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# charging a capacitor at a constant rate(C08)

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Introduction

School True Light Girls' College Mark Name Tam Teresa (???) Class (Class No.) 6S (22) /10 Group No. 3 Date (dd-mm-yy) 20-05-2009 ============================================================= AL Physics TAS - Experimental Worksheet C08 - Charging a capacitor at a constant rate Time Allowed 2 periods Objective To investigate how the charge on a capacitor is related to the potential difference applied across it by charging the capacitor at a constant rate. Apparatus * Capacitor (electrolytic type) 500�F * Microammeter 100�A * Potentiometer 100k? * Clip component holder * Stop-watch * CRO (Cathode Ray Oscillator) * Connecting leads Theory From definition, the capacitance C of a capacitor is found from C=Q/V where Q is the charge stored on the capacitor and V is the potential difference across it. ==> Q = CV ==> = C If a capacitor is charged up at a constant rate, i.e. = I , where I is a constant , then is also constant. Hence the potential difference across the capacitor increases linearly with time. Procedure 1. Connect up the circuit as shown in Fig. ...read more.

Middle

across the capacitor is directly proportional to time (V?t). Analysis For a constant current, the charge delivered to the capacitor is directly proportional to time. Q?t (Q = It) From the result of the experiment, the p.d. across the capacitor is directly proportional to time. V?t As a result, the charge delivered to the capacitor is directly proportional to the p.d. across it. Q?V Assumptions 1. The capacitor has been fully discharged before every charging. 2. The charging current is constant when the capacitor is charging. 3. The reading of times taken by the observer is exactly the same as the times for the p.d. across the capacitor to reach 1V, 2V, 3V, etc. Precautions of the experiment 1. A correct scale of ammeter should be chosen. The measured value of current may be out of range of ammeter if inappropriate ammeter is chosen. 2. As we use the stop-watch to take the times for the p.d. across the capacitor to reach 1V, 2V, 3V,etc, the reaction time of the observer plays large part. Therefore, we should have the same person for timing to prevent great time lag within same p.d. across capacitor. 3. ...read more.

Conclusion

across the capacitor to reach 1V, 2V, 3V, etc. 3. The capacitor may not fully discharged before charging, thus affect the results of times for the p.d. across the capacitor to reach 1V, 2V, 3V, etc. 4. It is too difficult to observe both stop-watch and reading of CRO trace. Thus, there may exist time lag and affects the results. Improvements 1. "Dry Runs" before actual measurement. 2. Use advance stop-watch which can take several readings at once. 3. Use video camera to capture the position of horizontal CRO trace against time. Thus, the reaction time of observer does not play a part in the experiment. The results of times are much more accurate. Limitations 1. The laboratory does not have advance stop-watch. 2. We do not have enough time to do several more times to take the average value of time to make the reaction time less significant. Comments This experiment has errors that make our experimental results inaccurate. Fortunately, I and my partner do cooperate well to minimize the error. Also, we conduct the experiment safely and the process is holistically fluent. Conclusion The charge on a capacitor is directly proportional to the potential difference applied across it by charging the capacitor at a constant rate. ...read more.

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