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Electric Currents
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The Electric Battery
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Electric Cell
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Electric Current
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When a continuous conducting path is connected between the terminals of a battery, we have an electric circuit.
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When such a circuit is formed, charge can flow through the wires of the circuit, from one terminal of the battery to the other. A flow of charge such as this is called as electric current.
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Units of Current
1 Ampere (A) = 1 C /1 s
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Conceptual Example: How to Connect a Battery.
What’s wrong with each of circuits shown below?
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Conventional Current
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When we speak of the current flowing in a circuit, we mean the direction positive charge would flow.
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Important: Current is not a vector, it’s a scalar!!!!
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Important : In any single circuit, the current at any instant is the same at one point as at any other point!!! This follows from the conservation of electric charge.
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Ohm’s Law: Resistance and Resistors
I~V
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Definition of Resistance
I=V/R
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Ohm’s Law
The current through a metal conductor is proportional to the applied voltage.
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Resistivity
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A
LR
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Conceptual Example: Stretching Changes Resistance
A wire of resistance R is stretched uniformly until it is twice its original length. What happens to its resistance?
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Effect of Temperature
))(1( 00 TTT
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– temperature coefficient of resistivity is positive for metals and negative for semiconductors.
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Superconductivity
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1. When you stack three flashlight batteries in the same direction, you get a voltage of 3x1 ½ volts = 4 ½ volts. What voltage do you get if one of the batteries is turned to face in the opposite direction?
2. What is the difference between a bulb burning out and removing the bulb from its socket?
3. If the resistance connected to a battery is cut in half, what happens to the current through the battery?