- Purpose Objectives To Investigate The Physical Properties Of Capacitors To Model A Parallel Plate Capacitor To Correctly 1 (71.05 KiB) Viewed 47 times
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The ability to control electricity is what underlies our entire modern society. Included in that is the ability to store electrical energy (and other quantities) in devices called capacitors. The teacher will first discuss some basic issues regarding electric energy and potential (voltage). Then we will do the following: Activity #1: Physical Properties of Capacitors To illustrate the basics of capacitance, we will simulate a parallel-plate capacitor (the simplest type). A pair of students will come to the front table as shown below. They will stand on opposite sides of the table. They will grasp opposite ends of a spring and pull it taught. Suppose the student on the left represents a positive charge and the student on the right represents a negative charge. 1. What does the spring represent in our simulation? (Hint: it can represent several, though related, quantities) 2. Sketch electric field lines (complete with arrows to indicate direction) in the figure at left. 3. Now more pairs of students are brought to the front table. Is there a limit to the number of pairs we can put around the table? If so, why?
4. It will take increasing amounts of electric energy to put electric charges on opposite sides of the plates. Besides the explanation cited in #3 above, suggest the reason why. 5. The springs don't pull the students into each other. Why not? 6. Similarly, the forces between opposite charges are attractive. Why don't the charges smash into each other for a real capacitor? 7. Suppose now the students use different springs. What might this represent? 8. Finally, construct an equation (with words) that shows the capacitance of a parallel-plate capacitor:
Activity #2: Electrical Properties of Capacitors Next, we will demonstrate electrical properties of capacitors (and relate them to their physical properties) by use of some simple circuits. 1. The instructor will construct a simple capacitor circuit as follows: Figure 2-1. A capacitor circuit with a double switch. The switch allows the capacitor to first be charged by the power supply, then discharged through the light bulb. As shown, the switch is open. 2. What is the meaning of each circuit symbol? Write each answer in the spaces provided. 4+ == 3. The initial voltage across the capacitor is 0 V. The instructor sets the voltage of the power supply to V. 4. The instructor closes the switch so that the capacitor and power supply are connected. Briefly describe what, if anything, happens to the light bulb.
6. Next, the instructor flips the switch so that the power supply is removed from the circuit and the light bulb is now connected with the capacitor. Briefly describe what, if anything, happens to the light bulb. 7. The instructor now resets the circuit and adjusts the voltage from the power supply to V. 8. Before the instructor reconnects the power supply to the capacitor, predict the value of the voltage that will be across the capacitor after waiting a few V. seconds: 9. The instructor reads the voltage across the capacitor. Does the result agree with your prediction? Briefly explain the reason for the value of the result. 10. Before the instructor reconnects the light bulb with the capacitor, predict what will happen to the light bulb:
11. The instructor reconnects the light bulb and the capacitor. Does the result agree with your prediction? Briefly explain the reason for the result. 12. The instructor resets the power supply to its original voltage ( now increases the value of the capacitance. Briefly describe what happens to the light bulb after it is connected to the circuit: V), but
13. It turns out that the amount of time it takes for the light to fade away is related to the amount of charge stored on the capacitor. (a) How does voltage affect the amount of charge stored? (b) How does capacitance affect the amount of charge stored? (a) (b) 14. Construct an equation (with words) that shows the relationship between charge, voltage, and capacitance: