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Gravitational Potential Energy (a review)
Assuming a uniform local gravitational field of 9.80 m/s2, what is the
change in gravitational potential energy per kg of a 10-kg mass when
it moves from a height of 6.0 m to 13.5 m?
A.
735 J/kg
B.
73.5 J/kg
C.
–73.5 J/kg
D.
–735 J/kg
E. None of the above.
(What if it had moved the other way?)
4/24/17
Oregon State University PH 213, Class #10
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Electric Potential Energy
We can store gravitational potential energy (UG) in a mass by moving
it against the force exerted by a uniform gravitational field…
UG = U0 + (mg)h
(where U0 is the potential energy value we assign at h = 0).
Likewise, we can store electric potential energy (UE) in a charge by
moving it against the force exerted by a uniform electrical field…
UE = U0 + (qE)d
(where U0 is the potential energy value we assign at d = 0).
For either object, if we then release it to move freely in response to
the field’s force, its potential energy is reduced; it “falls.”
4/24/17
Oregon State University PH 213, Class #10
2
Electric Potential Energy (UE)
and Electric Potential (“voltage”) (V)
In the study and harnessing of electrical energy, it is convenient to
express the electrical potential energy on a per-unit-charge basis.
This is called the electric potential or voltage, and is denoted by V.
Electric potential is a field—a point-by-point description of space—
but it’s an energy field, not a force field.
If a charge q is at a point in space that has an electric potential value
V, then its electrical potential energy is given by UE = qV
The units of V are Joules/Coulomb (also known as Volts). Note that
we speak of electric potential—like potential energy—in terms of its
changes.
4/24/17
Oregon State University PH 213, Class #10
3
Two test charges, initially at very
distant locations, are brought one
at a time, into the vicinity of a
charge +Q. First, test charge +q
is brought to a point a distance r
from +Q. Then this charge is removed and test charge –q is brought
to the same point. In which case does the test charge have a greater
electric potential energy?
A.
B.
C.
D.
4/24/17
+q
–q
Same for both.
Not enough information.
Oregon State University PH 213, Class #10
4
When a Charge Moves without a Change in Voltage
Suppose two point charges (A and B) are nearby one another. Charge
A is fixed, but charge B can move. What path could charge B follow
so that its UE would not change?
A.
Diagonally away from charge A.
B. A circular path around charge A.
4/24/17
C.
Directly away from charge A.
D.
None of the above.
E.
It can’t be done.
Oregon State University PH 213, Class #10
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