PHYS 012A Lecture Notes - Lecture 12: Farad, Electric Field, International System Of Units

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12 Jun 2018

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Chapter 26: Capacitance

Capacitors store electric charge

When you keep a negative and a positive charge apart, they start to store

charge. But when you want a current to flow, you can connect both

charges, and they will flow from negative to positive or vice versa. The

capacitor is made of two conductors called plates and the plates carry

charges of equal magnitude and opposite directions.

Definition of Capacitance

The capacitance, C of a capacitor is the ratio of the magnitude of the charge on either conductor to the

potential difference between the conductors.

The SI Unit is the Farad → 1C/1V

Parallel Plate Capacitor

- Each plate is connected to a terminal of the battery, and the battery is the source of a potential

difference.

- If the capacitor is initially uncharged, the battery establishes an electric field in the connecting

wires. The field applies a force on electrons which causes them to move to the negative plate

until equilibrium is achieved

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Capacitance of a Spherical Capacitor

Capacitance of a Cylindrical Capacitor

DV = -2ke



ln (b/a)



= Q/l

The capacitance is

( )

2 ln /

CV k b a



Circuit Symbols

- Represents actual circuit. Lines represent wires.

- Battery’s positie terial is idiated y the loger ire

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Document Summary

When you keep a negative and a positive charge apart, they start to store charge. But when you want a current to flow, you can connect both charges, and they will flow from negative to positive or vice versa. The capacitor is made of two conductors called plates and the plates carry charges of equal magnitude and opposite directions. The capacitance, c of a capacitor is the ratio of the magnitude of the charge on either conductor to the potential difference between the conductors. The si unit is the farad 1c/1v. Each plate is connected to a terminal of the battery, and the battery is the source of a potential difference. If the capacitor is initially uncharged, the battery establishes an electric field in the connecting wires. The field applies a force on electrons which causes them to move to the negative plate until equilibrium is achieved. Battery"s positi(cid:448)e ter(cid:373)i(cid:374)al is i(cid:374)di(cid:272)ated (cid:271)y the lo(cid:374)ger (cid:449)ire.

Related Questions

A parallel-plate capacitor is connected to a battery of electric potential difference V. If the plate separation is decreased, do the following quantities increase, decrease, or remain the same:

(a) the capacitor’s capacitance,

(b) the potential difference across the capacitor,

(d) the energy stored by the capacitor,

(e) the magnitude of the electric field between the plates,

(f ) the energy density of that electric field?

harlequingnat799

The SI unit of capacitance is a derived unit is known as the farad (named after Michel faraday) One farad is defined as one coulomb per volt 1F=1C/1V write an equation that relates capacitance, charge, and potential this equation for a charge. one coulomb per volt: F=1C/1V. Based on the definition of the unit. write an equation that relates capacitance, charge, and potential. and then solve this equation for the charge.

An engineer needs a 1.0 μF parallel-plate capacitor with a spacing of 0.050 mm of plates. She will use the capacitor in the design of a portable electronic device.

a. What would the area of the plates need to be to make this capacitor? Assume te εo = 8.85 *10^-12CNm).

b. Discuss the practicality of this capacitor in the context of its intended use.

c. If the portable electronic device is designed to use a 1.5 V battery, what is the maximum charge the capacitor can store?

cerisekoala77

To understand the meaning of capacitance and ways of calculating capacitance.

When a positive charge q is placed on a conductor that is insulated from ground, an electric field emanates from the conductor to ground, and the conductor will have a nonzero potential V relative to ground. If more charge is placed on the conductor, this voltage will increase proportionately. The ratio of charge to voltage is called the capacitance C of this conductor: C=q/V.

Capacitance is one of the central concepts in electrostatics, and specially constructed devices called capacitors are essential elements of electronic circuits. In a capacitor, a second conducting surface is placed near the first (they are often called electrodes), and the relevant voltage is the voltage between these two electrodes.

This tutorial is designed to help you understand capacitance by assisting you in calculating the capacitance of a parallel-plate capacitor, which consists of two plates each of area A separated by a small distanced with air or vacuum in between. In figuring out the capacitance of this configuration of conductors, it is important to keep in mind that the voltage difference is the line integral of the electric field between the plates.

Assume that charge -q is placed on the top plate, and +q is placed on the bottom plate. What is the magnitude of the electric field E between the plates?
What is the voltage V between the plates of the capacitor?

beigemouse1

PHYS 012A Lecture Notes - Lecture 12: Farad, Electric Field, International System Of Units

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