Physics Paper 3, Nov/Dec 2011

Questions:	1	2	3	Main

General Comments
Weakness/Remedies
Strength

Question3
The circuit above shows a solenoid and an ammeter connected in series with an a.c. source of variable voltage. A voltmeter is connected across the a.c. source. For five different low voltages from the a.c. source, the currents Ii and the corresponding potential differences Vi are read and recorded. Fig 3(a) and Fig 3(b) shows the ammeter readings Ii and the corresponding voltmeter readings Vi respectively, where i = 1,2,3,4 and 5 respectively. Read and record the values, I, of the current. Read and record the corresponding values, V, of the voltage Evaluate I-1 and V-1 in each case Tabulate your readings. Plot a graph with V-1 on the vertical axis and I-1 on the horizontal axis Determine the slope, s, of the graph. Determine the value of I when V is: 8V; 16V Determine the frequency, f, of the a.c.. source from the formula: f = 1 2πLs Where π = 22/7 and L = 0.024 H State two precautions that are necessary to ensure accurate results when performing this experiment. (b) (i) Give a reason why a moving-coil ammeter may not be suitable for measuring currents in this experiment. State one type of ammeter that could be used. (ii) Define inductive reactance and write an expression for the reactance of the circuit used for the experiment above
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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