1. Chp. 19 Current and Resistance

2. Current: the rate of charge movement ELECTRIC CURRENT I = Δ Q 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

3. 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

4. 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

5. 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

6. Materials that have no resistance below a critical temperature are called superconductors. There are thousands today: aluminum, tin, lead and zinc Figure 19-9 and Table 19-3 pg. 706 *Copper, gold and silver do not exhibit superconductivity Superconductors

7. 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.

8. Electric power is the rate of conversion of electrical energy P = Δ PE = q Δ V = I Δ V = ( Δ V ) 2 Δ t Δ t 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

9. 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. Δ V= 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