Rc time constant experiment

Physics Laboratory Manual n L ABOR AT ORY Loyd 33 The RC Time Constant OBJECTIVES o Investigate the time needed to discharge a capacitor in an RC circuit. o Measure the voltage across a resistor as a function of time in an RC circuit as a means to determine the RC time constant. o Determine the value of an unknown capacitor and resistor from the measurements. EQUIPMENT LIST . Voltmeter (at least 10 MO resistance-digital readout), laboratory timer . Direct current power supply (20 V), high quality unknown capacitor (5–10 mF) . Unknown resistor (approximately 10 MO), single-pole (double-throw) switch . Assorted connecting leads THEORY Consider the circuit shown in Figure 33-1 consisting of a capacitor C, a resistor R, a source of emf ", and a switch S. If the switch S is thrown to point A at time t ¼ 0 when the capacitor is initially uncharged, charge begins to flow in the series circuit consisting of ", R, and C, and it flows until the capacitor is fully charged. The current I has an initial value of "/R and decreases exponentially with time. The charge Q on the capacitor begins at zero and increases exponentially with time until it becomes equal to C". The equations that describe those events are COPYRIGHT ª 2008 Thomson Brooks/Cole Q ¼ C" ð1 # e#t=RC Þ and I ¼ "=R e#t=RC ðEq: 1Þ The quantity RC is called the time constant of the circuit, and it has units of seconds if R is in ohms and C is in farads. After a period of time that is long compared to the time constant RC, the charge Q is equal to C", and the current in the circuit is zero. If switch S is now thrown to position B, the capacitor discharges through the resistor. The charge on the capacitor and the current in the circuit both decay exponentially while the capacitor is discharging. The equations that describe the discharging process are Q ¼ C" e#t=RC and I ¼ "=R e#t=RC ðEq: 2Þ ª 2008 Thomson Brooks/Cole, a part of TheThomson Corporation.Thomson,the Star logo, and Brooks/Cole are trademarks used herein under license. ALL RIGHTSRESERVED.No part of this work covered by the copyright hereon may be reproduced or used in any form or by any meansçgraphic, electronic, or mechanical,including photocopying, recording, taping,web distribution, information storage and retrievalsystems,or in any other mannerçwithout the written permission of the publisher. 329 330 Physics Laboratory Manual n Loyd A C B ! R Figure 33-1 Simple series RC circuit. The current in the discharging case will be in the opposite direction from the current in the charging case, but the magnitude of the current is the same in both cases. Consider the circuit shown in Figure 33-2 consisting of a power supply of emf ", a capacitor C, a switch S, and a voltmeter with an input resistance of R. If initially the switch S is closed, the capacitor is charged almost immediately to ", the voltage of the power supply. When the switch is opened, the capacitor discharges through the resistance of the meter R with a time constant given by RC. With the switch open, the only elements in the circuit are the capacitor C and the voltmeter resistance R, and thus the voltage across the capacitor is equal to the voltage across the voltmeter. It is given by V ¼ " e#t=RC ðEq: 3Þ Rearranging and taking the natural log of both sides of the equation gives Inð"=VÞ ¼ ð1=RCÞt ðEq: 4Þ If the voltage across the capacitor is determined as a function of time, a graph of ln("/V) versus t will give a straight line with a slope of (1/RC). Thus RC can be determined, and if R the voltmeter resistance is known, then C can be determined. If an unknown resistor is placed in parallel with the voltmeter, it produces a circuit like that shown in Figure 33-3. The capacitor can again be charged and then discharged, but now the time constant will be equal to RtC where Rt is the parallel combination of R and RU. If the relationship between R, RU, and Rt is solved for RU, the result is RU ¼ RRt R # Rt ðEq: 5Þ Therefore, a measurement of the capacitor voltage as a function of time will produce a dependence like that given by Equation 4, except that the slope of the straight line will be (1/RtC). Thus if C is known and RtC is found from the slope, then Rt can be determined. Using Equation 5, RU can be found from R and Rt. S # Power Supply ! " C Capacitor Voltmeter R Figure 33-2 An RC circuit using a voltmeter as the resistance. Laboratory 33 n The RC Time Constant S 331 # Power Supply ! " C Capacitor Voltmeter R RU Figure 33-3 RC circuit using voltmeter and RU in parallel.