Physic II - M2A1 Experiment: Capacitors

PART I - The Parallel Plate Capacitor

The most easily illustrated kind of capacitor is a parallel plate capacitor.

· Start the simulation "Capacitor Lab" (if you haven't done so already) by clicking on the image below: Please follow the link below:

http://phet.colorado.edu/sims/capacitor-lab/capacitor-lab_en.jnlp

· The "Introduction Tab" is selected by default, but check to make sure you are working with this tab.

· Use the checkboxes to display

· Capacitance

· Plate Charge (Top)

· Stored Energy

· Voltmeter

· Set the battery voltage at or close to 1.0 V.

· Use the mouse and the plate separation and plate area controls to adjust the capacitor plate area and plate separation.

Observe the effect of adjusting the capacitor plate area and plate separation on the capacitance of the system. Record you observations in your laboratory notebook. These observations should include general observations expressed in English sentences as well as observations that are numerical in nature. In other words, take data. The expression for the capacitance of the parallel plate capacitor is C = Aε0/d. Do your observations support the idea that the capacitance for a parallel plate capacitor will increase linearly with increasing area? Do your observations support the idea that increasing the plate separation (d) will cause the capacitance to decrease as 1/d (read as "one over d")? Use the instructions from M1A1 Experiment: Charges and Fields on using a spreadsheet to plot data and for inserting a trend line into such plots. Again use the linear model for the changes in capacitance with respect to plate area (A) and the power law option for the plate separation (S).

Part II - Capacitors in Series and in Parallel

The rules for determining the effective capacitance of a network of capacitors were derived in the Module Notes. You should take the time to convince yourself that these expressions were derived correctly and that you understand them. Still, we would like to confirm these rules with direct observations and data.

· Select the tab labeled "Multiple Capacitors."

· From the meters, select the "Total Capacitance" display.

· From the circuits, select "3 in Series."

· Set the battery value to some reasonable non zero value.

· Use the slider to change the capacitance values so that no two capacitors are equal.

Try several different sets of values for the 3 capacitors. In your laboratory notebook, record the values of the individual capacitors and the total capacitance of the system. Calculate the equivalent capacitance of the three series capacitors using the appropriate formula. Does the measured value of total capacitance of the system match the calculated value? Now:

· From the circuits, select "3 in parallel."

· Set the battery voltage to some reasonable non zero value.

· Use the slider to change the capacitance values so that no two capacitors are equal.

Once again, try several different sets of values for the three capacitors. Record theses values and the total capacitance in your laboratory notebook. Calculate the equivalent capacitance of the parallel capacitors using the appropriate formula. Does the measured value of total capacitance of the system match the calculated value?

PART III - Dielectrics in Capacitors

· Select the tab labeled "Dielectric." Set the battery voltage to some reasonable non zero value.

· Under View, select "Plate Charges."

· Display the capacitance and the stored energy meters.

· Use the "Disconnect Battery" button to isolate the capacitor.

Observe what happens to the value of the capacitance in this isolated capacitor as you slide the dielectric block into and back out of the gap between the plates. In general, does the addition of a dielectric material to a capacitor increase or decrease the capacitance? What happens to the stored or potential energy of the system? Motion is related to potential energy. In a gravitational system, a mass released from some height seeks a lower potential energy state, e.g., it falls. It will help you to solidify this understanding of the connection between potential energy and motion if we apply the same consideration to the potential energy of the dielectric system. The energy stored (U) in the capacitor is given by

U = Q²/2C

For an isolated capacitor, the charge (Q) on the plates is not going to change. Inserting a dielectric material increases the capacitance of the device, and thus, lowers the potential energy of the system. Do your observations support this line of reasoning? Use what you know about the connection between potential energy and motion to answer this question. Would a dielectric block, free to move, positioned half in and half out of the capacitor, be (1) pulled inward or (2) expelled from between the plates?