1. Electric Fields
-Electric Field: the region around a charged particle through which a force is exerted on another charged particle
-Test Charge: an idealized model of an object whose physical properties (usually mass, charge, or size) are assumed to be negligible except for the property being studied.
- Electric Field Line: the imaginary line along which a positive test charge would move in an electric field (from positive to negative)

2. Electric fields are vector quantities
The following features are common to all electric field diagrams:
Field lines always begin on positively charged objects and terminate on negatively charged objects.
Field lines never intersect each other.
Field lines are perpendicular to the surface of the charged object they meet.

3. Electric Field Lines

4. Parallel Plates
When the distance between the plates is much smaller than the size of the plates and the charge on the plates is uniformly distributed, the field lines are parallel to each other and perpendicular to the plates (except near the edges).
If a positive test charge is released between the plates, it will be pushed in a straight line from the positive to the negative plate.

5. Since the field between the plates is uniformly intense, the magnitude of the force exerted on a test charge is the same everywhere in the region between the plates (except near the edges).
Outside the plates, the net force is practically zero so no field lines appear there.

6. Electric field strength: the force on a stationary positive test charge per unit charge in an electric field
The electric field strength is also referred to as the electric field intensity.
The closer the electric field lines, the stronger the field strength and vice versa.
The electric field strength is zero everywhere inside the conducting material.

7. E = Fe/q E = kq/r2 Fe = electrostatic force (N) q = charge (C)
E = electric field strength (N/C)
The electric field strength due to a single charge is given by
E = kq/r2
- The intensity decreases inversely with the square of the distance from the point charge.

8. What is the magnitude of the electric field strength at a point in a field where an electron experiences a 1.0N force?
A positive test charge of 4.0x10-5C is placed in an electric field. The force acting on it is 0.60N. What is the electric field strength at the location of the test charge?
The electric field strength at a point is 4.0x104N/C. What is the force on a charge of 0.50x10-6C placed at that point?
What charge exists on a test charge that experiences a force of 1.4x10-8 N at a point where the electric field intensity is 2.0x10-4N/C?

9. Millikan’s Oil Drop Experiment

10. Electric Potential
Like the gravitational force, the electric force, can pull an object through a distance and therefore do work. It can also push an object and do work.
A charged object has potential energy due to its position in an electric field.
If an electric field does work on any charged object, electric potential energy decreases
In this case PE is converted to KE or heat

11. If work is done against an electric field, such as when a positive charge is moved closer to another positive charge, electric potential energy increases. This is similar to lifting an object on earth (doing work against the gravitational field).
Electric potential energy is larger when two like charges are closer together and larger when two unlike charges are further apart.

12. Electric potential: the total amount of work required to bring one Coulomb of positive charge from infinity to that point.
Electric potential is positive if work must be done against the field to move a positive test charge from infinity to that point.
Lines of equal electric potential are perpendicular to the electric field lines.

13. 1eV = 1.60x10-19J W =Vq = 1V * (1.6x10-19C) = 1.60x10-19 J = 1eV
Potential difference: the work done (or change in potential energy) per unit charge as a charged particle is moved between the points.
V=W/q
V=potential difference (J/C or volt)
W=Work (J), q = charge (C)
If an elementary charge is moved against an electric field through a potential difference of one volt, the amount of work or gain in potential energy is called the electronvolt (eV)
1eV = 1.60x10-19J
W =Vq = 1V * (1.6x10-19C) = 1.60x10-19 J = 1eV

14. Example:Moving a point charge of 3
Example:Moving a point charge of 3.2x10-19 C between points A and B in an electric field requires 4.8x10-18J of energy. What is the potential difference between these points?
Example: A 12V battery does 1200J of work transferring a charge. How much charge is transferred?
Example: Two parallel plates are 0.500m apart. The electric field strength between them is 6.00x103N/C. What is the potential difference between the plates? How much work is done moving a charge equal to that of one electron from one plate to another?
Example: A voltmeter measures the potential difference between two parallel plates to be 60.0V. The plates are 0.030m apart. What is the magnitude of the electric field intensity?

15. Potential Difference between Parallel plates
Given: E = F/q , V = W/q, W=F*d
If we solve for q (charge)
F/E = q
W/V = q
Set the two equations equal to each other
F/E =W/V
F/E = (F*d)/V divide by F
Therefore V=E*d we can rewrite this as E = V/d.

16. Millikan’s Oil Experiment
A potential difference was created between two charged plates. A charged drop of oil was dropped between these plates.
Drop was suspended when Fg=Fe.
The magnitudes of the forces were equal so we can set Eq=mg.
This will allow us to find the charge on the drop.
Eventually Millikan realized that the charge was always a multiple of 1.6x10-19C.

17. Example: An oil drop weighs 1. 9x10-14N
Example: An oil drop weighs 1.9x10-14N. It is suspended in an electric field of intensity 4.0x104N/C. What is the charge on the oil drop? If the drop is attracted toward the positive plate, how many excess electrons does it have?
Example: A negatively charged oil drop weighs 8.5x10-15N. The drop is suspended in an electric field intensity of 5.3x103N/C. What is the charge on the drop? How many excess electrons does it have?