Lab 5

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Rensselaer Polytechnic Institute *

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1200

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Electrical Engineering

Date

Dec 6, 2023

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pdf

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Uploaded by CaptainAlligator3603

PHYS-1200/1250 Lab Manual Name_________________________ Section__________ 1 24A – Capacitance Concepts . A capacitor is represented in circuits as two parallel lines placed close to one another. (=>) Calculating capacitance from geometry 1. Consider two metal plates separated by a distance d . Shown schematically to the right. The plates have cross-sectional area A . The top plate is charged positively to + Q and the bottom to – Q . The area charge density on the capacitor plates is σ. a) Where do you expect the charge on the plate to be? A) Mostly on the inside (facing) surfaces, B) Mostly on the outside surfaces. C) Distributed throughout the metal. b) Explain your answer: 1a)___________ c) Is there a field inside the metal? A) Yes. B) No. 1c)___________ d) Explain. e) What is the area charge density σ on the bottom surface of the top plate in terms of some of the given variables charge Q, plate area A, and plate separation d ? e) ___________ A A capacitor is a device in which equal and opposite charge is stored on two plates, resulting in the storage of energy in the form of electric field. The capacitance is a property of the geometry and of the plates and the material between the plates. The capacitor is usually charged by supplying electrical energy from a battery to separate positive and negative charges onto one plate and the other. The capacitance serves to relate the charge stored on a pair of plates to the potential difference required to store that charge. Q C V =  Eq. 24a

PHYS-1200/1250 Lab Manual Name_________________________ Section__________ 2 2) Consider a disk-shaped Gaussian surface (right circular cylinder or “soup - can”) with the top surface inside the metal of the top plate and the bottom surface between the plates, as suggested by the sketch. Take the top surface area of the can to be A ’ . a) What is the charge Q ’ contained in the soup -can in terms of charge Q, plate area A, plate separation d , and can top area A ’ ? a)____________ b) Use Gauss’ Law to find the field between the plates in terms of charge Q ’ and top area A ’ ? b)____________ c) Now that you know the field, find the potential difference between the plates. c)____________ d) What is the capacitance (Q/ Δ V) in terms of d, A, and physical constants? d)____________ e) What must the area of a certain capacitor be if it has plate separation of d = 2x10 -6 m and a voltage of Δ V = 1V applied across the capacitor plates results in charge storage of Q = 10 -12 C. Show your work. e)____________ f) If the distance between the plates of the capacitor is halved ( d/2 ) while keeping the charge Q constant, does the potential difference change? (Yes or No) f)____________ g) If so what is the new potential difference? g)___________

PHYS-1200/1250 Lab Manual Name_________________________ Section__________ 3 24B – Simulation: Charge, Potential, Capacitance In this module you will explore the behavior of a simulated capacitor and check to see if the creators of the simulation did it correctly. This lab uses the simulation at the website: https://phet.colorado.edu/en/simulation/capacitor-lab-basics Some of the activities here have been adapted from the “Capacitor Lab Basic” document by Trish Loeblein at the Colorado PhET site. • Start the simulation and select the capacitance icon. The screen should look like this: • Enable all of the measurement capabilities of the simulation by selecting all of the boxes. Drag the voltmeter from its storage and touch the two leads to upper and lower capacitor plates. 1. Find the conditions (Battery Voltage, Plate Area, Plate Separation) that produce the largest capacitance. (Some of these parameters might not be relevant.) a. Describe these parameters (e.g. – “largest plate spacing) in words.

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