Conceptual Physics

(Sean Pound) #1

30,000 m/s (70,000 mi/h). The ions escape through small holes in the rear plate. (To
prevent the ions from being attracted back into the engine, a beam of electrons is fired
into the ion exhaust to neutralize it.)


NASA tested this engine system on the Deep Space 1 mission. Deep Space 1 was shut
down on December 18, 2001, after successfully validating the performance of the ion
propulsion system. The drive had provided more than 16,000 hours of thrust, using only
72 kg of xenon gas. This was less than 10% of the overall mass of the spacecraft. An
equivalent chemical rocket drive would have needed 720 kg of fuel to generate the
same amount of propulsion. This is equivalent to the difference between launching a
person and launching a small car.


The ion propulsion system is now in use in satellites that orbit the Earth, where it is
used to “tune” their orbits. Photovoltaic panels supply the energy required to ionize the


gas and charge the plates. An electric-field rocket engine


Electron bombardment ionizes xenon
gas
Field accelerates ions to 30,000 m/s
Low mass, high velocity system

23.11 - Conductors in electrostatic equilibrium


Electrostatic equilibrium: In


an isolated conductor, excess


charges quickly achieve a


state where there is no net


motion of charge.


An isolated, charged conducting object has several
interesting and perhaps unexpected properties. (By
isolated, we mean the object is attached neither to a
source of what is called a potential difference, such
as a battery, nor to a ground.) The excess charge on
such an object will rapidly reach a state called
electrostatic equilibrium, which means there is no
further net motion of the charge. In other words, any
excess charges in the object can be treated as
stationary. Many experiments have confirmed the
existence of electrostatic equilibrium and the properties of conductors described below.


What are some properties of a charged conductor in electrostatic equilibrium? First, it
creates an electric field that, just outside the conductor’s surface, is perpendicular to the
surface. You see this in the diagram for Concept 1, which shows a solid conducting
sphere.


Why is the field perpendicular? This question can be answered by asking another
question: What if the field were not perpendicular? If there were a component of the
field parallel to the surface, this would mean a charged particle there would have a force
exerted on it by the field, parallel to the surface. This force would cause the particle to
move along the surface, and this motion would contradict the assumption of
electrostatic equilibrium.


Second, all excess charge resides on the surface of a conducting object. One way to
explain this property is to note that such a configuration allows like charges to maximize
their distance from one another. If a charge remained in the middle of the conductor, it
would not be maximizing its distance from its fellows.


Third, there is no electric field within the bulk ofa conductor. Again, consider the
conducting sphere in Concept 1. If there were a field within the sphere, it would cause
movement of excess charges there, meaning the sphere would not be in electrostatic equilibrium.


In Concept 2, we show what occurs when a hollow conductor is placed in an external field. In this case, two charged plates cause the field. If
the sphere were not present, the electric field would be uniform, and a field diagram would represent the field with equally spaced, horizontal
lines.


When the sphere is placed in the field caused by the plates, an asymmetrical distribution of charge is induced on the outside of the sphere. You
can see the charge distribution and how the sphere alters the external field in the diagram.


There is another important point illustrated in the diagram: There is no field within the hollow conductor. This effect is quite useful because it
means a conducting shell will insulate its contents from electric fields. When you see electric or electronic circuits placed within a protective
metal box, one reason is to shield them from nearby electric fields that might distort the circuitry’s operation. The shielding effect occurs only


Charge resides on the surface of the inner metallic sphere in this toy.
An electric field extends radially outward, made visible by ionized gas.

Electrostatic equilibrium


Excess charge moves to surface
Electric field perpendicular to surface
No field inside material of conductor

Copyright 2007 Kinetic Books Co. Chapter 23^427

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