Transcript Chp. 19 Current and Resistance

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
Resistance
The opposition to the flow of current in a conductor.
The ratio of potential difference to current
R = ΔV
I
Ohm’s Law
R =resistance measured in ohms (Ω)
V= potential difference (Volts- V)
I= Current (Amps -A)
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
Superconductors
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
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.
Electrical Power
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
* Derivations pg. 709
P = Power in watts (W)
I = current in amps (A)
V = voltage in Volts (V)
R = resistance (Ω)
Example 19C & 19 D pg 710 & 712
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
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