Electric Currents

The Electric Battery

Electric Cell

Electric Current

When a continuous conducting path is connected between the terminals of a battery, we have an electric circuit.

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

Conceptual Example: How to Connect a Battery.

What’s wrong with each of circuits shown below?

Conventional Current

When we speak of the current flowing in a circuit, we mean the direction positive charge would flow.

Important: Current is not a vector, it’s a scalar!!!!

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.

Ohm’s Law: Resistance and Resistors

I~V

Definition of Resistance

I=V/R

Ohm’s Law

The current through a metal conductor is proportional to the applied voltage.

Resistivity

A

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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?

Effect of Temperature

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– temperature coefficient of resistivity is positive for metals and negative for semiconductors.

Superconductivity

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?