8: Electric Current and Resistance
- Page ID
- 47054
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- 8.2: 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.
- 8.3: 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.
- 8.4: 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.
- 8.5: 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.