Go to the simulation at http://phet.colorado.edu/en/simulation/capacitor-lab (or

Question

Go to the simulation at http://phet.colorado.edu/en/simulation/capacitor-lab (or http://phet.colorado.edu/sims/capacitor-lab/capacitor-lab_en.jnlp). On a Windows machine, you will probably need to click "Download" and then "Open". [If this will not run on your computer, please try another browser or computer. It should work on campus lab computers and the computers in the Learning Resource Center in Osmond 207.]

Use the "Introduction" Tab. Make sure the Capacitance Meter & Plate Charge Views are selected and the battery is turned on (set to 1.5 V).

(a) Vary the plate area and separation in the simulation. What do you find about the Capacitance of the plates?

A. To maximum C, have a small area and small separation

B. To maximum C, have a large area and large separation     

C. To maximum C, have a large area and small separation

D. To maximum C, have a small area and large separation

(b) As you vary C by varying the plate area and separation (while connected to the battery), what happens to the charge and potential difference across the plates? (You can use the voltmeter tool to measure potential difference if you aren't sure of what its value will be.)

A. Varying C changes Q not but V

B. Varying C doesn't change Q or V     

C. Varying C changes V and Q

D. Varying C changes V but not Q

(c) Now disconnect the battery (there's a white box above the battery for this purpose). What about the capacitor changed?

A. Didn't change either Q or V

B. Changed V not but Q     

C. Changed both Q and V

D. Changed Q not but V

(d) With the battery disconnected, separate the plates and/or change the plate area. What about the capacitor changes?

A. Neither Q nor V changes

B. Changes Q not but V     

C. Changes V not but Q

D. Changes both Q and V

(e) You want to maximize the stored energy on the capacitor (turn on the "Stored Energy" meter). What's the largest stored energy you can achieve with this setup? (Think outside the box on how to do this - it's not a trivial thing to determine.) [Note: you should still be in the "Introduction" tab.]

U =  J

(f) Now go to the "Dieletric" Tab. Turn on the "Capacitance"meter and "Plate Charge" view, and turn on the battery (1.5 V). You can also use the Voltmeter to measure the voltage across the two plates. Now insert the dielectric between the plates of the capacitor. What happens to the capacitor?

A. C increases, Q increases, V decreases

B. C increases, Q decreases, V remains fixed     

C. C decreases, Q increases, V remains fixed

D. C doesn't change, Q increases, V decreases

E. C decreases, Q decreases, V remains fixed

F. C decreases, Q increases, V decreases

G. C increases, Q increases, V remains fixed

H. C doesn't change, Q increases, V remains fixed

I. C doesn't change, Q decreases, V remains fixed

(g) What happened to the dielectric?

A. Dielectric became positively charged

B. Dielectric became negatively charged     

C. Dielectric became polarized (but still neutral)

(h) Remove the dielectric and then disconnect the battery. Then insert the dielectric again. What happens to the capacitor when you insert the dielectric?

A. C increases, Q decreases, V doesn't change

B. C increases, Q decreases, V doesn't change     

C. C increases, Q decreases, V decreases

D. C decreases, Q increases, V increases

E. C decreases, Q doesn't change, V decreases

F. C increases, Q doesn't change, V decreases

G. C decreases, Q decreases, V doesn't change

H.C decreases, Q decreases, V decreases

(i) You want to maximize the stored energy on the capacitor (turn on the "Stored Energy" meter). What's the largest stored energy you can achieve with this setup? [Note: you should still be in the "Dielectric" tab.] This is a bit of a challenge and requires some playing around to hit upon the solution (it's not trivial). U =  J

Explanation / Answer

a)
C = ebsoleneo*A/d
To maximise C, A should be large and d should be small
Answer: C. To maximum C, have a large area and small separation

b)
Since battery is connected, V will not change
Q = C*V
Varying C will change the Q
Answer: A. Varying C changes Q but not V

c)
Once battery is disconnected, charge on it will not chnage
V = Q/C
So, changing Q now will change V
Answer: B. Changed V but not Q

d)
Answer: C. Changes V not but Q
same as above

Only 4 parts at a time please

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