College Physics

# Introduction to Electric Current, Resistance, and Ohm's Law

College PhysicsIntroduction to Electric Current, Resistance, and Ohm's Law

Figure 20.1 Electric energy in massive quantities is transmitted from this hydroelectric facility, the Srisailam power station located along the Krishna River in India, by the movement of charge—that is, by electric current. (credit: Chintohere, Wikimedia Commons)

### Chapter Outline

20.1 Current
• Define electric current, ampere, and drift velocity
• Describe the direction of charge flow in conventional current.
• Use drift velocity to calculate current and vice versa.
20.2 Ohm’s Law: Resistance and Simple Circuits
• Explain the origin of Ohm’s law.
• Calculate voltages, currents, or resistances with Ohm’s law.
• Explain what an ohmic material is.
• Describe a simple circuit.
20.3 Resistance and Resistivity
• Explain the concept of resistivity.
• Use resistivity to calculate the resistance of specified configurations of material.
• Use the thermal coefficient of resistivity to calculate the change of resistance with temperature.
20.4 Electric Power and Energy
• Calculate the power dissipated by a resistor and power supplied by a power supply.
• Calculate the cost of electricity under various circumstances.
20.5 Alternating Current versus Direct Current
• Explain the differences and similarities between AC and DC current.
• Calculate rms voltage, current, and average power.
• Explain why AC current is used for power transmission.
20.6 Electric Hazards and the Human Body
• Define thermal hazard, shock hazard, and short circuit.
• Explain what effects various levels of current have on the human body.
20.7 Nerve Conduction–Electrocardiograms
• Explain the process by which electric signals are transmitted along a neuron.
• Explain the effects myelin sheaths have on signal propagation.
• Explain what the features of an ECG signal indicate.

The flicker of numbers on a handheld calculator, nerve impulses carrying signals of vision to the brain, an ultrasound device sending a signal to a computer screen, the brain sending a message for a baby to twitch its toes, an electric train pulling its load over a mountain pass, a hydroelectric plant sending energy to metropolitan and rural users—these and many other examples of electricity involve electric current, the movement of charge. Humankind has indeed harnessed electricity, the basis of technology, to improve our quality of life. Whereas the previous two chapters concentrated on static electricity and the fundamental force underlying its behavior, the next few chapters will be devoted to electric and magnetic phenomena involving current. In addition to exploring applications of electricity, we shall gain new insights into nature—in particular, the fact that all magnetism results from electric current.

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