Aim: To determine dielectric constant of a dielectric material by RC charging and discharging method.
Apparatus: 5V DC power supply, RC charging and discharging experimental setup consisting of digital stop clock 0.1 second resolution, digital dc voltmeter, set of resistors and set of capacitors of known dimensions, patch cords, etc.
Formula: The dielectric constant of a dielectric material of the given capacitor:
where, d is the thickness of the dielectric material (m)
A is the Area of the dielectric foil (m2)
T1/2 is the Time require for 50% charge or discharge (s)
0 is the permittivity of free space (F/m)
R is the Resistance of resistor in series with the capacitor (Ω)
Procedure: The required circuit is as shown in figure. The supply points are switched on. The 5V power supply is connected to the points A&B. One resistor (say R=100 k) among the given resistors is connected between the points M & N. Similarly one capacitor (say C2) among the given capacitors is connected between the points S & T. To begin with, the toggle in the switch S2 is set to halt position. The timer is set to zero by pressing the reset button. The digital voltmeter (DVM) is connected between the points P & Q in the circuit at which time it should read zero.
Charge mode studies: To start with
The toggle of the switch S1 is set to charge mode.
The toggle in switch S2 is flicked to start position at which instant the capacitor begins to get charged to higher voltage and the timer starts counting simultaneously.
Immediately start noting down the voltage readings “V” in the voltmeter at every 5 seconds interval, from 0th second, until Voltage (V) becomes partially constant. (i.e., say when two consecutive readings remain same). The readings for Voltage are entered in the tabular column under charge mode.
Discharge mode studies: Then
The toggle of the switch S2 is changed to halt position and the timer is reset to read zero,
The toggle in switch S1 is changed to discharge mode, and simultaneously,
The toggle switch in switch S2 is flicked to start position.
Immediately, start noting down the reading for V at every 5 secs interval till V becomes partially constant i.e., over two consecutive observations.
The readings for V are tabulated under discharge mode for C in which for T=0, V=maximum voltage value attained during charge mode studies.
From the readings, graph of (Time vs. Voltage) is plotted. The charge mode curve & discharge mode curve intersect at the point P, the position of which to the time axis i.e., T_(1/2) in secs is found out. By the given values, the value of the dielectric constant is calculated using the given formula.