Physics Paper 3, Nov/Dec 2011

Questions:	1	2	3	Main

General Comments
Weakness/Remedies
Strength

Question3
A standard resistor of resistance 2W is connected in the left gap of the metre bridge and a length, 1, of wire W1 is connected across the right gap of the bridge as shown in the diagram above. The balance point, B, is determined by touching the metre wire with the jockey. The balance length and are measured and recorded. The procedure is repeated for five other values of length The entire procedure is repeated using a second wire W2. The diameters d1 of W1 and d2 of W2 are measured and recorded. Fig 3(a) and Fig 3(b) represent sections of a micrometer screw gauge indicating the readings of the diameters d1 and d2 respectively. Fig 3(c) and Fig 3(d) show the positions Bi of the jockey on the metre wire for W1 and W2 respectively, where I = 1,2,3,4,5, and 6 (i) Read and record the diameters d1 and d2 (ii) Measure and record X and the corresponding for 1. (iii) Evaluate R1 = x R in each case. (iv) Measure and record and the corresponding Xni for 2 (v) Evaluate R2 = x R in each case. (vi) Tabulate your readings. (vii) Plot a graph with R2 on the vertical axis and R1 on the horizontal axis. (viii) Determine the slope, s, of the graph. (ix) Evaluate K = d2 x d1 (x) State two precautions that are necessary to ensure accurate results when performing this experiment. (b) (i) Explain why the resistance of a metallic conductor increases with increase in temperature. (ii) The rating of an electrical heater is 1000W, 200V. Calculate its resistance.
_____________________________________________________________________________________________________
Observation
This question on electricity was the least popular among the candidates. The performance was poor. Most candidates could not read the micro-meter screw gauge correctly. Candidates had no problem with the measurement of the lengths and but could not determine correctly the resistances R1 and R2 as most of the candidates mixed up values for wire 1 with that of wire 2. Hence most candidates could not get a composite table for the experiment. Their graphs were poorly drawn and most of them could not evaluate K = correctly. Precautions and part (b) were also poorly attempted. Candidates were expected to: - read and record values of d1 and d2 to 2 d.p in mm - measure and record six values of and = (100 – ) for W1 to at least 1 d.p in cm - measure and record six values of and = (100 – Xm) for W2 to at least 1 d.p in cm - evaluate R1 and R2 in each case - tabulate their readings - plot graph of R2 against R1 using reasonable scales. - draw line of best fit - determine the slope of graph - evaluate K = State any two of the following precautions Ensure tight/clear terminals Avoid parallax error in reading metre rule Zero error noted/corrected on micrometer-screw gauge/metre rule/galvanometer Open key when not taking readings Avoid sliding jockey on wire Ensure that readings are repeated. In part b, the expected responses are: b(i) A metal consists of free electrons in random motion. When the temperature of the metal is increased, the vibration of the atoms increased. This increases the number of collisions of the electrons with the atoms and therefore reduces the average velocity of electrons (conductivity). This results in the increase in the resistance of the metal. (ii) P = R = = 2002 1000 = 40 Ω