STPM Physics Practical Experiment 6 Capacitance and Time Constant (Second Term)

STPM Physics Experiment 6 (Second Term) Title: Capacitance and time constant Results: No. 1 2 3 4 5 Average

Initial Current, I0/mA 0.90 0.90 0.90 0.90 0.90 0.90

Current at time t, 1 I/mA 0.85 1.27 0.80 2.74 0.75 4.08 0.70 5.20 0.65 6.50 0.60 8.60 0.55 9.98 Centroid (5.46 , 0.26)

Resistance, R/Ω 6200 6200 6200 6200 6200 6200

Time, t/s 2

3

Average

1.29 2.71 4.00 5.50 6.32 8.57 9.60

1.24 2.74 4.30 5.36 6.35 8.55 9.75

1.27 2.74 4.13 5.35 6.39 8.57 9.78

I0/I

ln(I0/I)

1.06 1.13 1.20 1.29 1.38 1.50 1.64

0.06 0.12 0.18 0.25 0.32 0.41 0.49

Calculations: Since

,

,

( )

Gradient k for the graph plotted is the reciprocal of the time constant. ∴,

and

Based on the graph plotted, Hence,

While C = capacitance of capacitor in F and R = resistance of resistor in Ω

∴, Since

While I = current in A and e = Euler’s constant Discussions: (a) Precautions (i) The capacitor is fully discharged before starting the experiment. (ii) All connections of wire and devices are ensured tight. (iii) The experiment is carried without necessary delay in the process to prevent wires from heating up which would increase the resistance of the circuit, affecting accuracy of the results. (iv) All devices are checked working fine before beginning the experiment. (b) Sources of error (i) Deviance from true value of the results due to the assumption that wires and circuit elements have zero resistance. (ii) Use of nominal values of direct current rating instead of measuring the rating. (iii) Error due to reaction time for pressing the stopwatch. (c) Steps to overcome the weakness of the experiment (i) Repeat the experiment several times and obtain the average values of the data acquired to increase the accuracy of the results (minimize errors due to reaction time). (ii) Use a voltmeter to ensure the voltage supplied by the direct current supply is 6V.

Conclusion: For the R-C circuit used in this experiment, the capacitance of the capacitor, C is 3.37mF while the time constant, is 20.9s. The current I can be expressed as: . Note: For the graph, it should be a straight line passing through the origin.