Chp. 19 Current and Resistance

Current: the rate of charge movement

ELECTRIC CURRENT

I = ΔQ Δ t

electric current = charge passing through a given area time interval

I = Current in Amperes ( A= C/s) Q = charge (C)

t =time (sec)

* Example page 695: solving for time

Conventional Current: defined in terms of positive charge movement : positive to negative ( Table 19.1)

In reality: negative to positive

Sources of Current: Batteries and generators supply energy to charge carriers.Converting other forms of energy to electrical energy. Direct (dc) vs. Alternating (ac) Current:

In Direct the charges are moving in one direction while in Alternating the motion of the charges

continuously changes in the forward and reverse direction. (Fig. 19-5)

ac used in our homes dc used to transfer electricity Generators can produce both

The opposition to the flow of current in a conductor.The ratio of potential difference to current

R = ΔV I

R =resistance measured in ohms (Ω)V= potential difference (Volts- V)

I= Current (Amps -A)

Resistance

Ohm’s Law

Ohm’s Law does not hold for all materials

Resistance depends on length, cross-sectional area, material and temperature

Table 19-2 pg. 701

Resistors are used to control the amount of current in a conductor.

* Example 19B pg. 702

Materials that have no resistance below a critical temperature are called superconductors.

There are thousands today: aluminum, tin, lead and zincFigure 19-9 and Table 19-3 pg. 706

*Copper, gold and silver do not exhibit superconductivity

Superconductors

Meissner Effect: Causing a magnet to float in the air over a superconductor as it interacts between the current in the superconductor and the magnet.

Electric power is the rate of conversion of electrical energy

P = ΔPE = q ΔV = IΔV = (ΔV)2

Δt Δt R Electric power = current x potential difference

P = Power in watts (W) I = current in amps (A) V = voltage in Volts (V)

R = resistance (Ω)* Derivations pg. 709 Example 19C & 19 D pg 710 & 712

Electrical Power

Homework #3 Chp. 19 Bk (12)19A pg. 695 1,2,5

2. t=1 sec

19B pg. 703 1,2,4,5,6 1. I= .43 A

2. I= 1.8 A 4. ΔV= 110V

5. R= 46Ω 6. R= 220Ω I= .41 A I = .59A

19C pg. 710 1,3,4 4. I=6.25 A P= 312 W

19D pg. 712 1