connected in turns in a circuit as illustrated in the diagram above.
For each trial the rheostat is adjusted such that the same current flows through the wires.
The corresponding voltmeter readings are noted and recorded.
 
Fig. 3(a) represents the voltmeter readings Vi.
Fig. 3(b) represents the ammeter reading I.
Fig. 3(c) represents the diameters di of the wires J, K, L, M and N respectively
            where i = 1, 2, 3, 4 and 5.

  1. Read and record the values of I.
  1. Read and record the values of Vi.
  1. Record the corresponding values of di measured horizontally.
  1. Evaluate R  = V  in each case.
                        I
  1. Evaluate the cross-sectional area, A, of each constantan wire using its corresponding diameter di.  Express your answers in standard form

                                                πd2
                                              A =   4 
                  

  1. Evaluate A‑1
  2. Tabulate your readings.
  3. Plot a graph of R on the vertical axis against A-1 on the horizontal axis.
  4. State two precautions necessary to ensure accurate results when performing this experiment.

(b)  (i)  A bulb is rated 12 V, 6W, Calculate the time for which it can operate continuously from a 30 Ah 12 V battery.

(ii)  Explain why the e.m.f. of a cell is greater than the terminal p.d when the cell is used to supply current to an external resistor.

_____________________________________________________________________________________________________
Observation

This was a popular question and it was well attempted. Some candidates had difficulties in the determination of d, A and A-1.   Those that were able to determine A1 correctly have difficulties handling the exponential factors of the evaluated quantity A-1.  For instance a value of 2.705 x 10-7, the value of 2.7 was used and the 10-7 was not taken into account in evaluating the slope.
The (b) part was poorly attempted.

In part (a), candidates were required to:

  1. read and record at least 1 decimal place,  five values of V, I and d

(ii) convert using the given scale the five values of d                            

(iii)  evaluate correctly and record to at least 3 d.p or s s.f as appropriate five values of R, A and A-1                                              

(iv) record data in  composite table showing v, d, R, A and A-1

(v) plot five points correctly on a distinguished axes using reasonable scales with a line of best fit through the points.

(vi)       State any two of the following precautions:

    1. Tight connections were ensured;
    2. Avoid parallax errors in reading volt-meter/ammeter/meter rules;
    3. Key opened when readings are not taken
    4. Ensure clean terminals
    5. Zero error noted and corrected in reading voltmeter/ammeter/metre rule.


(b)(i)    Pt =  energy                  
            Pt  =  QV                                                                                                                   

            t      =      QV       =      30 x 12
                             p                    6

 =  60 hours                                                                                                                

(ii)  The internal resistance of the cell drops a certain amount of voltage across it [lost volts] whenever the cell is delivering current but this is not so when it is not delivering current.             
                                                                                                                                   
                                    OR

                       Terminal p.d (v) = I R
                       Emf (E)  =  iR + ir
                 ∴   E > V because of ir (lost volts)

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