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# 20: Electric Current, Resistance, and Ohm's Law

• 20.0: Prelude to Electric Current, Resistance, and Ohm's Law
• 20.1: Current
Electric current is defined to be the rate at which charge flows. A large current, such as that used to start a truck engine, moves a large amount of charge in a small time, whereas a small current, such as that used to operate a hand-held calculator, moves a small amount of charge over a long period of time.
• 20.2: Ohm’s Law - Resistance and Simple Circuits
What drives current? We can think of various devices—such as batteries, generators, wall outlets, and so on—which are necessary to maintain a current. All such devices create a potential difference and are loosely referred to as voltage sources. When a voltage source is connected to a conductor, it applies a potential difference V that creates an electric field. The electric field in turn exerts force on charges, causing current.
• 20.3: Resistance and Resistivity
The resistance of an object depends on its shape and the material of which it is composed. For a given shape, the resistance depends on the material of which the object is composed. Different materials offer different resistance to the flow of charge. We define the resistivity ρ of a substance so that the resistance R of an object is directly proportional to ρ.
• 20.4: 20.4 Electric Power and Energy
Electric energy depends on both the voltage involved and the charge moved.  Electric power ( P ) is simply the product of current times voltage. Power has familiar units of watts. Since the SI unit for potential energy (PE) is the joule, power has units of joules per second, or watts.
• 20.5: Alternating Current versus Direct Current
Direct current (DC) is the flow of electric charge in only one direction. It is the steady state of a constant-voltage circuit. Most well-known applications, however, use a time-varying voltage source. Alternating current (AC) is the flow of electric charge that periodically reverses direction. If the source varies periodically, particularly sinusoidally, the circuit is known as an alternating current circuit.
• 20.6: Electric Hazards and the Human Body
There are two known hazards of electricity—thermal and shock. A thermal hazard is one where excessive electric power causes undesired thermal effects, such as starting a fire in the wall of a house. A shock hazard occurs when electric current passes through a person. Shocks range in severity from painful, but otherwise harmless, to heart-stopping lethality. This section considers these hazards and the various factors affecting them in a quantitative manner.
• 20.7: Nerve Conduction–Electrocardiograms
Electric potentials in neurons and other cells are created by ionic concentration differences across semipermeable membranes. Stimuli change the permeability and create action potentials that propagate along neurons. Myelin sheaths speed this process and reduce the needed energy input. This process in the heart can be measured with an electrocardiogram (ECG).
• 20.E: Heat and Heat Transfer Methods (Exercise)

Thumbnail: Electrons flowing in a metal according to Drude model with electrons (shown here in blue) constantly bouncing among heavier, stationary crystal ions (shown in red). (CC-BY-SA 3.0, Rafaelgarcia).