CH26 Capacitors Lab In this experiment we will determine how voltages are distributed in capacitor circuits and explore series and parallel combinations of capacitors. The capacitance is a measure of a device’s ability to store charge. Capacitors are passive electronic devices which have fixed values of capacitance and negligible resistance. The capacitance, C, is the charge stored in the device, Q, divided by the voltage difference across the device, V: C = Q/V. (1) The SI unit of capacitance is the farad, 1 F = 1 C/V, In general, the capacitance can be calculated knowing the geometry of the device. For most practical devices, the capacitor consists of capacitor plates which are thin sheets of metal separated by a dielectric, insulating material. For this reason, the schematic symbol of a capacitor is has two vertical lines a small distance apart (representing the capacitor plates) connected to two lines representing the connecting wires (or leads). There are two ways to connect capacitors in an electronic circuit - series or parallel connection. In a series connection the components are connected at a single point, end to end as shown below: C2 C1 For a series connection, the charge on each capacitor will be the same and the voltage drops will add. We can find the equivalent capacitance, Ceq, from Q · 1/Ceq = V = V1 + V2 = Q/C1 + Q/C2 = Q [1/C1 + 1/C2] (3) So SERIES: 1/Ceq = 1/C1 + 1/C2 (4) In the parallel connection, the components are connected together at both ends as shown below: C1 C2 For a parallel connection, the voltage drops will be the same, but the charges will add. Then the equivalent capacitance can be calculated by adding the charges: CeqV = Q = Q1 + Q2 = C1V + C2V = [C1 + C2] V So PARALLEL: CeqC1 + C2 NOTE: Here we will use AC, the voltage is actually V = I/C, where I is the current and is the angular frequency. We don’t actually measure I or here, and the analysis is the same. This is covered in more detail in the chapter on AC circuits. (5) (6) Procedure This lab uses a Protoboard to build and test circuits. Connect one of the AC power supply outputs to the 2nd vertical line. Connect the other of the AC power supply outputs to the 5th vertical line. 1. Turn on the power supply and set the AC voltage to 10 V. Measure the accurate voltage with the multimeter and record it below: Vac = ___________________V The protoboard has vertical connections in the lines arranged in pairs and horizontal connections in the blocks of five. 2. Connect two 0.1 F capacitors in series. Measure V1 (across C1 which is the one in the box) with the meter and record it below. V1 (measured) = ___________________ V Compute the expected value of V1 using Vac, the values of C1 and C2 with equations 3 and 4. V1 (expected) = ___________________ V % difference = |measured - expected| / measured x 100 % = __________ 3. Connect a third 0.1 F capacitor in parallel with C1. Compute the equivalent capacitance of C1 (the 2 capacitors in the box). Ceq = _________ F. Measure and compute the voltage across the equivalent capacitance (the 2 capacitors in the box). V1 (measured) = ___________ V, V1 (expected) = _________V, % difference = __________ 4. Now remove the third capacitor and replace it with a 0.01 F capacitor. Compute the equivalent capacitance of C1 (the 2 capacitors in the box). Ceq = _________ F. Measure and compute the voltage across the equivalent capacitance. V1 (measured) = ___________ V, V1 (expected) = _________V, % difference = __________ 5. Now connect the 0.1 F and the 0.01 F capacitor in series as C1, followed by C2 in series. Compute the equivalent capacitance of C1 (the 2 capacitors in the box). Ceq = _________ F. Measure and compute the voltage across the equivalent capacitance. V1 (measured) = ___________ V, V1 (expected) = _________V, % difference = __________