1. 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 J/kg
B 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

2. 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

3. 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

4. Oregon State University PH 213, Class #10
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. +q B. –q C. Same for both. D. Not enough information.
4/24/17
Oregon State University PH 213, Class #10

5. 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.
C. Directly away from charge A.
D. None of the above.
E. It can’t be done.
4/24/17
Oregon State University PH 213, Class #10