1. Monday(02/08): Intro. to Ch.27, Electric Current, Resistance, and

Ohm's Law. HW: Read and study pages 831-40. Solve problems 2, 6, 7,

11, and 13 on page 852.

2. Tuesday(02/09): Electrical Energy and Power. HW: Read and Study

pages 841-47, then solve problems 15, 17, 23, and 29 on pages 852-3.

3. Wednesday(02/10): LAB on Resistivity of Nichrome. HW: Process lab

data. Lab Report is due Friday.

4. Thursday(02/11): Post-Lab Discussion. HW: Read and study pages

847-50, then solve problems 33, 37, 39, 41, and 47 on pages 853-4.

5. Friday(02/12): REVIEW I Chapter 27 - Current and Resistance.

HW: Finish all Review Handouts.

6. Monday(02/15): No School - Presidents Day. HW: Finish homework

assignments and study for test.

7. Tuesday(02/16): REVIEW II Chapter 27 - Current and Resistance.

HW: Finish all Review Handouts.

8. Wednesday(02/17): TEST on Ch.27 - Current and Resistance.

HW: Go to Website for notes and plans for Ch.28 - Direct Current (DC)

Circuits.

1. In electricity two fundamental concepts are current and voltage. For any

electrical element the voltage (V) across the element is the potential

difference between its two ends, while the current, I, (defined as I=dQ/dt)

through the element is the rate at which electrical charges are flowing.

2. For many devices (but not all) the voltage and the current are proportional

to each other, and we can write I = (1/R)∙V in which R is a constant of

proportionality known as the resistance.

3. The current in a conductor is related to the motion of charge carriers

through the equation I=nqv_dA, where n is the density of charge carriers, and

v_d, the drift velocity.

4. Current density, J, can be calculated three ways with J=I/A, J=nqv_d, and

J=σE. The quantity, σ, is referred to as conductivity.

5. We can also calculate resistance, R, with R = pL/A with p, rho, being the

resistivity. ( p = 1/σ)

6. Resistivity also has a temperature dependence which is given by the

equation, ρ=ρ_o[1 + α·(T -T_o)], with α being a temperature coefficient.

7. The equation, V = I·R is known as Ohm's Law, and devices which obey

Ohm's Law are known as linear or ohmic devices.

8. Familiar examples are resistors which are found in radios, TV sets,

computers, and other electronic systems; the filaments of light bulbs; and

 the heating elements of electrical ovens.

9. There are however other devices which do not obey Ohm's Law,

semiconductor devices such as transistors and diodes, and fluorescent

light bulbs. These are known as nonlinear devices.

10. Ohm's Law can be used to solve simple circuits. A complete circuit is

one which is a closed loop. It contains at least one source of voltage (thus

providing an increase in potential energy) and at least one potential drop

i.e., a place where potential energy decreases.

11. If a potential difference (voltage) is maintained across a resistor, the

power, can be calculated with P = V·I = I²·R = V²/R.

12. An increase of potential energy in a circuit causes a charge to move

from a lower to a higher potential (ie. voltage). Note the difference

between potential energy and potential.

13. Because of the electrostatic force, which tries to move a positive

charge from a higher to a lower potential, there must be another "force"

to move charge from a lower potential to a higher inside the battery.

14. This so-called force is called the electromotive force, or emf. The SI unit

for the emf is a volt (and thus this is not really a force, despite its name).

We will use a script E, the symbol , to represent the emf.

15. A decrease of potential energy can occur by various means. For example,

 heat lost in a circuit due to some electrical resistance could be one source

of energy drop.

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.

THE AP PHYSICS C ARCHIVES Ch.1: Physics Intro. Ch.2: Linear Motion. Ch.3: Vectors. Ch.4: 2-Dim Motion. Ch.5&6: Newton's Laws. Ch.7&8: Work&Energy. Ch.9: Momentum. Ch.10&11: Rotary Motion. Ch.12: Elasticity. Ch.13: Gravitation. Ch.15: SHM. Mechanics Review. Ch.23: Electric Fields. Ch.24: Gauss's Law. Ch.25: Electric Potential. Ch.26: Capacitance. Ch.27: Current/Resistance. Ch.28: DC Circuits.