Chapter%2019_Current%2C%20Resistance%2C%20and%20Direct-Current%20Circuits - 1 Q I t 19.1 Electric Current Definition the current is the rate at which


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2 19.1 Electric Current Definition : the current is the rate at which charge flows through this surface. When a net charge Q passes through a cross section of conductor during time t, the current is The SI units of current is the ampere (A) . 1 A = 1 C/s 1 A of current is equivalent to 1 C of charge passing through the area in a time interval of 1 s.
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3 Example: The amount of charge that passes through the filament of a certain light bulb in 2.00 s is 1.67 c. Find the current in the light bulb. Find no. of electrons?
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4 19.2 Resistance and Ohm s Law When a voltage (potential difference) is applied across the ends of a metallic conductor , the current is found to be proportional to the applied voltage . In situations where the proportionality is exact, one can write. The proportionality constant R is called resistance of the conductor .
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5 The resistance is defined as the ratio. In SI , resistance is expressed in volts per ampere. A special name is given: ohms Example: if a potential difference of 10 V applied across a conductor produces a 0.2 A current, then one concludes the conductors has a resistance of 10 V/0.2 A = 50 .
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6 Ohm s Law Resistance in a conductor arises because of collisions between electrons and fixed charges within the material. In many materials, including most metals, the resistance is constant over a wide range of applied voltages. This is a statement of Ohm’s law. Ohm’s Law
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7 Ohmic materials: the I-V curve for is linear . This device does obey Ohm’s law. None-Ohmic materials: the I-V curve for is nonlinear diode. This device does not obey Ohm’s law .
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8 Resistivity Electrons moving inside a conductor subject to an external potential constantly collide with atoms of the conductor. They lose energy and are repeated re-accelerated by the electric field produced by the external potential. The collision process is equivalent to an internal friction. This is the origin of a material’s resistance .
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9 The resistance R of an ohmic conductor is proportional to the its length, l , and inversely proportional to the cross section area, A , of the conductor . The constant of proportionality is called the resistivity of the material. Every material has a characteristic resistivity that depends on its electronic structure, and the temperature. Good conductors have low resistivity . Insulators have high resistivity .
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10 Resistivity - Units Resistance expressed in Ohms, Length in meter. Area are m 2 , Resistivity thus has units of  m.
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11 Resistivity of various materials at room temperature
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12 Example (a) Calculate the resistance per unit length of a nichrome wire of radius 0.321 mm. Cross section: Resistivity (Table): 1.5 x 10  m.
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  • Spring '17
  • hammed
  • Resistor, Electric charge, Electrical resistance, Electrical impedance

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