Download Conceptual Reading: Electric Fields, Electric Potential Energy, and

Static Charges 3: Electric Fields, Electric
Potential Energy, and Electric Potential
(Voltage)
Conceptual Physics, Hewitt
Introduction to Chapter 33, Electric Fields and Potential
1.
Based on the introduction, describe what a field is in your own
words. What examples of fields are given here?
33.1 Electric Fields
2.
If you throw an object into the air, does it come back down again
by interacting directly with the earth’s mass? Explain.
3.
How much of the space around a charge is occupied by the charge’s
electric field? What do stronger electric fields potentially do?
4.
An electric field has both strength (magnitude) and direction.
In other words, electric fields are
vector quantities. How is the
direction of the electric field
determined?
-
+
Draw arrows on the diagrams shown to represent the direction of the
electric fields around the positive and negative charges.
Section 33.4 Electrical Potential Energy
Review Concept: Doing work is the process of transferring energy.
The amount of work done determines the amount of energy
transferred.
5.
What determines how much gravitational potential energy is stored
when an object is elevated near Earth’s surface?
6.
How is an electric potential energy transferred to a pair of
charges? What must be done to the charges?
7.
In the situations below, draw an arrow to indicate the direction
in which the shaded charge must be pushed in order to increase its
electric potential energy stored in the charge pair:
+
8.
+
+
-
Under what circumstances will electric potential energy transform
into kinetic energy? (This, by the way, is the beginning of
understanding how batteries work.)
Section 33.5 Electric Potential
My note: it can be confusing to have two important concepts with
such similar names: electric potential energy and electric
potential. Don’t blame me for this. I will always refer to the
concept introduced in this section—electric potential—by its other
name: voltage. Of course, it doesn’t help that voltage is also
often referred to as electromotive force, or EMF. Who’s in charge
of this operation, anyway?
Review Concept: In physics, doing work is the process of
transferring energy. The amount of work done determines
the amount of energy transferred.
Section 33.4 Electrical Potential Energy
1. What determines how much gravitational potential energy an object
has when it is elevated?
2.
How is electric potential energy transferred to a charge? Where
does the charge have to be? What must be done to it to increase
the electric potential energy the charges store?
3.
In the situations below, draw an arrow to indicate the direction
in which the shaded charge must be pushed in order to increase the
electric potential energy stored in the charge pair:
+
4.
+
+
-
Under what circumstances will a charge’s electric potential
energy transform into kinetic energy? (This, by the way, is the
beginning of understanding how batteries work.)
Section 33.5 Electric Potential
5. The idea of voltage (electric potential) can be understood
visually. If the charge pairs below on the left store a certain
amount of electric potential energy, then adding a negative charge
would double the energy stored.
Three charges would result in three
times as much total stored energy, and so on.
+
+
-
+
-
-
Adding charge to an electric field is equivalent to adding mass to a
gravitational field—both processes add to the total potential energy
stored.
However, voltage is the total amount of electrical potential divided
by the total charge (the units end up being Joules per Coulomb, which
we call a VOLT). In each of the three situations above, the voltage
is the same, because the voltage is the amount of energy per charge,
not the total amount of energy present.
discuss batteries.
More on this later, when we
So, to summarize, electrical potential energy is the TOTAL energy
stored by all of the charges in the system, while voltage is the
energy stored PER CHARGE.
6.
What does the text say is the reason why the electric potential
(voltage) is the same in a given location, no matter how much
charge is present in that location?
OK.
Ready for this?
Let’s start with a positive charge.
+
Next, let’s place a smaller negative charge next to the
positive charge. Keep in mind that the SIZE of the charge does not
matter here, only the amount of charge present. The positive and
negative particles each carry the SAME amount of charge here, even
though they are different sizes.
+
-
This is very similar to mass at rest on the surface of the
earth. Now let’s move the negative charge away from the positive
charge. This requires applying a force along a distance, otherwise
known as doing work. When we do work on the negative charge, the
charge pair gains electric potential energy.
+
-
Force
The location of the charges with respect to
each other stores a certain amount of electric potential energy. The
total amount of electric potential energy depends on the strength of
the charges, and on the distance between them.
However, we do not even need to have the negative charge present to
talk about the electric potential (voltage) associated with the
particular location where it was placed. Talking about the voltage
is like saying, “every Coulomb of charge in this location will have X
Joules of potential energy.” But only one charge has to be present
for a voltage to exist.
This is why high voltages are not necessarily dangerous. The Van de
Graff generator we use in class charged to a very high voltage
(probably thousands of volts), and the voltage increases as you
approach the sphere. But the sphere doesn’t hold enough charge (only
a very small fraction of a Coulomb) to result in large amounts of
electric potential energy.
This is a bit like saying there is the POSSIBILITY of a large amount
of gravitational potential energy at the top of a hill. But if I
don’t park my car there, no gravitational potential energy is stored
yet. It might be in the future, but for now, there is only the
POTENTIAL for gravitational potential energy at the top of the hill,
and no ACTUAL energy is stored until a large mass is placed there.
7.
Here are four concepts we have encountered so far. Do your best
to explain each in your own words. Use drawings. Please use a
separate sheet.
a. Electric Field
b. Electrostatic Force
c. Electric Potential (voltage)
d. Electric Potential Energy