1. Electric Fields & Potential Chapter 33
Physics

2. Gravitational vs Electrical Force Fields
Both are force fields; Electrical fields have 2 kinds of charges while gravitational fields have only 1;
Can shield yourself from electrical field but cannot from gravity field – no antigravity devices possible.
Electrical field strength ∝ size of charge while Gravitational field strength ∝ mass of objects.
Electrical fields impt in atomic interactions while gravity is impt on the macroscopic scale.

4. Electric Fields Some important facts:
An electric field is depicted by vector lines
Always in direction that a POSITIVE “test” charge would move
Indicate amt of force PER “test” charge
By definition, the lines are “LINES OF FORCE”
If you placed a 2nd positive charge (test charge), near the positive charge shown above, it would move AWAY.
If you placed that same charge near the negative charge shown above it would move TOWARDS.

5. Electric Fields
The equation for ELECTRIC FIELD is symbolic of the equation for WEIGHT just like Coulomb’s law is symbolic of Newton’s Law of Gravitation.
The “q” in the equation is that of a “test charge”.
The symbol for Electric Field is, “E”. And since it is defined as a force per unit charge, the unit is Newtons per Coulomb, N/C.
NOTE: equations above ONLY help determine MAGNITUDE of field or force.

6. Electric Field of a Conductor
Suppose you bring a conductor NEAR a charged object. The side closest to which ever charge will be INDUCED to the opposite charge.
Charge on conductors ONLY exists on surface. Can’t have an electric field inside (as e- constantly arrange charges to equilibrium).
Insulators, however, can store charge inside.
There must be a negative charge on this side OR this side was induced positive due to the other side being negative.
There must be a positive charge on this side

7. Electric Potential Energy
A charged object can have potential energy depending upon its location in the electric field.
Work (d x F) is required to push a charged particle through an electric field.
The electric energy of a charged particle is increased when work is done to push it farther thru the field.
Electric potential energy = the potential energy gained by work performed on the charge.

10. Electric Potential
ELEC POTENTIAL ENERGY ≠ ELECTRIC POTENTIAL
Electric potential energy (J) per charge (C) is called the electric potential, or
Electric potential = electric potential energy / charge
The SI unit for electric potential is the VOLT.
Since potential energy is measured in joules and charge in coulombs then,
1 volt = 1 joule / 1 coulomb OR 1 J/C
Voltage is electric potential
A 12 volt battery has a difference of 12 joules as a charge moves from one terminal to the other. (voltage)

11. Answer to #14 Twice as much force means twice the work. W= f d
Twice the work means twice the change in electric potential energy.
However, electric potential equals the electric potential energy divided by the charge, and 
Twice the electric potential energy divided by twice the charge gives the SAME potential. Lets’ prove this mathematically:

12. Answer to #15
If V = EPE / q where V = electric potential (v) EPE = electric potential energy (J), and q = charge of the object (C)
then V = 12 J / C = 12,000 volts

13. Answer to #16
If V = EPE / q where V = electric potential (v) EPE = electric potential energy (J), and q = charge of the object (C)
then V = 24 J / C = 12,000 volts

14. Capacitors A capacitor stores electrical energy.
When a capacitor is connected to a charging device, the charge is transferred into the capacitor.
It continues to charge until potential difference across capacitor = the potential difference of the charging device.
Energy stored in a capacitor comes from the work required to charge it.

15. Batteries vs Capacitors
Batteries use chemical means to make & store voltage
Capacitors use magnetic fields to do the same
Short comparison (just read it, don’t write it).
Ultracapacitors are another story.

16. Van der Graaf Generator
Device that produces high voltages (high electric potential is voltage)
Doesn’t hurt you because there is high voltage in a large resistance so charge doesn’t build up & current remains low
Is also a band….

17. Answer: C Assessment Questions An electric field has no direction.
only magnitude.
both magnitude and direction.
a uniformed strength throughout.
Answer: C

18. Assessment Questions
In the electric field surrounding a group of charged particles, field strength is greater where field lines are
thickest.
longest.
farthest apart.
closest.
Answer: D

19. Assessment Questions
Electrons on the surface of a conductor will arrange themselves such that the electric field
inside cancels to zero.
follows the inverse-square law.
tends toward a state of minimum energy.
is shielded from external charges.
Answer: A

20. Assessment Questions Answer: C
The potential energy of a compressed spring and the potential energy of a charged object both depend
only on the work done on them.
only on their locations in their respective fields.
on their locations in their respective fields and on the work done on them.
on their kinetic energies exceeding their potential energies.
Answer: C

21. Assessment Questions Answer: B
Electric potential is related to electrical potential energy as
the two terms are different names for the same concept.
electric potential is the ratio of electrical potential energy per charge.
both are measured using the units of coulomb.
both are measured using only the units of joules.
Answer: B

22. Assessment Questions A capacitor cannot store charge.
cannot store energy.
can only store energy.
can store energy and charge.
Answer: D

23. Assessment Questions Answer: D
What happens to the electric field inside the conducting sphere of a Van de Graaff generator as it charges?
The field increases in magnitude as the amount of charge increases.
The field decreases in magnitude as the amount of charge increases.
The field will have a net force of one.
Nothing; the field is always zero.
Answer: D