Allesandro Volta (1745-1827)

Equipotential Lines are Perpendicular to  Electric Field Lines.

LNK2LRN™ 2010/11         Ch.19 - Electric Potential.      AP Physics B      January 27th to February 3rd.

Plans for the Week and Assignments:

1. Thursday(01/27): Intro. to Ch.19 - Electric Potential.

Potential Difference and Potential Energy Due to Point Charges.

 HW: Read and Study pages 549-54, then solve problems 4, 6, 8, 14,

and 15 on page 572.

2. Friday(01/28): Study of Capacitors and Dielectrics. HW: Read

and Study pages 555-57, then solve problems 16, 17, 19, and 23 on

pages 572-3.

3. Monday(01/31): Equipotential Surfaces and the Electric

Field. HW: Read and Study pages 558-59, then solve problems 27, 28,

29, and 34 on page 573.

4. Tuesday(02/01): Charged Particles Accelerated by a Voltage,

and Capacitance. HW: Read and Study pages 560-64, then solve

problems 36, 37, 38, 39, and 40 on page 574.

5. Wednesday(02/02): REVIEW Ch.19 - Electric Potential.

HW:  Complete All  Review Handouts.

6. Thursday(02/03): TEST on Ch.19. HW: Go to website and study

notes for Ch.20 - Electric Circuits.

Very Important: If you have any questions or miss a class, see

me before school (8:00 - 8:30 AM), during Lunch, or after school.

Best to send an email to rpersin@fau.edu.

Strange But True, Electrical Sensitivity in Fish: As hard as it may be to

be believed, sharks can sense an electrical differential of one billionth

of a volt. This is the equivalent of sensing the electrical current running

between two flashlight batteries set on the ocean floor 2,000 miles apart!

1. We know that an electric field is a region in space where an electric

force is exerted on a charge. Therefore, since a charge in an electric field

experiences a force, work is done by the field on the charge when the

charge moves a distance, d, along a field line.

2. Electric lines of force represent the direction that a positive test charge

would move in an electric field. By convention, they originate at positively

charged objects and terminate at negatively charged objects.

3. Work is done by the electric field if the electric force acting on the charge

causes it to move from one point to another, say A to B. In terms of an

equation, W_AB = EPE_A - EPE_B. We then say that these two points differ in

their Electric Potential Energy, EPE, or U. So W_AB = U_A - U_B.

4. The magnitude of the work done on the charge by the electric field is a

measure of this difference in potential. The electric potential difference (V),

which we call "voltage", is the work done per unit charge as a charge is

moved between two points in an electric field. Simply stated, W = q·V .

5. The Volt (V) is the unit used to measure electric potential difference.

Since a Volt measures work done per unit charge, 1 Volt = 1 Joule/Coulomb.

Other equivalent expressions for a Volt exist. For example V = U/q_o.

6. The electric potential difference between two points, A and B, in an

electric field is V_B - V_A = U_A/q_o - U_B/q_o = -W_AB/q_o.

7. As charge moves from one point to another in an electric circuit, energy

is released. We calculate this energy using the unit "electron-volt", where

1 eV = 1.60x10^-19 J. When energy is released, this results in a decrease

in electric potential. The decrease in electric potential implies that there

is an electric potential difference between the two points. This term,

"electric potential difference" is what we refer to as "voltage."

8. The potential of the earth is arbitrarily said to be zero. An object

connected directly to the ground can be described as being "grounded".

(The original expression was "earthed".) A ground may be a common

plane of zero voltage compared with the rest of the circuit.

9. The potential at any point in an electric field can be either positive or

negative with respect to the earth, depending on the nature of the charge.

The change in electric potential over this distance is defined through the

work done by this force, where potential is shorthand for change in electric

potential, or potential difference.

10. This is analogous to the definition of the gravitational potential energy

through the work done by the force of gravity in moving a mass through a

certain distance.

11. The units of potential difference, or simply potential, are Joules/Coulomb,

which are called Volts (V). Physically, potential difference has to do with how

much work the electric field does in moving a charge from one place to another.

12. Batteries, for example, are rated by the potential difference across their

terminals. In a nine volt battery the potential difference between the positive

and negative terminals is precisely nine volts. On the other hand the potential

difference across the power outlet in the wall of your home is 110 volts.

13. Consider a charge placed in an electric field. Let us chose some arbitrary

reference point in the field: at this point the electric potential energy of the

charge is defined to be zero.

14. This uniquely defines the electric potential energy of the charge at every

other point in the field. For instance, the electric potential energy at some

point is simply the work done in moving the charge from one point to another

along any path, and can be calculated using V = Ed or V = kΣq_i/r_i .

15. It is clear that potential depends on both the particular charge which we

place in the field and the magnitude and direction of the electric field along

some arbitrary route between points A and B. However, it is also clear that

it is directly proportional to the magnitude of the charge.

16. The total energy of a system is the sum of all the forms of energy in

that system. K_i + qV_i = K_f + qV_f. Here K = ½mv² and U = qV .

17. A capacitor is a device that stores charge. Capacitors are formed by a

pair of conductors separated by an insulator. They are found in computer

keyboards, automobile ignition systems, and flash cameras, for example.

18. The type of capacitor we are most interested in will have a charge Q and

-Q on each conductor. There will also be a resultant voltage, V, between the

two conductors.

19. This voltage is linearly dependent on the charge. If we triple the charge,

we triple the voltage. Because of this relationship, the ratio of Q / V is a

constant for that capacitor.

20. The value of Q/V for a given capacitor is known as its capacitance. This

gives the simple equation, C = Q/V .

21. The unit of capacitance is the Farad, named after Michael Faraday

(1791-1867). It is equivalent to one coulomb per volt.

22. One Farad is an extremely large capacitance; most capacitors come in

units of micro (μ), nano (n), or pico (p) farads.

23. The capacitance of a capacitor is determined by two factors: (i) the

geometry of the capacitor, and (ii) the material between the conductors.

This material is known as a dielectric.

24. In a parallel plate capacitor, capacitance can be calculated by using the

equation, C = ε_oA/d , where C is capacitance, ε_o is the permittivity of free

space, ε_o = 8.85x10^-12 C²/Nm², A is the area of a plate, and d is the

distance between the plates.

25. The phenomena of capacitance is a type of electrical energy storage

in the form of a field in an enclosed space. The energy (ability to do work, W)

stored between the plates is E = ½CV². A much more useful measure is

that of "energy density", which is given by the equation u = ½ε_oE².

26. And still, we need these steps to solve any problem in Physics:

(i) read the problem and identify the given variables

(ii) determine what you are asked to solve for

(iii) find the correct equation to use

(iv) use Algebra, Trigonometry, and/or Calculus to isolate the unknown

(v) substitute-in the given information and simplify.

View the Ch. 19 Slides.

THE AP PHYSICS B ARCHIVES Ch.1: Physics Intro. Ch.2: Linear Motion. Ch.3: 2-Dim Motion. Ch.4&5: Newton's Laws. Ch.6: Work/Energy. Ch.7: Momentum. Ch.8&9: Rotary Motion. Ch.10: SHM. Ch.11: Fluids. Ch.12&13: Temp.&Heat. Ch.14&15: Thermodynamics. Semester Review. Ch.16: Waves & Sound. Ch.17: Wave Interference. Ch.18: Electric Fields.