Resistance of a wire=directly proportional to its length and to a property of the material called its resistivity (rho) and inversely proportional to its cross-sectional area. R= rho L/A
resistivity (rho):increases with temperature for metals, but may decrease for semiconductors
Power=energy transformed/time = QV/t = IV or P = I^2R = V^2/R
1 Watt = 1 J/s
1 kilowatt-hour (1 kWh) = 3.6 x 10^6 J
direct current (DC)= current is steady in one direction
alternating current = current reverses direction at a particular frequency, typically 60 Hz. Alternating currents are often sinusoidal in time, I = Io sin 2 pi ft and are produced by alternating voltage.
rms values of sinusoidal alternating current and voltage are given by
Irms = Io/sqrt2 and Vrms=Vo/sqrt2; where Io and Vo are the peak values
Summary of Units:
Current = 1 A = 1 C/s
Potential difference = 1 V = 1 J/C
Power = 1 W = 1 J/s
Resistance = 1 ohm = 1 V/A
Effect of electric currents on the human body:
Current(A) Effect
0.001 Can be felt
0.005 Is painful
0.010 Causes involuntary muscle spasms
0.015 Causes loss of muscle control
0.070 Goes through the heart, serious disruption, probably fatal if current lasts for more than 1 s
Problem Assignment:
The following problems should be worked to assist you in meeting the Chapter Objectives.
Problem 1 Page 551
Problem 5 Page 551
Problem 11 Page 551
Problem 27 Page 552
Problem 41 Page 553
Tutorial Links
Physics Tutorial from Prentice Hall
Peter Jeschofnig, Ph.D., Professor of Science, Colorado Mountain College
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This page was created by Peter Jeschofnig and was
last updated: 1/17/2002
