1. Electrical Energy and Current
Chapter 17

2. Electrical Potential Energy
Results from the interaction of two objects’ charges
Electrical potential energy can be associated with a charge in a uniform field
A uniform field is a field that has the same value and direction at all points

3. Assume the charge is displaced at a constant velocity in the same direction as the electric field
There is a change in the electrical potential energy associated with the charge’s new position in the electric field
The change in the Peelectric depends on the charge, q, the strength of the electric field, E, and the displacement, d
Peelectric = -qEd

4. The negative sign indicates that the electrical potential energy will increase if the charge is negative and decrease if the charge is positive
It is the difference in electrical potential energy that is physically important
The SI unit for electrical potential energy is the joule (J)
This equation is valid only for a uniform electric field, such as that between two oppositely charged parallel plates
The electric field of a point charge is an example of a nonuniform electric field

5. Potential Difference
Electric potential is defined as the electrical potential associated with a charged particle in an electric field divided by the charge of the particle
V = Peelectric / q
Potential difference is a change in electric potential
∆V = Peelectric / q

6. Potential difference is a measure of the difference in the electrical potential energy between two positions in space divided by the charge
The SI unit for potential difference and for electric potential is the volt (V) and is equivalent to one joule per coulomb
Potential Difference in a Uniform Electric Field ∆V = -Ed
Remember that d is the displacement parallel to the field and that motion perpendicular to the field does not change the electrical potential energy

7. Potential difference between a point at infinity and a point near a point charge
∆V = K (q/r)
Potential difference = Coulomb constant x value of the point charge / distance to the point charge
The superposition principle can be used to calculate the electric potential for a group of charges
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8. Capacitance
A capacitor is a device that is used to store electrical potential energy
Capacitance is defined as the ratio of the net charge on each plate to the potential difference created by the separated charges
C = Q / ∆V
Q is the magnitude of charge on each plate

9. The material between a capacitor’s plates can change its capacitance
Discharging a capacitor releases its charge
A parallel-plate capacitor is often used in keyboards
Because the area of the plates and the distance between the plates can be controlled, the capacitance, and thus the electric field strength, can also be easily controlled

10. Energy and Capacitors
A charged capacitor stores electrical potential energy because it requires work to move charges through a circuit to the opposite plates of a capacitor
The work done on these charges is a measure of the transfer of energy

11. Electrical Potential Energy Stored in a Charged Capacitor
Peelectric = ½ Q ∆V
Q = charge on the plate, ∆V = final potential difference
By substituting capacitance
Pe = ½ C (∆V)2
Pe = Q2 / 2C

12. Current Current is the rate of charge movement
Exists whenever there is a net movement of electric charge through a medium
Electric current is the rate at which charges move through the cross section of the wire.
The direction of current is opposite the movement of the negative charges (p. 608)
I = Q / t
Electric current = charge / time
Unit for current is the ampere (Amp or A)

13. Resistance
The opposition to the motion of charge through a conductor is the conductor’s resistance
R = ∆V / I
Resistance = potential difference / current
SI unit is the ohm (Ω)

14. Ohm’s Law
For many materials including most metals the resistance is constant over a wide range of applied potential differences
∆V / I = constant
Common practice to express Ohm’s law as ∆V = IR

15. Ohm’s Law is not a fundamental law of nature
It is a behavior that is valid only for certain materials
Materials that have a constant resistance over a wide range of potential difference are said to be ohmic
Materials that do not function according to Ohm’s law are said to be nonohmic
Ex: diode
Factors that affect resistance
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