Section : On Your Own As seen, the bigger the capacitor, the greater its capacit

Question

Section : On Your Own As seen, the bigger the capacitor, the greater its capacitance. However, capacitance does not you've depend only on size. To illustrate, examine the two large circular plates that make up the "Variable Capacitor Setup" The two circular plates are supported by two clear posts that can be moved closer and farther apart. create a capacitor, place the two metal plates flat against each other, separated only by the plastic "dots" on one of the plates. Question #1. Without the plastic dots, the two metal plates will touch. Why is it important for them not to touch? With the two metal plates flat against each other (and without you touching the plates, so you aren't part of the capacitor), use the LCR meter to measure the capacitance of the two-plate capacitor by connecting the probes to the screws on the side of the clear posts. You should find that the capacitance is very small compared to the capacitance of the four capacitors observed previously, despite its relatively enormous size. To see why, slowly move the two plates away from each other while the LCR meter stays connected to the capacitor (so you can see how the plate separation influences the capacitance). You should find that the capacitance goes down when the plate separation goes up. The small capacitors you used before have the plates very, very close to each other. To prevent the plates from touching, a real capacitor uses a thin sheet of paper or other insulator between the plates. This raises the question as to whether the presence of the insulator affects the capacitance. Go ahead and insert the cardboard between the two plates. You may have to separate the two plates a little to allow the cardboard to be placed between the plates but be sure to keep the separation distance the same when you compare your measurements with and without the cardboard. 2. 3. 4. Question #2. What happens to the capacitance when a cardboard sheet is inserted between the two plates (displacing some of the air with cardboard)? Provide the values (with units) with and without the cardboard. In "capucitance language",the matertial between the plates is known as the dielectric. A dielectric that doubles the capacitance over nothing at all is said to have a dielectric constant of triples the capacitance over nothing at all is said to have a dielectric constant of 3 2. A dielectric that Question #3. Estimate the dielectric constant of the cardboard. what Show your work. (note: think about

Explanation / Answer

1. in case of no plastic dots, the two plates will touch and would form a closed circuit and current would flow easily, no charge would be stored anywhere, so there would be no capacitance.

2. when an insulator (cardboard) is placed between the plates of a capacitor, the electric field due to the charged plates induces a net dipole moment in the dielectric. this effect is called polarisation which induces a field in the opposite direction. the net electric field inside the insulator and hence the potential difference between the plates are reduced. consequently the capacitance increases. C=kCo(C = new capacitance after inserting insulator, Co= capacitance without insulator, k=dielectric constant of the material). its unit is farad (1 farad=1coloumb/1volt)

3. estimated value of dielectric constant of cardboard lies in range 2.0-2.6. so your capacitance can increase 2.0-2.6 times.

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