Name:____________________ Capacitor Lab Period:_______ Purpose: Investigate the charging of a capacitor. Background Information: A capacitor is a device that stores electrical charge. They are made of two conducting plates separated by air or another insulating material, often referred to as a dielectric. The capacitance of a capacitor is dependent upon the nature of the dielectric substance, the area of the plates, and the distance between the plates. The illustrations below show a circuit that consists of a capacitor, voltage source/power supply, ammeter to measure the flow of charge, and a resistor. A resistor is a simple device that resists or slows the flow of charge. All these components form one continuous loop; they are connected in series. When we measure the amount of charge flowing per time, this is called current. One coulomb per second = 1 Ampere. When our circuit is complete and turned on, charge accumulates on each plate of the capacitor, but no current flows through it due to the dielectric separating the plates. As charge accumulates in the capacitor, the electric potential difference increases until it equals the power supply voltage. At this point no additional charge can be transferred. The capacitance of any capacitor can be found by: C = q/V Where C is the capacitance in Farads, q is the charge in Coulombs, and V is the electric potential difference in volts. In this experiment we will measure the capacitance of the capacitor by finding the charge stored, and the voltage used. Materials: 1000µF capacitor 10 kΩ resistor 33 kΩ resistor power supply Digital multimeter connecting wires stopwatch Procedure: 1. Locate the power supply which will be our source of electric potential difference. Plug it in, and turn it on. Push the 0-24V DC button. Adjust the knob to read 15.0 volts. Turn it off for now. 2. Using the other components, create the circuit shown in the diagram and photo below. Try to follow the diagram and connection the components in order as you go along. See hints below too. Multimeter to measure current Voltage A source 33 kΩ resistor capacitor On the power supply, use the red and black 0-24V ports when connecting wires to it. On the digital multimeter that we’ll use to measure current, use the red and black ports, push in the DCA button, and turn the knob to 20mA. If you are using magnetic tipped wires, you might need to use alligator clips to attach to the resistor ends. The magnets will stick to the alligator clips. BE SURE to connect the capacitor in the right direction. There will be some indication of + or – on the capacitor. Make sure that end goes to the appropriate sign on the voltage source. There is no +/- end for resistors, they can be connected either way. The color of the wires makes no difference. The colors are just for organizing 3. Only turn on the voltage source power when you are ready to begin. When everything is connected, turn on the voltage and take an initial measurement of the charge flowing. This will be the time =0 reading. Continue every 5 seconds until the end of the chart, or there is no noticeable change in the current. Use a stopwatch if needed. 4. When finished, you can remove the capacitor and use a spare piece of wire to discharge it. Simply connect one end of the capacitor to the other with a wire. The accumulated charge will flow back. With our relatively small capacitors this is safe. 5. Repeat the procedure with the 10 kΩ resistor. Time (s) 33 kΩ resistor 10 kΩ resistor Current mA Current A Current mA Current A 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 Analysis/Calculations: 1. Using your data, plot two graphs for current ( in amps) as a function of time (in seconds). From the start button, programs, open Graphical Analysis. Input all of your X values into the appropriate column, double click on the X at the top of the column to name that axis, input units too. Input all of your Y values, double click on the Y at the top of the column to name that axis, input units too. 2. If you found the area under this curve, what would that represent? Consider the product of the two axes. Remember that 1 Ampere = 1 C/s. You would get a unit of charge. Find the charge stored in the two capacitor trials. In graphical analysis, go to analyze, then integral. Print one of these graphs. 3. Calculate the capacitance of your capacitor based on the charge stored, and the voltage applied. Show formulas, math work, and units below. 4. Compare your capacitance value to the value printed on the capacitor itself. Find a % difference. Show work below. 5. Calculate the energy stored for each case. Use the capacitance value you determined. Show formulas, math work, and units below. 33 kΩ resistor 10 kΩ resistor trial trial Charge stored, C Calculated capacitance, F Capacitance % difference E stored, J Questions: 1. Hypothesize why the charge flow starts at a high value, then drops towards zero. 2. Look at your data for the two different resistors. Hypothesize about the purpose of the resistor in a circuit. 3. Which would store more energy: your fully charged capacitor from this lab, or your 1.0 kg physics book lifted 1.0 m above the floor? Justify your answer. 4. Is a capacitor the same as a battery? Explain why or why not.
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